Patent Publication Number: US-2020280101-A1

Title: Storage battery, battery control unit, and electronic device

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an object, a method, or a manufacturing method. The present invention relates to a process, a machine, manufacture, or a composition of matter. In particular, one embodiment of the present invention relates to a semiconductor device, a display device, a light-emitting device, a power storage device, a storage device, a driving method thereof, or a manufacturing method thereof. In particular, one embodiment of the present invention relates to a power storage device and a manufacturing method thereof. 
     Note that a power storage device in this specification refers to every element and/or device having a function of storing electric power. 
     2. Description of the Related Art 
     In recent years, a variety of power storage devices, for example, secondary batteries such as lithium-ion secondary batteries, lithium-ion capacitors, and air cells have been actively developed. In particular, demand for lithium-ion secondary batteries with a high output and a high energy density has rapidly grown with the development of the semiconductor industry, for electronic devices, for example, portable information terminals such as mobile phones, smartphones, and laptop computers, portable music players, and digital cameras; medical equipment; next-generation clean energy vehicles such as hybrid electric vehicles (HEVs), electric vehicles (EVs), and plug-in hybrid electric vehicles (PHEVs); and the like. The lithium-ion secondary batteries are essential as rechargeable energy supply sources for today&#39;s information society. 
     In a storage battery that utilizes the reaction of carrier ions, such as a lithium-ion battery, the volume of an active material might be changed by charge and discharge process. For example, it is known that the interlayer distance of graphite increases from 0.336 nm to 0.370 nm as disclosed in Non-patent Document 1 (see Non-Patent Document 1, pp. 333-334). 
     As disclosed in Patent Document 1, for example, the shape or volume of an alloy-based material such as silicon might be changed by repeated charge and discharge cycles. 
     REFERENCE 
     
         
         [Patent Document 1] Japanese Published Patent Application No. 2013-214501 
         [Non-Patent Document 1] Masaki Yoshio et al., “Lithium-Ion Batteries Science and Technologies”, Springer, chapter 16, pp 333-334 
       
    
     SUMMARY OF THE INVENTION 
     An object of one embodiment of the present invention is to provide a novel electrode. Another object of one embodiment of the present invention is to provide a novel power storage device. 
     Another object of one embodiment of the present invention is to provide a power storage device in which a decrease in capacity caused by charge and discharge cycles is small. Another object of one embodiment of the present invention is to provide a long-life power storage device. Another object of one embodiment of the present invention is to provide a highly reliable power storage device. 
     Another object of one embodiment of the present invention is to provide an electrode with a high capacity. Another object of one embodiment of the present invention is to provide a power storage device with high energy density. Another object of one embodiment of the present invention is to provide a power storage device whose characteristics suffer little degradation when external force is repeatedly applied to the power storage device. 
     Note that the description of these objects does not disturb the existence of other objects. In one embodiment of the present invention, there is no need to achieve all the objects. Other objects will be apparent from and can be derived from the description of the specification, the drawings, the claims, and the like. 
     One embodiment of the present invention is a storage battery including a positive electrode, a negative electrode, and an electrolytic solution. The negative electrode includes a first element and carbon. The first element is any of silicon, tin, gallium, aluminum, germanium, lead, antimony, bismuth, silver, zinc, cadmium, and indium. The negative electrode includes an active material and a first layer in contact with a surface of the active material. The first layer includes a portion with a thickness larger than or equal to 10 nm and smaller than or equal to 1000 nm. The electrolytic solution contains a first cation and a second cation. The first cation is one or more of a lithium ion, a sodium ion, a calcium ion, and a magnesium ion. The second cation is an imidazolium cation or a tertiary sulfonium cation. 
     Another embodiment of the present invention is a storage battery including a positive electrode, a negative electrode, and an electrolytic solution. The negative electrode includes a first element and carbon. The first element is any of silicon, tin, gallium, aluminum, germanium, lead, antimony, bismuth, silver, zinc, cadmium, and indium. The negative electrode includes a first region, a second region in contact with a surface of the first region, and a third region in contact with a surface of the second region. The second region and the third region each have a shape of a layer. The thickness of the second region is larger than or equal to 10 nm and smaller than or equal to 500 nm. The thickness of the third region is larger than or equal to 10 nm and smaller than or equal to 1000 nm. The atomic ratio of carbon to the first element in the first region is x 1 :y 1 . The atomic ratio of carbon to the first element in the second region is x 2 :y 2 . The atomic ratio of carbon to the first element in the third region is x 3 :y 3 . x 1 /y 1  is smaller than or equal to 3. x 2 /y 2  is larger than or equal to 0.1 and smaller than 10. x 3 /y 3  is larger than or equal to 5. The electrolytic solution contains a first cation and a second cation. The first cation is one or more of a lithium ion, a sodium ion, a calcium ion, and a magnesium ion. The second cation is an imidazolium cation or a tertiary sulfonium cation. 
     In the above structure, an aromatic cation, an aliphatic onium cation, or the like can be used as the second cation. Examples of the aromatic cation include a pyridinium cation and an imidazolium cation. Examples of the aliphatic onium cation include a quaternary ammonium cation, a tertiary sulfonium cation, and a quaternary phosphonium cation. Furthermore, in the above structure, any of the cations represented by Chemical Formulas (1) to (17) described below is preferably used as the second cation. 
     In the above structure, as an anion, a monovalent amide-based anion, a monovalent methide-based anion, a fluorosulfonate anion, a perfluoroalkylsulfonate anion, a tetrafluoroborate anion, a perfluoroalkylborate anion, a hexafluorophosphate anion, a perfluoroalkylphosphate anion, or the like can be used. 
     Another embodiment of the present invention is an electronic device including the storage battery described in any of the above. The electronic device preferably includes a display device. It is preferred that the electronic device include an input-output terminal and the input-output terminal have a function of performing wireless communication. 
     Another embodiment of the present invention is a battery control unit including the storage battery described in any of the above. 
     One embodiment of the present invention can provide a novel electrode. Another embodiment of the present invention can provide a novel power storage device. 
     Another embodiment of the present invention can provide a power storage device in which a decrease in capacity with an increasing number of charge and discharge cycles is small. Another embodiment of the present invention can provide a long-life power storage device. Another embodiment of the present invention can provide a highly reliable power storage device. 
     Another embodiment of the present invention can provide an electrode with a high capacity. Another embodiment of the present invention can provide a power storage device with high energy density. Another embodiment of the present invention can provide a power storage device whose characteristics suffer little degradation when external force is repeatedly applied to the power storage device. 
     Note that the descriptions of these effects do not disturb the existence of other effects. One embodiment of the present invention does not necessarily have all the effects listed above. Other effects will be apparent from and can be derived from the descriptions of the specification, the drawings, the claims, and the like. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the accompanying drawings: 
         FIGS. 1A to 1E  each illustrate constituents of an electrolytic solution; 
         FIGS. 2A and 2B  each illustrate a particle of one embodiment of the present invention; 
         FIGS. 3A and 3B  are diagrams illustrating part of a cross section of an electrode; 
         FIG. 4  illustrates a storage battery; 
         FIGS. 5A and 5B  are each a cross-sectional view of a storage battery; 
         FIGS. 6A and 6B  illustrate a method for fabricating a storage battery; 
         FIGS. 7A and 7B  illustrate a method for fabricating a storage battery; 
         FIG. 8  illustrates a storage battery; 
         FIGS. 9A and 9B  illustrate a method for fabricating a storage battery; 
         FIGS. 10A and 10B  illustrate a method for fabricating a storage battery; 
         FIGS. 11A to 11C  illustrate a method for fabricating a storage battery; 
         FIGS. 12A to 12C  illustrate a method for fabricating a storage battery; 
         FIGS. 13A to 13C  illustrate the curvature radius of a surface; 
         FIGS. 14A to 14D  illustrate the curvature radius of a film; 
         FIGS. 15A and 15B  illustrate a coin-type storage battery; 
         FIGS. 16A and 16B  illustrate a cylindrical storage battery; 
         FIGS. 17A to 17C  are each a part of a cross-sectional view of a storage battery; 
         FIGS. 18A and 18B  are each a part of a cross-sectional view of a storage battery; 
         FIGS. 19A to 19C  are parts of cross-sectional views of a storage battery; 
         FIGS. 20A to 20C  illustrate an example of a storage battery; 
         FIGS. 21A to 21C  illustrate an example of a storage battery; 
         FIGS. 22A and 22B  illustrate an example of a power storage system; 
       FIGS.  23 A 1 ,  23 A 2 ,  23 B 1 , and  23 B 2  illustrate examples of power storage systems; 
         FIGS. 24A and 24B  illustrate an example of a power storage system, 
         FIGS. 25A to 25G  illustrate examples of electronic devices; 
         FIGS. 26A to 26C  illustrate an example of an electronic device; 
         FIG. 27  illustrates examples of electronic devices; 
         FIGS. 28A and 28B  illustrate examples of electronic devices; 
         FIG. 29  is a block diagram illustrating one embodiment of the present invention; 
         FIGS. 30A to 30C  are schematic views each illustrating one embodiment of the present invention; 
         FIG. 31  is a circuit diagram illustrating one embodiment of the present invention; 
         FIG. 32  is a circuit diagram illustrating one embodiment of the present invention; 
         FIGS. 33A to 33C  are schematic views each illustrating one embodiment of the present invention; 
         FIG. 34  is a block diagram illustrating one embodiment of the present invention; 
         FIG. 35  is a flow chart showing one embodiment of the present invention; 
         FIGS. 36A and 36B  each show CV measurement results; 
         FIGS. 37A and 37B  each show CV measurement results; 
         FIGS. 38A and 38R  each show CV measurement results; 
         FIG. 39  shows CV measurement results; 
         FIG. 40  shows CV measurement results; 
         FIGS. 41A and 41B  each show CV measurement results; 
         FIGS. 42A and 42B  each show charge and discharge characteristics; 
         FIG. 43  shows cycle performances; 
         FIGS. 44A and 44B  each show XPS analysis results; 
         FIGS. 45A and 45B  each show XPS analysis results; 
         FIG. 46  shows XPS analysis results; 
         FIG. 47  shows a cross-sectional TEM observation result; 
         FIG. 48  shows a cross-sectional TEM observation result; 
         FIG. 49A  shows a cross-sectional observation image and  FIG. 49B  shows EDX analysis results, 
         FIG. 50A  shows a cross-sectional observation image and  FIG. 50B  shows EDX analysis results; 
         FIGS. 51A and 51B  each show EDX analysis results; 
         FIG. 52A  shows a cross-sectional observation image and  FIG. 52B  shows EELS analysis results; 
         FIG. 53A  shows a cross-sectional observation image and  FIG. 53B  shows EELS analysis results; and 
         FIGS. 54A and 54B  illustrate a negative electrode of one embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Hereinafter, embodiments and examples of the present invention will be described in detail with reference to the accompanying drawings. However, the present invention is not limited to the descriptions of the embodiments and examples and it is easily understood by those skilled in the art that the mode and details can be changed variously. Accordingly, the present invention should not be interpreted as being limited to the descriptions of the embodiments below. 
     Note that in drawings used in this specification, the sizes, thicknesses, and the like of components such as films, layers, substrates, and regions are exaggerated for simplicity in some cases. Therefore, the sizes of the components are not limited to the sizes in the drawings and relative sizes between the components. 
     Note that the ordinal numbers such as “first” and “second” in this specification and the like are used for convenience and do not denote the order of steps, the stacking order of layers, or the like. Therefore, for example, description can be made even when “first” is replaced with “second” or “third”, as appropriate. In addition, the ordinal numbers in this specification and the like are not necessarily the same as those which specify one embodiment of the present invention. 
     Note that in structures of the present invention described in this specification and the like, the same portions or portions having similar functions are denoted by common reference numerals in different drawings, and descriptions thereof are not repeated. Further, the same hatching pattern is applied to portions having similar functions, and the portions are not especially denoted by reference numerals in some cases. 
     Note that in this specification and the like, a positive electrode and a negative electrode for a power storage device may be collectively referred to as a power storage device electrode; in this case, the power storage device electrode refers to at least one of the positive electrode and the negative electrode for the power storage device. 
     Embodiment 1 
     In this embodiment, a storage battery of one embodiments of the present invention will be described. 
     A storage battery of one embodiment of the present invention includes a positive electrode, a negative electrode, and an electrolytic solution. 
     The electrolytic solution in the storage battery of one embodiment of the present invention preferably contains an ionic liquid. Furthermore, the storage battery of one embodiment of the present invention preferably includes first cations in addition to ions contained in the ionic liquid. As the first cations, alkali metal ions, alkaline earth metal ions, or the like can be used, for example. Alkali metal ions and alkaline earth metal ions serve as carrier ions of the storage battery. 
     Examples of alkali metals include lithium, sodium, and potassium. Examples of alkaline earth metals include calcium, strontium, barium, beryllium, and magnesium. 
     In the case of using lithium ions as carrier ions, one of lithium salts such as LiPF 6 , LiClO 4 , LiAsF 6 , LiBF 4 , LiAlCl 4 , LiSCN, LiBr, LiBr, Li 2 SO 4 , Li 2 B 10 Cl 10 , Li 2 B 12 Cl 12 , LiCF 3 SO 3 , LiC 4 F 9 SO 3 , LiC(CF 3 SO 2 ) 3 , LiC(C 2 F 5 SO 2 ) 3 , LiN(CF 3 SO 2 ) 2 , LiN(C 4 F 9 SO 2 )(CF 3 SO 2 ), and LiN(C 2 F 5 SO 2 ) 2  can be dissolved in an ionic liquid, or two or more of these lithium salts can be dissolved in an ionic liquid in an appropriate combination at an appropriate ratio. 
     Here, the concentration of carrier ions is preferably higher than 0.1 mol/L and lower than 3 mol/L, more preferably higher than or equal to 0.3 mol/L and lower than or equal to 2.5 mol/L. 
     The electrolytic solution may contain a solvent other than the ionic liquid, such as an aprotic organic solvent. As an aprotic organic solvent, for example, one of ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate, chloroethylene carbonate, vinylene carbonate, γ-butyrolactone, γ-valerolactone, dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), methyl formate, methyl acetate, methyl butyrate, 1,3-dioxane, 1,4-dioxane, dimethoxyethane (DME), dimethyl sulfoxide, diethyl ether, methyl diglyme, acetonitrile, benzonitrile, tetrahydrofuran, sulfolane, and sultone can be used, or two or more of these solvents can be used in an appropriate combination in an appropriate ratio. 
     In charging and discharging the storage battery, a decomposition reaction of the electrolytic solution might occur. The electrolytic solution is decomposed in the vicinity of a surface of an electrode mainly by an electrical reaction. 
     In the storage battery using alkali metal ions, alkaline earth metal ions, or the like as carrier ions, for example, the reaction potential of a negative electrode is low and thus a decomposition reaction of the electrolytic solution easily occurs, in some cases. For example, the case where the electrolytic solution is decomposed in the vicinity of a surface of the negative electrode by a reduction reaction will be described. In the case where the decomposition of the electrolytic solution is an irreversible reaction, irreversible capacity might increase. Note that irreversible capacity is a difference between charge capacity and discharge capacity. An increase in irreversible capacity decreases the capacity of the storage battery. 
     To inhibit a decrease in the capacity of the storage battery, it is preferable to inhibit an irreversible reaction of the electrolytic solution in the vicinity of the electrode surface. For example, it is preferred that a reaction of main carrier ions, alkali metal ions or alkaline earth metal ions, of constituents of the electrolytic solution be promoted and reactions of other constituents such as cations and anions in the ionic liquid be inhibited. 
       FIG. 1A  is a schematic view illustrating constituents of an electrolytic solution near a surface of a negative electrode active material  671  of a storage battery. The electrolytic solution contains first cations  681  and an ionic liquid. The ionic liquid contains cations  682  and anions  683 . For example, lithium ions can be used as the first cations  681 , and 1-ethyl-3-methylimidazolium (EMI) can be used as the cations  682 . 
     Lithium has a significantly low redox potential, specifically, 3.045 V lower than a standard electrode potential. The reaction potential of a negative electrode active material is preferably as low as possible, in which case the voltage of a power storage device can be high. On the other hand, when the potential is low, power of reducing an electrolyte solution is high, so that an organic solvent or the like in an electrolyte solution might reductively decompose. The reaction potential of a negative electrode of a lithium-ion battery is preferably equal to or slightly higher than the redox potential of lithium, in which case the voltage of a power storage device can be high. Meanwhile, the reaction potential of cations in a solvent of an electrolytic solution, e.g., the ionic liquid is higher than the redox potential of lithium in many cases. 
     Charging the storage battery is accompanied by a decrease in the potential of the negative electrode active material  671 . Charge accumulates in the negative electrode until the potential of the negative electrode active material  671  reaches a potential at which reactions of the first cations  681 , the cations  682 , and the anions  683  noticeably occur. The first cations  681  preferably form an electric double layer on a surface of the negative electrode active material  671  as illustrated in  FIG. 1B  with accumulation of the charge. The anions  683  are arranged on a surface of the electric double layer formed by the first cations  681 . In  FIG. 1B , the cations  682  are prevented from reaching the surface of the negative electrode active material  671 . 
     In the case where the reaction potential of the cations  682  is higher than that of the first cations  681 , a noticeable reaction of the cations  682  occurs when the potential of the negative electrode decreases to the reaction potential of the cations  682 . In the case where the cations  682  are prevented from reaching a surface of the negative electrode as illustrated in  FIG. 1B , however, a reaction of the cations  682  can be inhibited until the potential of the negative electrode reaches the reaction potential of the first cations  681 . 
     Next, the case where the potential of the negative electrode decreases to a potential at which a noticeable reaction of the first cations  681  occurs will be described. The first cations  681  are consumed by the reduction reaction of the first cations  681 . The reduction of the first cations  681  might be accompanied by formation of a compound of the first cations  681  and the negative electrode active material  671 . Alternatively, the reduction might unionize the first cations  681  to form a deposition layer  681   b  on the surface of the negative electrode active material  671 . 
     The electric double layer formed on the surface of the negative electrode active material  671  disappears with the consumption of the first cations  681 , so that new cations reach the surface of the negative electrode active material  671 . When the cations  682  reach the surface of the negative electrode active material  671 , an irreversible reaction, e.g., reductive decomposition, of the cations  682  might occur to form a reaction product  684 . The reaction product  684  might be deposited to form a coating film  685  on the surface of the negative electrode active material  671 . 
     Thus, the first cations  681  preferably reach the surface of the negative electrode active material  671  faster than the cations  682 . For example, in the case where the diffusion rate of the first cations  681  in the electrolytic solution is higher than that of the cations  682  as illustrated in  FIG. 1D , a reaction of the cations  682  can be inhibited and a reaction of the first cations  681  serving as main carrier ions can be promoted. 
     Here, the case where an aprotic organic solvent is used as a solvent of the electrolytic solution will be described. The solvent coordinates to (or solvates) the first cations  681 . When the first cations  681  serving as main carrier ions move to the vicinity of the surface of the negative electrode to form an electric double layer, the solvent, which is neutral and thus is solvated, might move to the vicinity of the surface of the negative electrode, resulting in inhibition of formation of the electric double layer by the first cations  681 . 
     Meanwhile, in the case where an ionic liquid is used as the solvent of the electrolytic solution, the anions  683  appear to coordinate to the first cations  681 . When the first cations  681  move to the vicinity of the surface of the negative electrode to form an electric double layer, the anions  683 , which assume negative charge, receive an electric field in the direction in which the distance from the surface of the negative electrode increases. Thus, presumably, the coordination strength of the anions  683  decreases and the anions  683  become detached as the first cations  681  get closer to the surface of the negative electrode. This implies that the first cations  681  form an electric double layer easily compared with the case where an aprotic organic solvent is used as a solvent of an electrolytic solution. 
     [Diffusion Rate of Cations] 
     In the electrolytic solution of the storage battery of one embodiment of the present invention, the diffusion rate of the first cations  681  is preferably higher than that of the cations  682 . The diffusion rate of cations depends on molecular weight, uneven charge distribution, a three-dimensional structure, and the like. 
     For example, a large molecular weight of the cations  682  decreases the diffusion rate of the cations  682  but increases the viscosity of the electrolytic solution. As the viscosity of the electrolytic solution increases, the diffusion rate of the first cations  681  serving as main carrier ions decreases. As the diffusion rate of the first cations  681  decreases, the output characteristics of the storage battery are degraded. 
     Examples of the cations  682  include aromatic cations and aliphatic onium cations. Examples of aromatic cations include pyridinium cations and imidazolium cations Examples of aliphatic onium cations include quaternary ammonium cations, tertiary sulfonium cations, and quaternary phosphonium cations. 
     As an ionic liquid containing imidazolium cations, an ionic liquid represented by General Formula (G1) below can be used, for example. In General Formula (G1), R 1  represents an alkyl group having 1 to 4 carbon atoms, R 2  to R a  individually represent a hydrogen atom or an alkyl group having 1 to 4 carbon atoms, and R 5  represents an alkyl group or a main chain composed of atoms of two or more of C, O, Si, N, S, and P. The main chain represented by R 5  may have a substituent. Examples of the substituent include an alkyl group and an alkoxy group. 
     
       
         
         
             
             
         
       
     
     As an ionic liquid containing tertiary sulfonium cations, an ionic liquid represented by General Formula (G2) below can be used, for example. In General Formula (G2), R 25  to R 27  individually represent a hydrogen atom, an alkyl group having 1 to 4 carbon atoms, or a phenyl group. Alternatively, R 25  to R 27  may individually represent a main chain composed of atoms of two or more of C, O, Si, N, S. and P. 
     
       
         
         
             
             
         
       
     
     As an ionic liquid containing quaternary ammonium cations, an ionic liquid represented by General Formula (G4), (G5), or (G6) below can be used, for example. 
     
       
         
         
             
             
         
       
     
     In General Formula (G4), R 12  to R 17  individually represent an alkyl group, a methoxy group, a methoxymethyl group, or a methoxyethyl group each having 1 or more and 20 or less carbon atoms, or a hydrogen atom. 
     
       
         
         
             
             
         
       
     
     In General Formula (G5), R 18  to R 24  individually represent an alkyl group, a methoxy group, a methoxymethyl group, a methoxyethyl group each having 1 or more and 20 or less carbon atoms, or a hydrogen atom. 
     
       
         
         
             
             
         
       
     
     In General Formula (G6), n and m are greater than or equal to 1 and less than or equal to 3. Assume that α is greater than or equal to 0 and less than or equal to 6. When n is 1, α is greater than or equal to 0 and less than or equal to 4. When n is 2, a is greater than or equal to 0 and less than or equal to 5. When n is 3, α is greater than or equal to 0 and less than or equal to 6. Assume that β is greater than or equal to 0 and less than or equal to 6. When m is 1, β is greater than or equal to 0 and less than or equal to 4. When m is 2, β is greater than or equal to 0 and less than or equal to 5. When m is 3, β is greater than or equal to 0 and less than or equal to 6. When α or β is 0, at least one of two aliphatic rings is unsubstituted. Note that the case where both α and β are 0 is excluded. X or Y is a substituent such as a straight-chain or side-chain alkyl group having 1 to 4 carbon atoms, a straight-chain or side-chain alkoxy group having 1 to 4 carbon atoms, or a straight-chain or side-chain alkoxyalkyl group having 1 to 4 carbon atoms. 
     As an ionic liquid containing pyridinium cations, an ionic liquid represented by General Formula (G3) below may be used, for example. In General Formula (03), R 6  represents an alkyl group or a main chain composed of atoms of two or more of C, O, Si, N, S, and P atoms, and R 7  to R 11  individually represent a hydrogen atom or an alkyl group having 1 to 4 carbon atoms. The main chain represented by R 6  may have a substituent. Examples of the substituent include an alkyl group and an alkoxy group. 
     
       
         
         
             
             
         
       
     
     Anions that can be used as the anions  683  can be referred to for A −  shown in General Formulas (G1) to (G6). 
     When pyridinium cations with a molecular weight of 150 or less are used as the cations  682 , the cations  682  diffuse in the electrolytic solution fast and might reach the surface of the negative electrode, easily causing a decomposition reaction. In contrast, tertiary sulfonium cations with a molecular weight of 110 or more, imidazolium cations with a molecular weight of 100 or more, and quaternary ammonium cations with a molecular weight of 130 or more are preferably used as the cations  682 , in which case the diffusion rate of the cations  682  may be lower than that of the first cations  681 . Note that tertiary sulfonium cations with a molecular weight of 220 or less, and imidazolium cations with a molecular weight of 250 or less, preferably 175 or less, are more preferably used as the cations  682 , in which case an increase in the viscosity of the electrolytic solution can be small. 
     The ionic liquid of one embodiment of the present invention is preferably any of the ionic liquids represented by General Formulas (G1) to (G6), more preferably the ionic liquid represented by General Formula (G1) or (G2), still more preferably the ionic liquid represented by General Formula (G1). 
     As the anions  683 , monovalent amide-based anions, monovalent methide-based anions, fluorosulfonate anions, perfluoroalkylsulfonate anions, tetrafluoroborate anions, perfluoroalkylborate anions, hexafluorophosphate anions (PF 6   − ), perfluoroalkylphosphate anions, or the like can be used. 
     As the anions  683 , for example, monovalent amide-based anions, monovalent methide-based anions, fluorosulfonate anions (SO 3 F − ), fluoroalkylsulfonate anions, and the like are preferably used. 
     An example of monovalent amide-based anions is (C n F 2n+1 SO 2 ) 2 N −  (n=0 to 3). An example of monovalent cyclic amide-based anions is (CF 2 SO 2 ) 2 N − . An example of monovalent methide-based anions is (C n F 2n+1 SO 2 ) 3 C −  (n=0 to 3). An example of monovalent cyclic methide-based anions is (CF 2 SO 2 ) 2 C −  (CF 3 SO 2 ). An example of fluoroalkyl sulfonic acid anions is (C m F 2m+1 SO 3 ) −  (m=0 to 4). An example of fluoroalkylborate anions is {BF n (C m H k F 2m+1−k ) 4−n } −  (n=0 to 3, m=1 to 4, and k=0 to 2m). An example of fluoroalkylphosphate anions is {PF u (C m H k F 2m+1−k ) 6−n } −  (n=0 to 5, m=1 to 4, and k=0 to 2m). These anions are preferably used because interactions with the first cations  681  and the cations  682  may be small. The small interaction between the first cations  681  and the anions  683  may increase the diffusion rate of the first cations  681 . Furthermore, the small interaction between the cations  682  and the anions  683  may lower the melting point of the ionic liquid in the electrolytic solution. 
     Examples of monovalent amide-based anions include bis(fluorosulfonyl)amide anions and bis(trifluoromethanesulfonyl)amide anions. 
     As the anions  683 , tetrafluoroborate anions (BF 4   − ), fluoroalkylborate anions, or fluoroalkylphosphate anions may alternatively be used. 
     A compound used for the power storage device of one embodiment of the present invention includes a cation represented by General Formula (G7) and its counter anion. 
     
       
         
         
             
             
         
       
     
     In General Formula (G7), R 1  represents an alkyl group having 1 to 4 carbon atoms, R 2  to R 4  individually represent a hydrogen atom or an alkyl group having 1 to 4 carbon atoms, A 1  to A 4  individually represent a methylene group or an oxygen atom, and at least one of A 1  to A 4  represents an oxygen atom. 
     When substituents (including substituents represented by A 1  to A 4  in General Formula (G7)) are bonded to nitrogen of the imidazolium cation, the cation in the ionic liquid has a sterically bulky structure and thus side reactions (e.g., cation insertion into graphite and decomposition of the cation during charge, and gas generation associated with the insertion and the decomposition) in a battery can be inhibited. However, the viscosity of the ionic liquid is likely to be enhanced as the number of carbon atoms in A 1  to A 4  is larger; therefore, it is preferable to control the ionic liquid in accordance with desirable charge and discharge efficiency and desirable viscosity. 
     The anions contained in the ionic liquid are monovalent anions which form the ionic liquid with the imidazolium cations. Examples of the anions include monovalent amide-based anions, monovalent methide-based anions, fluorosulfonic acid anions (SO 3 F − ), fluoroalkyl sulfonic acid anions, tetrafluoroborate anions (BF 4   − ), fluoroalkylborate anions, hexafluorophosphate anions (PF 6   − ), and fluoroalkylphosphate anions. An example of monovalent amide-based anions is (C n F 2n+1 SO 2 ) 2 N −  (n=0 to 3). An example of monovalent cyclic amide-based anions is (CF 2 SO 2 ) 2 N − . An example of monovalent methide-based anions is (C n F 2n+1 SO 2 ) 3 C −  (n=0 to 3). An example of monovalent cyclic methide-based anions is (CF 2 SO 2 ) 2 C −  (CF 3 SO 2 ). An example of fluoroalkyl sulfonic acid anions is (C m F 2m+1 SO 3 ) −  (m=0 to 4). An example of fluoroalkylborate anions is {BF n (C m H k F 2m+1−k ) 4−n } −  (n=0 to 3, m=1 to 4, and k=0 to 2m). An example of fluoroalkylphosphate anions is {PF n (C m H k F 2m+1−k ) 6−n } −  (n=0 to 5, m=1 to 4, and k=0 to 2m). Note that the anions are not limited thereto. 
     The anions in the ionic liquid are preferably bis(fluorosulfonyl)amide anions, which are monovalent amide-based anions. An ionic liquid containing bis(fluorosulfonyl)amide anions and cations has high conductivity and relatively low viscosity. A power storage device including the ionic liquid and using graphite for a negative electrode can be charged and discharged. 
     A compound used for the power storage device of one embodiment of the present invention includes an anion and a cation represented by General Formula (G8). 
     
       
         
         
             
             
         
       
     
     In General Formula (G8), R 1  represents an alkyl group having 1 to 4 carbon atoms, R 2  to R 4  individually represent a hydrogen atom or an alkyl group having 1 to 4 carbon atoms. 
     The anions contained in the ionic liquid are monovalent anions which form the ionic liquid with the imidazolium cations. Examples of the anions include monovalent amide-based anions, monovalent methide-based anions, fluorosulfonic acid anions (SO 3 F − ), fluoroalkyl sulfonic acid anions, tetrafluoroborate anions (BF 4   − ), fluoroalkylborate anions, hexafluorophosphate anions (PF 6   − ), and fluoroalkylphosphate anions. An example of monovalent amide-based anions is (C n F 2n+1 SO 2 ) 2 N −  (n=0 to 3). An example of monovalent cyclic amide-based anions is (CF 2 SO 2 ) 2 N − . An example of monovalent methide-based anions is (C n F 2n+1 SO 2 ) 3 C −  (n=0 to 3). An example of monovalent cyclic methide-based anions is (CF 2 SO 2 ) 2 C −  (CF 3 SO 2 ). An example of fluoroalkyl sulfonic acid anions is (C m F 2m+1 SO 3 ) −  (m=0 to 4). An example of fluoroalkylborate anions is {BF n (C m H k F 2m+1−k ) 4−n } −  (n=0 to 3, m=1 to 4, and k=0 to 2m). An example of fluoroalkylphosphate anions is {PF n (C m H k F 2m+1−k ) 6−n } −  (n=0 to 5, m=1 to 4, and k=0 to 2m). Note that the anions are not limited thereto. 
     The anions in the ionic liquid are preferably monovalent amide-based anions. 
     A compound used for the power storage device of one embodiment of the present invention includes a cation represented by General Formula (G9) and its counter anion. 
     
       
         
         
             
             
         
       
     
     In General Formula (G9). R 1  represents an alkyl group having 1 to 4 carbon atoms, R 2  to R 4  individually represent a hydrogen atom or an alkyl group having 1 to 4 carbon atoms. 
     The anions contained in the ionic liquid are monovalent anions which form the ionic liquid with the imidazolium cations. Examples of the anions include monovalent amide-based anions, monovalent methide-based anions, fluorosulfonic acid anions (SO 3 F − ), fluoroalkyl sulfonic acid anions, tetrafluoroborate anions (BF 4   − ), fluoroalkylborate anions, hexafluorophosphate anions (PF 6   − ), and fluoroalkylphosphate anions. An example of monovalent amide-based anions is (C n F 2n+1 SO 2 ) 2 N −  (n=0 to 3). An example of monovalent cyclic amide-based anions is (CF 2 SO 2 ) 2 N − . An example of monovalent methide-based anions is (C n F 2n+1 SO 2 ) 3 C −  (n=0 to 3). An example of monovalent cyclic methide-based anions is (CF 2 SO 2 ) 2 C −  (CF 3 SO 2 ). An example of fluoroalkyl sulfonic acid anions is (C m F 2m+1 SO 3 ) −  (m=0 to 4). An example of fluoroalkylborate anions is {BF n (C m H k F 2m+1−k ) 4−n } −  (n=0 to 3, m=1 to 4, and k=0 to 2m). An example of fluoroalkylphosphate anions is {PF n (C m H k F 2m+1−k ) 6−n } −  (n=0 to 5, m=1 to 4, and k=0 to 2m). Note that the anions are not limited thereto. 
     The anions in the ionic liquid are preferably monovalent amide-based anions. 
     The alkyl groups in the ionic liquids represented by General Formulas (G7) to (G9) may each be either a straight-chain alkyl group or a branched-chain alkyl group, such as an ethyl group or a tert-butyl group. In the ionic liquid represented by General Formula (G7), it is preferred that A 1  to A 4  not have an oxygen-oxygen bond (peroxide). An oxygen-oxygen single bond breaks very easily and is highly reactive; thus, the ionic liquid with the bond might be explosive. Thus, the ionic liquid is not suitable for power storage devices. 
     Specific examples of the cation represented by General Formula (01) below include Structural Formulas (111) to (174) 
     
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
     
     Specific examples of the cation represented by General Formula (G2) below include Structural Formulas (201) to (215). 
     
       
         
         
             
             
         
       
       
         
         
             
             
         
       
     
     [Negative Electrode Active Material] 
     In the case where the active material is a negative electrode active material, for example, an alloy-based material, a carbon-based material, or the like can be used. 
     For the negative electrode active material, an element which enables charge-discharge reactions by an alloying reaction and a dealloying reaction with lithium can be used. For example, a material containing at least one of silicon, tin, gallium, aluminum, germanium, lead, antimony, bismuth, silver, zinc, cadmium, indium, and the like can be used. Such elements have higher capacity than carbon. In particular, silicon has a significantly high theoretical capacity of 4200 mAh/g. For this reason, silicon is preferably used as the negative electrode active material. Alternatively, a compound containing any of the above elements may be used. Examples of the compound include SiO, Mg 2 Si, Mg 2 Ge, SnO, SnO 2 , Mg 2 Sn, SnS 2 , V 2 Sn 3 , FeSn 2 , CoSn 2 , Ni 3 Sn 2 , Cu 6 Sn 5 , Ag 3 Sn, Ag 3 Sb, Ni 2 MnSb, CeSb 3 , LaSn 3 , La 3 Co 2 Sn 7 , CoSb 2 , InSb, SbSn, and the like. Here, an element that enables charge-discharge reactions by an alloying reaction and a dealloying reaction with lithium, a compound containing the element, and the like may be referred to as an alloy-based material. 
     It is known that the volume of an alloy-based material of silicon or the like expands due to an alloying reaction with lithium as disclosed in Patent Document 1. The expanded volume contracts due to a dealloying reaction with lithium. Charge and discharge of the storage battery are accompanied by expansion and contraction of the negative electrode active material. 
     In the state where the negative electrode active material expands, the coating film  685  is formed on the surface of the negative electrode active material. After that, the negative electrode active material whose surface is provided with the coating film  685  contracts. In the contraction, stress from the coating film  685  is imposed on the negative electrode active material, easily forming a crack or the like in the negative electrode active material, for example. Furthermore, in the contraction, the coating film  685  might be caught in the crack, expanding the crack or causing pulverization of the active material. A mixed region of the pulverized negative electrode active material and the coating film caught in the crack formed in the negative electrode active material is formed on the surface of the negative electrode active material. The pulverized negative electrode active material might lose electrical conduction. In that case, no charge and discharge reactions might occur, resulting in a decrease in the capacity of the storage battery. Thus, the negative electrode active material is preferably prevented from being cracked and pulverized. 
     In this specification and the like. SiO refers, for example, to silicon monoxide. SiO can alternatively be expressed as SiOx. Here, x preferably has an approximate value of 1. For example, x is preferably 0.2 or more and 1.5 or less, more preferably 0.3 or more and 1.2 or less. 
     As the carbon-based material, graphite, graphitizing carbon (soft carbon), non-graphitizing carbon (hard carbon), a carbon nanotube, graphene, carbon black, or the like can be used. 
     Examples of graphite include artificial graphite and natural graphite Examples of artificial graphite include meso-carbon microbeads (MCMB), coke-based artificial graphite, and pitch-based artificial graphite. As artificial graphite, spherical graphite having a spherical shape can be used. For example, MCMB is preferably used because it may have a spherical shape. Moreover. MCMB may preferably be used because it can relatively easily have a small surface area. Examples of natural graphite include flake graphite and spherical natural graphite. 
     Graphite has a low potential substantially equal to that of a lithium metal (higher than or equal to 0.1 V and lower than or equal to 0.3 V vs. Li/Li + ) when lithium ions are intercalated into the graphite (while a lithium-graphite intercalation compound is formed). For this reason, a lithium-ion secondary battery can have a high operating voltage. In addition, graphite is preferred because of its advantages such as a relatively high capacity per unit volume, relatively small volume expansion, low cost, and higher level of safety than that of a lithium metal. 
     Note that it is known that when lithium ions are intercalated in graphite, the interlayer distance of graphite increases from 0.336 nm to 0.370 nm, for example (see Non-patent Document 1, pp. 333-334). That is, the interlayer distance increases by approximately 11%. 
     Alternatively, for the negative electrode active materials, an oxide such as titanium dioxide (TiO 2 ), lithium titanium oxide (Li 4 Ti 5 O 12 ), lithium-graphite intercalation compound (Li x C 6 ), niobium pentoxide (Nb 2 O 5 ), tungsten oxide (WO 2 ), or molybdenum oxide (MoO 2 ) can be used. 
     Still alternatively, for the negative electrode active materials, Li 3−x M x N (M=Co, Ni, or Cu) with a Li 3 N structure, which is a nitride containing lithium and a transition metal, can be used. For example, Li 2.6 Co 0.4 N 3  is preferable because of high charge and discharge capacity (900 mAh/g and 1890 mAh/cm 3 ). 
     A nitride containing lithium and a transition metal is preferably used, in which case lithium ions are contained in the negative electrode active materials and thus the negative electrode active materials can be used in combination with a material for a positive electrode active material which does not contain lithium ions, such as V 2 O 5  or Cr 3 O 8 . In the case of using a material containing lithium ions as a positive electrode active material, the nitride containing lithium and a transition metal can be used for the negative electrode active material by extracting the lithium ions contained in the positive electrode active material in advance. 
     Alternatively, a material which causes a conversion reaction can be used for the negative electrode active materials; for example, a transition metal oxide which does not tbform an alloy with lithium, such as cobalt oxide (CoO), nickel oxide (NiO), and iron oxide (FeO), may be used. Other examples of the material which causes a conversion reaction include oxides such as Fe 2 O 3 , CuO, Cu 2 O, RuO 2 , and Cr 2 O 3 , sulfides such as CoS 0.89 , NiS, and CuS, nitrides such as Zn 3 N 2 , Cu 3 N, and Ge 3 N 4 , phosphides such as NiP 2 , FeP 2 , and CoP 3 , and fluorides such as FeF 3  and BiF 3 . 
     [Predoping] 
     In the case where a coating film is formed in the initial charge and discharge cycle, an irreversible reaction occurs. For example, in the case where one of an irreversible reaction at the positive electrode and an irreversible reaction at the negative electrode is greater, the balance between charge and discharge might be disrupted, resulting in a decrease in the capacity of the storage battery. Replacing an electrode used as a counter electrode after charge and discharge using the counter electrode are performed can inhibit a decrease in capacity. For example, charge or charge and discharge are performed using a positive electrode in combination with a negative electrode, and then, the positive electrode is removed to be replaced with another positive electrode in the storage battery. This may inhibit a decrease in the capacity of the storage battery. This method may be called predoping or preaging. 
     Inhibiting deposition of the coating film  685  on the surface of the negative electrode active material can prevent the negative electrode active material from being cracked and pulverized in some cases. 
     The negative electrode of one embodiment of the present invention preferably includes a first element and carbon. The first element is preferably any of silicon, tin, gallium, aluminum, germanium, lead, antimony, bismuth, silver, zinc, cadmium, and indium. 
     [Layers Included in Negative Electrode] 
     The negative electrode of one embodiment of the present invention includes a negative electrode active material and a first layer on the surface of the negative electrode active material. The first layer may be called a coating film. The thickness of the first layer is preferably larger than or equal to 10 nm and smaller than or equal to 1000 nm, more preferably larger than or equal to 50 nm and smaller than or equal to 200 nm, still more preferably larger than or equal to 50 nm and smaller than or equal to 100 nm. 
     Alternatively, the negative electrode of one embodiment of the present invention includes a first region, a second region in contact with a surface of the first region, and a third region in contact with a surface of the second region. The second region and the third region each have a shape of a layer. The thickness of the second region is preferably larger than or equal to 10 nm and smaller than or equal to 500 nm, more preferably larger than or equal to 50 nm and smaller than or equal to 200 nm, still more preferably larger than or equal to 50 nm and smaller than or equal to 100 nm. The thickness of the third region is preferably larger than or equal to 10 nm and smaller than or equal to 1000 nm, more preferably larger than or equal to 50 nm and smaller than or equal to 200 nm, still more preferably larger than or equal to 50 nm and smaller than or equal to 100 nm. The atomic ratio of carbon to the first element in the first region is x 1 :y 1 . The atomic ratio of carbon to the first element in the second region is x 2 :y 2 . The atomic ratio of carbon to the first element in the third region is x 3 :y 3 . x 1 /y 1  is preferably smaller than or equal to 3, more preferably smaller than or equal to 1.5. x 2 /y 2  is preferably larger than or equal to 0.1 and smaller than 10, more preferably larger than or equal to 0.3 and smaller than or equal to 5. x 3 /y 3  is preferably larger than or equal to 5, more preferably larger than or equal to 10, still more preferably larger than or equal to 20. 
     When the intensity ratio of carbon to the first element in the first region, which is obtained by EDX analysis, is x 1 :y 1 , the intensity ratio of carbon to the first element in the second region is x 2 :y 2 , and the intensity ratio of carbon to the first element in the third region is x 3 :y 3 , x 1 /y 1  is preferably smaller than or equal to 0.3, x 2 /y 2  is preferably larger than or equal to 0.1 and smaller than or equal to 5, more preferably larger than or equal to 0.3 and smaller than or equal to 3, and x 3 /y 3  is preferably larger than or equal to 2, more preferably larger than or equal to 5, still more preferably larger than or equal to 10. 
     When the intensity ratio of carbon to the first element in the first region, which is obtained by EELS analysis, is x 1 :y 1 , the intensity ratio of carbon to the first element in the second region is x 2 :y 2 , and the intensity ratio of carbon to the first element in the third region is x 3 :y 3 , x 1 /y 1  is preferably smaller than or equal to 0.3, x 2 /y 2  is preferably larger than or equal to 0.1 and smaller than 5, more preferably larger than or equal to 0.3 and smaller than or equal to 3, and x 3 /y 3  is preferably larger than or equal to 2, more preferably larger than or equal to 5, still more preferably larger than or equal to 10. 
       FIG. 54A  illustrates an example where a negative electrode includes a first region  551 , a second region  552 , and a third region  553 . As illustrated in  FIG. 54B , the second region  552  may be a mixed region of a material of the first region  551  and a material of the third region  553 . For example, the first region  551  is a negative electrode active material, and the third region  553  is a coating film formed by deposition of a decomposition product of an electrolytic solution. In the case where the negative electrode active material includes the first element, e.g., silicon, and the decomposition product of the electrolytic solution includes carbon, the first region  551  includes silicon and carbon such that the amount of carbon is smaller than that of silicon. The third region  553  includes silicon and carbon such that the amount of carbon is larger than that of silicon. That is, x 3 /y 3  is larger than x 1 /y 1 . 
     For the S2p spectrum of XPS analysis of the negative electrode of one embodiment of the present invention, the spectral intensity at around 168 eV is two or more times, three or more times, or four or more times the spectral intensity at around 163 eV, for example. 
     A current collector included in each of the positive electrode and the negative electrode can be formed using a material that has high conductivity, such as a metal of stainless steel, gold, platinum, aluminum, titanium, or the like, or an alloy thereof. In the case where the current collector is used in the positive electrode, it is preferred that it not dissolve at the potential of the positive electrode. In the case where the current collector is used in the negative electrode, it is preferred that it not be alloyed with carrier ions such as lithium ions. Alternatively, the current collector can be formed using an aluminum alloy to which an element that improves heat resistance, such as silicon, titanium, neodymium, scandium, or molybdenum, is added. Still alternatively, a metal element that forms silicide by reacting with silicon can be used. Examples of the metal element that forms silicide by reacting with silicon include zirconium, titanium, hafnium, vanadium, niobium, tantalum, chromium, molybdenum, tungsten, cobalt, nickel, and the like. The current collector can have any of various shapes including a foil-like shape, a plate-like shape (sheet-like shape), a net-like shape, a punching-metal shape, and an expanded-metal shape. The current collector preferably has a thickness of 5 μm to 30 μm inclusive. 
     Examples of a positive electrode active material include a composite oxide with an olivine crystal structure, a composite oxide with a layered rock-salt crystal structure, and a composite oxide with a spinel crystal structure. 
     As the positive electrode active material, a compound such as LiFeO 2 , LiCoO 2 , LiNiO 2 , LiMn 2 O 4 , V 2 O 5 , Cr 2 O 5 , or MnO 2  can be used. LiCoO 2  is particularly preferable because it has a high capacity, stability in the air higher than that of LiNiO 2 , and thermal stability higher than that of LiNiO 2 , for example. It is preferable to add a small amount of lithium nickel oxide (LiNiO 2  or LiNi 1−x M x O 2  (M=Co, Al, or the like)) to a lithium-containing material with a spinel crystal structure which contains manganese such as LiMn 2 O 4  because the characteristics of a secondary battery using such a material can be improved. 
     The average diameter of primary particles of the positive electrode active material is preferably greater than or equal to 5 nm and less than or equal to 50 μm, more preferably greater than or equal to 100 nm and less than or equal to 500 nm, for example. Furthermore, the specific surface area is preferably greater than or equal to 5 m 2 /g and less than or equal to 15 m 2 /g. Furthermore, the average diameter of secondary particles is preferably greater than or equal to 5 μm and less than or equal to 50 μm. Note that the average particle sizes can be measured with a particle size distribution analyzer or the like using a laser diffraction and scattering method or by observation with a scanning electron microscope (SEM) or a transmission electron microscope (TEM). The specific surface area can be measured by a gas adsorption method. 
     Another example of the positive electrode active material is a lithium-manganese composite oxide that is represented by a composition formula Li a Mn b M c O d . Here, the element M is preferably a metal element other than lithium and manganese, or silicon or phosphorus, more preferably nickel. Furthermore, in the case where the whole particle of a lithium-manganese composite oxide is measured, it is preferable to satisfy the following at the time of discharging: 0&lt;a/(b+c)&lt;2; c&gt;0; and 0.26≤(b+c)/d&lt;0.5. To achieve a high capacity, the lithium-manganese composite oxide preferably includes a region where the surface portion and the middle portion are different in the crystal structure, the crystal orientation, or the oxygen content. In order that such a lithium-manganese composite oxide can be obtained, the composition formula is preferably Li a Mn b Ni c O d  satisfying the following: 1.6≤a≤1.848; 0.19≤c/b≤0.935; and 2.5≤d≤3. Furthermore, it is particularly preferable to use a lithium-manganese composite oxide represented by a composition formula Li 1.68 Mn 0.8062 Ni 0.318 O 3 . In this specification and the like, a lithium-manganese composite oxide represented by a composition formula Li 1.68 Mn 0.8062 Ni 0.318 O 3  refers to that formed at a ratio (molar ratio) of the amounts of raw materials of Li 2 CO 3 :MnCO 3 :NiO=0.84:0.8062:0.318. Although this lithium-manganese composite oxide is represented by a composition formula Li 1.68 Mn 0.8062 Ni 0.318 O 3 , the composition might deviate from this. 
     Note that the ratios of metal, silicon, phosphorus, and other elements to the total composition in the whole particle of a lithium-manganese composite oxide can be measured with, for example, an inductively coupled plasma mass spectrometer (ICP-MS). The ratio of oxygen to the total composition in the whole particle of a lithium-manganese composite oxide can be measured by, for example, energy dispersive X-ray spectroscopy (EDX). Alternatively, the ratio of oxygen to the total composition in the whole particle of a lithium-manganese composite oxide can be measured by ICP-MS combined with fusion gas analysis and valence evaluation of X-ray absorption fine structure (XAFS) analysis. Note that the lithium-manganese composite oxide is an oxide containing at least lithium and manganese, and may contain at least one selected from chromium, cobalt, aluminum, nickel, iron, magnesium, molybdenum, zinc, indium, gallium, copper, titanium, niobium, silicon, phosphorus, and the like. 
       FIGS. 2A and 2B  each illustrate an example of a cross-sectional view of a particle of a lithium-manganese composite oxide having a region with different crystal structures, crystal orientations, or oxygen contents. 
     As illustrated in  FIG. 2A , the lithium-manganese composite oxide having a region with different crystal structures, crystal orientations, or oxygen contents preferably include a region  331 , a region  332 , and a region  333 . The region  332  is in contact with at least part of the outer side of the region  331 . Here, the term “outer side” refers to the side closer to a surface of a particle. The region  333  preferably includes a region corresponding to a surface of a particle containing the lithium-manganese composite oxide. 
     As shown in  FIG. 2B , the region  331  may include a region not covered with the region  332 . The region  332  may include a region not covered with the region  333 . Furthermore, the region  331  may include a region in contact with the region  333 , for example. Furthermore, the region  331  may include a region covered with neither the region  332  nor the region  333 . 
     The region  332  preferably has composition different from that of the region  331 . 
     For example, the case will be described where the composition of the region  331  and that of the region  332  are separately measured and the region  331  and the region  332  each contain lithium, manganese, the element M, and oxygen; the atomic ratio of lithium to manganese, the element M, and oxygen in the region  331  is represented by a1:b1:c1:d1; and the atomic ratio of lithium to manganese, the element M, and oxygen in the region  332  is represented by a2:b2:c2:d2. Note that the composition of each of the region  331  and the region  332  can be measured by, for example, EDX using a TEM. In measurement by EDX, the proportion of lithium is sometimes difficult to measure. Thus, a difference between the region  331  and the region  332  in composition except for lithium will be described below. Here, d1/(b1+c1) is preferably greater than or equal to 2.2, more preferably greater than or equal to 2.3, still more preferably greater than or equal to 2.35 and less than or equal to 3. Furthermore, d2/(b2+c2) is preferably less than 2.2, more preferably less than 2.1, much more preferably greater than or equal to 1.1 and less than or equal to 1.9. In this case, the composition of the whole particle of lithium-manganese composite oxide including the region  331  and the region  332  also preferably satisfies the above inequality: 0.26≤(b+c)/d&lt;0.5. 
     The valence of manganese in the region  332  may be different from that of manganese in the region  331 . The valence of the element M in the region  332  may be different from that of the element M in the region  331 . 
     Specifically, the region  331  is preferably a lithium-manganese composite oxide having a layered rock-salt crystal structure. The region  332  is preferably a lithium-manganese composite oxide having a spinel crystal structure. 
     Here, in the case where the compositions of the regions or valences of elements in the regions are spatially distributed, the compositions or valences in a plurality of portions are obtained, the average values thereof are calculated, and the average values are regarded as the compositions or valences of the regions, for example. 
     A transition layer may be provided between the region  332  and the region  331 . The transition layer is a region where the composition, crystal structure, or crystal lattice constant changes continuously or stepwise. A mixed layer may be provided between the region  332  and the region  331 . The mixed layer is a layer in which, for example, two or more crystals having different crystal orientations are mixed, two or more crystals having different crystal structures are mixed, or two or more crystals having different compositions are mixed. 
     The region  333  preferably contains carbon or a metal compound. Examples of the metal include cobalt, aluminum, nickel, iron, manganese, titanium, zinc, and lithium. Examples of the metal compound include an oxide and a fluoride of the metal. 
     It is particularly preferable that the region  333  contain carbon. Since carbon has high conductivity, the particle covered with carbon in the electrode of the power storage device can reduce the resistance of the electrode, for example. The region  333  preferably includes a graphene compound. The use of the graphene compound in the region  333  allows the lithium-manganese composite oxide particle to be efficiently coated with the region  333 . The graphene compound will be described later. The region  333  may include, specifically, graphene or graphene oxide, for example. Furthermore, graphene formed by reducing graphene oxide is preferably used as graphene. Graphene has excellent electrical characteristics of high conductivity and excellent physical properties of high flexibility and high mechanical strength. When graphene oxide is used for the region  333  and is reduced, the region  332  in contact with the region  333  is oxidized in some cases. 
     When the region  333  includes a graphene compound, the secondary battery using the lithium-manganese composite oxide as a positive electrode material can have improved cycle performance. 
     The thickness of the region  333  is preferably greater than or equal to 0.4 nm and less than or equal to 40 nm. 
     Furthermore, the average diameter of primary particles of the lithium-manganese composite oxide is preferably greater than or equal to 5 nm and less than or equal to 50 mm, more preferably greater than or equal to 100 nm and less than or equal to 500 nm, for example. Furthermore, the specific surface area is preferably greater than or equal to 5 m 2 /g and less than or equal to 15 m 2 /g. Furthermore, the average diameter of secondary particles is preferably greater than or equal to 5 μm and less than or equal to 50 μm. 
     The positive electrode and the negative electrode may include a conductive additive. Examples of the conductive additive include a carbon material, a metal material, and a conductive ceramic material. Alternatively, a fiber material may be used as the conductive additive. The content of the conductive additive in the active material layer is preferably greater than or equal to 1 wt % and less than or equal to 10 wt %, more preferably greater than or equal to 1 wt % and less than or equal to 5 wt %. 
     A network for electric conduction can be formed in the electrode by the conductive additive. The conductive additive also allows maintaining of a path for electric conduction between the positive electrode active material particles. The addition of the conductive additive to the active material layer increases the electric conductivity of the active material layer. 
     Examples of the conductive additive include natural graphite, artificial graphite such as mesocarbon microbeads, and carbon fiber. Examples of carbon fiber include mesophase pitch-based carbon fiber, isotropic pitch-based carbon fiber, carbon nanofiber, and carbon nanotube. Carbon nanotube can be formed by, for example, a vapor deposition method. Other examples of the conductive additive include carbon materials such as carbon black (e.g., acetylene black (AB)), graphite (black lead) particles, graphene, and fullerene. Alternatively, metal powder or metal fibers of copper, nickel, aluminum, silver, gold, or the like, a conductive ceramic material, or the like can be used. 
     Alternatively, a graphene compound may be used as the conductive additive 
     A graphene compound may have excellent electrical characteristics of high conductivity and excellent physical properties of high flexibility and high mechanical strength. A graphene compound has a planar shape and enables low-resistant surface contact. Furthermore, a graphene compound has extremely high conductivity even with a small thickness in some cases and thus allows a conductive path to be formed in an active material layer efficiently even with a small amount. Thus, a graphene compound is preferably used as a conductive additive, in which case the area where an active material and the conductive additive are in contact with each other can be increased and electrical resistance may be reduced. Here, it is particularly preferred that graphene, multilayer graphene, or reduced graphene oxide (hereinafter referred to as RGO) be used as a graphene compound. Note that RGO refers to a compound obtained by reducing graphene oxide (GO), for example. 
     In the case where an active material with a small particle diameter (e.g., 1 μm or less) is used, the specific surface area of the active material is large and thus more conductive paths for the active material particles are needed. In such a case, it is particularly preferred that a graphene compound with extremely high conductivity that can efficiently form a conductive path even in a small amount is used. 
     A cross-sectional structure example of the active material layer containing a graphene compound as a conductive additive will be described below. 
       FIG. 3A  is a longitudinal sectional view of the active material layer. The active material layer includes the active material particles  103 , graphene compounds  321  as a conductive additive, and a binder (not illustrated). Here, graphene or multilayer graphene can be used as the graphene compound  321 , for example. The graphene compound  321  preferably has a sheet-like shape. The graphene compound  321  may have a sheet-like shape formed of a plurality of sheets of multilayer graphene and/or a plurality of sheets of graphene that partly overlap with each other. 
     The longitudinal section of the active material layer in  FIG. 3A  shows substantially uniform dispersion of the graphene compounds  321  in the active material layer. The graphene compounds  321  are schematically shown by thick lines in  FIG. 3A  but are actually thin films each having a thickness corresponding to the thickness of a single layer or a multi-layer of carbon molecules. The plurality of graphene compounds  321  are formed in such a way as to wrap, coat, or adhere to the surfaces of the plurality of active material particles  103 , so that the graphene compounds  321  make surface contact with the active material particles  103 . Furthermore, the graphene compounds  321  are also in surface contact with each other, consequently, the plurality of graphene compounds  321  form a three-dimensional network for electric conduction. 
     Here, a plurality of graphene compounds are bonded to each other to form net-like graphene compound sheet (hereinafter referred to as a graphene compound net or a graphene net). The graphene net covering the active material can function as a binder for binding active materials. The amount of the binder can thus be reduced, or the binder does not have to be used. This can increase the proportion of the active material in the electrode volume or weight. That is to say, the capacity of the power storage device can be increased. 
     Here, it is preferable to perform reduction after a layer to be the active material layer is formed in such a manner that graphene oxide flakes are used as the graphene compounds  321  and mixed with an active material. When graphene oxide flakes with extremely high dispersibility in a polar solvent are used for the formation of the graphene compounds  321 , the graphene compounds  321  can be substantially uniformly dispersed in the active material layer. The solvent is removed by volatilization from a dispersion medium in which graphene oxide flakes are uniformly dispersed, and the graphene oxide is reduced; hence, the graphene compounds  321  remaining in the active material layer partly overlap with each other and are dispersed such that surface contact is made, thereby forming a three-dimensional conduction path. Note that graphene oxide flakes can be reduced either by heat treatment or with the use of a reducing agent, for example. 
     Unlike a conductive additive in the form of particles, such as acetylene black, which makes point contact with an active material, the graphene compound  321  is capable of making low-resistance surface contact; accordingly, the electrical conduction between the active material particles  103  and the graphene compounds  321  can be improved with a smaller amount of the graphene compounds  321  than that of a normal conductive additive. Thus, the proportion of the active material particles  103  in the active material layer can be increased. Accordingly, the discharge capacity of a power storage device can be increased. 
     The positive electrode and the negative electrode may each include a binder. As the binder, a rubber material such as styrene-butadiene rubber (SBR), styrene-isoprene-styrene rubber, acrylonitrile-butadiene rubber, butadiene rubber, or ethylene-propylene-diene copolymer can be used. Alternatively, fluororubber can be used as the binder. 
     For the binder, for example, water-soluble polymers are preferably used. As the water-soluble polymers, a polysaccharide or the like can be used. As the polysaccharide, a cellulose derivative such as carboxymethyl cellulose (CMC), methyl cellulose, ethyl cellulose, hydroxypropyl cellulose, diacetyl cellulose, or regenerated cellulose, starch, or the like can be used. It is more preferred that such water-soluble polymers be used in combination with any of the above rubber materials. 
     Alternatively, as the binder, a material such as polystyrene, poly(methyl acrylate), poly(methyl methacrylate) (PMMA), sodium polyacrylate, polyvinyl alcohol (PVA), polyethylene oxide (PEO), polypropylene oxide, polyimide, polyvinyl chloride, polytetrafluoroethylene, polyethylene, polypropylene, polyisobutylene, polyethylene terephthalate, nylon, polyvinylidene fluoride (PVdF), polyacrylonitrile (PAN), ethylene-propylene-diene polymer, polyvinyl acetate, or nitrocellulose is preferably used. 
     Two or more of the above materials may be used in combination for the binder. 
     For example, a material having a significant viscosity modifying effect and another material may be used in combination. For example, a rubber material or the like has high adhesion or high elasticity but may have difficulty in viscosity modification when mixed in a solvent. In such a case, a rubber material or the like is preferably mixed with a material having a significant viscosity modifying effect, for example. As a material having a significant viscosity modifying effect, for example, a water-soluble polymer is preferably used. An example of a water-soluble polymer having an especially significant viscosity modifying effect is the above-mentioned polysaccharide; for example, a cellulose derivative such as carboxymethyl cellulose (CMC), methyl cellulose, ethyl cellulose, hydroxypropyl cellulose, diacetyl cellulose, or regenerated cellulose, or starch can be used. 
     Note that a cellulose derivative such as carboxymethyl cellulose obtains a higher solubility when converted into a salt such as a sodium salt or an ammonium salt of carboxymethyl cellulose, and accordingly, easily exerts an effect as a viscosity modifier. The high solubility can also increase the dispersibility of an active material and other components in the formation of slurry for an electrode. In this specification, cellulose and a cellulose derivative used as a binder of an electrode include salts thereof. 
     The water-soluble polymers stabilize viscosity by being dissolved in water and allow stable dispersion of the active material and another material combined as a binder such as styrene-butadiene rubber in an aqueous solution. Furthermore, a water-soluble polymer is expected to be easily and stably adsorbed to an active material surface because it has a functional group Many cellulose derivatives such as carboxymethyl cellulose have functional groups such as a hydroxyl group and a carboxyl group. Because of functional groups, polymers are expected to interact with each other and cover an active material surface in a large area. 
     In the case where the binder covering or being in contact with the active material surface forms a film, the film is expected to serve as a passivation film to suppress the decomposition of the electrolytic solution. Here, the passivation film refers to a film without electric conductivity or a film with extremely low electric conductivity, and can inhibit the decomposition of an electrolytic solution at a potential at which a battery reaction occurs in the case where the passivation film is formed on the active material surface, for example. It is preferred that the passivation film can conduct lithium ions while suppressing electric conduction. 
     [Method for Fabricating Electrode] 
     In examples of methods for fabricating negative and positive electrodes, a slurry is formed and an electrode is fabricated by application of the slurry. A method for forming a slurry used for electrode fabrication will be described. 
     A polar solvent is preferably used as the solvent used for formation of the slurry Examples of the polar solvent include water, methanol, ethanol, acetone, tetrahydrofuran (THF), dimethylformamide (DMF), N-methylpyrrolidone (NMP), dimethyl sulfoxide (DMSO), and a mixed solution of any two or more of the above. 
     First, the active material, the conductive additive, and the binder are mixed to form Mixture A (Step S 110 ). Next, the solvent is added to Mixture A and kneading (mixing with a high viscosity) is performed, so that Mixture B is formed (Step S 120 ). Here, Mixture B is preferably in a paste form, for example. In the case where a second binder is added in a later step S 141 , a first binder is not necessarily added in Step S 110  in some cases. 
     Next, the solvent is added to Mixture B and kneading is performed, so that Mixture C is formed (Step  130 ). 
     Next, in the case where the second binder is used, the second binder is added to form Mixture D (Step S 141 ). At this time, a solvent may be added. In the case where the second binder is not used, a solvent is added as needed to form Mixture E (Step S 142 ). 
     Then, Mixture D or Mixture E is mixed in a reduced-pressure atmosphere to form Mixture F (Step S 150 ). At this time, a solvent may be added. In the mixing and kneading steps in Steps S 110  to S 150 , a mixer may be used, for example. 
     Next, the viscosity of Mixture F is measured (Step S 160 ). After that, a solvent is added as needed to adjust the viscosity. Through the above steps, slurry for application of the active material layer is obtained. 
     Here, for example, the higher the viscosity of Mixtures C to F in Steps S 130  to S 160  is, the higher the dispersibility of the active material, the binder, and the conductive additive in the mixtures is (the better they are mixed together), in some cases. Thus, the viscosity F is preferably higher. However, an excessively high viscosity of Mixture F is not preferred in terms of productivity because it might reduce the electrode application speed. 
     Next, a method for forming the active material layer on the current collector with the use of the formed slurry will be described. 
     First, the slurry is applied to the current collector. Before the application of the slurry, surface treatment may be performed on the current collector. Examples of surface treatment include corona discharge treatment, plasma treatment, and undercoat treatment. Here, the “undercoat” refers to a film formed over a current collector before application of slurry onto the current collector for the purpose of reducing the interface resistance between an active material layer and the current collector or increasing the adhesion between the active material layer and the current collector. Note that the undercoat is not necessarily formed in a film shape, and may be formed in an island shape. In addition, the undercoat may serve as an active material to have capacity. For the undercoat, a carbon material can be used, for example. Examples of the carbon material include graphite, carbon black such as acetylene black and ketjen black (registered trademark), and a carbon nanotube. 
     For the application of the slurry, a slot die method, a gravure method, a blade method, or combination of any of them can be used. Furthermore, a continuous coater or the like may be used for the application. 
     Then, the solvent of the slurry is volatilized to form the active material layer. The steps for volatilizing the solvent of the slurry are as follows, for example. 
     The step of volatilizing the solvent of the slurry is preferably performed at a temperature in the range from 50° C. to 200° C. inclusive, more preferably from 60° C. to 150° C. inclusive. 
     Heat treatment is performed using a hot plate at 30° C. or higher and 70° C. or lower in an air atmosphere for longer than or equal to 10 minutes, and then, for example, another heat treatment is performed at room temperature or higher and 100° C. or lower in a reduced-pressure environment for longer than or equal to 1 hour and shorter than or equal to 10 hours. 
     Alternatively, heat treatment may be performed using a drying furnace or the like. In the case of using a drying furnace, the heat treatment is performed at 30° C. or higher and 120° C. or lower for longer than or equal to 30 seconds and shorter than or equal to 20 minutes, for example. 
     The temperature may be increased in stages. For example, after heat treatment is performed at 60° C. or lower for shorter than or equal to 10 minutes, another heat treatment may further be performed at higher than or equal to 65° C. for longer than or equal to 1 minute. 
     The thickness of the active material layer formed through the above steps is, for example, preferably greater than or equal to 5 μm and less than or equal to 300 μm, more preferably greater than or equal to 10 μm and less than or equal to 150 μm. Furthermore, the loading of the active material in the active material layer is, for example, preferably greater than or equal to 2 mg/cm 2  and less than or equal to 50 mg/cm 2 . 
     The active material layer may be formed over only one surface of the current collector, or the active material layers may be formed such that the current collector is sandwiched therebetween. Alternatively, the active material layers may be formed such that part of the current collector is sandwiched therebetween. 
     After the volatilization of the solvent from the active material layer, pressing may be performed by a compression method such as a roll press method or a flat plate press method. In performing pressing, heat may be applied. 
     Note that the active material layer may be predoped. There is no particular limitation on the method for predoping the active material layer. For example, the active material layer may be predoped electrochemically. For example, before a battery is assembled, the active material layer can be predoped with lithium in an electrolytic solution described later with the use of a lithium metal as a counter electrode. Alternatively, predoping may be performed using a positive electrode for predoping as a counter electrode of a negative electrode, and then, the positive electrode for predoping may be removed. Predoping can particularly inhibit a decrease in initial charge and discharge efficiency, leading to an increase in the capacity of the storage battery. 
     This embodiment can be implemented in combination with any of the other embodiments as appropriate. 
     Embodiment 2 
     In this embodiment, power storage devices of embodiments of the present invention will be described. 
     Examples of the power storage device of one embodiment of the present invention include a secondary battery that utilizes an electrochemical reaction, such as such as a lithium-ion battery, an electrochemical capacitor such as an electric double-layer capacitor or a redox capacitor, an air battery, and a fuel battery. 
     &lt;Thin Storage Battery&gt; 
       FIG. 4  illustrates a thin storage battery as an example of a power storage device. When a flexible thin storage battery is used in an electronic device at least part of which is flexible, the storage battery can be bent as the electronic device is bent. 
       FIG. 4  is an external view of a storage battery  500 , which is a thin storage battery.  FIG. 5A  is a cross-sectional view along dashed-dotted line A 1 -A 2  in  FIG. 4 , and FIG. SB is a cross-sectional view along dashed-dotted line B 1 -B 2  in  FIG. 4 . The storage battery  500  includes a positive electrode  503  including a positive electrode current collector  501  and a positive electrode active material layer  502 , a negative electrode  506  including a negative electrode current collector  504  and a negative electrode active material layer  505 , a separator  507 , an electrolytic solution  508 , and an exterior body  509 . The separator  507  is provided between the positive electrode  503  and the negative electrode  506  in the exterior body  509 . The electrolytic solution  508  is contained in the exterior body  509 . 
     The electrolytic solution used for a power storage device is preferably highly purified and contains a small amount of dust particles and elements other than the constituent elements of the electrolytic solution (hereinafter, also simply referred to as impurities). Specifically, the weight ratio of impurities to the electrolytic solution is less than or equal to 1%, preferably less than or equal to 0.1%, and more preferably less than or equal to 0.01%. 
     Furthermore, an additive agent such as vinylene carbonate, propane sultone (PS), tert-butylbenzene (TBB), fluoroethylene carbonate (FEC), or LiBOB may be added to the electrolytic solution. The concentration of such an additive agent in the whole solvent is, for example, higher than or equal to 0.1 wt % and lower than or equal to 5 wt %. 
     Alternatively, a gel polymer electrolyte obtained in such a manner that a polymer is swelled with an electrolytic solution may be used. 
     Examples of host polymers include a polymer having a polyalkylene oxide structure, such as polyethylene oxide (PEO); PVDF; polyacrylonitrile; and a copolymer containing any of them. For example, PVDF-HFP, which is a copolymer of PVDF and hexafluoropropylene (HFP) can be used. The formed polymer may be porous. 
     Instead of the electrolytic solution, a solid electrolyte including an inorganic material such as a sulfide-based inorganic material or an oxide-based inorganic material, or a solid electrolyte including a macromolecular material such as a polyethylene oxide (PEO)-based macromolecular material may alternatively be used. When the solid electrolyte is used, a separator and a spacer are not necessary. Furthermore, the battery can be entirely solidified; therefore, there is no possibility of liquid leakage and thus the safety of the battery is dramatically increased. 
     As the separator  507 , paper, nonwoven fabric: glass fiber, ceramics; synthetic fiber containing nylon (polyamide), vinylon (polyvinyl alcohol-based fiber), polyester, acrylic, polyolefin, or polyurethane; or the like can be used. 
     The separator  507  is preferably formed to have a bag-like shape to surround one of the positive electrode  503  and the negative electrode  506 . For example, as illustrated in  FIG. 6A , the separator  507  is folded in two so that the positive electrode  503  is sandwiched, and sealed with a sealing member  514  in a region outside the region overlapping with the positive electrode  503 ; thus, the positive electrode  503  can be reliably supported inside the separator  507 . Then, as illustrated in  FIG. 6D , the positive electrodes  503  surrounded by the separators  507  and the negative electrodes  506  are alternately stacked and provided in the exterior body  509 , whereby the storage battery  500  can be formed. 
     Next, aging after fabrication of a storage battery will be described. Aging is preferably performed after fabrication of a storage battery. The aging can be performed under the following conditions, for example. Charge is performed at a rate of 0.001 C or more and 0.2 C or less at a temperature higher than or equal to room temperature and lower than or equal to 50° C. In the case where the reaction potential of the positive electrode or the negative electrode is out of the range of the potential window of the electrolytic solution  508 , the electrolytic solution is decomposed by charge and discharge operations of a storage battery in some cases. In the case where the electrolytic solution is decomposed and a gas is generated and accumulated in the cell, the electrolytic solution cannot be in contact with a surface of the electrode in some regions. That is to say, an effectual reaction area of the electrode is reduced and effectual resistance is increased. 
     When the resistance is extremely increased, the negative electrode potential is lowered. Consequently, lithium is intercalated into graphite and lithium is deposited on the surface of graphite. The lithium deposition might reduce capacity. For example, if a film or the like is grown on the surface after lithium deposition, lithium deposited on the surface cannot be dissolved again. This lithium cannot contribute to capacity. In addition, when deposited lithium is physically collapsed and conduction with the electrode is lost, the lithium also cannot contribute to capacity. Therefore, the gas is preferably released before the negative electrode potential reaches the potential of lithium because of an increase in a charging voltage. 
     After the release of the gas, the charging state may be maintained at a temperature higher than room temperature, preferably higher than or equal to 30° C. and lower than or equal to 60° C., more preferably higher than or equal to 35° C. and lower than or equal to 50° C. for, for example, 1 hour or more and 100 hours or less. In the initial charge, an electrolytic solution decomposed on the surface forms a film on a surface of graphite. The formed coating film may thus be densified when the charging state is held at a temperature higher than room temperature after the release of the gas, for example. 
       FIGS. 7A and 7B  illustrate an example where current collectors are welded to a lead electrode. As illustrated in  FIG. 7A , the positive electrodes  503  each wrapped by the separator  507  and the negative electrodes  506  are alternately stacked. Then, the positive electrode current collectors  501  are welded to the positive electrode lead electrode  510 , and the negative electrode current collectors  504  are welded to the negative electrode lead electrode  511 .  FIG. 7B  illustrates an example in which the positive electrode current collectors  501  are welded to the positive electrode lead electrode  510 . The positive electrode current collectors  501  are welded to the positive electrode lead electrode  510  in a welding region  512  by ultrasonic welding or the like. The positive electrode current collector  501  includes a bent portion  513  as illustrated in  FIG. 7B , and it is therefore possible to relieve stress due to external force applied after fabrication of the storage battery  500 . The reliability of the storage battery  500  can be thus increased. 
     In the storage battery  500  illustrated in  FIG. 4  and  FIGS. 5A and 5B , the positive electrode current collectors  501  in the positive electrode  503  and the negative electrode current collectors  504  in the negative electrode  506  are welded to the positive electrode lead electrode  510  and a negative electrode lead electrode  511 , respectively, by ultrasonic welding. The positive electrode current collector  501  and the negative electrode current collector  504  can double as terminals for electrical contact with the outside. In that case, the positive electrode current collector  501  and the negative electrode current collector  504  may be arranged so that part of the positive electrode current collector  501  and part of the negative electrode current collector  504  are exposed to the outside of the exterior body  509  without using lead electrodes. 
     Although the positive electrode lead electrode  510  and the negative electrode lead electrode  511  are provided on the same side in  FIG. 4 , the positive electrode lead electrode  510  and the negative electrode lead electrode  511  may be provided on different sides as illustrated in  FIG. 8 . The lead electrodes of a storage battery of one embodiment of the present invention can be freely positioned as described above; therefore, the degree of freedom in design is high. Accordingly, a product including a storage battery of one embodiment of the present invention can have a high degree of freedom in design. Furthermore, a yield of products each including a storage battery of one embodiment of the present invention can be increased. 
     As the exterior body  509  in the storage battery  500 , for example, a film having a three-layer structure in which a highly flexible metal thin film of aluminum, stainless steel, copper, nickel, or the like is provided over a film formed of a material such as polyethylene, polypropylene, polycarbonate, ionomer, or polyamide, and an insulating synthetic resin film of a polyamide-based resin, a polyester-based resin, or the like is provided as the outer surface of the exterior body over the metal thin film can be used. 
     Although the examples in  FIGS. 5A and 5B  each include five positive electrode active material layer-negative electrode active material layer pairs (the positive and negative electrode active material layers of each pair face each other), it is needless to say that the number of pairs of electrode active material layers is not limited to five, and may be more than five or less than five. In the case of using a large number of electrode active material layers, the storage battery can have a high capacity. In contrast, in the case of using a small number of electrode active material layers, the storage battery can have a small thickness and high flexibility. 
     [Predoping] 
     Here, an example of the case where the negative electrode  506  included in the storage battery  500  is predoped will be described.  FIG. 9A  illustrates the top surface of a stack in which positive electrodes, a negative electrode, and separators are stacked, and  FIG. 9B  is a perspective view of  FIG. 9A . A separator  507   i  is stacked over the negative electrode  506 , a positive electrode  503   i  for predoping is stacked over the separator  507   i , the separator  507  is stacked over the positive electrode  503   i , and the positive electrode  503  is stacked over the separator  507 . Note that for the separator  507   i , the description of the separator  507  can be referred to. The positive electrode  503   i  includes a positive electrode active material layer  502   i  and a positive electrode current collector  501   i . For the positive electrode  503   i , the positive electrode active material layer  502   i , and the positive electrode current collector  50   i , the respective descriptions of the positive electrode  503 , the positive electrode active material layer  502 , and the positive electrode current collector  501  can be referred to. The positive electrode  503  and the positive electrode  503   i  may include different positive electrode active materials. 
     The stack illustrated in  FIGS. 9A and 9B  is sandwiched by a sheet  509   a  serving as an exterior body as in a perspective view of  FIG. 10A . 
     Then, three sides of the sheet  509   a  are sealed with heat or the like to form the exterior body  509 , so that the storage battery  500 , which is a thin storage battery, is fabricated, as in a top view of  FIG. 10B .  FIG. 11A  is a cross-sectional view of the storage battery along the dashed-dotted line B 1 -B 2  in  FIG. 10B . 
     Then, predoping is performed using the negative electrode  506  and the positive electrode  503   i  of the fabricated storage battery  500 . For the predoping, only charge or both charge and discharge can be performed, for example. 
     After the predoping, one side of the exterior body  509  is cut open. Then, as in a cross-sectional view of  FIG. 11B , the positive electrode  503   i  and the separator  507   i  are taken out from the opened exterior body  509 . At this time, the separator  507  may be taken out instead of the separator  507   i . The separator  507   i  is not necessarily taken out and may be left in the storage battery  500 . 
     After that, the opened side of the exterior body  509  is sealed as in a cross-sectional view of  FIG. 11C . Through the above process, predoping can be performed. 
     Although the example in  FIGS. 9A to 9C  to  FIGS. 11A to 11C  includes one positive-negative electrode active material layer pair (the positive and negative electrode active material layers face each other), the number of positive-negative electrode active material layer pairs when predoping is performed is not limited to one. The example in  FIGS. 12A to 12C  includes three positive-negative electrode active material layer pairs (the positive and negative electrode active material layers of each pair face each other). In  FIG. 12A , the positive electrode  503   i  is located between the positive electrode  503  and the negative electrode  506  facing each other. First, predoping is performed using the positive electrode  503   i  and the negative electrode  506 . After that, the positive electrode  503   i  are removed as illustrated in  FIG. 12B , so that the storage battery is obtained in which three positive-negative electrode active material layer pairs are included and the positive and negative electrode active material layers of each pair face each other as illustrated in  FIG. 12C . 
     In the above structure, the exterior body  509  of the secondary battery can change its form such that the smallest curvature radius is greater than or equal to 3 mm and less than or equal to 30 mm, preferably greater than or equal to 3 mm and less than or equal to 10 mm. One or two films are used as the exterior body of the storage battery. In the case where the storage battery has a layered structure, the storage battery has a cross section sandwiched by two curved surfaces of the films when it is bent. 
     Description will be given of the radius of curvature of a surface with reference to  FIGS. 13A to 13C . In  FIG. 13A , on a plane  1701  along which a curved surface  1700  is cut, part of a curve  1702  of the curved surface  1700  is approximate to an arc of a circle, and the radius of the circle is referred to as a radius  1703  of curvature and the center of the circle is referred to as a center  1704  of curvature.  FIG. 13B  is a top view of the curved surface  1700 .  FIG. 13C  is a cross-sectional view of the curved surface  1700  taken along the plane  1701 . When a curved surface is cut by a plane, the radius of curvature of a curve in a cross section differs depending on the angle between the curved surface and the plane or on the cut position, and the smallest radius of curvature is defined as the radius of curvature of a surface in this specification and the like. 
     In the case of bending a secondary battery in which a component  1805  including electrodes, an electrolytic solution, and the like is sandwiched between two films as exterior bodies, a radius  1802  of curvature of a film  1801  close to a center  1800  of curvature of the secondary battery is smaller than a radius  1804  of curvature of a film  1803  far from the center  1800  of curvature ( FIG. 14A ). When the secondary battery is curved and has an arc-shaped cross section, compressive stress is applied to a surface of the film on the side closer to the center  1800  of curvature and tensile stress is applied to a surface of the film on the side farther from the center  1800  of curvature ( FIG. 14B ). However, by forming a pattern including projections or depressions on surfaces of the exterior bodies, the influence of a strain can be reduced to be acceptable even when compressive stress and tensile stress are applied. For this reason, the secondary battery can change its form such that the exterior body on the side closer to the center of curvature has the smallest curvature radius greater than or equal to 3 mm and less than or equal to 30 mm, preferably greater than or equal to 3 mm and less than or equal to 10 mm. 
     Note that the cross-sectional shape of the secondary battery is not limited to a simple are shape, and the cross section can be partly arc-shaped; for example, a shape illustrated in  FIG. 14C , a wavy shape illustrated in  FIG. 14D , or an S shape can be used. When the curved surface of the secondary battery has a shape with a plurality of centers of curvature, the secondary battery can change its form such that a curved surface with the smallest radius of curvature among radii of curvature with respect to the plurality of centers of curvature, which is a surface of the exterior body on the side closer to the center of curvature, has the smallest curvature radius, for example, greater than or equal to 3 mm and less than or equal to 30 mm, preferably greater than or equal to 3 mm and less than or equal to 10 mm. 
     Next, a variety of examples of the stack of the positive electrode, the negative electrode, and the separator will be described. 
       FIG. 17A  illustrates an example where six positive electrodes  111  and six negative electrodes  115  are stacked. One surface of a positive electrode current collector  121  included in a positive electrode Ill is provided with a positive electrode active material layer  122 . One surface of a negative electrode current collector  125  included in a negative electrode  115  is provided with a negative electrode active material layer  126 . 
     In the structure illustrated in  FIG. 17A , the positive electrodes  111  and the negative electrodes  115  are stacked so that surfaces of the positive electrodes  111  on each of which the positive electrode active material layer  122  is not provided are in contact with each other and that surfaces of the negative electrodes  115  on each of which the negative electrode active material layer  126  is not provided are in contact with each other. When the positive electrodes  111  and the negative electrodes  115  are stacked in this manner, contact surfaces between metals can be formed; specifically, the surfaces of the positive electrodes  111  on each of which the positive electrode active material layer  122  is not provided can be in contact with each other, and the surfaces of the negative electrodes  115  on each of which the negative electrode active material layer  126  is not provided can be in contact with each other. The coefficient of friction of the contact surface between metals can be lower than that of a contact surface between the active material and the separator. 
     Therefore, when the storage battery  500  is curved, the surfaces of the positive electrodes  111  on each of which the positive electrode active material layer  122  is not provided slide on each other, and the surfaces of the negative electrodes  115  on each of which the negative electrode active material layer  126  is not provided slide on each other, thus, the stress due to the difference between the inner diameter and the outer diameter of a curved portion can be relieved. Here, the inner diameter of the curved portion refers to the radius of curvature of the inner surface of the curved portion in the exterior body  509  of the storage battery  500  in the case where the storage battery  500  is curved, for example. Therefore, the deterioration of the storage battery  500  can be inhibited. Furthermore, the storage battery  500  can have high reliability. 
       FIG. 17R  illustrates an example of a stack of the positive electrodes  111  and the negative electrodes  115  which is different from that in  FIG. 17A . The structure illustrated in  FIG. 17B  is different from that in  FIG. 17A  in that the positive electrode active material layers  122  are provided on both surfaces of the positive electrode current collector  121 . When the positive electrode active material layers  122  are provided on both the surfaces of the positive electrode current collector  121  as illustrated in  FIG. 17B , the capacity per unit volume of the storage battery  500  can be increased. 
       FIG. 17C  illustrates an example of a stack of the positive electrodes  111  and the negative electrodes  115  which is different from that in  FIG. 17B . The structure illustrated in  FIG. 17C  is different from that in  FIG. 17B  in that the negative electrode active material layers  126  are provided on both surfaces of the negative electrode current collector  125 . When the negative electrode active material layers  126  are provided on both the surfaces of the negative electrode current collector  125  as illustrated in  FIG. 17C , the capacity per unit volume of the storage battery  500  can be further increased. 
     In the structures illustrated in  FIGS. 17A to 17C , the separator  123  has a bag-like shape by which the positive electrodes  11   l  are surrounded; however, one embodiment of the present invention is not limited thereto.  FIG. 18A  illustrates an example in which the separator  123  has a different structure from that in  FIG. 17A . The structure illustrated in  FIG. 18A  is different from that in  FIG. 17A  in that the separator  123 , which is sheet-like, is provided between every pair of the positive electrode active material layer  122  and the negative electrode active material layer  126 . In the structure illustrated in  FIG. 18A , six positive electrodes  111  and six negative electrodes  115  are stacked, and six separators  123  are provided. 
       FIG. 18B  illustrates an example in which the separator  123  different from that in  FIG. 18A  is provided. The structure illustrated in  FIG. 18B  is different from that in  FIG. 18A  in that one sheet of separator  123  is folded more than once to be interposed between every pair of the positive electrode active material layer  122  and the negative electrode active material layer  126 . It can be said that the structure illustrated in  FIG. 18R  is a structure in which the separators  123  in the respective layers which are illustrated in  FIG. 18A  are extended and connected together between the layers. In the structure of  FIG. 18B , six positive electrodes  111  and six negative electrodes  115  are stacked and thus the separator  123  needs to be folded at least five times. The separator  123  is not necessarily provided so as to be interposed between every pair of the positive electrode active material layer  122  and the negative electrode active material layer  126 , and the plurality of positive electrodes  111  and the plurality of negative electrodes  115  may be bound together by extending the separator  123 . 
     Note that the positive electrode, the negative electrode, and the separator may be stacked as illustrated in  FIGS. 19A to 19C .  FIG. 19A  is a cross-sectional view of a first electrode assembly  130 , and  FIG. 19B  is a cross-sectional view of a second electrode assembly  131 .  FIG. 19C  is a cross-sectional view taken along the dashed-dotted line A 1 -A 2  in  FIG. 4 . In  FIG. 19C , the first electrode assembly  130 , the second electrode assembly  131 , and the separator  123  are selectively illustrated for the sake of clarity. 
     As illustrated in  FIG. 19C , the storage battery  500  includes a plurality of first electrode assemblies  130  and a plurality of second electrode assemblies  131 . 
     As illustrated in  FIG. 19A , in each of the first electrode assemblies  130 , a positive electrode  111   a  including the positive electrode active material layers  122  on both surfaces of a positive electrode current collector  121 , the separator  123 , a negative electrode  115   a  including the negative electrode active material layers  126  on both surfaces of a negative electrode current collector  125 , the separator  123 , and the positive electrode  11   a  including the positive electrode active material layers  122  on both surfaces of the positive electrode current collector  121  are stacked in this order. As illustrated in  FIG. 19B , in each of the second electrode assemblies  131 , the negative electrode  115   a  including the negative electrode active material layers  126  on both surfaces of the negative electrode current collector  125 , the separator  123 , the positive electrode  111   a  including the positive electrode active material layers  122  on both surfaces of the positive electrode current collector  121 , the separator  123 , and the negative electrode  115   a  including the negative electrode active material layers  126  on both surfaces of the negative electrode current collector  125  are stacked in this order. 
     As illustrated in  FIG. 19C , the plurality of first electrode assemblies  130  and the plurality of second electrode assemblies  131  are covered with the wound separator  123 . 
     [Coin-Type Storage Battery] 
     Next, an example of a coin-type storage battery will be described as an example of a power storage device with reference to  FIGS. 15A and 15B .  FIG. 15A  is an external view of a coin-type (single-layer flat type) storage battery, and  FIG. 15B  is a cross-sectional view thereof. 
     In a coin-type storage battery  300 , a positive electrode can  301  doubling as a positive electrode terminal and a negative electrode can  302  doubling as a negative electrode terminal are insulated from each other and sealed by a gasket  303  made of polypropylene or the like. A positive electrode  304  includes a positive electrode current collector  305  and a positive electrode active material layer  306  provided in contact with the positive electrode current collector  305 . 
     A negative electrode  307  includes a negative electrode current collector  308  and a negative electrode active material layer  309  provided in contact with the negative electrode current collector  308 . 
     The description of the positive electrode  503  can be referred to for the positive electrode  304 . The description of the positive electrode active material layer  502  can be referred to for the positive electrode active material layer  306 . The description of the negative electrode  506  can be referred to for the negative electrode  307 . The description of the negative electrode active material layer  505  can be referred to for the negative electrode active material layer  309 . The description of the separator  507  can be referred to for the separator  310 . The description of the electrolytic solution  508  can be referred to for the electrolytic solution. 
     Note that only one surface of each of the positive electrode  304  and the negative electrode  307  used for the coin-type storage battery  300  is provided with an active material layer. 
     For the positive electrode can  301  and the negative electrode can  302 , a metal having a corrosion-resistant property to an electrolytic solution, such as nickel, aluminum, or titanium, an alloy of such a metal, or an alloy of such a metal and another metal (e.g., stainless steel or the like) can be used. Alternatively, the positive electrode can  301  and the negative electrode can  302  are preferably covered with nickel, aluminum, or the like in order to prevent corrosion due to the electrolytic solution. The positive electrode can  301  and the negative electrode can  302  are electrically connected to the positive electrode  304  and the negative electrode  307 , respectively. 
     The negative electrode  307 , the positive electrode  304 , and the separator  310  are immersed in the electrolytic solution. Then, as illustrated in  FIG. 15B , the positive electrode  304 , the separator  310 , the negative electrode  307 , and the negative electrode can  302  are stacked in this order with the positive electrode can  301  positioned at the bottom, and the positive electrode can  301  and the negative electrode can  302  are subjected to pressure bonding with the gasket  103  interposed therebetween. In such a manner, the coin-type storage battery  300  can be manufactured. 
     [Cylindrical Storage Battery] 
     Next, an example of a cylindrical storage battery will be described as an example of a power storage device with reference to  FIGS. 16A and 16B . As illustrated in  FIG. 16A , a cylindrical storage battery  600  includes a positive electrode cap (battery cap)  601  on the top surface and a battery can (outer can)  602  on the side surface and bottom surface. The positive electrode cap  601  and the battery can  602  are insulated from each other by a gasket (insulating gasket)  610 . 
       FIG. 16B  is a diagram schematically illustrating a cross section of the cylindrical storage battery. Inside the battery can  602  having a hollow cylindrical shape, a battery element in which a strip-like positive electrode  604  and a strip-like negative electrode  606  are wound with a separator  605  interposed therebetween is provided. Although not illustrated, the battery element is wound around a center pin. One end of the battery can  602  is close and the other end thereof is open. For the battery can  602 , a metal having a corrosion-resistant property to an electrolytic solution, such as nickel, aluminum, or titanium, an alloy of such a metal, or an alloy of such a metal and another metal (e.g., stainless steel or the like) can be used. Alternatively, the battery can  602  is preferably covered with nickel, aluminum, or the like in order to prevent corrosion due to the electrolytic solution. Inside the battery can  602 , the battery element in which the positive electrode, the negative electrode, and the separator are wound is provided between a pair of insulating plates  608  and  609  which face each other. Furthermore, a nonaqueous electrolytic solution (not illustrated) is injected inside the battery can  602  provided with the battery element. As the nonaqueous electrolytic solution, a nonaqueous electrolytic solution that is similar to those of the coin-type storage battery can be used. 
     The description of the positive electrode  503  can be referred to for the positive electrode  604 . The description of the negative electrode  506  can be referred to for the negative electrode  606 . The description of the method for fabricating an electrode that is described in Embodiment 1 can be referred to for the positive electrode  604  and the negative electrode  606 . Since the positive electrode and the negative electrode of the cylindrical storage battery are wound, active materials are preferably formed on both sides of the current collectors. A positive electrode terminal (positive electrode current collecting lead)  603  is connected to the positive electrode  604 , and a negative electrode terminal (negative electrode current collecting lead)  607  is connected to the negative electrode  606 . Both the positive electrode terminal  603  and the negative electrode terminal  607  can be formed using a metal material such as aluminum. The positive electrode terminal  603  and the negative electrode terminal  607  are resistance-welded to a safety valve mechanism  612  and the bottom of the battery can  602 , respectively. The safety valve mechanism  612  is electrically connected to the positive electrode cap  601  through a positive temperature coefficient (PTC) element  611 . The safety valve mechanism  612  cuts off electrical connection between the positive electrode cap  601  and the positive electrode  604  when the internal pressure of the battery exceeds a predetermined threshold value. The PTC element  611 , which serves as a thermally sensitive resistor whose resistance increases as temperature rises, limits the amount of current by increasing the resistance, in order to prevent abnormal heat generation. Note that barium titanate (BaTiO 3 )-based semiconductor ceramic can be used for the PTC element. 
     In the case where an electrode is wound as in the cylindrical storage battery illustrated in  FIGS. 16A and 16B , a great stress is caused at the time of winding the electrode. In addition, an outward stress from an axis of winding is applied to the electrode all the time in the case where a wound body of the electrode is provided in a housing. However, the active material can be prevented from being cleaved even when such a great stress is applied to the electrode. 
     Note that in this embodiment, the coin-type storage battery, the cylindrical storage battery, and the thin storage battery are given as examples of the storage battery; however, any of storage batteries with a variety of shapes, such as a sealed storage battery and a square-type storage battery, can be used. Furthermore, a structure in which a plurality of positive electrodes, a plurality of negative electrodes, and a plurality of separators are stacked or wound may be employed. For example,  FIGS. 20A to 20C  to  FIGS. 24A and 24B  illustrate examples of other storage batteries. 
     &lt;Structural Example of Thin Storage Battery&gt; 
       FIGS. 20A to 20C  and  FIGS. 21A to 21C  illustrate structural examples of thin storage batteries. A wound body  993  illustrated in  FIG. 20A  includes a negative electrode  994 , a positive electrode  995 , and a separator  996 . 
     The wound body  993  is obtained by winding a sheet of a stack in which the negative electrode  994  overlaps with the positive electrode  995  with the separator  996  therebetween. The wound body  993  is covered with a rectangular sealed container or the like; thus, a rectangular secondary battery is fabricated. 
     Note that the number of stacks each including the negative electrode  994 , the positive electrode  995 , and the separator  996  is determined as appropriate depending on capacity and element volume which are required. The negative electrode  994  is connected to a negative electrode current collector (not illustrated) via one of a lead electrode  997  and a lead electrode  998 . The positive electrode  995  is connected to a positive electrode current collector (not illustrated) via the other of the lead electrode  997  and the lead electrode  998 . 
     In a storage battery  990  illustrated in  FIGS. 20B and 20C , the wound body  993  is packed in a space formed by bonding a film  981  and a film  982  having a depressed portion that serve as exterior bodies by thermocompression bonding or the like. The wound body  993  includes the lead electrode  997  and the lead electrode  998 , and is soaked in an electrolytic solution inside a space surrounded by the film  981  and the film  982  having a depressed portion. 
     For the film  981  and the film  982  having a depressed portion, a metal material such as aluminum or a resin material can be used, for example. With the use of a resin material for the film  981  and the film  982  having a depressed portion, the film  981  and the film  982  having a depressed portion can be changed in their forms when external force is applied; thus, a flexible storage battery can be fabricated. 
     Although  FIGS. 20B and 20C  illustrate an example where a space is formed by two films, the wound body  993  may be placed in a space formed by bending one film. 
     Furthermore, in fabricating a flexible power storage device, a resin material or the like can be used for an exterior body and a sealed container of the power storage device. In that case, a resin material or the like is used for the exterior body and the sealed container. Note that in the case where a resin material is used for the exterior body and the sealed container, a conductive material is used for a portion connected to the outside. 
     For example,  FIGS. 21B and 21C  illustrate another example of a flexible thin storage battery. The wound body  993  illustrated in  FIG. 21A  is the same as that illustrated in  FIG. 20A , and the detailed description thereof is omitted. 
     In the storage battery  990  illustrated in  FIGS. 21B and 21C , the wound body  993  is packed in an exterior body  991 . The wound body  993  includes the lead electrode  997  and the lead electrode  998 , and is soaked in an electrolytic solution inside a space surrounded by the exterior body  991  and an exterior body  992 . For example, a metal material such as aluminum or a resin material can be used for the exterior bodies  991  and  992 . With the use of a resin material for the exterior bodies  991  and  992 , the exterior bodies  991  and  992  can be changed in their forms when external force is applied; thus, a flexible thin storage battery can be fabricated. 
     When the electrode including the active material of one embodiment of the present invention is used in the flexible thin storage battery, the active material can be prevented from being cleaved even if a stress caused by repeated bending of the thin storage battery is applied to the electrode. 
     When the active material in which at least part of the cleavage plane is covered with graphene is used for an electrode as described above, a decrease in the voltage and discharge capacity of a battery can be prevented. Accordingly, the cycle performance of the battery can be improved. 
     &lt;Structural Example of Power Storage System&gt; 
     Structural examples of power storage systems will be described with reference to  FIGS. 22A and 22B  to  FIGS. 24A and 24B . Here, a power storage system refers to, for example, a device including a power storage device. 
       FIGS. 22A and 22B  are external views of a power storage system. The power storage system includes a circuit board  900  and a storage battery  913 . A label  910  is attached to the storage battery  913 . As shown in  FIG. 22B , the power storage system further includes a terminal  951 , a terminal  952 , an antenna  914 , and an antenna  915 . 
     The circuit board  900  includes terminals  911  and a circuit  912 . The terminals  911  are connected to the terminals  951  and  952 , the antennas  914  and  915 , and the circuit  912 . Note that a plurality of terminals  911  serving as a control signal input terminal, a power supply terminal, and the like may be provided. 
     The circuit  912  may be provided on the rear surface of the circuit board  900 . The shape of each of the antennas  914  and  915  is not limited to a coil shape and may be a linear shape or a plate shape. Further, a planar antenna, an aperture antenna, a traveling-wave antenna, an EH antenna, a magnetic-field antenna, or a dielectric antenna may be used. Alternatively, the antenna  914  or the antenna  915  may be a flat-plate conductor. The flat-plate conductor can serve as one of conductors for electric field coupling. That is, the antenna  914  or the antenna  915  can serve as one of two conductors of a capacitor. Thus, electric power can be transmitted and received not only by an electromagnetic field or a magnetic field but also by an electric field. 
     The line width of the antenna  914  is preferably larger than that of the antenna  915 . This makes it possible to increase the amount of electric power received by the antenna  914 . 
     The power storage system includes a layer  916  between the storage battery  913  and the antennas  914  and  915 . The layer  916  may have a function of blocking an electromagnetic field by the storage battery  913 . As the layer  916 , for example, a magnetic body can be used. 
     Note that the structure of the power storage system is not limited to that shown in  FIGS. 22A and 22B . 
     For example, as shown in FIGS.  23 A 1  and  23 A 2 , two opposite surfaces of the storage battery  913  in  FIGS. 22A and 22B  may be provided with respective antennas. FIG.  23 A 1  is an external view showing one side of the opposite surfaces, and FIG.  23 A 2  is an external view showing the other side of the opposite surfaces. For portions similar to those in  FIGS. 22A and 22B , the description of the power storage system illustrated in  FIGS. 22A and 22B  can be referred to as appropriate 
     As illustrated in FIG.  23 A 1 , the antenna  914  is provided on one of the opposite surfaces of the storage battery  913  with the layer  916  interposed therebetween, and as illustrated in FIG.  23 A 2 , the antenna  915  is provided on the other of the opposite surfaces of the storage battery  913  with a layer  917  interposed therebetween. The layer  917  may have a function of preventing an adverse effect on an electromagnetic field by the storage battery  913 . As the layer  917 , for example, a magnetic body can be used. 
     With the above structure, both of the antennas  914  and  915  can be increased in size. 
     Alternatively, as illustrated in  FIGS. 2381  and  23 B 2 , two opposite surfaces of the storage battery  913  in  FIGS. 22A and 228  may be provided with different types of antennas. FIG.  23 B 1  is an external view showing one side of the opposite surfaces, and FIG.  23 B 2  is an external view showing the other side of the opposite surfaces. For portions similar to those in  FIGS. 22A and 22B , the description of the power storage system illustrated in  FIGS. 22A and 22B  can be referred to as appropriate. 
     As illustrated in  FIG. 2311 , the antenna  914  is provided on one of the opposite surfaces of the storage battery  913  with the layer  916  interposed therebetween, and as illustrated in FIG.  23 B 2 , an antenna  918  is provided on the other of the opposite surfaces of the storage battery  913  with the layer  917  interposed therebetween. The antenna  918  has a function of communicating data with an external device, for example. An antenna with a shape that can be applied to the antennas  914  and  915 , for example, can be used as the antenna  918 . As a system for communication using the antenna  918  between the power storage system and another device, a response method that can be used between the power storage system and another device, such as NFC, can be employed. 
     Alternatively, as illustrated in  FIG. 24A , the storage battery  913  in  FIGS. 22A and 22B  may be provided with a display device  920 . The display device  920  is electrically connected to the terminal  911  via a terminal  919 . It is possible that the label  910  is not provided in a portion where the display device  920  is provided. For portions similar to those in  FIGS. 22A and 22B , the description of the power storage system illustrated in  FIGS. 22A and 22B  can be referred to as appropriate. 
     The display device  920  can display, for example, an image showing whether charge is being carried out, an image showing the amount of stored power, or the like. As the display device  920 , electronic paper, a liquid crystal display device, an electroluminescent (EL) display device, or the like can be used. For example, the use of electronic paper can reduce power consumption of the display device  920 . 
     Alternatively, as illustrated in  FIG. 24B , the storage battery  913  illustrated in  FIGS. 22A and 22B  may be provided with a sensor  921 . The sensor  921  is electrically connected to the terminal  911  via a terminal  922 . For portions similar to those in  FIGS. 22A and 22B , the description of the power storage system illustrated in  FIGS. 22A and 22B  can be referred to as appropriate. 
     As the sensor  921 , a sensor that has a function of measuring, for example, force, displacement, position, speed, acceleration, angular velocity, rotational frequency, distance, light, liquid, magnetism, temperature, chemical substance, sound, time, hardness, electric field, electric current, voltage, electric power, radiation, flow rate, humidity, gradient, oscillation, odor, or infrared rays can be used. With the sensor  921 , for example, data on an environment (e.g., temperature) where the power storage system is placed can be determined and stored in a memory inside the circuit  912 . 
     The electrode of one embodiment of the present invention is used in the storage battery and the power storage system that are described in this embodiment. The capacity of the storage battery and the power storage system can thus be high. Furthermore, energy density can be high. Moreover, reliability can be high, and life can be long. 
     This embodiment can be implemented in appropriate combination with any of the other embodiments. 
     Embodiment 3 
     In this embodiment, an example of an electronic device including a flexible power storage device will be described. 
       FIGS. 25A to 25G  illustrate examples of electronic devices including the flexible power storage devices described in Embodiment 2. Examples of electronic devices each including a flexible power storage device include television devices (also referred to as televisions or television receivers), monitors of computers or the like, cameras such as digital cameras and digital video cameras, digital photo frames, mobile phones (also referred to as mobile phones or mobile phone devices), portable game machines, portable information terminals, audio reproducing devices, and large game machines such as pachinko machines. 
     In addition, a flexible power storage device can be incorporated along a curved inside/outside wall surface of a house or a building or a curved interior/exterior surface of a car. 
       FIG. 25A  illustrates an example of a mobile phone. A mobile phone  7400  is provided with a display portion  7402  incorporated in a housing  7401 , an operation button  7403 , an external connection port  7404 , a speaker  7405 , a microphone  7406 , and the like. Note that the mobile phone  7400  includes a power storage device  7407 . 
       FIG. 25B  illustrates the mobile phone  7400  that is bent. When the whole mobile phone  7400  is bent by the external force, the power storage device  7407  included in the mobile phone  7400  is also bent.  FIG. 25C  illustrates the bent power storage device  7407 . The power storage device  7407  is a thin storage battery. The power storage device  7407  is fixed in a state of being bent. Note that the power storage device  7407  includes a lead electrode  7408  electrically connected to a current collector  7409 . The current collector  7409  is, for example, copper foil, and partly alloyed with gallium; thus, adhesion between the current collector  7409  and an active material layer in contact with the current collector  7409  is improved and the power storage device  7407  can have high reliability even in a state of being bent. 
       FIG. 25D  illustrates an example of a bangle display device. A portable display device  7100  includes a housing  7101 , a display portion  7102 , an operation button  7103 , and a power storage device  7104 .  FIG. 25E  illustrates the bent power storage device  7104 . When the display device is worn on a user&#39;s arm while the power storage device  7104  is bent, the housing changes its form and the curvature of a part or the whole of the power storage device  7104  is changed. Note that the radius of curvature of a curve at a point refers to the radius of the circular are that best approximates the curve at that point. The reciprocal of the radius of curvature is curvature. Specifically, a part or the whole of the housing or the main surface of the power storage device  7104  is changed in the range of radius of curvature from 40 mm to 150 mm inclusive. When the radius of curvature at the main surface of the power storage device  7104  is greater than or equal to 40 mm and less than or equal to 150 mm, the reliability can be kept high. 
       FIG. 25F  illustrates an example of a watch-type portable information terminal. A portable information terminal  7200  includes a housing  7201 , a display portion  7202 , a band  7203 , a buckle  7204 , an operation button  7205 , an input output terminal  7206 , and the like. 
     The portable information terminal  7200  is capable of executing a variety of applications such as mobile phone calls, e-mailing, viewing and editing texts, music reproduction, Internet communication, and a computer game. 
     The display surface of the display portion  7202  is curved, and images can be displayed on the curved display surface. In addition, the display portion  7202  includes a touch sensor, and operation can be performed by touching the screen with a finger, a stylus, or the like. For example, by touching an icon  7207  displayed on the display portion  7202 , application can be started. 
     With the operation button  7205 , a variety of functions such as time setting, power on/off, on/off of wireless communication, setting and cancellation of a silent mode, and setting and cancellation of a power saving mode can be performed. For example, the functions of the operation button  7205  can be set freely by setting the operation system incorporated in the portable information terminal  7200 . 
     The portable information terminal  7200  can employ near field communication that is a communication method based on an existing communication standard. In that case, for example, mutual communication between the portable information terminal  7200  and a headset capable of wireless communication can be performed, and thus hands-free calling is possible. 
     Moreover, the portable information terminal  7200  includes the input output terminal  7206 , and data can be directly transmitted to and received from another information terminal via a connector. In addition, charging via the input output terminal  7206  is possible. Note that the charging operation may be performed by wireless power feeding without using the input output terminal  7206 . 
     The display portion  7202  of the portable information terminal  7200  is provided with a power storage device including the electrode of one embodiment of the present invention. For example, the power storage device  7104  illustrated in  FIG. 25E  that is in the state of being curved can be provided in the housing  7201 . Alternatively, the power storage device  7104  illustrated in  FIG. 25E  can be provided in the band  7203  such that it can be curved. 
     A portable information terminal  7200  preferably includes a sensor. As the sensor, for example a human body sensor such as a fingerprint sensor, a pulse sensor, or a temperature sensor, a touch sensor, a pressure sensitive sensor, an acceleration sensor, or the like is preferably mounted. 
       FIG. 250  illustrates an example of an armband display device. A display device  7300  includes a display portion  7304  and the power storage device of one embodiment of the present invention. The display device  7300  can include a touch sensor in the display portion  7304  and can serve as a portable information terminal. 
     The display surface of the display portion  7304  is bent, and images can be displayed on the bent display surface. A display state of the display device  7300  can be changed by, for example, near field communication, which is a communication method based on an existing communication standard. 
     The display device  7300  includes an input output terminal, and data can be directly transmitted to and received from another information terminal via a connector. In addition, charging via the input output terminal is possible. Note that the charging operation may be performed by wireless power feeding without using the input output terminal. 
     This embodiment can be implemented in appropriate combination with any of the other embodiments. 
     Embodiment 4 
     In this embodiment, examples of electronic devices that can include power storage devices will be described. 
       FIGS. 26A and 26B  illustrate an example of a tablet terminal that can be folded in half. A tablet terminal  9600  illustrated in  FIGS. 26A and 26B  includes a housing  9630   a , a housing  9630   b , a movable portion  9640  connecting the housings  9630   a  and  9630   b , a display portion  9631  including a display portion  9631   a  and a display portion  9631   b , a display mode changing switch  9626 , a power switch  9627 , a power saving mode changing switch  9625 , a fastener  9629 , and an operation switch  9628 .  FIG. 26A  illustrates the tablet terminal  9600  that is opened, and  FIG. 26B  illustrates the tablet terminal  9600  that is closed. 
     The tablet terminal  9600  includes a power storage unit  9635  inside the housings  9630   a  and  9630   h  The power storage unit  9635  is provided across the housings  9630   a  and  9630   b , passing through the movable portion  9640 . 
     Part of the display portion  9631   a  can be a touch panel region  9632   a , and data can be input by touching operation keys  9638  that are displayed. Note that half of the area of the display portion  9631   a  has only a display function and the other half of the area has a touch panel function. However, the structure of the display portion  9631   a  is not limited to this, and all the area of the display portion  9631   a  may have a touch panel function. For example, all the area of the display portion  9631   a  can display a keyboard and serve as a touch panel while the display portion  9631   b  can be used as a display screen. 
     As in the display portion  9631   a , part of the display portion  9631   b  can be a touch panel region  9632   h . When a keyboard display switching button  9639  displayed on the touch panel is touched with a finger, a stylus, or the like, a keyboard can be displayed on the display portion  9631   b.    
     Touch input can be performed in the touch panel region  9632   a  and the touch panel region  9632   b  at the same time. 
     The display mode changing switch  9626  allows switching between a landscape mode and a portrait mode, color display and black-and-white display, and the like. The power saving mode changing switch  9625  can control display luminance in accordance with the amount of external light in use of the tablet terminal  9600 , which is measured with an optical sensor incorporated in the tablet terminal  9600 . In addition to the optical sensor, other detecting devices such as sensors for determining inclination, such as a gyroscope or an acceleration sensor, may be incorporated in the tablet terminal. 
     Although the display portion  9631   a  and the display portion  9631   b  have the same area in  FIG. 26A , one embodiment of the present invention is not limited to this example. The display portion  9631   a  and the display portion  9631   b  may have different areas or different display quality. For example, one of the display portions  9631   a  and  9631   b  may display higher definition images than the other. 
     The tablet terminal is closed in  FIG. 26B . The tablet terminal includes the housing  9630 , a solar cell  9633 , and a charge and discharge control circuit  9634  including a DCDC converter  9636 . The power storage unit of one embodiment of the present invention is used as the power storage unit  9635 . 
     The tablet terminal  9600  can be folded such that the housings  9630   a  and  9630   b  overlap with each other when not in use. Thus, the display portions  9631   a  and  9631   b  can be protected, which increases the durability of the tablet terminal  9600 . In addition, the power storage unit  9635  of one embodiment of the present invention has flexibility and can be repeatedly bent without a significant decrease in charge and discharge capacity. Thus, a highly reliable tablet terminal can be provided. 
     The tablet terminal illustrated in  FIGS. 26A and 26B  can also have a function of displaying various kinds of data (e.g., a still image, a moving image, and a text image), a function of displaying a calendar, a date, or the time on the display portion, a touch-input function of operating or editing data displayed on the display portion by touch input, a function of controlling processing by various kinds of software (programs), and the like. 
     The solar cell  9633 , which is attached on the surface of the tablet terminal, supplies electric power to a touch panel, a display portion, an image signal processing portion, and the like. Note that the solar cell  9633  can be provided on one or both surfaces of the housing  9630  and the power storage unit  9635  can be charged efficiently. The use of a lithium-ion battery as the power storage unit  9635  brings an advantage such as reduction in size. 
     The structure and operation of the charge and discharge control circuit  9634  illustrated in  FIG. 26B  will be described with reference to a block diagram in  FIG. 26C . The solar cell  9633 , the power storage unit  9635 , the DCDC converter  9636 , a converter  9637 , switches SW 1  to SW 3 , and the display portion  9631  are illustrated in  FIG. 26C , and the power storage unit  9635 , the DCDC converter  9636 , the converter  9637 , and the switches SW 1  to SW 3  correspond to the charge and discharge control circuit  9634  in  FIG. 26B . 
     First, an example of operation when electric power is generated by the solar cell  9633  using external light will be described. The voltage of electric power generated by the solar cell is raised or lowered by the DCDC converter  9636  to a voltage for charging the power storage unit  9635 . When the display portion  9631  is operated with the electric power from the solar cell  9633 , the switch SW 1  is turned on and the voltage of the electric power is raised or lowered by the converter  9637  to a voltage needed for operating the display portion  9631 . When display on the display portion  9631  is not performed, the switch SW 1  is turned off and the switch SW 2  is turned on, so that the power storage unit  9635  can be charged. 
     Note that the solar cell  9633  is described as an example of a power generation means; however, one embodiment of the present invention is not limited to this example. The power storage unit  9635  may be charged using another power generation means such as a piezoelectric element or a thermoelectric conversion element (Peltier element). For example, the power storage unit  9635  may be charged with a non-contact power transmission module capable of performing charging by transmitting and receiving electric power wirelessly (without contact), or any of the other charge means used in combination. 
       FIG. 27  illustrates other examples of electronic devices. In  FIG. 27 , a display device  8000  is an example of an electronic device including a power storage device  8004  of one embodiment of the present invention. Specifically, the display device  8000  corresponds to a display device for TV broadcast reception and includes a housing  8001 , a display portion  8002 , speaker portions  8003 , and the power storage device  8004 . The power storage device  8004  of one embodiment of the present invention is provided in the housing  8001 . The display device  8000  can receive electric power from a commercial power supply. Alternatively, the display device  8000  can use electric power stored in the power storage device  8004 . Thus, the display device  8000  can be operated with the use of the power storage device  8004  of one embodiment of the present invention as an uninterruptible power supply even when electric power cannot be supplied from a commercial power supply due to power failure or the like. 
     A semiconductor display device such as a liquid crystal display device, a light-emitting device in which a light-emitting element such as an organic EL element is provided in each pixel, an electrophoresis display device, a digital micromirror device (DMD), a plasma display panel (PDP), or a field emission display (FED) can be used for the display portion  8002 . 
     Note that the display device includes, in its category, all of information display devices for personal computers, advertisement displays, and the like besides TV broadcast reception. 
     In  FIG. 27 , an installation lighting device  8100  is an example of an electronic device including a power storage device  8103  of one embodiment of the present invention. Specifically, the lighting device  8100  includes a housing  8101 , a light source  8102 , and the power storage device  8103 . Although  FIG. 27  illustrates the case where the power storage device  8103  is provided in a ceiling  8104  on which the housing  8101  and the light source  8102  are installed, the power storage device  8103  may be provided in the housing  8101 . The lighting device  8100  can receive electric power from a commercial power supply. Alternatively, the lighting device  8100  can use electric power stored in the power storage device  8103 . Thus, the lighting device  8100  can be operated with the use of power storage device  8103  of one embodiment of the present invention as an uninterruptible power supply even when electric power cannot be supplied from a commercial power supply due to power failure or the like. 
     Note that although the installation lighting device  8100  provided in the ceiling  8104  is illustrated in  FIG. 27  as an example, the power storage device of one embodiment of the present invention can be used in an installation lighting device provided in, for example, a wall  8105 , a floor  8106 , a window  8107 , or the like other than the ceiling  8104 . Alternatively, the power storage device of one embodiment of the present invention can be used in a tabletop lighting device or the like. 
     As the light source  8102 , an artificial light source which emits light artificially by using electric power can be used. Specifically, an incandescent lamp, a discharge lamp such as a fluorescent lamp, and light-emitting elements such as an LED and an organic EL element are given as examples of the artificial light source. 
     In  FIG. 27 , an air conditioner including an indoor unit  8200  and an outdoor unit  8204  is an example of an electronic device including a power storage device  8203  of one embodiment of the present invention. Specifically, the indoor unit  8200  includes a housing  8201 , an air outlet  8202 , and the power storage device  8203 . Although  FIG. 27  illustrates the case where the power storage device  8203  is provided in the indoor unit  8200 , the power storage device  8203  may be provided in the outdoor unit  8204 . Alternatively, the power storage devices  8203  may be provided in both the indoor unit  8200  and the outdoor unit  8204 . The air conditioner can receive electric power from a commercial power supply. Alternatively, the air conditioner can use electric power stored in the power storage device  8203 . Particularly in the case where the power storage devices  8203  are provided in both the indoor unit  8200  and the outdoor unit  8204 , the air conditioner can be operated with the use of the power storage device  8203  of one embodiment of the present invention as an uninterruptible power supply even when electric power cannot be supplied from a commercial power supply due to power failure or the like. 
     Note that although the split-type air conditioner including the indoor unit and the outdoor unit is illustrated in  FIG. 27  as an example, the power storage device of one embodiment of the present invention can be used in an air conditioner in which the functions of an indoor unit and an outdoor unit are integrated in one housing. 
     In  FIG. 27 , an electric refrigerator-freezer  8300  is an example of an electronic device including a power storage device  8304  of one embodiment of the present invention. Specifically, the electric refrigerator-freezer  8300  includes a housing  8301 , a door for a refrigerator  8302 , a door for a freezer  8303 , and the power storage device  8304 . The power storage device  8304  is provided in the housing  8301  in  FIG. 27 . The electric refrigerator-freezer  8300  can receive electric power from a commercial power supply. Alternatively, the electric refrigerator-freezer  8300  can use electric power stored in the power storage device  8304 . Thus, the electric refrigerator-freezer  8300  can be operated with the use of the power storage device  8304  of one embodiment of the present invention as an uninterruptible power supply even when electric power cannot be supplied from a commercial power supply due to power failure or the like. 
     Note that among the electronic devices described above, a high-frequency heating apparatus such as a microwave oven and an electronic device such as an electric rice cooker require high power in a short time. The tripping of a breaker of a commercial power supply in use of an electronic device can be prevented by using the power storage device of one embodiment of the present invention as an auxiliary power supply for supplying electric power which cannot be supplied enough by a commercial power supply. 
     In addition, in a time period when electronic devices are not used, particularly when the proportion of the amount of electric power which is actually used to the total amount of electric power which can be supplied from a commercial power supply source (such a proportion referred to as a usage rate of electric power) is low, electric power can be stored in the power storage device, whereby the usage rate of electric power can be reduced in a time period when the electronic devices are used. For example, in the case of the electric refrigerator-freezer  8300 , electric power can be stored in the power storage device  8304  in night time when the temperature is low and the door for a refrigerator  8302  and the door for a freezer  8303  are not often opened or closed. On the other hand, in daytime when the temperature is high and the door for a refrigerator  8302  and the door for a freezer  8303  are frequently opened and closed, the power storage device  8304  is used as an auxiliary power supply; thus, the usage rate of electric power in daytime can be reduced. 
     This embodiment can be implemented in combination with any of the other embodiments as appropriate 
     Embodiment 5 
     In this embodiment, examples of vehicles using power storage devices will be described. 
     The use of power storage devices in vehicles enables production of next-generation clean energy vehicles such as hybrid electric vehicles (HEVs), electric vehicles (EVs), and plug-in hybrid electric vehicles (PHEVs). 
       FIGS. 28A and 28B  each illustrate an example of a vehicle using one embodiment of the present invention. An automobile  8400  illustrated in  FIG. 28A  is an electric vehicle that runs on the power of an electric motor. Alternatively, the automobile  8400  is a hybrid electric vehicle capable of driving appropriately using either an electric motor or an engine. One embodiment of the present invention can provide a high-mileage vehicle. The automobile  8400  includes the power storage device. The power storage device is used not only for driving the electric motor  8406 , but also for supplying electric power to a light-emitting device such as a headlight  8401  or a room light (not illustrated). 
     The power storage device can also supply electric power to a display device of a speedometer, a tachometer, or the like included in the automobile  8400 . Furthermore, the power storage device can supply electric power to a semiconductor device included in the automobile  8400 , such as a navigation system. 
       FIG. 28B  illustrates an automobile  8500  including the power storage device. The automobile  8500  can be charged when the power storage device is supplied with electric power through external charging equipment by a plug-in system, a contactless power feeding system, or the like. In  FIG. 28B , a power storage device  8024  included in the automobile  8500  is charged with the use of a ground-based charging apparatus  8021  through a cable  8022 . In charging, a given method such as CHAdeMO (registered trademark) or Combined Charging System may be employed as a charging method, the standard of a connector, or the like as appropriate. The ground-based charging apparatus  8021  may be a charging station provided in a commerce facility or a power source in a house. For example, with the use of a plug-in technique, the power storage device  8024  included in the automobile  8500  can be charged by being supplied with electric power from outside. The charging can be performed by converting AC electric power into DC electric power through a converter such as an AC-DC converter. 
     Furthermore, although not illustrated, the vehicle may include a power receiving device so that it can be charged by being supplied with electric power from an above-ground power transmitting device in a contactless manner. In the case of the contactless power feeding system, by fining a power transmitting device in a road or an exterior wall, charging can be performed not only when the electric vehicle is stopped but also when driven. In addition, the contactless power feeding system may be utilized to perform transmission and reception of electric power between vehicles. Furthermore, a solar cell may be provided in the exterior of the automobile to charge the power storage device when the automobile stops or moves. To supply electric power in such a contactless manner, an electromagnetic induction method or a magnetic resonance method can be used. 
     According to one embodiment of the present invention, the power storage device can have improved cycle performance and reliability. Furthermore, according to one embodiment of the present invention, the power storage device itself can be made more compact and lightweight as a result of improved characteristics of the power storage device. The compact and lightweight power storage device contributes to a reduction in the weight of a vehicle, and thus increases the driving distance. Furthermore, the power storage device included in the vehicle can be used as a power source for supplying electric power to products other than the vehicle. In such a case, the use of a commercial power source can be avoided at peak time of electric power demand. 
     This embodiment can be implemented in appropriate combination with any of the other embodiments. 
     Embodiment 6 
     A battery control unit (battery management unit: BMU) that can be used in combination with battery cells each including the materials described in the above embodiment and transistors that are suitable for a circuit included in the battery control unit will be described with reference to  FIG. 29  to  FIG. 35 . In this embodiment, in particular, a battery control unit of a power storage device including battery cells connected in series will be described. 
     When the plurality of battery cells connected in series are repeatedly charged and discharged, there occurs variations in charge and discharge characteristics among the battery cells, which causes variations in capacity (output voltage) among the battery cells. The discharge capacity of all the plurality of battery cells connected in series depends on the capacity of the battery cell that is low. The variations in capacity among the battery cells reduce the discharge capacity of all the battery cells. Furthermore, when charge is performed based on the capacity of the battery cell that is low, the battery cells might be undercharged. In contrast, when charge is performed based on the capacity of the battery cell that is high, the battery cells might be overcharged. 
     Thus, the battery control unit of the power storage device including the battery cells connected in series has a function of reducing variations in capacity among the battery cells, which cause an undercharge and an overcharge. Examples of a circuit configuration for reducing variations in capacity among battery cells include a resistive type, a capacitive type, and an inductive type, and a circuit configuration that can reduce variations in capacity among battery cells using transistors with a low off-state current will be explained here as an example. 
     A transistor including an oxide semiconductor in its channel formation region (an OS transistor) is preferably used as the transistor with a low off-state current. When an OS transistor with a low off-state current is used in the circuit of the battery control unit of the power storage device, the amount of charge that leaks from a battery can be reduced, and reduction in capacity with the lapse of time can be suppressed. 
     As the oxide semiconductor used in the channel formation region, an In-M-Zn=oxide (M is Ga, Sn, Y, Zr, La, Ce, or Nd) is used. In the case where the atomic ratio of the metal elements of a target for forming an oxide semiconductor film is In:M:Zn=x 1 :y 1 :z 1 , x 1 /y 1  is preferably greater than or equal to 1/3 and less than or equal to 6, more preferably greater than or equal to 1 and less than or equal to 6, and z 1 /y 1  is preferably greater than or equal to 1/3 and less than or equal to 6, more preferably greater than or equal to 1 and less than or equal to 6. Note that when z 1 /y 1  is greater than or equal to 1 and less than or equal to 6, a CAAC-OS film as the oxide semiconductor film is easily formed. 
     Here, the details of the CAAC-OS film will be described. 
     A CAAC-OS film is one of oxide semiconductor films having a plurality of c-axis aligned crystal parts. 
     In a combined analysis image (also referred to as a high-resolution TEM image) of a bright-field image and a diffraction pattern of a CAAC-OS film, which is obtained using a TEM, a plurality of crystal parts can be observed. However, in the high-resolution TEM image, a boundary between crystal parts, that is, a grain boundary is not clearly observed. Thus, in the CAAC-OS film, a reduction in electron mobility due to the grain boundary is less likely to occur. 
     According to the high-resolution cross-sectional TEM image of the CAAC-OS film observed in the direction substantially parallel to the sample surface, metal atoms are arranged in a layered manner in the crystal parts. Each metal atom layer reflects unevenness of a surface over which the CAAC-OS film is formed (hereinafter, a surface over which the CAAC-OS film is formed is referred to as a formation surface) or the top surface of the CAAC-OS film, and is arranged parallel to the formation surface or the top surface of the CAAC-OS film. 
     On the other hand, according to the plan high-resolution TEM image of the CAAC-OS film observed in the direction substantially perpendicular to the sample surface, metal atoms are arranged in a triangular or hexagonal arrangement in the crystal parts. However, there is no regularity of arrangement of metal atoms between different crystal parts. 
     For example, when the structure of a CAAC-OS including an InGaZnO 4  crystal is analyzed by an out-of-plane method using an X-ray diffraction (XRD) apparatus, a peak may appear at a diffraction angle (2θ) of around 31°. This peak is derived from the (009) plane of the InGaZnO 4  crystal, which indicates that crystals in the CAAC-OS film have c-axis alignment, and that the c-axes are aligned in the direction substantially perpendicular to the formation surface or the top surface of the CAAC-OS film. 
     Note that in analysis of the CAAC-OS film with an InGaZnO 4  crystal by an out-of-plane method, another peak may appear when 201s around 36°, in addition to the peak at 2θ of around 31°. The peak at 2θ of around 36° indicates that a crystal having no c-axis alignment is included in part of the CAAC-OS film. It is preferable that in the CAAC-OS film, a peak appear when 28 is around 31° and that a peak not appear when 2θ is around 360. 
     The CAAC-OS film is an oxide semiconductor film with low impurity concentration. The impurity is an element other than the main components of the oxide semiconductor film, such as hydrogen, carbon, silicon, or a transition metal element. In particular, an element (specifically, silicon or the like) having higher strength of bonding to oxygen than a metal element included in an oxide semiconductor film extracts oxygen from the oxide semiconductor film, which results in disorder of the atomic arrangement and reduced crystallinity of the oxide semiconductor film. Furthermore, a heavy metal such as iron or nickel, argon, carbon dioxide, or the like has a large atomic radius (molecular radius), and thus disturbs the atomic arrangement of the oxide semiconductor film and causes a decrease in crystallinity when it is contained in the oxide semiconductor film. Note that the impurity contained in the oxide semiconductor might serve as a carrier trap or a carrier generation source. 
     The CAAC-OS film is an oxide semiconductor having a low density of defect states. In some cases, oxygen vacancies in the oxide semiconductor film serve as carrier traps or serve as carrier generation sources when hydrogen is captured therein, for example. 
     The state in which impurity concentration is low and density of defect states is low (the number of oxygen vacancies is small) is referred to as a “highly purified intrinsic” or “substantially intrinsic” state. A highly purified intrinsic or substantially intrinsic oxide semiconductor has few carrier generation sources, and thus can have a low carrier density. Therefore, a transistor including the oxide semiconductor film rarely has negative threshold voltage (is rarely normally on). The highly purified intrinsic or substantially intrinsic oxide semiconductor film has few carrier traps. Accordingly, the transistor including the oxide semiconductor film has little variation in electrical characteristics and high reliability. Charge trapped by the carrier traps in the oxide semiconductor film takes a long time to be released and might behave like fixed charge. Thus, the transistor including the oxide semiconductor film having high impurity concentration and a high density of defect states has unstable electrical characteristics in some cases. 
     With the use of the CAAC-OS film in a transistor, variation in the electrical characteristics of the transistor due to irradiation with visible light or ultraviolet light is small. 
     Since the OS transistor has a wider band gap than a transistor including silicon in its channel formation region (a Si transistor), dielectric breakdown is unlikely to occur when a high voltage is applied. Although a voltage of several hundreds of volts is generated when battery cells are connected in series, the above-described OS transistor is suitable for a circuit of a battery control unit which is used for such battery cells in the power storage device. 
       FIG. 29  is an example of a block diagram of the power storage device A power storage device BT 00  illustrated in  FIG. 29  includes a terminal pair BT 01 , a terminal pair BT 02 , a switching control circuit BT 03 , a switching circuit BT 04 , a switching circuit BT 05 , a voltage transformation control circuit BT 06 , a voltage transformer circuit BT 07 , and a battery portion BT 08  including a plurality of battery cells BT 09  connected in series. 
     In the power storage device BT 00  illustrated in  FIG. 29 , a portion including the terminal pair BT 01 , the terminal pair BT 02 , the switching control circuit BT 03 , the switching circuit BT 04 , the switching circuit BT 05 , the voltage transformation control circuit BT 06 , and the voltage transformer circuit BT 07  can be referred to as a battery control unit. 
     The switching control circuit BT 03  controls operations of the switching circuits BT 04  and BT 05 . Specifically, the switching control circuit BT 03  selects battery cells to be discharged (a discharge battery cell group) and battery cells to be charged (a charge battery cell group) in accordance with voltage measured for every battery cell BT 09 . 
     Furthermore, the switching control circuit BT 03  outputs a control signal S 1  and a control signal S 2  on the basis of the selected discharge battery cell group and the selected charge battery cell group. The control signal S 1  is output to the switching circuit BT 04 . The control signal S 1  controls the switching circuit BT 04  so that the terminal pair BT 01  and the discharge battery cell group are connected. In addition, the control signal S 2  is output to the switching circuit BT 05 . The control signal S 2  controls the switching circuit BT 05  so that the terminal pair BT 02  and the charge battery cell group are connected. 
     The switching control circuit BT 03  generates the control signal S 1  and the control signal S 2  on the basis of the connection relation of the switching circuit BT 04 , the switching circuit BT 05 , and the voltage transformer circuit BT 07  so that terminals having the same polarity of the terminal pair BT 01  and the discharge battery cell group are connected with each other, or terminals having the same polarity of the terminal pair BT 02  and the charge battery cell group are connected with each other. 
     The operations of the switching control circuit BT 03  will be described in detail. 
     First, the switching control circuit BT 03  measures the voltage of each of the plurality of battery cells BT 09 . Then, the switching control circuit BT 03  determines that the battery cell BT 09  having a voltage higher than a predetermined threshold value is a high-voltage battery cell (high-voltage cell) and that the battery cell BT 09  having a voltage lower than the predetermined threshold value is a low-voltage battery cell (low-voltage cell), for example. 
     As a method to determine whether a battery cell is a high-voltage cell or a low-voltage cell, any of various methods can be employed. For example, the switching control circuit BT 03  may determine whether each battery cell BT 09  is a high-voltage cell or a low-voltage cell on the basis of the voltage of the battery cell BT 09  having the highest voltage or the lowest voltage among the plurality of battery cells BT 09 . In this case, the switching control circuit BT 03  can determine whether each battery cell BT 09  is a high-voltage cell or a low-voltage cell by, for example, determining whether or not the ratio of the voltage of each battery cell BT 09  to the reference voltage is the predetermined value or more. Then, the switching control circuit BT 03  determines a charge battery cell group and a discharge battery cell group on the basis of the determination result. 
     Note that high-voltage cells and low-voltage cells are mixed in various states in the plurality of battery cells BT 09 . For example, the switching control circuit BT 03  selects a portion having the largest number of high-voltage cells connected in series as the discharge battery cell group of mixed high-voltage cells and low-voltage cells. Furthermore, the switching control circuit BT 03  selects a portion having the largest number of low-voltage cells connected in series as the charge battery cell group. In addition, the switching control circuit BT 03  may preferentially select the battery cells BT 09  which are almost overcharged or over-discharged as the discharge battery cell group or the charge battery cell group 
     Here, operation examples of the switching control circuit BT 03  in this embodiment will be described with reference to  FIGS. 30A to 30C .  FIGS. 30A to 30C  illustrate the operation examples of the switching control circuit BT 03 . Note that  FIGS. 30A to 30C  each illustrate the case where four battery cells BT 09  are connected in series as an example for convenience of explanation. 
       FIG. 30A  shows the case where the relation of voltages Va, Vb, Vc, and Vd is Va=Vb=Vc&gt;Vd where the voltages Va, Vb, Vc, and Vd are the voltages of a battery cell a, a battery cell b, a battery cell c, and a battery cell d, respectively. That is, a series of three high-voltage cells a to c and one low-voltage cell d are connected in series. In this case, the switching control circuit BT 03  selects the series of three high-voltage cells a to c as the discharge battery cell group. In addition, the switching control circuit BT 03  selects the low-voltage cell d as the charge battery cell group. 
     Next,  FIG. 30B  shows the case where the relation of the voltages is Vc&gt;Va=Vb&gt;&gt;Vd. That is, a series of two low-voltage cells a and b, one high-voltage cell c, and one low-voltage cell d which is almost over-discharged are connected in series. In this case, the switching control circuit BT 03  selects the high-voltage cell c as the discharge battery cell group. Since the low-voltage cell d is almost over-discharged, the switching control circuit BT 03  preferentially selects the low-voltage cell d as the charge battery cell group instead of the series of two low-voltage cells a and b. 
     Lastly,  FIG. 30C  shows the case where the relation of the voltages as Va&gt;Vb=Vc=Vd. That is, one high-voltage cell a and a series of three low-voltage cells b to d are connected in series In this case, the switching control circuit BT 03  selects the high-voltage cell a as the discharge battery cell group. In addition, the switching control circuit BT 03  selects the series of three low-voltage cells b to d as the charge battery cell group. 
     On the basis of the determination result shown in the examples of  FIGS. 30A to 30C , the switching control circuit BT 03  outputs the control signal S 1  and the control signal S 2  to the switching circuit BT 04  and the switching circuit BT 05 , respectively. Information showing the discharge battery cell group, which is the connection destination of the switching circuit BT 04 , is set in the control signal S 1 . Information showing the charge battery cell group, which is the connection destination of the switching circuit BT 05  is set in the control signal S 2 . 
     The above is the detailed description of the operations of the switching control circuit BT 03 . 
     The switching circuit BT 04  sets the connection destination of the terminal pair BT 01  at the discharge battery cell group selected by the switching control circuit BT 03 , in response to the control signal S 1  output from the switching control circuit BT 03 . 
     The terminal pair BT 01  includes a pair of terminals A 1  and A 2 . The switching circuit BT 04  connects one of the pair of terminals A 1  and A 2  to a positive electrode terminal of the battery cell BT 09  positioned on the most upstream side (on the high potential side) of the discharge battery cell group, and the other to a negative electrode terminal of the battery cell BT 09  positioned on the most downstream side (on the low potential side) of the discharge battery cell group. Note that the switching circuit BT 04  can recognize the position of the discharge battery cell group on the basis of the information set in the control signal S 1 . 
     The switching circuit BT 05  sets the connection destination of the terminal pair BT 02  at the charge battery cell group selected by the switching control circuit BT 03 , in response to the control signal S 2  output from the switching control circuit BT 03 . 
     The terminal pair BT 02  includes a pair of terminals B 1  and B 2 . The switching circuit BT 05  sets the connection destination of the terminal pair BT 02  by connecting one of the pair of terminals B 1  and B 2  to a positive electrode terminal of the battery cell BT 09  positioned on the most upstream side (on the high potential side) of the charge battery cell group, and the other to a negative electrode terminal of the battery cell BT 09  positioned on the most downstream side (on the low potential side) of the charge battery cell group. Note that the switching circuit BT 05  can recognize the position of the charge battery cell group on the basis of the information set in the control signal S 2 . 
       FIG. 31  and  FIG. 32  are circuit diagrams showing configuration examples of the switching circuits BT 04  and BT 05 . 
     In  FIG. 31 , the switching circuit BT 04  includes a plurality of transistors BT 10 , a bus BT 11 , and a bus BT 12 . The bus BT 11  is connected to the terminal A 1 . The bus BT 12  is connected to the terminal A 2 . Sources or drains of the plurality of transistors BT 10  are connected alternately to the bus BT 11  and the bus BT 12 . The sources or drains which are not connected to the bus BT 11  and the bus BT 12  of the plurality of transistors BT 10  are each connected between two adjacent battery cells BT 09 . 
     The source or drain of the transistor BT 10  on the most upstream side of the plurality of transistors BT 10  is connected to the positive electrode terminal of the battery cell BT 09  on the most upstream side of the battery portion BT 08 . The source or drain of the transistor BT 10  on the most downstream side of the plurality of transistors BT 10  is connected to the negative electrode terminal of the battery cell BT 09  on the most downstream side of the battery portion BT 08 . 
     The switching circuit BT 04  connects the discharge battery cell group to the terminal pair BT 01  by bringing one of the plurality of transistors BT 10  which are connected to the bus BT 11  and one of the plurality of transistors BT 10  which are connected to the bus BT 12  into an on state in response to the control signal S 1  supplied to gates of the plurality of transistors BT 10 . Accordingly, the positive electrode terminal of the battery cell BT 09  on the most upstream side of the discharge battery cell group is connected to one of the pair of terminals A 1  and A 2 . In addition, the negative electrode terminal of the battery cell BT 09  on the most downstream side of the discharge battery cell group is connected to the other of the pair of terminals A 1  and A 2  (i.e., a terminal which is not connected to the positive electrode terminal). 
     An OS transistor is preferably used as the transistor BT 10 . Since the off-state current of the OS transistor is low, the amount of charge that leaks from the battery cell which does not belong to the discharge battery cell group can be reduced, and reduction in capacity with the lapse of time can be suppressed. In addition, dielectric breakdown is unlikely to occur in the OS transistor when a high voltage is applied. Therefore, the battery cell BT 09  and the terminal pair BT 01 , which are connected to the transistor BT 10  in an off state, can be insulated from each other even when the output voltage of the discharge battery cell group is high. 
     In  FIG. 31 , the switching circuit BT 05  includes a plurality of transistors BT 13 , a current control switch BT 14 , a bus BT 15 , and a bus BT 16 . The bus BT 15  and the bus BT 16  are provided between the plurality of transistors BT 13  and the current control switch BT 14 . Sources or drains of the plurality of transistors BT 13  are connected alternately to the bus BT 15  and the bus BT 16 . The sources or drains which are not connected to the bus BT 15  and the bus BT 16  of the plurality of transistors BT 13  are each connected between two adjacent battery cells BT 09 . 
     The source or drain of the transistor BT 13  on the most upstream side of the plurality of transistors BT 13  is connected to the positive electrode terminal of the battery cell BT 09  on the most upstream side of the battery portion BT 08 . The source or a drain of the transistor BT 13  on the most downstream side of the plurality of transistors BT 13  is connected to the negative electrode terminal of the battery cell BT 09  on the most downstream side of the battery portion BT 08 . 
     An OS transistor is preferably used as the transistors BT 13  like the transistors BT 10 . Since the off-state current of the OS transistor is low, the amount of charge that leaks from the battery cells which do not belong to the charge battery cell group can be reduced, and reduction in capacity with the lapse of time can be suppressed. In addition, dielectric breakdown is unlikely to occur in the OS transistor when a high voltage is applied. Therefore, the battery cell BT 09  and the terminal pair BT 02 , which are connected to the transistor BT 13  in an off state, can be insulated from each other even when a voltage for charging the charge battery cell group is high. 
     The current control switch BT 14  includes a switch pair BT 17  and a switch pair BT 18 . Terminals on one end of the switch pair BT 17  are connected to the terminal B 1 . Terminals on the other end of the switch pair BT 17  branch off from two switches. One switch is connected to the bus BT 15 , and the other switch is connected to the bus BT 16 . Terminals on one end of the switch pair BT 18  are connected to the terminal B 2 . Terminals on the other end of the switch pair BT 8  extend from two switches. One switch is connected to the bus BT 15 , and the other switch is connected to the bus BT 16 . 
     OS transistors are preferably used for the switches included in the switch pair BT 17  and the switch pair  1 T 18  like the transistors BT 10  and BT 13 . 
     The switching circuit BT 05  connects the charge battery cell group and the terminal pair BT 02  by controlling the combination of on and off states of the transistors BT 13  and the current control switch BT 14  in response to the control signal S 2 . 
     For example, the switching circuit BT 05  connects the charge battery cell group and the terminal pair BT 02  in the following manner. 
     The switching circuit BT 05  brings a transistor BT 13  connected to the positive electrode terminal of the battery cell BT 09  on the most upstream side of the charge battery cell group into an on state in response to the control signal S 2  supplied to gates of the plurality of transistors BT 13 . In addition, the switching circuit BT 05  brings a transistor BT 13  connected to the negative electrode terminal of the battery cell BT 09  on the most downstream side of the charge battery cell group into an on state in response to the control signal S 2  supplied to the gates of the plurality of transistors BT 13 . 
     The polarities of voltages applied to the terminal pair BT 02  can vary in accordance with the configurations of the voltage transformer circuit BT 07  and the discharge battery cell group connected to the terminal pair DT  01 . In order to supply a current in the direction for charging the charge battery cell group, terminals with the same polarity of the terminal pair BT 02  and the charge battery cell group are required to be connected to each other. In view of this, the current control switch BT 14  is controlled by the control signal S 2  so that the connection destination of the switch pair BT 17  and that of the switch pair BT 18  are changed in accordance with the polarities of the voltages applied to the terminal pair BT 02 . 
     The state where voltages are applied to the terminal pair BT 02  so as to make the terminal B 1  a positive electrode and the terminal B 2  a negative electrode will be described as an example. Here, in the case where the battery cell BT 09  positioned on the most downstream side of the battery portion BT 08  is in the charge battery cell group, the switch pair BT 17  is controlled to be connected to the positive electrode terminal of the battery cell BT 09  in response to the control signal S 2 . That is, the switch of the switch pair BT 17  connected to the bus BT 16  is turned on, and the switch of the switch pair BT 17  connected to the bus BT 15  is turned off. In contrast, the switch pair BT 18  is controlled to be connected to the negative electrode terminal of the battery cell BT 09  positioned on the most downstream side of the battery portion BT 08  in response to the control signal S 2 . That is, the switch of the switch pair BT 18  connected to the bus BT 15  is turned on, and the switch of the switch pair BT 18  connected to the bus BT 16  is turned off. In this manner, terminals with the same polarity of the terminal pair BT 02  and the charge battery cell group are connected to each other. In addition, the current which flows from the terminal pair BT 02  is controlled to be supplied in a direction so as to charge the charge battery cell group. 
     In addition, instead of the switching circuit BT 05 , the switching circuit BT 04  may include the current control switch BT 14 . 
       FIG. 32  is a circuit diagram illustrating configuration examples of the switching circuit BT 04  and the switching circuit BT 05  which are different from those of  FIG. 31 . 
     In  FIG. 32 , the switching circuit BT 04  includes a plurality of transistor pairs BT 21 , a bus BT 24 , and a bus BT 25 . The bus BT 24  is connected to the terminal A 1 . The bus BT 25  is connected to the terminal A 2 . Terminals on one end of each of the plurality of transistor pairs BT 21  branch off from a transistor BT 22  and a transistor BT 23 . Sources or drains of the transistors BT 22  are connected to the bus BT 24 . Sources or drains of the transistors BT 23  are connected to the bus BT 25 . In addition, terminals on the other end of each of the plurality of transistor pairs are connected between two adjacent battery cells BT 09 . The terminals on the other end of the transistor pair BT 21  on the most upstream side of the plurality of transistor pairs BT 21  are connected to the positive electrode terminal of the battery cell BT 09  on the most upstream side of the battery portion BT 08 . The terminals on the other end of the transistor pair BT 21  on the most downstream side of the plurality of transistor pairs BT 21  are connected to a negative electrode terminal of the battery cell BT 09  on the most downstream side of the battery portion BT 08 . 
     The switching circuit BT 04  switches the connection destination of the transistor pair BT 21  to one of the terminal A 1  and the terminal A 2  by turning on or off the transistors BT 22  and BT 23  in response to the control signal S. Specifically, when the transistor BT 22  is turned on, the transistor BT 23  is turned off, so that the connection destination of the transistor pair BT 21  is the terminal A 1 . On the other hand, when the transistor BT 23  is turned on, the transistor BT 22  is turned off so that the connection destination of the transistor pair BT 21  is the terminal A 2 . Which of the transistors BT 22  and BT 23  is turned on is determined by the control signal S 1 . 
     Two transistor pairs BT 21  are used to connect the terminal pair BT 01  and the discharge battery cell group. Specifically, the connection destinations of the two transistor pairs BT 21  are determined on the basis of the control signal S 1 , and the discharge battery cell group and the terminal pair BT 01  are connected. The connection destinations of the two transistor pairs BT 21  are controlled by the control signal S 1  so that one of the connection destinations is the terminal A 1  and the other is the terminal A 2 . 
     The switching circuit BT 05  includes a plurality of transistor pairs BT 31 , a bus BT 34 , and a bus BT 35 . The bus BT 34  is connected to the terminal B 1 . The bus BT 35  is connected to the terminal B 2 . Terminals on one end of each of the plurality of transistor pairs BT 31  branch off from a transistor BT 32  and a transistor BT 33 . One terminal extending from the transistor BT 32  is connected to the bus BT 34 . The other terminal extending from the transistor BT 33  is connected to the bus BT 35 . Terminals on the other end of each of the plurality of transistor pairs BT 31  are connected between two adjacent battery cells BT 09 . The terminal on the other end of the transistor pair BT 31  on the most upstream side of the plurality of transistor pairs BT 31  is connected to the positive electrode terminal of the battery cell BT 09  on the most upstream side of the battery portion BT 08 . The terminal on the other end of the transistor pair BT 31  on the most downstream side of the plurality of transistor pairs BT 31  is connected to the negative electrode terminal of the battery cell BT 09  on the most downstream side of the battery portion BT 08 . 
     The switching circuit BT 05  switches the connection destination of the transistor pair BT 31  to one of the terminal B 1  and the terminal B 2  by turning on or off the transistors BT 32  and BT 33  in response to the control signal S 2 . Specifically, when the transistor BT 32  is turned on, the transistor BT 33  is turned off, so that the connection destination of the transistor pair BT 31  is the terminal B 1 . On the other hand, when the transistor BT 33  is turned on, the transistor BT 32  is turned off, so that the connection destination of the transistor pair BT 31  is the terminal B 2 . Which of the transistors BT 32  and BT 33  is turned on is determined by the control signal S 2 . 
     Two transistor pairs BT 31  are used to connect the terminal pair BT 02  and the charge battery cell group. Specifically, the connection destinations of the two transistor pairs BT 31  are determined on the basis of the control signal S 2 , and the charge battery cell group and the terminal pair BT 02  are connected. The connection destinations of the two transistor pairs BT 31  are controlled by the control signal S 2  so that one of the connection destinations is the terminal B 1  and the other is the terminal B 2 . 
     The connection destinations of the two transistor pairs BT 31  are determined by the polarities of the voltages applied to the terminal pair BT 02 . Specifically, in the case where voltages which make the terminal B 1  a positive electrode and the terminal B 2  a negative electrode are applied to the terminal pair BT 02 , the transistor pair BT 31  on the upstream side is controlled by the control signal S 2  so that the transistor BT 32  is turned on and the transistor BT 33  is turned off. In contrast, the transistor pair BT 31  on the downstream side is controlled by the control signal S 2  so that the transistor BT 33  is turned on and the transistor BT 32  is turned off. In the case where voltages which make the terminal B 1  a negative electrode and the terminal B 2  a positive electrode are applied to the terminal pair BT 02 , the transistor pair BT 31  on the upstream side is controlled by the control signal S 2  so that the transistor BT 33  is turned on and the transistor BT 32  is turned off. In contrast, the transistor pair BT 31  on the downstream side is controlled by the control signal S 2  so that the transistor BT 32  is turned on and the transistor BT 33  is turned off. In this manner, terminals with the same polarity of the terminal pair BT 02  and the charge battery cell group are connected to each other. In addition, the current which flows from the terminal pair BT 02  is controlled to be supplied in the direction for charging the charge battery cell group. 
     The voltage transformation control circuit BT 06  controls the operation of the voltage transformer circuit BT 07 . The voltage transformation control circuit BT 06  generates a voltage transformation signal S 3  for controlling the operation of the voltage transformer circuit BT 07  on the basis of the number of the battery cells BT 09  included in the discharge battery cell group and the number of the battery cells BT 09  included in the charge battery cell group and outputs the voltage transformation signal S 3  to the voltage transformer circuit BT 07 . 
     In the case where the number of the battery cells BT 09  included in the discharge battery cell group is larger than that included in the charge battery cell group, it is necessary to prevent a charging voltage which is too high from being applied to the charge battery cell group. Thus, the voltage transformation control circuit BT 06  outputs the voltage transformation signal S 3  for controlling the voltage transformer circuit BT 07  so that a discharging voltage (Vdis) is lowered within a range where the charge battery cell group can be charged. 
     In the case where the number of the battery cells BT 09  included in the discharge battery cell group is less than or equal to that included in the charge battery cell group, a charging voltage necessary for charging the charge battery cell group needs to be ensured. Therefore, the voltage transformation control circuit BT 06  outputs the voltage transformation signal S 3  for controlling the voltage transformer circuit BT 07  so that the discharging voltage (Vdis) is raised within a range where a charging voltage which is too high is not applied to the charge battery cell group. 
     The voltage value of the charging voltage which is too high is determined in the light of product specifications and the like of the battery cell BT 09  used in the battery portion BT 08 . The voltage which is raised or lowered by the voltage transformer circuit BT 07  is applied as a charging voltage (Vcha) to the terminal pair BT 02 . 
     Here, operation examples of the voltage transformation control circuit BT 06  in this embodiment will be described with reference to  FIGS. 33A to 33C .  FIGS. 33A  to  33 C are conceptual diagrams for explaining the operation examples of the voltage transformation control circuits BT 06  for the discharge battery cell groups and the charge battery cell groups described in  FIGS. 30A to 30C .  FIGS. 33A to 33C  each illustrate a battery control unit BT 41 . The battery control unit BT 41  includes the terminal pair BT 01 , the terminal pair BT 02 , the switching control circuit BT 03 , the switching circuit BT 04 , the switching circuit BT 05 , the voltage transformation control circuit BT 06 , and the voltage transformer circuit BT 07 . 
     In an example illustrated in  FIG. 33A , the series of three high-voltage cells a to c and one low-voltage cell d are connected in series as described in  FIG. 30A . In this case, as described using  FIG. 30A , the switching control circuit BT 03  determines the high-voltage cells a to c as the discharge battery cell group, and determines the low-voltage cell d as the charge battery cell group. The voltage transformation control circuit BT 06  calculates a conversion ratio N for converting the discharging voltage (Vdis) into the charging voltage (Vcha) based on the ratio of the number of the battery cells BT 09  included in the charge battery cell group to the number of the battery cells BT 09  included in the discharge battery cell group. 
     In the case where the number of the battery cells BT 09  included in the discharge battery cell group is larger than that included in the charge battery cell group, when a discharging voltage is applied to the terminal pair BT 02  without transforming the voltage, an overvoltage may be applied to the battery cells BT 09  included in the charge battery cell group through the terminal pair BT 02 . Thus, in the case of  FIG. 33A , it is necessary that a charging voltage (Vcha) applied to the terminal pair BT 02  be lower than the discharging voltage. In addition, in order to charge the charge battery cell group, it is necessary that the charging voltage be higher than the total voltage of the battery cells BT 09  included in the charge battery cell group. Thus, the voltage transformation control circuit BT 06  sets the conversion ratio N for raising or lowering voltage larger than the ratio of the number of the battery cells BT 09  included in the charge battery cell group to the number of the battery cells BT 09  included in the discharge battery cell group. 
     Thus, the voltage transformation control circuit BT 06  preferably sets the conversion ratio N for raising or lowering voltage larger than the ratio of the number of the battery cells BT 09  included in the charge battery cell group to the number of the battery cells BT 09  included in the discharge battery cell group by about 1% to 10%. Here, the charging voltage is made higher than the voltage of the charge battery cell group, but the charging voltage is equal to the voltage of the charge battery cell group in reality. Note that the voltage transformation control circuit BT 06  feeds a current for charging the charge battery cell group in accordance with the conversion ratio N for raising or lowering voltage in order to make the voltage of the charge battery cell group equal to the charging voltage. The value of the current is set by the voltage transformation control circuit BT 06 . 
     In the example illustrated in  FIG. 33A , since the number of the battery cells BT 09  included in the discharge battery cell group is three and the number of the battery cells BT 09  included in the charge battery cell group is one, the voltage transformation control circuit BT 06  calculates a value which is slightly larger than 1/3 as the conversion ratio N for raising or lowering voltage. Then, the voltage transformation control circuit BT 06  outputs the voltage transformation signal S 3 , which lowers the discharging voltage in accordance with the conversion ratio N for raising or lowering voltage and converts the voltage into a charging voltage, to the voltage transformer circuit BT 07 . The voltage transformer circuit BT 07  applies the charging voltage which is obtained by transformation in response to the voltage transformation signal S 3  to the terminal pair BT 02 . Then, the battery cells BT 09  included in the charge battery cell group are charged with the charging voltage applied to the terminal pair BT 02 . 
     In each of examples illustrated in  FIGS. 33B and 33C , the conversion ratio N for raising or lowering voltage is calculated in a manner similar to that of  FIG. 33A . In each of the examples illustrated in  FIGS. 33B and 33C , since the number of the battery cells BT 09  included in the discharge battery cell group is less than or equal to the number of the battery cells BT 09  included in the charge battery cell group, the conversion ratio N for raising or lowering voltage in 1 or more. Therefore, in this case, the voltage transformation control circuit BT 06  outputs the voltage transformation signal S 3  for raising the discharging voltage and converting the voltage into the charging voltage. 
     The voltage transformer circuit BT 07  converts the discharging voltage applied to the terminal pair BT 01  into a charging voltage in response to the voltage transformation signal S 3 . The voltage transformer circuit BT 07  applies the charging voltage to the terminal pair BT 02 . Here, the voltage transformer circuit BT 07  electrically insulates the terminal pair BT 01  from the terminal pair BT 02 . Accordingly, the voltage transformer circuit BT 07  prevents a short circuit due to a difference between the absolute voltage of the negative electrode terminal of the battery cell BT 09  on the most downstream side of the discharge battery cell group and the absolute voltage of the negative electrode terminal of the battery cell BT 09  on the most downstream side of the charge battery cell group. Furthermore, the voltage transformer circuit BT 07  converts the discharging voltage, which is the total voltage of the discharge battery cell group, into the charging voltage in response to the voltage transformation signal S 3  as described above. 
     An insulated direct current (DC)-DC converter or the like can be used in the voltage transformer circuit BT 07 . In that case, the voltage transformation control circuit BT 06  controls the charging voltage converted by the voltage transformer circuit BT 07  by outputting a signal for controlling the on/off ratio (the duty ratio) of the insulated DC-DC converter as the voltage transformation signal S 3 . 
     Examples of the insulated DC-DC converter include a flyback converter, a forward converter, a ringing choke converter (RCC), a push-pull converter, a half-bridge converter, and a full-bridge converter, and a suitable converter is selected in accordance with the value of the intended output voltage. 
     The configuration of the voltage transformer circuit BT 07  including the insulated DC-DC converter is illustrated in  FIG. 34 . An insulated DC-DC converter BT 51  includes a switch portion BT 52  and a transformer BT 53 . The switch portion BT 52  is a switch for switching on/off of the insulated DC-DC converter, and a metal oxide semiconductor field-effect transistor (MOSFET), a bipolar transistor, or the like is used as the switch portion BT 52 . The switch portion BT 52  periodically turns on and off the insulated DC-DC converter BT 51  in response to the voltage transformation signal S 3  for controlling the on/off ratio which is output from the voltage transformation control circuit BT 06 . The switch portion BT 52  can have any of various structures in accordance with the type of the insulated DC-DC converter which is used. The transformer BT 53  converts the discharging voltage applied from the terminal pair BT 01  into the charging voltage. In detail, the transformer BT 53  operates in conjunction with the on/off state of the switch portion BT 52  and converts the discharging voltage into the charging voltage in accordance with the on/off ratio. As the time during which the switch portion BT 52  is on becomes longer in its switching period, the charging voltage is increased. On the other hand, as the time during which the switch portion BT 52  is on becomes shorter in its switching period, the charging voltage is decreased. In the case where the insulated DC-DC converter is used, the terminal pair BT 01  and the terminal pair BT 02  can be insulated from each other inside the transformer BT 53 . 
     A flow of operations of the power storage device BT 00  in this embodiment will be described with reference to  FIG. 35 .  FIG. 35  is a flow chart showing the flow of the operations of the power storage device BT 00 . 
     First, the power storage device BT 00  obtains a voltage measured for each of the plurality of battery cells BT 09  (step S 101 ). Then, the power storage device BT 00  determines whether or not the condition for starting the operation of reducing variations in voltage of the plurality of battery cells BT 09  is satisfied (step S 102 ). For example, the condition that the difference between the maximum value and the minimum value of the voltage measured for each of the plurality of battery cells BT 09  is higher than or equal to the predetermined threshold value can be used. In the case where the condition is not satisfied (step S 102 : NO), the power storage device BT 00  does not perform the following operation because voltages of the battery cells BT 09  are well balanced. In contrast, in the case where the condition is satisfied (step S 102 : YES), the power storage device BT 00  performs the operation of reducing variations in the voltage of the battery cells BT 09 . In this operation, the power storage device BT 00  determines whether each battery cell BT 09  is a high voltage cell or a low voltage cell on the basis of the measured voltage of each cell (step S 103 ). Then, the power storage device BT 00  determines a discharge battery cell group and a charge battery cell group on the basis of the determination result (step S 104 ). In addition, the power storage device BT 00  generates the control signal S 1  for setting the connection destination of the terminal pair BT 01  to the determined discharge battery cell group, and the control signal S 2  for setting the connection destination of the terminal pair BT 02  to the determined charge battery cell group (step S 105 ). The power storage device BT 00  outputs the generated control signals S 1  and S 2  to the switching circuit BT 04  and the switching circuit BT 05 , respectively. Then, the switching circuit BT 04  connects the terminal pair BT 01  and the discharge battery cell group, and the switching circuit BT 05  connects the terminal pair BT 02  and the discharge battery cell group (step S 106 ). The power storage device BT 00  generates the voltage transformation signal S 3  based on the number of the battery cells BT 09  included in the discharge battery cell group and the number of the battery cells BT 09  included in the charge battery cell group (step S 107 ). Then, the power storage device BT 00  converts, in response to the voltage transformation signal S 3 , the discharging voltage applied to the terminal pair BT 01  into a charging voltage and applies the charging voltage to the terminal pair BT 02  (step S 108 ). In this way, charge of the discharge battery cell group is transferred to the charge battery cell group. 
     Although the plurality of steps are shown in order in the flow chart of  FIG. 35 , the order of performing the steps is not limited to the order. 
     According to the above embodiment, when charge is transferred from the discharge battery cell group to the charge battery cell group, a structure where charge from the discharge battery cell group is temporarily stored, and the stored charge is sent to the charge battery cell group is unnecessary, unlike in the a capacitive type circuit. Accordingly, the charge transfer efficiency per unit time can be increased. In addition, the switching circuit BT 04  and the switching circuit BT 05  determine which battery cell in the discharge battery cell group and the charge battery cell group to be connected to the voltage transformer circuit. 
     Furthermore, the voltage transformer circuit BT 07  converts the discharging voltage applied to the terminal pair BT 01  into the charging voltage based on the number of the battery cells BT 09  included in the discharge battery cell group and the number of the battery cells BT 09  included in the charge battery cell group, and applies the charging voltage to the terminal pair BT 02 . Thus, charge can be transferred without any problems regardless of how the battery cells BT 09  are selected as the discharge battery cell group and the charge battery cell group. 
     Furthermore, the use of OS transistors as the transistor BT 10  and the transistor BT 13  can reduce the amount of charge that leaks from the battery cells BT 09  not belonging to the charge battery cell group or the discharge battery cell group. Accordingly, a decrease in capacity of the battery cells BT 09  which do not contribute to charging or discharging can be suppressed. In addition, the variations in characteristics of the OS transistor due to heat are smaller than those of a S 1  transistor. Accordingly, even when the temperature of the battery cells BT 09  is increased, an operation such as turning on or off the transistors in response to the control signals S 1  and S 2  can be performed normally. 
     This embodiment can be implemented in combination with any of the other embodiments as appropriate. 
     Example 1 
     In this example, electrolytic solutions were subjected to cyclic voltammetry (CV) measurement. 
     Four kinds of electrolytic solutions, Electrolytic Solution A-1, Electrolytic Solution A-2, Electrolytic Solution A-3, and Electrolytic Solution A-4, were prepared. In each of the four kinds of electrolytic solutions, 1-ethyl-3-methylimidazolium bis(fluorosulfonyl)amide (EMI-FSA) was used as a solvent. In each of Electrolytic Solutions A-2, A-3, and A-4, lithium bis(fluorosulfonyl)amide (LiFSA) was used as an electrolyte. In Electrolytic Solution A-1, an electrolyte was not used. The electrolyte concentrations are 1 mol/L in Electrolytic Solution A-2, 0.1 mol/L in Electrolytic Solution A-3, and 0.01 mol/L in Electrolytic Solution A-4. 
     Lithium, copper, and platinum were used as a reference electrode, a working electrode, and a counter electrode, respectively. The area of the working electrode was 0.02 cm 2 . In an electrolytic solution for the reference electrode, N-methyl-N-propylpiperidinium bis(trifluoromethanesulfonyl)amide (PP13-TFSA) was used as a solvent and lithium bis(trifluoromethanesulfonyl)amide (LiTFSA) was used as an electrolyte. The electrolyte concentration in the electrolytic solution was 0.4 mol/L. 
     The scanning rate was 50 mV/s, and the voltage range was from 3 V to −0.5 V.  FIGS. 36A and 36B  and  FIGS. 37A and 37B  show CV measurement results of Electrolytic Solutions A-1, A-2, A-3, and A-4, respectively. The horizontal axis represents potential (vs. Li/Li + ), and the vertical axis represents current density. 
     For Electrolytic Solution A-1, which does not use a lithium salt, a reduction current was observed at approximately 0.7 V, which suggests a reduction reaction of EMI-FSA. For Electrolytic Solution A-2, a reduction current at approximately 0.7 V observed in the case of Electrolytic Solution A-1 was inhibited and a reduction current and an oxidation current at approximately 0 V, which suggest lithium deposition and lithium dissolution, respectively, were observed. These results imply that in Electrolytic Solution A-2, lithium ions form an electric double layer on a surface of the electrode, resulting in inhibition of a reduction reaction of either or both of EMI cations and FSA anions. 
     For Electrolytic Solution A-3, a noticeable reduction current was not observed until approximately 0 V, which suggests inhibition of decomposition of EMI; however, an oxidation current, which suggests lithium deposition, was not observed. In contrast, for Electrolytic Solution A-4, a reduction current was observed at a potential higher than 0 V, which suggests that a reduction reaction of either or both of EMI cations and FSA anions occurs. 
       FIGS. 38A and 38B  show results of second and third cycles of CV measurement of Electrolytic Solution A-4. The waveforms of the second and third cycles of CV measurement of Electrolytic Solution A-4 were almost the same as the waveform of CV measurement of Electrolytic Solution A-1, which does not use a lithium salt. Presumably, in Electrolytic Solution A-4, the concentration of a lithium salt is too low to allow sufficient formation of an electric double layer on the electrode surface, so that reduction reactions of EMI cations and the like cannot be inhibited. 
     The CV measurement was performed on Electrolytic Solution A-4 at a sweep rate of 0.5 mV/s within the voltage range from 3 V to −0.3 V, and it was found that a reduction reaction is inhibited.  FIG. 39  shows the results. 
     Next, Electrolytic Solutions A-5 and A-6 were prepared. In each of Electrolytic Solutions A-5 and A-6, EMI-TFSA was used as a solvent and LiTFSA was used as an electrolyte. The electrolyte concentrations were 1 mol/L in Electrolytic Solution A-5 and 2 mol/l, in Electrolytic Solution A-6 
       FIG. 40  shows results of CV measurement of Electrolytic Solution A-5 at a sweep rate of 0.5 mV/s within the voltage range from 3 V to −0.15 V.  FIG. 41A  shows results of CV measurement of Electrolytic Solution A-6 at a sweep rate of 0.5 mV/s within the voltage range from 3 V to 0 V.  FIG. 41B  shows results of CV measurement of Electrolytic Solution A-6 at a sweep rate of 0.1 mV/s within the voltage range from 3 V to 0 V. Electrolytic Solution A-6, in which the concentration of a lithium salt is 2 mol/L, shows a peak suggesting lithium dissolution, which indicates that a formed electric double layer might inhibit reduction reactions of EMI cations and the like. 
     Example 2 
     In this example, storage batteries were fabricated and the charge and discharge cycles thereof were measured. 
     First, Electrolytic Solutions A-7 and AC-1 were prepared. In Electrolytic Solution A-7, BMI-FSA was used as a solvent and LiFSA was used as an electrolyte. The electrolyte concentration in Electrolytic Solution A-7 was 1.5 mol/L. In Electrolytic Solution AC-1, which is a comparative electrolytic solution, a solvent obtained by mixing EC and DEC at a volume ratio of EC:DEC=3:7 was used and LiPF 6  was used as an electrolyte. The electrolyte concentration in Electrolytic Solution AC-1 was 1 mol/L. 
     [Fabrication of Electrodes] 
     Next, Negative Electrode E1 and Positive Electrodes E2 and E3 were prepared. 
     The compounding and fabricating method of Negative Electrode E1 will be described. Carbon-coated SiO was used as an active material, polyimide was used as a binder, and acetylene black was used as a conductive additive. In a slurry for fabricating the electrode, the compounding ratio of SiO to acetylene black and polyimide was 80:5:15 (wt %). 
     First, acetylene black and NMP serving as a solvent were mixed with a mixer, so that a first mixture was obtained. 
     Next, the active material was added to the first mixture, so that a second mixture was obtained. 
     After that, an NMP solution of polyimide was added to the second mixture and mixing was performed with a mixer. Through the above steps, the slurry was formed. 
     The formed slurry was applied to one surface of a 10-μm-thick stainless-steel current collector. The application was performed with a continuous coater at a coating speed of 0.15 m/min. After that, drying was performed using a drying furnace at 40° C. for 20 minutes. 
     Then, the fabricated electrode was subjected to heat treatment for imidization. The heat treatment was performed with a muffle furnace at 400° C. in a nitrogen atmosphere for 5 hours. Through the above steps, the negative electrode was fabricated. 
     Next, Positive Electrodes E2 and E3 were prepared. 
     The compounding and fabricating conditions of Positive Electrode E2 will be described. LiCoO 2  with an average particle size of 10 μm was used as an active material, PVdF was used as a binder, and acetylene black was used as a conductive additive. In a slurry for fabricating the electrode, the compounding ratio of LiCoO 2  to acetylene black and PVdF was 90:5:5 (wt %). 
     The active material, the binder, and the conductive additive were mixed to form a slurry. The slurry was applied to one surface of a 20-μm-thick aluminum current collector. Then, the solvent was volatilized. After that, the positive electrode active material layer was pressed. Through the above steps, Positive Electrode E2 was fabricated. 
     Next, the compounding and manufacturing conditions of Positive Electrode E3 will be described. LiFePO 4  with a specific surface area of 15.6 m 2 /g was used as an active material, PVdF was used as a binder, and acetylene black was used as a conductive additive. In a slurry for fabricating the electrode, the compounding ratio of LiFePO 4  to acetylene black and PVdF was 85:8:7 (wt %). 
     First, acetylene black and PVdF serving as a binder were mixed with a mixer, so that a first mixture was obtained. 
     Next, the active material was added to the first mixture, so that a second mixture was obtained. 
     After that, a solvent NMP was added to the second mixture and mixing was performed with a mixer. Through the above steps, the slurry was formed. 
     Then, mixing was performed with a large-sized mixer. 
     Then, the formed slurry was applied to one surface of a 20-μm-thick aluminum current collector subjected to undercoating in advance. The application was performed with a continuous coater at a coating speed of 0.2 m/min. Then, the solvent was volatilized with a drying furnace. The solvent volatilization in the drying furnace was performed at 70° C. for 7.5 minutes and then further performed at 90° C. for 7.5 minutes. 
     Subsequently, the positive electrode active material layer was pressed by a roll press method so as to be consolidated. Through the above steps, Positive Electrode E3 was fabricated. 
     [Fabrication of Storage Batteries] 
     Next, Negative Electrode E1, Positive Electrode E2, and Positive Electrode E3 were cut, and lead electrodes were bonded to respective tab regions by welding. The area of Negative Electrode E1 was 23.8 cm 2 , and the area of each of Positive Electrodes E2 and E3 was 20.5 cm 2 . 
     Then, Negative Electrode E1 and Positive Electrode E2 were provided so as to face each other with a first separator therebetween, a second separator was provided over Positive Electrode E2, and Positive Electrode E3 was provided over the second separator. The second separator. Positive Electrode E2, and the first separator are located closer to Positive Electrode E3 in this order between Negative Electrode E1 and Positive Electrode E3. As each separator. TF40 made with cellulose having a thickness of 40 μm was used. 
     As a sheet for forming an exterior body, an aluminum sheet both surfaces of which are covered with a resin was prepared. The stack of Negative Electrode E1, the first separator, Positive Electrode E2, the second separator, and Positive Electrode E3 was wrapped with the sheet folded in half. 
     Next, two of three sides of the sheet folded in half were sealed with heat, so that the exterior body was formed. End portions of the lead electrodes bonded to the respective electrodes were located outside the sheet. After that, drying was performed at 80° C. 
     Then, an electrolytic solution was injected from a side that was not sealed, in a reduced-pressure atmosphere of −60 kPa or less. The amount of the injected electrolytic solution was approximately 0.6 ml. A storage battery using Electrolytic Solution A-7 is referred to as Storage Battery BA, and a comparative storage battery using Electrolytic Solution AC-1, which is a comparative electrolytic solution, is referred to as Storage Battery BC. 
     The loadings of Negative Electrode E1, Positive Electrode E2, and Positive Electrode E3 used in Storage Battery BA were 2.0 mg/cm 2 , 20 mg/cm 2 , and 9.3 mg/cm 2 , respectively. The densities of Negative Electrode E1, Positive Electrode E2, and Positive Electrode E3 in Storage Battery BA were 0.55 g/cc, approximately 2.2 g/cc, and 1.0 g/cc, respectively. 
     The loadings of Negative Electrode E1, Positive Electrode E2, and Positive Electrode E3 used in Storage Battery BC were 2.0 mg/cm 2 , 20 mg/cm 2 , and 9.4 mg/cm 2 , respectively. The densities of Negative Electrode E1, Positive Electrode E2, and Positive Electrode E3 in Storage Battery BC were 0.63 g/cc, approximately 2.1 g/cc, and 1.1 g/cc, respectively. Note that loading means the weight of an active material per unit area. 
     Then, the side which was not sealed was sealed with heat. Through the above steps, each storage battery was fabricated. 
     [Predoping] 
     Constant current charge was performed at 25° C. at a current density of 17.9 mA/g (corresponding to approximately 0.01 C with respect to the negative electrode active material capacity) using Positive Electrode E2 as a positive electrode and Negative Electrode E1 as a negative electrode up to 600 mAh/g. A 0.5-hour break was taken after the charge. 
     After the charge, one side of the exterior body was cut. Then. Positive Electrode E2 and the second separator were taken out from the side that was cut. 
     Next, 0.3 ml of Electrolytic Solution A-7 was injected into Storage Battery BA, and 0.3 ml of Electrolytic Solution AC-1 was injected into Storage Battery BC. 
     After that, the side cut open was sealed with heat. 
     [Aging] 
     Next, an aging step was performed. First, constant current charge was performed at 25° C. using Positive Electrode E3 as a positive electrode and Negative Electrode E1 as a negative electrode. The charging conditions was as follows: the current density was 1.7 mA/g (corresponding to approximately 0.01 C with respect to the positive electrode active material capacity), and the upper voltage limit was 3.2 V A 2-hour break was taken after the charge. 
     After the charge, one side of the exterior body was cut, and degasification was performed. After that the side cut open was sealed with heat. 
     Then, constant current charge was performed under the conditions that the current density was 8.5 mA/g (corresponding to approximately 0.05 C) and the upper voltage limit was 4 V. After that, constant current discharge was performed under the conditions that the current density was 34 mA/g (corresponding to approximately 0.2 C) and the lower voltage limit was 2 V. After each of the charge and discharge, a 2-hour break was taken. 
     Then, two cycles of constant current charge and discharge were performed at a current density of 34 mA/g.  FIGS. 42A and 42B  show charge and discharge curves of the two cycles. In each of the graphs, the solid lines show the initial charge and discharge, and dotted lines show the second charge and discharge.  FIG. 42A  shows charge and discharge cycles of Storage Battery BC, and  FIG. 42B  shows charge and discharge cycles of Storage Battery BA. The upper charging voltage limit was 4 V, and the lower discharging voltage limit was 2 V. After each of the charge and discharge, a 2-hour break was taken. Through the above steps, aging was performed. 
     [Charge and Discharge Cycles] 
     The charge and discharge cycle performances of Storage Batteries BA and BC subjected to the aging were evaluated. Charge and discharge were performed at a constant current at a current density of 51 mA/g (corresponding to approximately 0.3 C). The upper charging voltage limit was 4 V, and the lower discharging voltage limit was 2 V. After each of the charge and discharge, a 0.5-hour break was taken.  FIG. 43  shows the charge and discharge cycle performances. The horizontal axis represents the number of cycles, and the vertical axis represents discharge capacity. The discharge capacity in the 500th cycle of Storage Battery BC was as low as approximately 60% of the initial discharge capacity, whereas the discharge capacity in the 500th cycle of Storage Battery BA was over 80%, which indicates excellent charge and discharge cycle performance of Storage Battery BA. 
     Example 3 
     Surfaces of the negative electrodes of Storage Batteries BA and BC fabricated in Example 2 were subjected to XPS analysis after charge and discharge cycles. 
     After charge and discharge cycles, Storage Batteries BA and BC were disassembled. After the disassembly, deuterated acetonitrile was added to be mixed with the electrolytic solution left in each of the storage batteries, and the mixed solution was extracted with a pipette. Then, the negative electrode was immersed in DMC, which was put in a petri dish, to be washed. DMC was renewed twice, and washing was performed three times in total. After that, the solvent was volatilized in a reduced-pressure atmosphere. 
     Next, the negative electrodes of Storage Batteries BA and BC were subjected to XPS analysis. Table 1 shows the proportions of C, O, F, S, Li, P, N, Al, and Si (atomic %). 
     
       
         
           
               
               
               
               
               
               
               
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 C 
                 O 
                 F 
                 S 
                 Li 
                 P 
                 N 
                 Al 
                 Si 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
            
               
                 Storage 
                 32 
                 31 
                 7.9 
                 — 
                 28 
                 0.3 
                 0.3 
                 — 
                 0.7 
               
               
                 Battery BC 
               
               
                 Storage 
                 31 
                 38 
                 1.4 
                 3.8 
                 24 
                 — 
                 2.1 
                 — 
                 — 
               
               
                 Battery BA 
               
               
                   
               
            
           
         
       
     
       FIG. 44A  shows C1s spectra.  FIG. 44B  shows O1s spectra.  FIG. 45A  shows F s spectra.  FIG. 45B  shows S2p spectra.  FIG. 46  shows Li1s spectra. 
     A prominent difference was observed between peaks of the S2p spectrum. The SO x  peak of the negative electrode of Storage Battery BA is three or more times the metal-S peak. 
     Example 4 
     Cross sections of the negative electrodes of Storage Batteries BA and BC fabricated in Example 2 were observed after charged and discharge cycles. 
     After charge and discharge cycles, Storage Batteries BA and BC were disassembled. After the disassembly, deuterated acetonitrile was added to be mixed with the electrolytic solution left in each of the storage batteries, and the mixed solution was extracted with a pipette. Then, the negative electrode was immersed in DMC, which was put in a petri dish, to be washed. DMC was renewed twice, and washing was performed three times in total. After that, the solvent was volatilized in a reduced-pressure atmosphere. 
     Next, to observe the cross sections of the negative electrodes, the negative electrodes were sliced by FIB. Then, the cross sections of the negative electrodes were observed with a TEM.  FIGS. 47 and 48  show the observed negative electrodes of Storage Batteries BC and BA, respectively. 
     Then, the cross sections of the negative electrodes were observed with a STEM, and STEM-EDX line analyses were performed as elementary analyses of the following six elements: carbon, oxygen, fluorine, silicon, phosphorus, and sulfur.  FIG. 49A  shows a STEM image of a portion of Storage Battery BC on which line analysis was performed.  FIG. 49B  and  FIG. 51A  each show spectra obtained by the line analysis.  FIG. 50A  shows a STEM image of a portion of Storage Battery BA on which line analysis was performed.  FIG. 501  and  FIG. 518  each show spectra obtained by the line analysis. Note that the vertical axis in  FIG. 49B  and  FIG. 50B  represents signal intensity, and the vertical axis in  FIG. 51A  and  FIG. 51B  represents element content (atomic %). 
     In  FIG. 49B  and  FIG. 51A , a region from the surface to a depth of approximately 0.2 μm is referred to as a third region, a region from a depth of approximately 0.2 μm to a depth of approximately 0.45 μm is referred to as a second region, and a region deeper than a depth of approximately 0.45 μm is referred to as a first region. In the first region, constituents of SiO, which is a negative electrode active material, appear to be mainly detected. In the third region, constituents of a coating film formed by decomposition of the electrolytic solution appear to be mainly detected. The second region is presumably a mixed region of the negative electrode active material and the coating film. As shown in the EDX analysis results of  FIG. 49B , the intensity of carbon is 10 or more times that of silicon in the third region, whereas the intensity of carbon is approximately 1 time that of silicon in the second region and the intensity of carbon is one sixth or less of that of silicon in the first region. As shown in the EDX analysis results of  FIG. 11A , the number of carbon atoms is 20 or more times that of silicon in the third region, whereas the number of carbon atoms is approximately 4 times that of silicon in the second region and the number of carbon atoms is approximately 1 time that of silicon in the first region. 
     In  FIG. 50B  and  FIG. 51B , a region from the surface to a depth of approximately 0.1 μm is referred to as a third region, a region from a depth of approximately 0.1 μm to a depth of approximately 0.2 μm is referred to as a second region, and a region deeper than a depth of approximately 0.2 μm is referred to as a first region. As shown in the EDX analysis results of  FIG. 50B , the intensity of carbon is 10 or more times that of silicon in the third region, whereas the intensity of carbon is approximately 1 time that of silicon in the second region and the intensity of carbon is one sixth or less of that of silicon in the first region. As shown in the EDX analysis results of  FIG. 51B , the number of carbon atoms is 20 or more times that of silicon in the third region, whereas the number of carbon atoms is approximately 3 or more times that of silicon in the second region and the number of carbon atoms is approximately 1 time that of silicon in the first region. 
     Next, EELS line analyses were performed as elementary analyses.  FIG. 52A  shows a SEM image of a portion of Storage Battery BC on which line analysis was performed.  FIG. 52B  shows spectra obtained by the line analysis.  FIG. 53A  shows a SEM image of a portion of Storage Battery BA on which line analysis was performed.  FIG. 53B  shows spectra obtained by the line analysis. Note that the vertical axis represents signal intensity. 
     In  FIG. 52B , a region from the surface to a depth of approximately 0.2 μm is referred to as a third region, a region from a depth of approximately 0.2 μm to a depth of approximately 0.45 μm is referred to as a second region, and a region deeper than a depth of approximately 0.45 μm is referred to as a first region. As shown in the EELS analysis results of  FIG. 52B , the intensity of carbon is 10 or more times that of silicon in the third region, whereas the intensity of carbon is approximately half of that of silicon in the second region and the intensity of carbon is one tenth or less of that of silicon in the first region. 
     In  FIG. 53B , a region from the surface to a depth of approximately 0.1 μm is referred to as a third region, a region from a depth of approximately 0.1 m to a depth of approximately 0.2 μm is referred to as a second region, and a region deeper than a depth of approximately 0.2 μm is referred to as a first region. As shown in the EELS analysis results of  FIG. 53B , the intensity of carbon is 10 or more times that of silicon in the third region, whereas the intensity of carbon is approximately one tenth or less of that of silicon in the first region. 
       FIG. 47  to  FIGS. 53A and 53B  show that the second region in the negative electrode of Storage Battery BA is thinner than that in the negative electrode of Storage Battery BC, which suggests that the surface of the active material is prevented from being cracked and pulverized. 
     This application is based on Japanese Patent Application serial no. 2016-016346 filed with Japan Patent Office on Jan. 29, 2016, the entire contents of which are hereby incorporated by reference.