Patent Publication Number: US-9406978-B2

Title: Power storage device

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional of U.S. application Ser. No. 12/719,629, filed Mar. 8, 2010, now allowed, which claims the benefit of a foreign priority application filed in Japan as Serial No. 2009-054519 on Mar. 9, 2009, both of which are incorporated by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention disclosed in this specification relates to power storage devices. 
     2. Description of the Related Art 
     In recent years, power storage devices, such as lithium-ion secondary batteries, which are power storage devices where carbon or lithium metal oxide is used as a battery material and which are charged and discharged by the movement of lithium ions as carrier ions between a positive electrode and a negative electrode, and electrochemical capacitors, have been actively developed (see References 1 to 3). 
     REFERENCES 
     
         
         [Reference 1] Japanese Published Patent Application No. 2008-294314 
         [Reference 2] Japanese Published Patent Application No. 2002-289174 
         [Reference 3] Japanese Published Patent Application No. 2007-299580 
       
    
     SUMMARY OF THE INVENTION 
     In order to obtain a power storage device with high capacity, the surface areas of a positive electrode and a negative electrode should be increased. The surface areas of a positive electrode and a negative electrode can be increased by providing a surface of each of the positive electrode and the negative electrode with projections and depressions. 
     A high-capacity power storage device can be obtained by interposing a separator between a positive electrode and a negative electrode having projections and depressions and by providing an electrolyte between the positive electrode and the negative electrode. 
     However, the positive or negative electrode may expand due to charging, and with that pressure, the separator may be broken and defective short circuit may be caused. 
     In addition, if pressure is applied to a separator between a positive electrode and a negative electrode in a thin, small-sized power storage device, the separator may easily be broken. 
     In the present invention, a surface of each of a positive-electrode active material and a negative-electrode active material is provided with a plurality of projections, and on an end of each of the projections, an insulator is disposed to relieve pressure that is to be applied to a separator. 
     An embodiment of the present invention relates to a power storage device including: a positive electrode having a positive-electrode current collector, a positive-electrode active material with a plurality of first projections on the positive-electrode current collector, and a first insulator on an end of each of the plurality of first projections; a negative electrode having a negative-electrode current collector, a negative-electrode active material with a plurality of second projections on the negative-electrode current collector, and a second insulator on an end of each of the plurality of second projections; a separator between the positive electrode and the negative electrode; and an electrolyte provided in a space between the positive electrode and the negative electrode and containing carrier ions. In each of the first projections and the second projections, a ratio of the height to the width is 3 or more and 1000 or less to 1, i.e. (3 to 1000):1. 
     Each of the first insulator and the second insulator may be any one of, or a stacked layer of two or more of, an acrylic resin, a polyimide resin, a polyimide amide resin, a phenol resin, an epoxy resin, a resist, a silicon oxide film, a silicon oxide film containing nitrogen, a silicon nitride film containing oxygen, and a silicon nitride film. 
     The carrier ions may be alkali metal ions or alkaline earth metal ions. The alkali metal ions may be lithium (Li) ions or sodium (Na) ions, and the alkaline earth metal ions may be magnesium (Mg) ions or calcium (Ca) ions. 
     Because the surface of each of the positive-electrode active material and the negative-electrode active material is provided with the plurality of projections, the surface area is increased and a thin, small-sized power storage device with high capacity can be obtained. 
     Furthermore, because the insulator is provided on each of the plurality of projections, even when pressure is applied between the positive electrode and the negative electrode, the insulator absorbs or disperses the pressure so as to prevent breaking of the separator. Accordingly, a highly reliable power storage device can be obtained. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross-sectional view illustrating a manufacturing process of a power storage device. 
         FIGS. 2A to 2D  are cross-sectional views illustrating a manufacturing process of a power storage device. 
         FIGS. 3A to 3D  are cross-sectional views illustrating a manufacturing process of a power storage device. 
         FIGS. 4A and 4B  are a perspective view and a cross-sectional view of a power storage device. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Embodiments of the invention disclosed in this specification will be hereinafter described with reference to the accompanying drawings. Note that the invention disclosed in this specification can be carried out in a variety of different modes, and it is easily understood by those skilled in the art that the modes and details of the invention disclosed in this specification can be changed in various ways without departing from the spirit and scope thereof. Therefore, the invention disclosed in this specification should not be interpreted as being limited to the description in the embodiments. Note that in the accompanying drawings, the same portions or portions having similar functions are denoted by the same reference numerals, and repetitive description thereof is omitted. 
     [Embodiment 1] 
     This embodiment is described with reference to  FIG. 1 ,  FIGS. 2A to 2D ,  FIGS. 3A to 3D , and  FIGS. 4A and 4B . 
     First, a plate-like positive-electrode current collector  111  is prepared (see  FIG. 2A ). As the positive-electrode current collector  111 , a simple substance, such as aluminum (Al) or titanium (Ti), or a compound thereof may be used. 
     Next, a plate-like positive-electrode active material  101  which is a material of a positive-electrode active material  112  is formed over the positive-electrode current collector  111  (see  FIG. 2B ). 
     As the plate-like positive-electrode active material  101 , a metal compound (oxide, sulfide, or nitride) having a layered structure can be used. In addition, as the positive-electrode active material  112  for a capacitor, activated carbon can be used. Furthermore, as the positive-electrode active material  112  for a lithium-ion secondary battery where lithium ions are used as carrier ions, a lithium-containing composite oxide represented by a chemical formula Li x M y O 2  (note that M represents Co, Ni, Mn, V, Fe, or Ti, and x is in the range of from 0.2 to 2.5 and y is in the range of from 0.8 to 1.25), such as LiCoO 2  or LiNiO 2 , may be used. Note that in the case where the aforementioned lithium-containing composite oxide represented by the chemical formula Li x M y O 2  is used as the positive-electrode active material  112  of a lithium-ion secondary battery, M may include either one element or two or more elements. In other words, as the positive-electrode active material  112  of a lithium-ion secondary battery, a multi-element, lithium-containing composite oxide may be used. 
     Over the plate-like positive-electrode active material  101 , a plurality of insulators  113  serving as a mask in an etching step are formed (see  FIG. 2C ). 
     Examples of the insulators  113  include organic resins such as an acrylic resin, a polyimide resin, a polyimide amide resin, a phenol resin, an epoxy resin, and a resist. The insulators  113  may be formed with such an organic resin by a printing method, a spin-coating method, or the like. For example, the insulators  113  may be formed as follows: unexposed photosensitive acrylic is formed over a surface of the plate-like positive-electrode active material  101  by a printing method and regions where the insulators  113  are to be formed are exposed to light. 
     Alternatively, an inorganic insulating material, such as a silicon oxide film, a silicon oxide film containing nitrogen, a silicon nitride film containing oxygen, or a silicon nitride film, may be used for the insulators  113 . 
     Further alternatively, a single layer of the aforementioned organic resin or inorganic insulating material, a stacked layer of two or more of the organic resins or a stacked layer of two or more of the inorganic insulating materials, or a stacked layer of two or more of the organic resins and the inorganic insulating materials may be used for the insulators  113 . 
     Next, with the use of the insulators  113  as a mask, the plate-like positive-electrode active material  101  is anisotropically etched by a dry etching method. Accordingly, the positive-electrode active material  112  is formed, which includes a plurality of projections  115  in which a ratio of height h to width a is 3 or more and 1000 or less to 1, i.e. (3 to 1000):1, preferably 10 or more and 1000 or less to 1, i.e. (10 to 1000):1. For example, each of the projections  115  has a width a of 1 μm to 10 μm and a height b of 3 μm to 1000 μm, preferably, a width a of 1 μm to 10 μm and a height b of 10 μm to 100 μm, or a width a of 1 μm and a height b of 10 μm (see  FIG. 2D ).  FIG. 2D  is a cross-sectional view, in which the positive-electrode active material  112  is illustrated as having a comb-like shape. However, the projections  115  are also formed in rows behind those illustrated, and thus, the positive-electrode active material  112  has a shape like a pin frog (spikes). 
     In the case of using a plate-like positive-electrode active material  101  which is difficult to etch by dry etching, the projections  115  may be formed by a different method such as mechanical processing, screen printing, electroplating, or hot embossing. Even in the case of using a plate-like positive-electrode active material  101  which can be etched by dry etching, the projections  115  may be formed by any of these methods. In the above manner, a positive electrode  117  is formed. 
     On the other hand, a plate-like negative-electrode current collector  121  is prepared (see  FIG. 3A ). As the negative-electrode current collector  121 , a simple substance, such as copper (Cu), aluminum (Al), nickel (Ni), or titanium (Ti), or a compound thereof may be used. 
     Next, a plate-like negative-electrode active material  105  which is a material of a negative-electrode active material  122  is formed over the negative-electrode current collector  121  (see  FIG. 3B ). 
     As the plate-like negative-electrode active material  105 , a lithium-ion holding body such as a carbon material, a silicon material, or a silicon alloy material, which is capable of occluding and releasing lithium ions, is used. As such a carbon material, powdered or fibrous graphite or the like can be used. As such a silicon material, a material obtained by depositing microcrystalline silicon and then removing amorphous silicon from the microcrystalline silicon by etching may be used. When amorphous silicon is removed from microcrystalline silicon, the surface area of the remaining microcrystalline silicon is increased. In a lithium-ion capacitor where lithium ions are used as carrier ions, for example, a material obtained by impregnating the aforementioned lithium-ion holding body with metallic lithium may be used. In other words, a material obtained by impregnating the aforementioned carbon material, silicon material, silicon alloy material, or the like with metallic lithium may be used as the negative-electrode active material  122 . 
     Next, over the plate-like negative-electrode active material  105 , a plurality of insulators  123  serving as an etching mask are formed (see  FIG. 3C ). The insulators  123  may be formed with a material and by a manufacturing method which are similar to those of the insulators  113 . 
     Next, with the use of the insulators  123  as a mask, the plate-like negative-electrode active material  105  which can be etched by dry etching is anisotropically etched by a dry etching method. Accordingly, the negative-electrode active material  122  is formed, which includes a plurality of projections  125  in which a ratio of height d to width c is 3 or more and 1000 or less to 1, i.e. (3 to 1000):1, preferably 10 or more and 1000 or less to 1, i.e. (10 to 1000):1. For example, each of the projections  125  has a width c of 1 μm to 10 μm and a height d of 3 μm to 1000 μm, preferably, a width c of 1 μm to 10 μm and a height d of 10 μm to 100 μm, or a width c of 1 μm and a height d of 10 μm (see  FIG. 3D ).  FIG. 3D  is a cross-sectional view, in which the negative-electrode active material  122  is illustrated as having a comb-like shape. However, the projections  125  are also formed in rows behind those illustrated, and thus, the negative-electrode active material  122  has a shape like a pin frog (spikes). 
     In the case of using a plate-like negative-electrode active material  105  which is difficult to etch by dry etching, the projections  125  may be formed by a different method such as mechanical processing, screen printing, electroplating, or hot embossing. Even in the case of using a plate-like negative-electrode active material  105  which can be etched by dry etching, the projections  125  may be formed by any of these methods. In the above manner, a negative electrode  127  is formed. 
     Next, the positive electrode  117  and the negative electrode  127  are disposed to face each other, and a separator  131  is provided between the positive electrode  117  and the negative electrode  127 . 
     As the separator  131 , paper, nonwoven fabric, a glass fiber, a synthetic fiber such as nylon (polyamide), vinylon (also called vinalon) (a polyvinyl alcohol based fiber), polyester, acrylic, polyolefin, or polyurethane, or the like may be used. Note that a material which does not dissolve in an electrolyte  132  mentioned below should be selected. 
     More specific examples of materials of the separator  131  are high-molecular compounds based on fluorine-based polymer, polyether such as polyethylene oxide and polypropylene oxide, polyolefin such as polyethylene and polypropylene, polyacrylonitrile, polyvinylidene chloride, polymethyl methacrylate, polymethylacrylate, polyvinyl alcohol, polymethacrylonitrile, polyvinyl acetate, polyvinylpyrrolidone, polyethyleneimine, polybutadiene, polystyrene, polyisoprene, and polyurethane, derivatives thereof, cellulose, paper, and nonwoven fabric, which can be used either alone or in combination. 
     The separator  131  is interposed between the projections  115  of the positive-electrode active material  112  and the projections  125  of the negative-electrode active material  122  and thus may be broken by application of pressure. However, because the insulators  113  are provided on the projections  115  and the insulators  123  are provided on the projections  125 , the insulators  113  and the insulators  123  absorb or suppress pressure and can prevent the separator  131  from being broken. Accordingly, the positive electrode  117  and the negative electrode  127  can be prevented from being in contact with each other and short-circuited. 
     The electrolyte  132  is provided in a space between the positive electrode  117  and the negative electrode  127 . Through the above process, a power storage device  135  is manufactured (see  FIG. 1 ). 
     The electrolyte  132  contains alkali metal ions as carrier ions, such as lithium ions, and the lithium ions are responsible for electrical conduction. The electrolyte  132  includes a solvent and a lithium salt which dissolves in the solvent. Examples of lithium salts include LiPF 6  (lithium hexafluorophosphate), LiClO 4 , LiBF 4 , LiAlCl 4 , LiSbF 6 , LiSCN, LiCl, LiCF 3 SO 3 , LiCF 3 CO 2 , Li(CF 3 SO 2 ) 2 , LiAsF 6 , LiN(CF 3 SO 2 ) 2 , LiB 10 Cl 10 , LiN(C 2 F 5 SO 2 ), LiPF 3 (CF 3 ) 3 , LiPF 3 (C 2 F 5 ) 3 , and the like, which can be used for the electrolyte  132 , either alone or in combination. 
     Note that in the description of this specification, alkali metal ions such as lithium (Li) ions are used as carrier ions; instead of lithium ions, alkali metal ions such as sodium (Na) ions may be used. Furthermore, alkaline earth metal ions such as magnesium (Mg) ions or calcium (Ca) ions may be used. 
     In the case of manufacturing a capacitor where such carrier ions are used and the negative-electrode active material  122  is impregnated with a metal of the same kind as that of the carrier ions, the aforementioned carbon material, silicon material, silicon alloy material, or the like, which is capable of occluding and releasing the carrier ions, may be impregnated with the metal. 
     Examples of the solvent of the electrolyte  132  include: cyclic carbonates such as ethylene carbonate (hereinafter abbreviated as EC), propylene carbonate (PC), butylene carbonate (BC), and vinylene carbonate (VC); acyclic carbonates such as dimethyl carbonate (DMC), diethyl carbonate (DEC), ethylmethyl carbonate (hereinafter abbreviated as EMC), methylpropyl carbonate (MPC), methylisobutyl carbonate (MIPC), and dipropyl carbonate (DPC); aliphatic carboxylic acid esters such as methyl formate, methyl acetate, methyl propionate, and ethyl propionate; γ-lactones such as γ-butyrolactone; acyclic ethers such as 1,2-dimethoxyethane (DME), 1,2-diethoxyethane (DEE), and ethoxymethoxy ethane (EME); cyclic ethers such as tetrahydrofuran and 2-methyltetrahydrofuran; dimethylsulfoxide; 1,3-dioxolane; alkyl phosphate esters such as trimethyl phosphate, triethyl phosphate, and trioctyl phosphate and fluorides thereof. These materials can be used either alone or in combination. 
     The power storage device  135  manufactured in the above manner may be used with a substrate  137  attached to its surface, if necessary (see  FIG. 4B ). As the substrate  137 , a substrate which functions as a sealing layer may be selected, or a substrate which functions as a protector may be selected. Furthermore, a substrate which functions as both a sealing layer and a protector may be selected, or a substrate which functions as a sealing layer and a substrate which functions as a protector may be stacked. 
     The power storage device  135  may be used in a long plate-like shape, or the power storage device  135  in a long plate-like shape may be rolled into a cylindrical power storage device  138 , if necessary (see  FIG. 4A ). Note that  FIG. 4B  is a cross-sectional view taken along a line A-A′ of  FIG. 4A . 
     This application is based on Japanese Patent Application serial no. 2009-054519 filed with Japan Patent Office on Mar. 9, 2009, the entire contents of which are hereby incorporated by reference.