Patent Publication Number: US-10790490-B2

Title: Battery, battery can, battery pack, electronic device, electric vehicle, electricity storage device, and electric power system

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application claims the benefit of International Application No. PCT/JP2016/003743, filed Aug. 15, 2016, which claims priority to Japanese Application No. 2015-177379, filed Sep. 9, 2015, the disclosures of which are incorporated herein by reference. 
     BACKGROUND 
     The present technology relates to a battery, a battery can, a battery pack, an electronic device, an electric vehicle, an electricity storage device, and an electric power system. 
     BACKGROUND ART 
     In recent years, a lithium ion secondary battery is used inmost electronic devices. In the lithium ion secondary battery, for example, when abnormal heat is applied in an overcharged state, a gas pressure abnormally increases on a can bottom side (bottom side), and rupture of the battery may occur. Particularly in a lithium ion secondary battery with high capacity and high output, not only the amount of gas generated when abnormal heat is applied is large, but also the diameter of a center hole of an electrode body is also reduced. 
     Therefore, release of gas to a sealing portion side (top side) of the battery decreases, and a gas pressure tends to abnormally increase on a can bottom side. 
     In order to prevent such rupture of a battery as described above, there is proposed a battery in which a groove is disposed in a can bottom of a battery can such that the groove portion is broken when abnormal heat is applied to the battery and generated gas is discharged from the can bottom. For example, Patent Document 1 describes that a breaking pressure of a thin wall portion of a bottom portion of a battery case due to a gas pressure generated in a battery is larger than a breaking pressure of a valve body of an explosion-proof sealing plate and is smaller than a withstanding pressure of a battery sealing portion. 
     CITATION LIST 
     Patent Document 
     
         
         Patent Document 1: Japanese Patent Application Laid-Open No. 6-333548 
       
    
     SUMMARY 
     Problems to be Solved by the Invention 
     An object of the present technology is to provide a battery, a battery can, a battery pack, an electronic device, an electric vehicle, an electricity storage device, and an electric power system capable of improving safety when abnormal heat is applied while a decrease in mechanical strength of a bottom portion of the battery can is suppressed. 
     Solutions to Problems 
     In order to solve the above problems, a first aspect of the present technology is a battery including: an electrode body; and a battery can housing the electrode body and having a bottom portion, in which the bottom portion has an arc-shaped groove, an opening angle of the groove with respect to a center of the arc is 0.5 degrees or more and 56 degrees or less, and a ratio of an inner diameter of the groove with respect to an outer diameter of the bottom portion is 44% or more and 77% or less. 
     A second aspect of the present technology is a battery pack including the battery of the first aspect of the present technology and a control unit for controlling the battery. 
     A third aspect of the present technology is an electronic device including the battery of the first aspect of the present technology and receiving electric power from the battery. 
     A fourth aspect of the present technology is an electric vehicle including the battery of the first aspect of the present technology, a converter for converting electric power supplied from the battery into a driving force of a vehicle, and a control device for performing information processing regarding vehicle control on the basis of information regarding the battery. 
     A fifth aspect of the present technology is an electricity storage device including the battery of the first aspect of the present technology and supplying electric power to an electronic device connected to the battery. 
     A sixth aspect of the present technology is an electric power system including the battery of the first aspect of the present technology and receiving electric power from the battery. 
     A seventh aspect of the present technology is a battery can including a bottom portion with an arc-shaped groove, in which an opening angle of the groove with respect to a center of the arc is 0.5 degrees or more and 56 degrees or less, and a ratio of an inner diameter of the groove with respect to an outer diameter of the bottom portion is 44% or more and 77% or less. 
     Effects of the Invention 
     As described above, the present technology can improve safety when abnormal heat is applied while a decrease in mechanical strength of a bottom portion of a battery can is suppressed. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a cross-sectional view illustrating a configuration example of a non-aqueous electrolyte secondary battery according to a first embodiment of the present technology. 
         FIG. 2A  is a plan view exemplifying a can bottom with an arc-shaped groove.  FIG. 2B  is a cross-sectional view taken along line IIB-IIB in  FIG. 2A .  FIG. 2C  is an enlarged cross-sectional view of a portion surrounded by a two-dot chain line in  FIG. 2B . 
         FIG. 3  is a schematic diagram for explaining a flow of heat when abnormal heat is applied to a battery. 
         FIG. 4  is an enlarged cross-sectional view of a part of a wound electrode body illustrated in  FIG. 1 . 
         FIG. 5  is a schematic diagram for explaining a flow of generated gas when abnormal heat is applied to a battery. 
         FIG. 6A  is a cross-sectional view illustrating a configuration example of a can bottom of a non-aqueous electrolyte secondary battery according to Modification Example 1 of the first embodiment of the present technology.  FIG. 6B  is a cross-sectional view illustrating a configuration example of a can bottom of a non-aqueous electrolyte secondary battery according to Modification Example 2 of the first embodiment of the present technology. 
         FIG. 7  is a block diagram illustrating a configuration example of an electronic device according to a second embodiment of the present technology. 
         FIG. 8  is a schematic diagram illustrating a configuration example of an electricity storage system according to a third embodiment of the present technology. 
         FIG. 9  is a schematic diagram illustrating a configuration example of an electric vehicle according to a fourth embodiment of the present technology. 
         FIG. 10A  is a graph illustrating a relationship between an opening angle θ of a groove  11 Gv with respect to a center of a can bottom and a test passing ratio.  FIG. 10B  is a graph illustrating a relationship between a ratio Ra of an inner diameter R in  of a groove with respect to an outer diameter R out  of a can bottom and a test passing ratio. 
         FIG. 11A  is a graph illustrating a relationship between a thickness t of a can bottom at a groove bottom and a test passing ratio.  FIG. 11B  is a graph illustrating a relationship between a width w of a groove and a test passing ratio. 
         FIG. 12  is a graph illustrating a relationship between a volume energy density of a battery and a test passing ratio. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments of the present technology will be described in the following order. 
     1. First embodiment (example of cylinder type battery) 
     2. Second embodiment (examples of battery pack and electronic device) 
     3. Third embodiment (example of electricity storage system) 
     4. Fourth embodiment (example of electric vehicle) 
     1. First Embodiment 
     [Configuration of Battery] 
     Hereinafter, a configuration example of a non-aqueous electrolyte secondary battery (hereinafter, simply referred to as a “battery”) according to a first embodiment of the present technology will be described with reference to  FIG. 1 . For example, this battery is a so-called lithium ion secondary battery in which a capacity of a negative electrode is expressed by a capacity component due to occlusion of lithium (Li) which is an electrode reactant and release thereof. This battery is a so-called cylinder type battery, and includes a wound electrode body  20  obtained by stacking a pair of strip-shaped positive electrode  21  and strip-shaped negative electrode  22  through a separator  23  and winding the resulting stacked body in an approximately hollow cylinder-shaped battery can  11 . The battery can  11  is constituted by nickel (Ni)-plated iron (Fe), and one end thereof is closed and the other end thereof is open. An electrolytic solution as an electrolyte is injected into the battery can  11 , and the positive electrode  21 , the negative electrode  22 , and the separator  23  are impregnated with the electrolytic solution. In addition, a pair of insulating plates  12  and  13  is disposed perpendicularly to a winding peripheral surface so as to sandwich the wound electrode body  20 . Note that in the following description, of both end sides of the battery, a closed end side of the battery can  11  may be referred to as a “bottom side”, and an open end side of the battery can  11  on the opposite side thereof may be referred to as a “top side”. 
     A battery lid  14 , a safety valve mechanism  15  disposed inside the battery lid  14 , and a positive temperature coefficient element (PTC element)  16  are attached to the open end of the battery can  11  by being crimped through a sealing gasket  17 . This seals an inside of the battery can  11 . For example, the battery lid  14  is constituted by a material similar to the battery can  11 . The safety valve mechanism  15  cleaves, for example, in a case where gas is generated inside the battery can  11  at the time of abnormality, and discharges the gas from a top side of the battery. In addition, the safety valve mechanism  15  is electrically connected to the battery lid  14 . In a case where an internal pressure of the battery becomes a certain level or more by internal short circuit, heating from an outside, or the like, a disk plate  15 A is reversed to cut electrical connection between the battery lid  14  and the wound electrode body  20 . For example, the sealing gasket  17  is constituted by an insulating material, and a surface thereof is coated with asphalt. 
     The wound electrode body  20  has a substantially cylindrical shape. The wound electrode body  20  has a center hole  20 H penetrating the wound electrode body  20  from a center of one end surface thereof toward a center of the other end surface thereof. A center pin  24  is inserted into the center hole  20 H. The center pin  24  has a tubular shape with both ends open. Therefore, in a case where gas is generated in the battery can  11 , the center pin  24  functions as a flow path guiding the gas from a bottom side to a top side. 
     A positive electrode lead  25  constituted by aluminum (Al) or the like is connected to the positive electrode  21  of the wound electrode body  20 . A negative electrode lead  26  constituted by nickel or the like is connected to the negative electrode  22 . The positive electrode lead  25  is electrically connected to the battery lid  14  by being welded to the safety valve mechanism  15 . The negative electrode lead  26  is welded and electrically connected to the battery can  11 . 
     In the battery according to the first embodiment, an open circuit voltage (that is, battery voltage) in a fully charged state per pair of the positive electrode  21  and the negative electrode  22  may be 4.2 V or less, but may be designed so as to be higher than 4.2 V, preferably 4.4 V or more and 6.0 V or less, and more preferably 4.4 V or more and 5.0 V or less. By a higher battery voltage, a higher energy density can be obtained. 
     Hereinafter, the battery can  11 , the positive electrode  21 , the negative electrode  22 , the separator  23 , and an electrolytic solution constituting the battery according to the first embodiment will be sequentially described. 
     (Battery Can) 
     The battery can  11  has a can bottom  11 Bt as a bottom portion on a bottom side. When the can bottom  11 Bt is viewed from a vertical direction, the can bottom  11 Bt has a circular shape as illustrated in  FIG. 2A . Of both surfaces of the can bottom  11 Bt, a surface inside the battery can  11  (hereinafter, simply referred to as an “inner surface of the can bottom  11 Bt”) has one groove  11 Gv as illustrated in  FIGS. 2A and 2B . The groove  11 Gv has an arc shape, more specifically a C shape or an inverted C shape. A center of the arc of the groove  11 Gv preferably coincides with a center of the can bottom  11 Bt. That is, the arc of the groove  11 Gv is preferably concentric with an outer periphery of the can bottom  11 Bt. 
     The battery can  11  in the first embodiment is preferably used for a battery having a volume energy density of more than 380 Wh/L, and more preferably used for a battery having a volume energy density of 430 Wh/L or more. This is because the amount of gas generated when abnormal heat is applied in an overcharged state or the like is particularly large in a battery having such a volume energy density, and a gas pressure in the battery tends to rise to a level higher than a gas release pressure (operation pressure) of the safety valve mechanism  15 . 
     An opening angle θ of the groove  11 Gv with respect to a center of the arc is 0.5 degrees or more and 56 degrees or less. Here, as illustrated in  FIG. 2A , the opening angle θ of the groove  11 Gv with respect to the center of the arc is obtained as an angle formed by two straight lines connecting both ends of an outer periphery of the groove  11 Gv and a center O of the can bottom  11 Bt. 
     If the opening angle θ is less than 0.5 degrees, since the opening angle θ is small, when abnormal heat is externally applied to a battery in an overcharged state or the like, the entire portion of the groove  11 Gv of the can bottom  11 Bt opens, and the contents of the battery may jump out. Furthermore, when a battery in an overcharged state or the like drops, the entire portion of the groove  11 Gv of the can bottom  11 Bt opens, and the contents of the battery may jump out of the battery can  11 . Meanwhile, if the opening angle θ is more than 56 degrees, since the opening angle θ is large, when abnormal heat is externally applied to a battery in an overcharged state or the like, an opening area of the can bottom  11 Bt is small, and the battery may rupture. 
     Incidentally, in a case where the groove  11 Gv is annular, when abnormal heat is applied to a battery in an overcharged state or the like, or when a battery in an overcharged state or the like drops, the entire groove  11 Gv largely opens, and therefore the contents of the battery may jump out of the battery can  11 . 
     A ratio Ra of an inner diameter (diameter) R in  of the groove  11 Gv with respect to an outer diameter (diameter) R out  of the can bottom  11 Bt (=(R in /R out )×100) is 44% or more and 77% or less. If the ratio Ra is less than 44%, since the groove  11 Gv is too far from an outer periphery of the can bottom  11 Bt, when abnormal heat is applied to a battery in an overcharged state or the like, the battery may rupture. If the ratio Ra is more than 77%, when a battery in an overcharged state or the like drops, since an opening area of the can bottom  11 Bt is large, the contents of the battery may jump out. 
     Here, a reason for setting the ratio Ra to 44% or more will be described more specifically with reference to  FIG. 3 . When abnormal heat is externally applied to a battery in an overcharged state or the like, heat (flame) is generated from an outer peripheral portion of the wound electrode body  20 . The heat has a function of softening the groove  11 Gv of the can bottom  11 Bt, and softens the groove  11 Gv more easily as the groove  11 Gv is closer to the outer peripheral portion of the wound electrode body  20 . When the ratio Ra is 44% or more, since the groove  11 Gv is close to the outer peripheral portion of the wound electrode body  20 , when abnormal heat is externally applied to a battery in an overcharged state or the like, the groove  11 Gv of the can bottom  11 Bt is easily softened. Therefore, the groove  11 Gv of the can bottom  11 Bt cleaves due to an increase in gas pressure on a bottom side by generated gas, and the gas can be released to an outside. Meanwhile, when the ratio Ra is less than 44%, since the groove  11 Gv is far away from the outer peripheral portion of the wound electrode body  20 , when abnormal heat is externally applied to a battery in an overcharged state or the like, the groove  11 Gv is hardly softened. Therefore, even if the gas pressure on the bottom side rises due to the generated gas, the can bottom  11 Bt does not cleave, and it may be impossible to release the gas to an outside. 
     In a case where the groove  11 Gv has a tapered shape narrowing from an opening side of the groove  11 Gv toward a bottom portion side, the inner diameter Rin of the groove  11 Gv is determined at a corner position formed at a boundary between an inner side surface  11 Sa of the groove  11 Gv and a bottom surface  11 Sb of the can bottom  11 Bt. Furthermore, as illustrated in  FIG. 2C , in a case where the boundary between the inner side surface  11 Sa of the groove  11 Gv and the bottom surface  11 Sb of the can bottom  11 Bt has an R shape or the like, the inner diameter Rin of the groove  11 Gv is determined at an intersection P between a surface obtained by imaginarily extending the inner side surface  11 Sa of the groove  11 Gv and a surface obtained by imaginarily extending the bottom surface  11 Sb of the can bottom  11 Bt. 
     The thickness t of the can bottom  11 Bt at the bottom of the groove  11 Gv (hereinafter, simply referred to as a “bottom thickness of the groove  11 Gv”) is preferably 0.020 mm or more and 0.150 mm or less. If the bottom thickness t of the groove  11 Gv is less than 0.020 mm, when a battery such in an overcharged state or the like drops, the contents of the battery may jump out of the battery can  11 . If the bottom thickness t of the groove  11 Gv is more than 0.150 mm, when abnormal heat is applied to a battery in an overcharged state or the like, the battery will rupture. Incidentally, in a case where the bottom of the groove  11 Gv is curved, for example, and the bottom thickness of the groove  11 Gv is not uniform, the thickness of the thinnest portion of the bottom thickness of the groove  11 Gv is defined as the bottom thickness of the groove  11 Gv. 
     The width w of the groove  11 Gv is preferably 0.10 mm or more and 1.00 mm or less. If the width w is less than 0.10 mm, when abnormal heat is applied to a battery in an overcharged state or the like, the battery will rupture. If the width w is more than 1.00 mm, when a battery in an overcharged state or the like drops, the contents of the battery may jump out of the battery can  11 . Incidentally, in a case where a side surface of the groove  11 Gv is a slope, a curved surface, or the like, the width of the widest portion of the width of the groove  11 Gv displaced in a thickness direction of the can bottom  11 Bt is defined as the width of the groove  11 Gv. The opening angle φ of the slope of the groove  11 Gv is, for example, 0 degrees or more and 90 degrees or less. 
     A gas release pressure (cleavage pressure) of the groove  11 Gv is preferably higher than a gas release pressure (operation pressure) of the safety valve mechanism  15 . This is because the groove  11 Gv of the can bottom  11 Bt is intended to release gas to an outside of a battery in an overcharged state or the like when abnormal heat is applied to the battery, and therefore it is necessary to prevent cleavage of the groove  11 Gv during normal use. The gas release pressure of the groove  11 Gv is preferably lower than a battery internal pressure at which a sealing portion of a battery in an overcharged state or the like is broken. This is because, when abnormal heat is applied to a battery in an overcharged state or the like, gas can be discharged to an outside of the battery by cleavage of the groove  11 Gv before the battery ruptures. Specifically, the gas release pressure of the groove  11 Gv is preferably within a range of 20 kgf/cm 2  or more and 100 kgf/cm 2  or less. 
     A cross-sectional shape of the groove  11 Gv is, for example, a substantially polygonal shape, a substantially partially circular shape, a substantially partially elliptical shape, or an indefinite shape, but is not limited thereto. A curvature R or the like may be given to a top of the polygonal shape. Examples of the polygonal shape include a triangular shape, a quadrangular shape such as a trapezoidal shape or a rectangular shape, and a pentagonal shape. Here, the “partially circular shape” is a part of a circular shape, for example, a semicircular shape. The partially elliptical shape is a part of an elliptical shape, for example, a semielliptical shape. In a case where the groove  11 Gv has a bottom surface, the bottom surface may be, for example, a flat surface, an uneven surface having a step, a curved surface having waviness, or a composite surface obtained by combining two or more of these surfaces. 
     (Positive Electrode) 
     For example, as illustrated in  FIG. 4 , the positive electrode  21  has a structure in which positive electrode active material layers  21 B are disposed on both surfaces of a positive electrode current collector  21 A. Note that the positive electrode active material layer  21 B may be disposed only on one surface of the positive electrode current collector  21 A although not illustrated. For example, the positive electrode current collector  21 A is constituted by a metal foil such as an aluminum foil, a nickel foil, or a stainless steel foil. For example, the positive electrode active material layer  21 B contains a positive electrode active material capable of occluding and releasing lithium (Li) which is an electrode reactant. The positive electrode active material layer  21 B may further contain an additive, if necessary. As the additive, for example, at least one of a conductive agent and a binder can be used. 
     (Positive Electrode Active Material) 
     As the positive electrode active material, for example, a lithium-containing compound such as a lithium oxide, a lithium phosphorus oxide, a lithium sulfide, or a lithium-containing interlayer compound may be suitably used, and two or more kinds thereof may be mixed and used. In order to increase an energy density, a lithium-containing compound containing lithium, a transition metal element, and oxygen (O) is preferable. Examples of such a lithium-containing compound include a lithium composite oxide having a layered rock-salt type structure illustrated in formula (A), and a lithium composite phosphate having an olivine type structure illustrated in formula (B). The lithium-containing compound more preferably contains at least one selected from the group consisting of cobalt (Co), nickel (Ni), manganese (Mn), and iron (Fe) as a transition metal element. Examples of such a lithium-containing compound include a lithium composite oxide having a layered rock-salt type structure represented by formula (C), (D), or (E), a lithium composite oxide having a spinel type structure represented by formula (F), and a lithium complex phosphate having an olivine type structure represented by formula (G). Specific examples thereof include LiNi 0.50 Co 0.20 Mn 0.30 O 2 , Li a CoO 2  (a≈1), Li b NiO 2  (b≈1), Li c1 Ni c2 CO 1-c2 O 2  (c1≈1, 0&lt;c2&lt;1), Li d Mn 2 O 4  (d≈1), and Li e FePO 4  (e≈1).
 
Li p Ni (1-q-r) Mn q Mi r O (2-y) X z   (A)
 
     (In formula (A), M1 represents at least one element selected from Group 2 to Group 15 excluding nickel (Ni) and manganese (Mn). X represents at least one of a group 16 element and a group 17 element other than oxygen (O). p, q, y, and z are values within ranges of 0≤p≤1.5, 0≤q≤1.0, 0≤r≤1.0, −0.10≤y≤0.20, and 0≤z≤0.2.)
 
Li a M2 b PO 4   (B)
 
     (In formula (B), M2 represents at least one element selected from Group 2 to Group 15. a and b are values within ranges of 0≤a≤2.0 and 0.5≤b≤2.0.)
 
Li f Mn (1-g-h) Ni g M3 h O (2-j) F k   (C)
 
     (Provided that in formula (C), M3 represents at least one selected from the group consisting of cobalt (Co), magnesium (Mg), aluminum (Al), boron (B), titanium (Ti), vanadium (V), chromium (Cr), iron (Fe), copper (Cu), zinc (Zn), zirconium (Zr), molybdenum (Mo), tin (Sn), calcium (Ca), strontium (Sr), and tungsten (W). f, g, h, j, and k are values within ranges of 0.8≤f≤1.2, 0&lt;g&lt;0.5, 0≤h≤0.5, g+h&lt;1, −0.1≤j≤0.2, and 0≤k≤0.1. Note that a composition of lithium varies depending on a state of charge-discharge, and a value of f represents a value in a fully discharged state.)
 
Li m Ni (1-n) M4 n O (2-p) F q   (D)
 
     (Provided that in formula (D), M4 represents at least one selected from the group consisting of cobalt (Co), manganese (Mn), magnesium (Mg), aluminum (Al), boron (B), titanium (Ti), vanadium (V), chromium (Cr), iron (Fe), copper (Cu), zinc (Zn), molybdenum (Mo), tin (Sn), calcium (Ca), strontium (Sr), and tungsten (W). m, n, p, and q are values within ranges of 0.8≤m≤1.2, 0.005≤n≤0.5, −0.1≤p≤0.2, and 0≤q≤0.1. Note that a composition of lithium varies depending on a state of charge-discharge, and a value of m represents a value in a fully discharged state.)
 
Li r Co (1-s) M5 s O (2-t) F u   (E)
 
     (Provided that in formula (E), M5 represents at least one selected from the group consisting of nickel (Ni), manganese (Mn), magnesium (Mg), aluminum (Al), boron (B), titanium (Ti), vanadium (V), chromium (Cr), iron (Fe), copper (Cu), zinc (Zn), molybdenum (Mo), tin (Sn), calcium (Ca), strontium (Sr), and tungsten (W). r, s, t, and u are values within ranges of 0.8≤r≤1.2, 0≤s&lt;0.5, −0.1≤t≤0.2, and 0≤u≤0.1. Note that a composition of lithium varies depending on a state of charge-discharge, and a value of r represents a value in a fully discharged state.)
 
Li v Mn 2-w M6 w O x F y   (F)
 
     (Provided that in formula (F), M6 represents at least one selected from the group consisting of cobalt (Co), nickel (Ni), magnesium (Mg), aluminum (Al), boron (B), titanium (Ti), vanadium (V), chromium (Cr), iron (Fe), copper (Cu), zinc (Zn), molybdenum (Mo), tin (Sn), calcium (Ca), strontium (Sr), and tungsten (W). v, w, x, and y are values within ranges of 0.9≤v≤1.1, 0≤w≤0.6, 3.7≤x≤4.1, and 0≤y≤0.1. Note that a composition of lithium varies depending on a state of charge-discharge, and a value of v represents a value in a fully discharged state.)
 
Li z M7PO 4   (G)
 
     (Provided that in formula (G), M7 represents at least one selected from the group consisting of cobalt (Co), manganese (Mn), iron (Fe), nickel (Ni), magnesium (Mg), aluminum (Al), boron (B), titanium (Ti), vanadium (V), niobium (Nb), copper (Cu), zinc (Zn), molybdenum (Mo), calcium (Ca), strontium (Sr), tungsten (W), and zirconium (Zr). z is a value within a range of 0.9≤z≤1.1. Note that a composition of lithium varies depending on a state of charge-discharge, and a value of z represents a value in a fully discharged state.) 
     The lithium-containing compound containing nickel (Ni) preferably has a Ni content of 80% or more. This is because the Ni content of 80% or more makes it possible to obtain a high battery capacity. Use of such a lithium-containing compound having a high Ni content makes the battery capacity high as described above, but makes the amount of gas generation (oxygen release amount) in the positive electrode  21  very large when abnormal heat is applied. The battery according to the first embodiment exhibits a particularly excellent safety improvement effect in a case where such an electrode with the large amount of gas generation is used. 
     The lithium-containing compound having a Ni content of 80% or more is preferably a positive electrode material represented by formula (H).
 
Li v Ni w M8 x M9 y O z   (H)
 
(In the formula, 0&lt;v&lt;2, w+x+y≤1, 0.8≤w≤1, 0≤x≤0.2, 0≤y≤0.2, and 0&lt;z&lt;3 are satisfied, and each of M8 and M9 represents at least one selected from cobalt (Co), iron (Fe), manganese (Mn), copper (Cu), zinc (Zn), aluminum (Al), chromium (Cr), vanadium (V), titanium (Ti), magnesium (Mg), and zirconium (Zr).)
 
     Other examples of the positive electrode material capable of occluding and releasing lithium include an inorganic compound containing no lithium, such as MnO 2 , V 2 O 5 , V 5 O 13 , NiS, or MoS. 
     The positive electrode material capable of occluding and releasing lithium may be a material other than the materials described above. In addition, two or more kinds of the positive electrode materials exemplified above may be mixed in any combination. 
     (Binder) 
     As a binding material, for example, at least one selected from resin materials such as polyvinylidene fluoride (PVdF), polytetrafluoroethylene (PTFE), polyacrylonitrile (PAN), styrene-butadiene rubber (SBR), and carboxymethyl cellulose (CMC), a copolymer mainly containing these resin materials, and the like is used. 
     (Conductive Agent) 
     Examples of the conductive agent include a carbon material such as graphite, carbon black, or Ketjen black. These materials are used singly or in mixture of two or more kinds thereof. Furthermore, in addition to the carbon material, a metal material, a conductive polymer material, or the like may be used as long as having conductivity. 
     (Negative Electrode) 
     For example, as illustrated in  FIG. 4 , the negative electrode  22  has a structure in which negative electrode active material layers  22 B are disposed on both surfaces of a negative electrode current collector  22 A. Note that the negative electrode active material layer  22 B may be disposed only on one surface of the negative electrode current collector  22 A although not illustrated. For example, the negative electrode current collector  22 A is constituted by a metal foil such as a copper foil, a nickel foil, or a stainless steel foil. 
     The negative electrode active material layer  22 B contains one or more kinds of negative electrode materials capable of occluding and releasing lithium as a negative electrode active material. The negative electrode active material layer  22 B may further contain an additive such as a binder, if necessary. 
     Note that, in the battery according to the first embodiment, an electrochemical equivalent of a negative electrode material capable of occluding and releasing lithium is larger than that of the positive electrode  21 , and a lithium metal is not precipitated on the negative electrode  22  during charging. 
     Examples of the negative electrode material capable of occluding and releasing lithium include a material capable of occluding and releasing lithium and containing at least one of metal elements and metalloid elements as a constituent element. Here, the negative electrode  22  containing such a negative electrode material is referred to as an alloy-based negative electrode. This is because use of such a material makes it possible to obtain a high energy density. Particularly, use of such a material together with a carbon material is more preferable because a high energy density and an excellent cycle characteristic can be obtained simultaneously. This negative electrode material may be a simple substance of a metal element or a metalloid element, an alloy thereof, or a compound thereof, and may partially contain one or more kinds of phases thereof. Incidentally, in the present technology, the alloy includes an alloy constituted by one or more kinds of metal elements and one or more kinds of metalloid elements in addition to an alloy constituted by two or more kinds of metal elements. In addition, a nonmetallic element may be contained. A structure thereof includes a solid solution, a eutectic (eutectic mixture), an intermetallic compound, and coexistence of two or more kinds thereof. 
     Examples of the metal element or the metalloid element constituting the negative electrode material include magnesium (Mg), boron (B), aluminum (Al), gallium (Ga), indium (In), silicon (Si), germanium (Ge), tin (Sn), lead (Pb), bismuth (Bi), cadmium (Cd), silver (Ag), zinc (Zn), hafnium (Hf), zirconium (Zr), yttrium (Y), palladium (Pd), and platinum (Pt). These elements maybe crystalline or amorphous. 
     Among these elements, as this negative electrode material, an element containing a metal element or a metalloid element of Group 4B in the short period periodic table as a constituent element is preferable, and an element containing at least one of (Si) and tin (Sn) as a constituent element is particularly preferable. This is because silicon (Si) and tin (Sn) have a high ability to occlude and release lithium (Li), and a high energy density can be obtained. 
     Examples of an alloy of tin (Sn) include an alloy containing at least one of the group consisting of silicon (Si), nickel (Ni), copper (Cu), iron (Fe), cobalt (Co), manganese (Mn), zinc (Zn), indium (In), silver (Ag), titanium (Ti), germanium (Ge), bismuth (Bi), antimony (Sb), and chromium (Cr) as a second constituent element other than tin (Sn). Examples of an alloy of silicon (Si) include an alloy containing at least one of the group consisting of tin (Sn), nickel (Ni), copper (Cu), iron (Fe), cobalt (Co), manganese (Mn), zinc (Zn), indium (In), silver (Ag), titanium (Ti), germanium (Ge), bismuth (Bi), antimony (Sb), and chromium (Cr) as a second constituent element other than silicon (Si). 
     Examples of a compound of tin (Sn) or a compound of silicon (Si) include a compound containing oxygen (O) or carbon (C). The compound of tin (Sn) or the compound of silicon (Si) may contain the above second constituent element in addition to tin (Sn) or silicon (Si). Specific examples of the compound of tin (Sn) include a silicon oxide represented by SiO v  (0.2&lt;v&lt;1.4). 
     Examples of the negative electrode material capable of occluding and releasing lithium include a carbon material such as hardly graphitizable carbon, easily graphitizable carbon, graphite, pyrolytic carbon, coke, glassy carbon, an organic polymer compound fired body, carbon fiber, or activated carbon. Preferable examples of the graphite include natural graphite subjected to a spheroidization treatment or the like, and substantially spherical artificial graphite. Preferable examples of the artificial graphite include artificial graphite obtained by graphitizing mesocarbon microbeads (MCMB) and artificial graphite obtained by graphitizing and grinding coke raw materials. Examples of the coke include pitch coke, needle coke, and petroleum coke. The organic polymer compound fired body is obtained by firing a polymer material such as a phenol resin or a furan resin at an appropriate temperature to be carbonized. Some organic polymer compound fired bodies are classified into hardly graphitizable carbon or easily graphitizable carbon. In addition, examples of the polymer material include polyacetylene and polypyrrole. These carbon materials are preferable because a change in a crystal structure occurring during charging and discharging is very small, a high charge-discharge capacity can be obtained, and an excellent cycle characteristic can be obtained. Particularly, graphite is preferable because a high energy density can be obtained due to a large electrochemical equivalent thereof. In addition, the hardly graphitizable carbon is preferable because an excellent characteristic can be obtained. Furthermore, a material having a low charge-discharge potential, specifically having a charge-discharge potential close to a lithium metal is preferable because a high energy density of a battery can be realized easily. 
     Other examples of the negative electrode material capable of occluding and releasing lithium include other metal compounds and a polymer material. Examples of the other metal compounds include an oxide such as MnO 2 , V 2 O 5 , or V 6 O 13 , a sulfide such as NiS or MoS, and a lithium nitride such as LiN 3 . Examples of the polymer material include polyacetylene, polyaniline, and polypyrrole. 
     (Binder) 
     As the binder, for example, at least one selected from resin materials such as polyvinylidene fluoride (PVdF), polytetrafluoroethylene (PTFE), polyacrylonitrile (PAN), styrene-butadiene rubber (SBR), and carboxymethyl cellulose (CMC), a copolymer mainly containing these resin materials, and the like is used. 
     (Separator) 
     The separator  23  isolates the positive electrode  21  and the negative electrode  22  from each other to prevent short circuit of a current due to contact between both the electrodes, and allows a lithium ion to pass therethrough. For example, the separator  23  is constituted by a synthetic resin porous film constituted by polytetrafluoroethylene, polypropylene, polyethylene, or the like, or a ceramic porous film, and may have a structure obtained by stacking two or more kinds of these porous films. Among these films, a polyolefin porous film is preferable because the polyolefin porous film exhibits an excellent effect for preventing short circuit, and can improve safety of a battery due to a shutdown effect. Particularly, polyethylene is preferable as a material constituting the separator  23  because polyethylene can obtain a shutdown effect within a range of 100° C. or higher and 160° C. or lower and has excellent electrochemical stability. In addition, polypropylene is preferable. Furthermore, a resin having chemical stability can be used by copolymerizing the resin with polyethylene or polypropylene or blending the resin with polyethylene or polypropylene. 
     (Electrolytic Solution) 
     The separator  23  is impregnated with an electrolytic solution which is a liquid electrolyte. The electrolytic solution contains a solvent and an electrolyte salt dissolved in this solvent. The electrolytic solution may contain a known additive in order to improve a battery characteristic. 
     As the solvent, a cyclic carbonate such as ethylene carbonate or propylene carbonate can be used. It is preferable to use one of ethylene carbonate and propylene carbonate, and particularly preferable to mix and use both thereof. This is because a cycle characteristic can be improved. 
     In addition, as the solvent, it is preferable to mix and use chain carbonates such as diethyl carbonate, dimethyl carbonate, ethylmethyl carbonate, and methyl propyl carbonate in addition to these cyclic carbonates. This is because a high ionic conductivity can be obtained. 
     The solvent preferably further contains 2,4-difluoro anisole or vinylene carbonate. This is because 2,4-difluoro anisole can improve a discharge capacity, and vinylene carbonate can improve a cycle characteristic. Therefore, use of these compounds in mixture thereof is preferable because the discharge capacity and the cycle characteristic can be improved. 
     In addition to these compounds, examples of the solvent include butylene carbonate, γ-butyrolactone, γ-valerolactone, 1,2-dimethoxyethane, tetrahydrofuran, 2-methyltetrahydrofuran, 1,3-dioxolane, 4-methyl-1,3 dioxolane, methyl acetate, methyl propionate, acetonitrile, glutaronitrile, adiponitrile, methoxy acetonitrile, 3-methoxy propylonitrile, N,N-dimethylformamide, N-methylpyrrolidinone, N-methyl-oxazolidinone, N,N-dimethyl imidazolidinone, nitromethane, nitroethane, sulfolane, dimethyl sulfoxide, and trimethyl phosphate. 
     Incidentally, a compound obtained by replacing at least a part of hydrogen atoms in these non-aqueous solvents with a fluorine atom may be preferable because the compound may improve reversibility of an electrode reaction with some types of combined electrodes. 
     Examples of the electrolyte salt include a lithium salt. The lithium salt can be used singly or in mixture of two or more kinds thereof. Examples of the lithium salt include LiPF 6 , LiBF 4 , LiAsF 6 , LiClO 4 , LiB(C 6 H 5 ) 4r  LiCH 3 SO 3 , LiCF 3 SO 3 , LiN(SO 2 CF 3 ) 2 , LiC(SO 2 CF 3 ) 3 , LiAlCl 4 , LiSiF 6 , LiCl, difluoro [oxalate —O,O′] lithium borate, lithium bis-oxalate borate, and LiBr. Among these lithium salts, LiPF 6  is preferable because LiPF 6  can obtain a high ionic conductivity and can improve a cycle characteristic. 
     In the battery having the above configuration, when charging is performed, for example, a lithium ion is released from the positive electrode active material layer  21 B, and is occluded by the negative electrode active material layer  22 B through an electrolytic solution. In addition, when discharging is performed, for example, a lithium ion is released from the negative electrode active material layer  22 B, and is occluded by the positive electrode active material layer  21 B through an electrolytic solution. 
     [Function of Battery] 
     In the battery having the above configuration, when abnormal heat is externally applied to the battery, as illustrated in  FIG. 5 , gas is generated from a heating portion, the vicinity thereof, and the like in the electrode, and the generated gas flows to a top side and a bottom side of the battery. The gas which has flowed to the top side is discharged to an outside via a safety valve mechanism which has cleaved (not illustrated). Meanwhile, the gas which has flowed to the bottom side goes around to the top side via the center hole  20 H of the wound electrode body  20  and is discharged to an outside via the safety valve mechanism  15  which has cleaved. 
     In a case where the amount of generated gas is small and the center hole  20 H of the wound electrode body  20  has a sufficient size, the gas which has flowed to the bottom side smoothly can go around to the top side, and can be discharged to an outside via the safety valve mechanism  15  which has cleaved. Therefore, a gas pressure on the bottom side of the battery is hardly abnormally increased. Meanwhile, in a case where the amount of generated gas is large and the center hole  20 H of the wound electrode body  20  does not have a sufficient size, the amount of gas flowing to the bottom side increases, and it is difficult for the gas which has flowed to the bottom side to smoothly go around to the top side via the center hole  20 H. Therefore, a gas pressure on the bottom side of the battery easily increases abnormally. Particularly, in a battery with high capacity and high output in an overcharged state, a gas pressure easily increases abnormally on the bottom side of the battery. 
     In the battery having the above configuration, the groove  11 Gv cleaves appropriately in response to an abnormally increased gas pressure on the bottom side, and gas accumulated in the can bottom  11 Bt can be discharged. At this time, gas accumulated in the can bottom  11 Bt can be discharged from the can bottom  11 Bt while jumping of the contents of the battery out of the can bottom  11 Bt which has cleaved is suppressed. 
     [Method for Manufacturing Battery] 
     Next, a method for manufacturing the battery according to the first embodiment of the present technology will be exemplified. 
     First, for example, a first positive electrode active material, a second positive electrode active material, a conductive agent, and a binder are mixed to prepare a positive electrode mixture, and this positive electrode mixture is dispersed in a solvent such as N-methyl-2-pyrrolidone (NMP) to manufacture a paste-like positive electrode mixture slurry. Subsequently, this positive electrode mixture slurry is applied to the positive electrode current collector  21 A, the solvent is dried, and the resulting product is subjected to compression molding with a roll press machine or the like to form the positive electrode active material layer  21 B and form the positive electrode  21 . 
     In addition, for example, a negative electrode active material and a binder are mixed to prepare a negative electrode mixture, and this negative electrode mixture is dispersed in a solvent such as N-methyl-2-pyrrolidone to manufacture a paste-like negative electrode mixture slurry. Subsequently, this negative electrode mixture slurry is applied to the negative electrode current collector  22 A, the solvent is dried, and the resulting product is subjected to compression molding with a roll press machine or the like to form the negative electrode active material layer  22 B and form the negative electrode  22 . 
     Subsequently, the positive electrode lead  25  is attached to the positive electrode current collector  21 A by welding or the like, and the negative electrode lead  26  is attached to the negative electrode current collector  22 A by welding or the like. Subsequently, the positive electrode  21  and the negative electrode  22  are wound through the separator  23 . Subsequently, an end of the positive electrode lead  25  is welded to the safety valve mechanism  15 , and an end of the negative electrode lead  26  is welded to the battery can  11 . The wound positive electrode  21  and negative electrode  22  are sandwiched by the pair of insulating plates  12  and  13 , and are housed in the battery can  11 . Subsequently, the positive electrode  21  and the negative electrode  22  are housed in the battery can  11 . Thereafter, the electrolytic solution is injected into the battery can  11 , and the separator  23  is impregnated therewith. Subsequently, the battery lid  14 , the safety valve mechanism  15 , and the positive temperature coefficient element  16  are fixed to an open end of the battery can  11  by being crimped through the sealing gasket  17 . The secondary battery illustrated in  FIG. 1  is thereby obtained. 
     [Effect] 
     According to the above first embodiment, the inner surface of the can bottom  11 Bt has the arc-shaped groove  11 Gv. In addition, the opening angle θ of the groove  11 Gv with respect to a center of the arc is 0.5 degrees or more and 56 degrees or less. The ratio Ra of the inner diameter R in  of the groove  11 Gv with respect to the outer diameter R out  of the can bottom  11 Bt is 44% or more and 77% or less. As a result, when abnormal heat is applied to a battery in an overcharged state or the like, the groove  11 Gv appropriately cleaves in accordance with an abnormal increase in the gas pressure in the battery can  11  in order to prevent the contents of the battery from jumping out of the battery can  11 , and rupture of the battery can be suppressed. In addition, when a battery in an overcharged state or the like drops, the groove  11 Gv cleaves due to an impact of drop, and it is also possible to suppress jumping of the contents of the battery out of the battery can  11 . Therefore, it is possible to improve safety when abnormal heat is applied to a battery in an overcharged state or the like while a decrease in mechanical strength (that is, cleavage strength of the groove  11 Gv) of the can bottom  11 Bt of the battery can  11  is suppressed. 
     As described above, the center pin  24  has a tubular shape, and functions as a flow path for guiding generated gas from the bottom side of the battery to the top side at the time of gas generation. If the center pin  24  is disposed, crushing of the center hole  20 H of the wound electrode body  20  can be suppressed. However, the center pin  24  is crushed by expansion of the wound electrode body  20 , the center hole  20 H of the wound electrode body  20  does not have a sufficient size, and a gas pressure on the bottom side may abnormally increase. Particularly, in a battery with high capacity and high output, the wound electrode body  20  is largely expanded at the time of charging and discharging or application of abnormal heat. Therefore, the center hole  20 H of the wound electrode body  20  easily loses a sufficient size, and therefore a gas pressure on the bottom side easily increases abnormally. Therefore, regardless of presence or absence of the center pin  24 , it is effective to dispose the arc-shaped groove  11 Gv in the can bottom  11 Bt as described above for improving safety of the battery. 
     Modification Example 
     Of both surfaces of the can bottom  11 Bt, a surface outside the battery can  11  (hereinafter, simply referred to as an “outer surface of the can bottom  11 Bt”) may have the arc-shaped groove  11 Gv as illustrated in  FIG. 6A . Furthermore, as illustrated in  FIG. 6B , each of the inner surface and the outer surface of the can bottom  11 Bt may have the arc-shaped groove  11 Gv. However, the groove  11 Gv is preferably disposed on the inner surface of the can bottom  11 Bt as in the first embodiment from a viewpoint of suppressing corrosion of the groove  11 Gv due to outside air. 
       FIG. 6B  illustrates an example in which the grooves  11 Gv disposed on the inner surface and the outer surface overlap each other in a thickness direction of the can bottom  11 Bt. However, the grooves  11 Gv disposed on the inner surface and the outer surface may be disposed so as to be deviated in an in-plane direction of the can bottom  11 Bt while the grooves  11 Gv do not overlap each other in the thickness direction of the can bottom  11 Bt. 
     In the first embodiment described above, the battery having the center pin  24  has been described, but a battery without the center pin  24  may be used. In a battery having such a configuration, the center hole  20 H of the wound electrode body  20  tends to have an insufficient size due to expansion of the wound electrode body  20 , and therefore an effect of improving safety by the groove  11 Gv is remarkably exhibited. 
     2. Second Embodiment 
     In a second embodiment, a battery pack and an electronic device each including the battery according to the first embodiment will be described. 
     [Configurations of Battery Pack and Electronic Device] 
     Hereinafter, configuration examples of a battery pack  300  and an electronic device  400  according to the second embodiment of the present technology will be described with reference to  FIG. 7 . The electronic device  400  includes an electronic circuit  401  of an electronic device main body and the battery pack  300 . The battery pack  300  is electrically connected to the electronic circuit  401  through a positive electrode terminal  331   a  and a negative electrode terminal  331   b . For example, in the electronic device  400 , the battery pack  300  is attachable and removable by a user. Note that the structure of the electronic device  400  is not limited thereto, but the battery pack  300  may be incorporated in the electronic device  400  such that a user cannot remove the battery pack  300  from the electronic device  400 . 
     During charging of the battery pack  300 , the positive electrode terminal  331   a  of the battery pack  300  and the negative electrode terminal  331   b  thereof are connected to a positive electrode terminal of a charger (not illustrated) and a negative electrode terminal thereof, respectively. On the other hand, during discharging of the battery pack  300  (during use of the electronic device  400 ), the positive electrode terminal  331   a  of the battery pack  300  and the negative electrode terminal  331   b  thereof are connected to a positive electrode terminal of the electronic circuit  401  and a negative electrode terminal thereof, respectively. 
     Examples of the electronic device  400  include a notebook personal computer, a tablet computer, a mobile phone (for example, a smart phone), a personal digital assistant (PDA), a display device (LCD, an EL display, electronic paper, or the like), an imaging device (for example, a digital still camera or a digital video camera), an audio device (for example, a portable audio player), a game device, a cordless handset phone machine, an electronic book, an electronic dictionary, a radio, a headphone, a navigation system, a memory card, a pacemaker, a hearing aid, an electric tool, an electric shaver, a refrigerator, an air conditioner, a television, a stereo, a water heater, a microwave oven, a dishwasher, a washing machine, a dryer, a lighting device, a toy, a medical device, a robot, a load conditioner, and a traffic signal. However, the electronic device  400  is not limited thereto. 
     (Electronic Device) 
     For example, the electronic circuit  401  includes CPU, a peripheral logic unit, an interface unit, and a storage unit, and controls the entire electronic device  400 . 
     (Battery Pack) 
     The battery pack  300  includes an assembled battery  301  and a charge-discharge circuit  302 . The assembled battery  301  is formed by connecting a plurality of secondary batteries  301   a  to each other in series or in parallel. For example, the plurality of secondary batteries  301   a  is connected to each other in n parallel m series (each of n and m is a positive integer). Note that  FIG. 7  illustrates an example in which six secondary batteries  301   a  are connected to each other in 2 parallel 3 series (2P3S). As the secondary battery  301   a , the battery according to the first embodiment is used. 
     The charge-discharge circuit  302  is a control unit for controlling charge-discharge of the assembled battery  301 . Specifically, during charging, the charge-discharge circuit  302  controls charging to the assembled battery  301 . Meanwhile, during discharging (that is, during use of the electronic device  400 ), the charge-discharge circuit  302  controls discharging to the electronic device  400 . 
     Modification Example 
     In the second embodiment described above, the case where the battery pack  300  includes the assembled battery  301  including the plurality of secondary batteries  301   a  has been exemplified. However, a configuration in which the battery pack  300  includes one secondary battery  301   a  in place of the assembled battery  301  may be adopted. 
     3. Third Embodiment 
     In a third embodiment, an electricity storage system including the battery according to the first embodiment in an electricity storage device will be described. This electricity storage system may be any system as long as using electric power, and includes a simple electric power device. Examples of this electric power system include a smart grid, a home energy management system (HEMS), and a vehicle. The electric power system can also store electricity. 
     [Configuration of Electricity Storage System] 
     Hereinafter, a configuration example of an electricity storage system (electric power system)  100  according to the third embodiment will be described with reference to  FIG. 8 . The electricity storage system  100  is a residential electricity storage system, and electric power is supplied from a centralized electric power system  102  such as thermal power generation  102   a , nuclear power generation  102   b , or hydroelectric power generation  102   c  to an electricity storage device  103  via an electric power network  109 , an information network  112 , a smart meter  107 , a power hub  108 , or the like. At the same time, electric power is supplied from an independent power source such as a home power generating device  104  to the electricity storage device  103 . Electric power supplied to the electricity storage device  103  is stored. Electric power used in a residence  101  is supplied using the electricity storage device  103 . Not only the residence  101  but also a building can use a similar electricity storage system. 
     The residence  101  is provided with the home power generating device  104 , an electric power consumption device  105 , the electricity storage device  103 , a control device  110  for controlling devices, the smart meter  107 , the power hub  108 , and a sensor  111  for acquiring various information. The devices are connected to each other via the electric power network  109  and the information network  112 . As the home power generating device  104 , a solar cell, a fuel cell, or the like is used, and generated electric power is supplied to the electric power consumption device  105  and/or the electricity storage device  103 . The electric power consumption device  105  is a refrigerator  105   a , an air conditioner  105   b , a television receiver  105   c , a bath  105   d , or the like. Furthermore, the electric power consumption device  105  further includes an electric vehicle  106 . The electric vehicle  106  is an electric car  106   a , a hybrid car  106   b , an electric motorcycle  106   c , or the like. 
     The electricity storage device  103  includes the battery according to the first embodiment. The smart meter  107  measures a use amount of commercial electric power, and transmits the measured use amount to an electric power company. The electric power network  109  may be any one of DC power supply, AC power supply, and non-contact power supply, or a combination of two or more thereof. 
     Examples of the various sensors  111  include a human sensor, an illuminance sensor, an object detection sensor, a consumed electric power sensor, a vibration sensor, a contact sensor, a temperature sensor, and an infrared sensor. Information acquired by the various sensors  111  is transmitted to the control device  110 . A weather condition, a human condition, or the like is understood due to the Information from the sensors  111 , and energy consumption can be minimized by automatic control of the electric power consumption device  105 . Furthermore, the control device  110  can transmit information regarding the residence  101  to an external electric power company or the like via internet. 
     The power hub  108  performs processing such as branching of an electric power line or DC-AC conversion. A communication method of the information network  112  connected to the control device  110  includes a method of using a communication interface such as universal asynchronous receiver-transceiver (UART) and a method of using a sensor network by a wireless communication standard, such as Bluetooth (registered trademark), ZigBee, or Wi-Fi. The Bluetooth (registered trademark) method is applied to multimedia communication and perform one-to-many communication. ZigBee uses a physical layer of IEEE (Institute of Electrical and Electronics Engineers) 802.15.4. IEEE802.15.4 is a name of a short-distance wireless network standard called personal area network (PAN) or wireless (W) PAN. 
     The control device  110  is connected to an external server  113 . This server  113  may be managed by any one of the residence  101 , an electric power company, and a service provider. For example, information transmitted or received by the server  113  is consumption electric power information, life pattern information, electric power charge, weather information, natural disaster information, or information regarding electric power transaction. A home electric power consumption device (for example, a television receiver) may transmit or receive the information, but an outside-home device (for example, a mobile phone) may transmit or receive the information. A device having a display function, such as a television receiver, a mobile phone, or a personal digital assistant (PDA), may display the information. 
     The control device  110  for controlling units is constituted by a central processing unit (CPU), a random access memory (RAM), a read only memory (ROM), and the like, and is housed in the electricity storage device  103  in this example. The control device  110  is connected to the electricity storage device  103 , the home power generating device  104 , the electric power consumption device  105 , the various sensors  111 , and the server  113  via the information network  112 , and for example, adjusts a use amount of commercial electric power and a power generation amount. Note that the control device  110  may perform electric power transaction in an electric power market or the like. 
     As described above, the electricity storage device  103  can store not only electric power from the centralized electric power system  102  such as the thermal power generation  102   a , the nuclear power generation  102   b , or the hydroelectric power generation  102   c  but also electric power generated by the home power generating device  104  (solar power generation or wind power generation). Therefore, even when the electric power generated by the home power generating device  104  fluctuates, control for keeping the amount of electric power to be sent to an outside constant or discharging by a necessary amount of electric power can be performed. For example, the following method of use is possible. That is, electric power obtained by solar power generation is stored in the electricity storage device  103 , midnight electric power the charge of which is low at night is stored in the electricity storage device  103 , and electric power stored in the electricity storage device  103  is used by discharging in daytime in which electric power charge is high. 
     Note that, in this example, the control device  110  housed in the electricity storage device  103  has been exemplified, but the control device  110  may be housed in the smart meter  107 , or may be formed alone. Furthermore, the electricity storage system  100  may be used for a plurality of homes in a multiple dwelling house or a plurality of detached houses. 
     4. Fourth Embodiment 
     In a fourth embodiment, an electric vehicle including the battery according to the first embodiment will be described. 
     [Configuration of Electric Vehicle] 
     Hereinafter, a configuration example of the electric vehicle according to the fourth embodiment of the present technology will be described with reference to  FIG. 9 . A hybrid vehicle  200  is a hybrid vehicle using a series hybrid system. The series hybrid system is a car traveling with an electric power driving force converter  203  using electric power generated by a generator driven by an engine or electric power obtained by temporarily storing the generated electric power in a battery. 
     An engine  201 , a generator  202 , the electric power driving force converter  203 , a driving wheel  204   a , a driving wheel  204   b , a wheel  205   a , a wheel  205   b , a battery  208 , a vehicle control device  209 , various sensors  210 , and a charging port  211  are mounted in this hybrid vehicle  200 . As the battery  208 , the battery according to the first embodiment is used. 
     The hybrid vehicle  200  travels using the electric power driving force converter  203  as a power source. An example of the electric power driving force converter  203  is a motor. The electric power driving force converter  203  acts by electric power of the battery  208 , and a rotating force of the electric power driving force converter  203  is transmitted to the driving wheels  204   a  and  204   b . Note that the electric power driving force converter  203  can be applied to both an AC motor and a DC motor by using DC-AC or reverse conversion (AC-DC conversion) at necessary portions. The various sensors  210  control an engine speed through the vehicle control device  209 , or control an opening degree (throttle opening degree) of a throttle valve (not illustrated). The various sensors  210  include a velocity sensor, an acceleration sensor, an engine speed sensor, and the like. 
     A rotating force of the engine  201  is transmitted to the generator  202 , and electric power generated by the generator  202  can be stored in the battery  208  by the rotating force. 
     When the hybrid vehicle  200  is decelerated by a brake mechanism (not illustrated), a resistance force during the deceleration is added to the electric power driving force converter  203  as a rotating force, and regenerative electric power generated by the electric power driving force converter  203  due to this rotating force is stored in the battery  208 . 
     By being connected to an external power source of the hybrid vehicle  200  through the charging port  211 , the battery  208  receives electric power from the external power source by using the charging port  211  as an input port, and can store the received electric power. 
     Although not illustrated, an information processing device for performing information processing regarding vehicle control on the basis of information regarding a battery may be included. Examples of such an information processing device include an information processing device for displaying a battery remaining amount based on information regarding the battery remaining amount. 
     Note that the above description has been made by exemplifying a series hybrid car traveling with a motor using electric power generated by a generator driven by an engine or electric power obtained by temporarily storing the generated electric power in a battery. However, the present technology can be applied effectively also to a parallel hybrid car using both an engine and a motor as a driving source and appropriately switching three methods of traveling only by the engine, traveling only by the motor, and traveling by both the engine and the motor to be used. Furthermore, the present technology can be applied effectively also to a so-called electric vehicle traveling by driving only with a driving motor without use of an engine. 
     EXAMPLES 
     Hereinafter, the present technology will be described specifically with Examples, but the present technology is not limited only to these Examples. 
     Examples of the present technology will be described in the following order. 
     i Samples with ratio Ra and opening angle θ changed 
     ii Samples with thickness t of groove bottom or width w of groove changed 
     iii Samples with volume energy density changed 
     i Samples with Ratio Ra and Opening Angle θ Changed 
     Examples 1-1 to 1-3 and Comparative Examples 1-1 to 1-3 
     (Step of Manufacturing Positive Electrode) 
     A positive electrode was manufactured as follows. First, lithium carbonate (Li 2 CO 3 ) and cobalt carbonate (CoCO 3 ) were mixed at a molar ratio of 0.5:1, and then the resulting mixture was calcined in air at 900° C. for 5 hours to obtain a lithium cobalt composite oxide (LiCoO 2 ) as a positive electrode active material. Subsequently, 91 parts by mass of the lithium cobalt composite oxide obtained as described above, 6 parts by mass of graphite as a conductive agent, and 3 parts by mass of polyvinylidene fluoride as a binder were mixed to obtain a positive electrode mixture. Thereafter, the positive electrode mixture was dispersed in N-methyl-2-pyrrolidone to obtain a paste-like positive electrode mixture slurry. Subsequently, the positive electrode mixture slurry was applied to both surfaces of a positive electrode current collector constituted by a strip-shaped aluminum foil (thickness of 12 μm), and was dried. Thereafter, the resulting product was compression-molded with a roll press machine to form a positive electrode active material layer. Subsequently, a positive electrode lead constituted by aluminum was welded and attached to one end of the positive electrode current collector. 
     (Step of Manufacturing Negative Electrode) 
     A negative electrode was manufactured as follows. First, 97 parts by mass of artificial graphite powder as a negative electrode active material and 3 parts by mass of polyvinylidene fluoride as a binder were mixed to obtain a negative electrode mixture. Thereafter, the negative electrode mixture was dispersed in N-methyl-2-pyrrolidone to obtain a paste-like negative electrode mixture slurry. Subsequently, the negative electrode mixture slurry was applied to both surfaces of a negative electrode current collector constituted by a strip-shaped copper foil (thickness of 15 μm), and was dried. Thereafter, the resulting product was compression-molded with a roll press machine to forma negative electrode active material layer. Subsequently, a negative electrode lead constituted by nickel was attached to one end of the negative electrode current collector. 
     (Step of Assembling Battery) 
     A battery was assembled as follows. First, the positive electrode and the negative electrode obtained as described above were stacked through a separator constituted by a microporous polyethylene stretched film having a thickness of 23 μm in order of the negative electrode, the separator, the positive electrode, and the separator, and the resulting stacked body was wound many times to obtain a jelly roll type wound electrode body. 
     Subsequently, a battery can with a can bottom having the following configuration was prepared. 
     Groove forming surface at can bottom: inner surface of can bottom 
     Groove shape: arc shape 
     Number of grooves: one 
     Outer diameter (diameter) of can bottom R out : 18.20 mm 
     Inner diameter (diameter) of groove R in : 4 mm to 16 mm 
     Ratio Ra (=(R in /R out )×100): 22% to 88% 
     Opening angle θ of groove with respect to center of can bottom: 0.5 degrees 
     Thickness t of can bottom at bottom of groove: 0.075 mm 
     Width w of groove: 0.4 mm 
     Opening angle φ of slope of groove: 30 degrees 
     Volume energy density: 430 Wh/L 
     Subsequently, the wound electrode body was sandwiched by a pair of insulating plates, the negative electrode lead was welded to the battery can, and the positive electrode lead was welded to the safety valve mechanism to house the wound electrode body inside the battery can. Subsequently, a non-aqueous electrolytic solution was prepared by dissolving LiPF 6  as an electrolyte salt in a solvent obtained by mixing ethylene carbonate and methylethyl carbonate at a volume ratio of 1:1 so as to have a concentration of 1 mol/dm 3 . 
     Finally, an electrolytic solution was injected into the above battery can housing the wound electrode body. Thereafter, a safety valve, a PTC element, and a battery lid were fixed by crimping the battery can via an insulating sealing gasket to manufacture a cylindrical battery having an outer diameter (diameter) of 18.20 mm and a height of 65 mm. Note that this battery was designed to have an open circuit voltage (that is, battery voltage) of 4.2 V at the time of full charge by adjusting the amount of a positive electrode active material and the amount of a negative electrode active material. However, the battery was evaluated at 4.4 V (in an overcharged state exceeding a usual use range voltage) in a test described later. 
     Examples 2-1 to 2-3 and Comparative Examples 2-1 to 2-3 
     A battery was manufactured in a similar manner to Examples 1-1 to 1-3 and Comparative Examples 1-1 to 1-3 except that the opening angle θ of the groove with respect to the center of the can bottom was changed to 6 degrees. 
     Examples 3-1 to 3-3 and Comparative Examples 3-1 to 3-3 
     A battery was manufactured in a similar manner to Examples 1-1 to 1-3 and Comparative Examples 1-1 to 1-3 except that the opening angle θ of the groove with respect to the center of the can bottom was changed to 30 degrees. 
     Examples 4-1 to 4-3 and Comparative Examples 4-1 to 4-3 
     A battery was manufactured in a similar manner to Examples 1-1 to 1-3 and Comparative Examples 1-1 to 1-3 except that the opening angle θ of the groove with respect to the center of the can bottom was changed to 45 degrees. 
     Examples 5-1 to 5-3 and Comparative Examples 5-1 to 5-3 
     A battery was manufactured in a similar manner to Examples 1-1 to 1-3 and Comparative Examples 1-1 to 1-3 except that the opening angle θ of the groove with respect to the center of the can bottom was changed to 56 degrees. 
     Comparative Examples 6-1 to 6-6 
     A battery was manufactured in a similar manner to Examples 1-1 to 1-3 and Comparative Examples 1-1 to 1-3 except that the shape of the groove at the can bottom was changed from an arc shape to a circular shape (the opening angle θ of the groove with respect to the center of the can bottom=0 degrees). 
     Comparative Examples 7-1 to 7-6 
     A battery was manufactured in a similar manner to Examples 1-1 to 1-3 and Comparative Examples 1-1 to 1-3 except that the opening angle θ of the groove with respect to the center of the can bottom was changed to 80 degrees. 
     (Evaluation) 
     The batteries of Examples 1-1 to 5-3 and Comparative Examples 1-1 to 7-6 obtained as described above were subjected to the following battery burning test and battery drop test. Note that these tests are in conformity to public tests. 
     (Battery Burning Test) 
     First, batteries were charged so as to be in an overcharged state with an open circuit voltage of 4.4 V. Subsequently, the central portions of the charged batteries were burned with a burner, and the number of batteries in which the contents did not jump out of the batteries and did not rupture was counted. Subsequently, a passing ratio r1 of the battery burning test was determined from the following formula.
 
(Passing ratio  r 1 of battery burning test)=((Number of batteries in which the contents did not jump out of the batteries and did not rupture)/(Number of batteries subjected to burning test))×100[%]
 
     (Battery Drop Test) 
     First, batteries were charged so as to be in an overcharged state with an open circuit voltage of 4.4 V. Subsequently, the charged batteries were caused to drop thirty times from a height of 10 m, and the number of batteries in which the contents did not jump out of the batteries was counted. Subsequently, a passing ratio r2 of the battery drop test was determined from the following formula.
 
(Passing ratio  r 2 of battery drop test)=((Number of batteries in which the contents did not jump out of the batteries)/(Number of batteries subjected to drop test))×100[%]
 
     Table 1 illustrates test results of the batteries of Examples 1-1 to 5-3 and Comparative Examples 1-1 to 7-6. 
     
       
         
           
               
               
               
               
               
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                   
                 θ: 0° 
                 θ: 0.5° 
                 θ: 6° 
                 θ: 30° 
                 θ: 45° 
                 θ: 56° 
                 θ: 80° 
               
               
                   
               
             
            
               
                 Rin: 16 mm 
                 (CEx. 6-1) 
                 (CEx. 1-1) 
                 (CEx. 2-1) 
                 (CEx. 3-1) 
                 (CEx. 4-1) 
                 (CEx. 5-1) 
                 (CEx. 7-1) 
               
               
                 Ra: 88% 
                 r1: 20%  
                 r1: 100% 
                 r1: 100% 
                 r1: 100% 
                 r1: 100% 
                 r1: 100% 
                  r1: 80%  
               
               
                   
                 (jumping  
                  r2: 60% 
                  r2: 60% 
                  r2: 60% 
                  r2: 60% 
                  r2: 60% 
                 (rupture) 
               
               
                   
                 out)  
                   
                   
                   
                   
                   
                 r2: 60% 
               
               
                   
                 r2: 60% 
               
               
                   
               
               
                 Rin: 14 mm 
                 (CEx. 6-2) 
                 (Ex. 1-1) 
                 (Ex. 2-1) 
                 (Ex. 3-1) 
                 (Ex. 4-1) 
                 (Ex. 5-1) 
                 (CEx. 7-2) 
               
               
                 Ra: 77% 
                 r1: 20%  
                 r1: 100% 
                 r1: 100% 
                 r1: 100% 
                 r1: 100% 
                 r1: 100% 
                  r1: 80%  
               
               
                   
                 (jumping  
                 r2: 100% 
                 r2: 100%  
                 r2: 100% 
                 r2: 100% 
                 r2: 100% 
                 (rupture) 
               
               
                   
                 out)  
                   
                   
                   
                   
                   
                 r2: 100% 
               
               
                   
                 r2: 60% 
               
               
                   
               
               
                 Rin: 14 mm 
                 (CEx. 6-3) 
                 (Ex. 1-2) 
                 (Ex. 2-2) 
                 (Ex. 3-2) 
                 (Ex. 4-2) 
                 (Ex. 5-2) 
                 (CEx. 7-3) 
               
               
                 Ra: 50% 
                 r1: 20%  
                 r1: 100% 
                 r1: 100% 
                 r1: 100% 
                 r1: 100% 
                 r1: 100% 
                  r1: 80%  
               
               
                   
                 (jumping  
                 r2: 100% 
                 r2: 100%  
                 r2: 100% 
                 r2: 100% 
                 r2: 100% 
                 (rupture) 
               
               
                   
                 out)  
                   
                   
                   
                   
                   
                 r2: 100% 
               
               
                   
                 r2: 60% 
               
               
                   
               
               
                 Rin: 8 mm 
                 (CEx. 6-4) 
                 (Ex. 1-3) 
                 (Ex. 2-3) 
                 (Ex. 3-3) 
                 (Ex. 4-3) 
                 (Ex. 5-3) 
                 (CEx. 7-4) 
               
               
                 Ra: 44% 
                 r1: 20%  
                 r1: 100% 
                 r1: 100% 
                 r1: 100% 
                 r1: 100% 
                 r1: 100% 
                  r1: 80%  
               
               
                   
                 (jumping  
                 r2: 100% 
                 r2: 100% 
                 r2: 100% 
                 r2: 100% 
                 r2: 100% 
                 (rupture) 
               
               
                   
                 out)  
                   
                   
                   
                   
                   
                 r2: 100% 
               
               
                   
                 r2: 60% 
               
               
                   
               
               
                 Rin: 6 mm 
                 (CEx. 6-5) 
                 (CEx. 1-2) 
                 (CEx. 2-2) 
                 (CEx. 3-2) 
                 (CEx. 4-2) 
                 (CEx. 5-2) 
                 (CEx. 7-5) 
               
               
                 Ra: 33% 
                 r1: 20%  
                  r1: 80% 
                  r1: 80% 
                  r1: 80% 
                  r1: 80% 
                  r1: 80% 
                  r1: 80%  
               
               
                   
                 (rupture)  
                 (rupture) 
                 (rupture) 
                 (rupture) 
                 (rupture) 
                 (rupture) 
                 (rupture) 
               
               
                   
                 r2: 80% 
                 r2: 100% 
                 r2: 100% 
                 r2: 100% 
                 r2: 100% 
                 r2: 100% 
                 r2: 100% 
               
               
                   
               
               
                 Rin: 4 mm 
                 (CEx. 6-6) 
                 (CEx. 1-3) 
                 (CEx. 2-3) 
                 (CEx. 3-3) 
                 (CEx. 4-3) 
                 (CEx. 5-3) 
                 (CEx. 7-6) 
               
               
                 Ra: 22% 
                 r1: 20%  
                  r1: 60% 
                  r1: 60% 
                  r1: 60% 
                  r1: 60% 
                  r1: 60% 
                  r1: 60%  
               
               
                   
                 (rupture)  
                 (rupture) 
                 (rupture) 
                 (rupture) 
                 (rupture) 
                 (rupture) 
                 (rupture) 
               
               
                   
                 r2: 100% 
                 r2: 100% 
                 r2: 100% 
                 r2: 100% 
                 r2: 100% 
                 r2: 100% 
                 r2: 100% 
               
               
                   
               
            
           
         
       
     
     The symbols listed in Table 1 mean the following. 
     R in : inner diameter of groove [mm] 
     Ra: ratio of inner diameter R in  of groove with respect to outer diameter R out  of can bottom [%] 
     θ: opening angle of groove with respect to center of can bottom [degree] 
     Ex.: Example 
     CEx.: Comparative Example 
     r1: Passing ratio of battery burning test [%] 
     r2: Passing ratio of battery drop test [%] 
     Of the above test results, the test results of the batteries of Examples 1-2, 2-2, 3-2, 4-2, and 5-2, and Comparative Examples 6-3 and 7-3 are illustrated representatively in  FIG. 10A . In addition, the test results of the batteries of Examples 2-1 to 2-3 and Comparative Examples 2-1 to 2-3 are illustrated representatively in  FIG. 10B . 
     Table 1 and  FIGS. 10A and 10B  indicate the following. 
     If the opening angle θ is less than 0.5 degrees, the passing ratio of the burning test tends to decrease. This is because the whole groove portion of the can bottom opens at the time of the burning test due to the small opening angle θ, and the contents of the battery easily jump out. In addition, if the opening angle θ is less than 0.5 degrees, the passing ratio of the drop test tends to decrease. This is because the whole groove portion of the can bottom opens at the time of the drop test due to the small opening angle θ, and the contents of the battery easily jump out. 
     If the opening angle θ is more than 56 degrees, the passing ratio of the burning test tends to decrease. This is because an opening area of the can bottom is small at the time of the burning test due to the large opening angle θ, and the battery easily ruptures. 
     If the ratio Ra is less than 44%, the passing ratio of the burning test tends to decrease. This is because the groove is too far from an outer periphery of the can bottom, the groove is hardly softened due to heat generation at the time of the burning test, the can bottom does not cleave, and the battery easily ruptures. 
     If the ratio Ra is more than 77%, the passing ratio of the drop test tends to decrease. This is because the contents of the battery easily jump out due to a large opening area of the can bottom at the time of the drop test. 
     Therefore, in order to suppress a decrease in the passing ratios of the drop test and the burning test, the opening angle θ is 0.5 degrees or more and 56 degrees or less, and the ratio Ra is 44% or more and 77% or less. 
     ii Samples with Thickness t of Groove Bottom or Width w of Groove Changed 
     Examples 6-1 to 6-6 
     As illustrated in Table 2, a battery was obtained in a similar manner to Example 2-2 except that the thickness t of the groove bottom was changed within a range of 0.010 mm to 0.200 mm. 
     Examples 7-1 to 7-7 
     As illustrated in Table 3, a battery was obtained in a similar manner to Example 2-2 except that the width w of the groove was changed within a range of 0.05 mm to 2.00 mm. 
     (Evaluation) 
     The batteries of Examples 6-1 to 6-6 and 7-1 to 7-7 obtained as described above were subjected to a battery burning test and a battery drop test in a similar manner to Examples 1-1 to 5-3 and Comparative Examples 1-1 to 7-6 described above. 
     Table 2 and  FIG. 11A  illustrate test results of Examples 2-2 and 6-1 to 6-6. 
     
       
         
           
               
               
               
               
               
             
               
                   
                 TABLE 2 
               
               
                   
                   
               
               
                   
                   
                 thickness t of 
                 passing ratio 
                 passing ratio  
               
               
                   
                   
                 groove  
                 of burning 
                 of drop  
               
               
                   
                   
                 bottom [mm] 
                 test [%] 
                 test [%] 
               
               
                   
                   
               
             
            
               
                   
                 Example 2-2 
                 0.075 
                 100 
                 100 
               
               
                   
                 Example 6-1 
                 0.010 
                 100 
                  60 
               
               
                   
                 Example 6-2 
                 0.020 
                 100 
                 100 
               
               
                   
                 Example 6-3 
                 0.050 
                 100 
                 100 
               
               
                   
                 Example 6-4 
                 0.100 
                 100 
                 100 
               
               
                   
                 Example 6-5 
                 0.150 
                 100 
                 100 
               
               
                   
                 Example 6-6 
                 0.200 
                  60 
                 100 
               
               
                   
                   
               
            
           
         
       
     
     Table 3 and  FIG. 11B  illustrate test results of Examples 2-2 and 7-1 to 7-7. 
     
       
         
           
               
               
               
               
               
               
             
               
                   
                 TABLE 3 
               
               
                   
                   
               
               
                   
                   
                   
                 width w  
                 passing ratio  
                 passing ratio  
               
               
                   
                   
                   
                 of groove 
                 of burning  
                 of drop  
               
               
                   
                   
                   
                 [mm] 
                 test [%] 
                 test [%] 
               
               
                   
                   
               
             
            
               
                   
                   
                 Example 2-2 
                 0.40 
                 100 
                 100 
               
               
                   
                   
                 Example 7-1 
                 0.05 
                  60 
                 100 
               
               
                   
                   
                 Example 7-2 
                 0.10 
                 100 
                 100 
               
               
                   
                   
                 Example 7-3 
                 0.50 
                 100 
                 100 
               
               
                   
                   
                 Example 7-4 
                 0.70 
                 100 
                 100 
               
               
                   
                   
                 Example 7-5 
                 1.00 
                 100 
                 100 
               
               
                   
                   
                 Example 7-6 
                 1.50 
                 100 
                  90 
               
               
                   
                   
                 Example 7-7 
                 2.00 
                 100 
                  60 
               
               
                   
                   
               
            
           
         
       
     
     Table 2 and  FIG. 11A  indicate the following. 
     If the thickness t of the groove bottom is less than 0.020 mm, the passing ratio of the drop test tends to decrease. This is because the groove cleaves at the time of the drop test due to too small cleavage strength of the groove, and the contents of the battery easily jump out. 
     If the thickness t of the groove bottom is more than 0.150 mm, the passing ratio of the burning test tends to decrease. This is because the cleavage strength of the groove (that is, gas cleavage pressure of the groove) is too high, a side surface of the battery and a sealing portion rupture before cleavage of the groove, and the contents of the battery easily jump out of the portions which have ruptured. 
     Therefore, the thickness t of the groove bottom is preferably 0.020 mm or more than 0.150 mm or less in order to suppress a decrease in the passing ratios of the drop test and the burning test. 
     Table 3 and  FIG. 11B  indicate the following. 
     If the width w of the groove  11 Gv is less than 0.10 mm, the passing ratio of the burning test tends to decrease. This is because the cleavage strength of the groove (that is, gas cleavage pressure of the groove) is too high, a side surface of the battery and a sealing portion rupture before cleavage of the groove, and the contents of the battery easily jump out of the portions which have ruptured. 
     If the width w of the groove  11 Gv is more than 1.00 mm, the passing ratio of the drop test tends to decrease. This is because the groove cleaves at the time of the drop test due to too small cleavage strength of the groove, and the contents of the battery easily jump out. 
     Therefore, the width w of the groove is preferably 0.10 mm or more and 1.00 mm or less in order to suppress a decrease in the passing ratios of the drop test and the burning test. 
     iii Sample with Volume Energy Density Changed 
     Comparative Examples 8-1 to 8-7 
     A battery can having a can bottom without a groove was prepared. Furthermore, as illustrated in Table 4, the volume energy density was changed within a range of 280 Wh/L to 580 Wh/L. A battery was obtained in a similar manner to Example 1-1 except for this change. 
     (Evaluation) 
     The batteries of Comparative Examples 8-1 to 8-7 obtained as described above were subjected to a battery burning test in a similar manner to Examples 1-1 to 5-3 and Comparative Examples 1-1 to 7-6 described above. 
     Table 4 and  FIG. 12  illustrate test results of Comparative Examples 8-1 to 8-7. 
     
       
         
           
               
               
               
               
             
               
                   
                 TABLE 4 
               
               
                   
                   
               
               
                   
                   
                 volume  
                 passing ratio  
               
               
                   
                   
                 energy density 
                 of burning 
               
               
                   
                   
                 [Wh/L] 
                 test [%] 
               
               
                   
                   
               
             
            
               
                   
                 Comparative 
                 280 
                 100 
               
               
                   
                 Example 8-1 
                   
                   
               
               
                   
                 Comparative 
                 330 
                 100 
               
               
                   
                 Example 8-2 
                   
                   
               
               
                   
                 Comparative 
                 380 
                 100 
               
               
                   
                 Example 8-3 
                   
                   
               
               
                   
                 Comparative 
                 430 
                  60 
               
               
                   
                 Example 8-4 
                   
                   
               
               
                   
                 Comparative 
                 480 
                  40 
               
               
                   
                 Example 8-5 
                   
                   
               
               
                   
                 Comparative 
                 530 
                  20 
               
               
                   
                 Example 8-6 
                   
                   
               
               
                   
                 Comparative 
                 580 
                  20 
               
               
                   
                 Example 8-7 
               
               
                   
                   
               
            
           
         
       
     
     Table 4 and  FIG. 12  indicate the following. 
     Within a range in which the volume energy density of a battery is more than 380 Wh/L and 530 Wh/L or less, the passing ratio of the burning test tends to decrease as the volume energy density increases. This is because the gas generation amount increases at the time of the burning test as the volume energy density of a battery increases. Within a range in which the volume energy density of a battery is more than 380 Wh/L and 430 Wh/L or less, a decrease in the passing ratio of the burning test is more significant than a case within a range in which the volume energy density of a battery is more than 430 Wh/L and 530 Wh/L or less. 
     Therefore, a configuration in which the opening angle of the groove with respect to the center of the arc is 0.5 degrees or more and 56 degrees or less and the ratio of the inner diameter of the groove with respect to the outer diameter of the can bottom is 44% or more and 77% or less is preferably used for a battery having a volume energy density of more than 380 Wh/L, and more preferably used for a battery having a volume energy density of 430 Wh/L or more. 
     Hereinabove, the embodiments of the present technology, Modification Examples thereof, and Examples have been described specifically. However, the present technology is not limited to the above embodiments, Modification Examples thereof, and Examples, and various modifications based on a technical idea of the present technology can be made. 
     For example, the configurations, the methods, the steps, the shapes, the materials, the numerical values, and the like exemplified in the above embodiment, Modification Example thereof, and Examples are only examples, and a configuration, a method, a step, a shape, a material, a numerical value, and the like different therefrom may be used, if necessary. 
     In addition, the configurations, the methods, the steps, the shapes, the materials, the numerical values, and the like in the above embodiment, Modification Example thereof, and Examples can be combined to each other as long as not departing from the gist of the present technology. 
     It should be understood that various changes and modifications to the presently preferred embodiments described herein will be apparent to those skilled in the art. Such changes and modifications can be made without departing from the spirit and scope of the present subject matter and without diminishing its intended advantages. It is therefore intended that such changes and modifications be covered by the appended claims. 
     Furthermore, in the above embodiments, an example in which the present technology is applied to a lithium ion secondary battery has been described. However, the present technology can also be applied to a secondary battery other than the lithium ion secondary battery and a primary battery. However, it is particularly effective to apply the present technology to a lithium ion secondary battery. 
     In addition, the present technology can adopt the following configurations. 
     (1) A battery including: 
     an electrode body; and 
     a battery can housing the electrode body and having a bottom portion, in which 
     the bottom portion has an arc-shaped groove, 
     an opening angle of the groove with respect to a center of the arc is 0.5 degrees or more and 56 degrees or less, and 
     a ratio of an inner diameter of the groove with respect to an outer diameter of the bottom portion is 44% or more and 77% or less. 
     (2) The battery according to (1), in which the groove is disposed inside the battery can. 
     (3) The battery according to (1) or (2), in which a volume energy density of the battery is 430 Wh/L or more. 
     (4) The battery according to any one of (1) to (3), in which the bottom portion at a bottom of the groove has a thickness of 0.020 mm or more and 0.150 mm or less. 
     (5) The battery according to any one of (1) to (4), in which the groove has a width of 0.10 mm or more and 1.00 mm or less. 
     (6) The battery according to any one of (1) to (5), further including a safety valve for releasing gas when the gas is generated in the battery can. 
     (7) The battery according to (6), in which a gas release pressure of the groove is higher than a gas release pressure of the safety valve. 
     (8) The battery according to any one of (1) to (7), in which a center of the arc coincides with a center of the bottom portion. 
     (9) The battery according to any one of (1) to (8), in which a cross-sectional shape of the groove is a substantially trapezoidal shape, a substantially rectangular shape, a substantially triangular shape, a substantially partially circular shape, a substantially partially elliptical shape, or an indefinite shape. 
     (10) The battery according to any one of (1) to (9), in which 
     the electrode body includes a positive electrode and a negative electrode, and 
     an open circuit voltage in a fully charged state per pair of the positive electrode and the negative electrode is 4.4 V or more and 6.00 V or less. 
     (11) The battery according to any one of (1) to (10), in which the electrode body includes a positive electrode containing a positive electrode active material having an average composition represented by the following formula (1),
 
Li v Ni w M′ x M″ y O z   (1)
 
     (where, 0&lt;v&lt;2, w+x+y≤1, 0.8≤w≤1, 0≤0.2, 0≤y≤0.2, and 0&lt;z&lt;3 are satisfied, and each of M′ and M′ represents at least one selected from cobalt (Co), iron (Fe), manganese (Mn), copper (Cu), zinc (Zn), aluminum (Al), chromium (Cr), vanadium (V), titanium (Ti), magnesium (Mg), and zirconium (Zr)). 
     (12) A battery pack including: 
     the battery according to any one of (1) to (11); and 
     a control unit for controlling the battery. 
     (13) An electronic device including the battery according to any one of (1) to (11), in which 
     the electronic device receives electric power from the battery. 
     (14) An electric vehicle including: 
     the battery according to any one of (1) to (11); 
     a converter for converting electric power supplied from the battery into a driving force of the vehicle; and 
     a control device for performing information processing regarding vehicle control on the basis of information regarding the battery. 
     (15) An electricity storage device including the battery according to any one of (1) to (11), in which 
     the electricity storage device supplies electric power to an electronic device connected to the battery. 
     (16) The electricity storage device according to (15), including an electric power information control device for transmitting a signal to or receiving a signal from another device via a network, in which 
     the electricity storage device performs charge-discharge control of the battery on the basis of information received by the electric power information control device. 
     (17) An electric power system including the battery according to any one of (1) to (11), in which 
     the electric power system receives electric power from the battery. 
     (18) The electric power system according to (17), in which electric power is supplied to the battery from a power generating device or an electric power network. 
     (19) A battery can including a bottom portion with an arc-shaped groove, in which 
     an opening angle of the groove with respect to a center of the arc is 0.5 degrees or more and 56 degrees or less, and 
     a ratio of an inner diameter of the groove with respect to an outer diameter of the bottom portion is 44% or more and 77% or less. 
     REFERENCE SIGNS LIST 
     
         
           11  Battery can 
           11 Bt Can bottom (bottom portion) 
           11 Gv Groove 
           12 ,  13  Insulating plate 
           14  Battery lid 
           15  Safety valve mechanism 
           15 A Disk plate 
           16  Positive temperature coefficient element 
           17  Gasket 
           20  Wound electrode body 
           21  Positive electrode 
           21 A Positive electrode current collector 
           21 B Positive electrode active material layer 
           22  Negative electrode 
           22 A Negative electrode current collector 
           22 B Negative electrode active material layer 
           23  Separator 
           24  Center pin 
           25  Positive electrode lead 
           26  Negative electrode lead