Lithium secondary battery

Abstract

A method for generating graphene includes the following steps: forming a layer containing graphene oxide on a first conductive layer; and providing the first conductive layer with a potential capable of reducing the graphene oxide in an electrolyte impregnated with a first conductive layer as a working electrode and a second conductive layer as a counter electrode. A method for manufacturing an energy storage device having at least a positive electrode, a negative electrode, an electrolyte, and an insulator, wherein graphene is generated using the above-described manufacturing method as an active material layer serving as one or both of the positive and negative electrodes.

Description

Lithium secondary battery The present invention relates to a method for manufacturing graphene, an electrode containing the graphene, and an energy storage device having the electrode. In addition, the present invention relates to graphene, an electrode, and an energy storage device manufactured by the manufacturing method. In addition, in this specification, the energy storage device refers to all elements and devices having a power storage function, such as a lithium primary battery, a lithium secondary battery, and a lithium ion capacitor. In recent years, due to the excellent electrical properties such as high conductivity and the excellent physical properties such as high flexibility and mechanical strength of graphene, research and development on applying it to various products have been continuously carried out. Applying graphene to energy storage devices such as lithium secondary batteries and lithium ion capacitors is one of its applications. For example, in order to improve the conductivity of an electrode material for a lithium secondary battery, graphene can be coated on the electrode material. In addition, as a method for manufacturing graphene, a method of reducing graphite oxide or graphene oxide in the presence of a base can be cited. In this method for manufacturing graphene, as a method for forming graphite oxide, methods such as using sulfuric acid, nitric acid, and potassium chlorate as oxidants; using sulfuric acid and potassium permanganate as oxidants; and using potassium chlorate and fuming nitric acid as oxidants can be cited (see Patent Document 1). The Modified Hummers method is a method for forming graphite oxide using sulfuric acid and potassium permanganate as oxidants. Here, a method for manufacturing graphene using the Modified Hummers method will be described with reference to FIG. 14. In a solvent, graphite is oxidized using an oxidant such as potassium permanganate to form a mixed solution 1 containing graphite oxide. Then, in order to remove the oxidant remaining in the mixed solution 1, hydrogen peroxide and water are added to the mixed solution 1 to form a mixed solution 2 (step S101). In addition, here, the unreacted potassium permanganate by hydrogen peroxide is reduced and reacts with sulfuric acid to form manganese sulfate. Next, graphite oxide is recovered from the mixed solution 2 (step S102). Next, in order to further remove the oxidant remaining or attached to the recovered graphite oxide, the graphite oxide is washed with an acidic solution and then washed with water (step S103). In addition, the washing process of step S103 is repeated. Next, the graphite oxide is diluted with a large amount of water and centrifuged to separate the acid from the graphite oxide, and the graphite oxide is recovered (step S104). Next, ultrasonic waves are applied to the mixed solution containing the recovered graphite oxide to peel the oxidized carbon layers constituting the graphite oxide to form graphene oxide (step S105). Next, by performing a reduction treatment of the graphene oxide, graphene can be generated (step S106). As a method for generating graphene by reducing graphene oxide, heat treatment can be used. [Patent Document 1] Japanese Patent Application Laid-Open No. 2011-500488 The conductivity of graphene sometimes generated from reduced graphene oxide varies depending on the bonding state in the graphene. Accordingly, an object of one aspect of the present invention is to provide graphene generated from graphene oxide with improved conductivity and a method for manufacturing the graphene. In addition, an electrode in an energy storage device includes a current collector and an active material layer. In a conventional electrode, the active material layer contains a conductive agent, a binder, and the like in addition to the active material. Therefore, it is difficult to effectively increase only the weight of the active material as an electrode, and thus it is difficult to increase the charge and discharge capacity per unit electrode weight or electrode volume. Further, a conventional electrode has a problem that when the binder contained in the active material layer comes into contact with an electrolytic solution, it swells and the electrode is deformed and vulnerable. Accordingly, an object of one aspect of the present invention is to provide an energy storage device with improved charge and discharge capacity, reliability, durability, etc. per unit electrode weight or electrode volume and a method for manufacturing the energy storage device. Oxides such as graphite oxide and graphene oxide can be reduced by heat treatment. However, in the present invention, electrical energy is used to generate graphene by electrochemically reducing graphene oxide. In addition, in the present specification, the reduction treatment that provides a potential for promoting the reduction reaction of the active material layer is sometimes referred to as electrochemical reduction. In the present specification, graphene refers to a thin sheet composed of a single atomic layer of carbon molecules having a double bond (also referred to as sp 2 bond) having voids through which ions pass, or a laminate formed by laminating two to 100 such thin sheets. In addition, the laminate may also be referred to as multilayer graphene. In addition, it is preferable that the ratio of hydrogen and elements other than carbon in the graphene is 15 atomic% or less, or the ratio of elements other than carbon is 30 atomic% or less. In addition, the graphene may be added with an alkali metal such as potassium. Therefore, graphene analogs also belong to the graphene. In addition, in this specification, graphene oxide refers to graphene in which a six-membered ring or polycyclic ring composed of carbon is bonded to an oxygen atom. Specifically, it refers to graphene in which a six-membered ring or polycyclic ring composed of carbon is bonded to a carbonyl group such as an epoxy group or a carboxyl group, or a hydroxyl group. In addition, graphene oxide sometimes forms a graphene oxide salt according to the manufacturing method of graphene oxide. The graphene oxide salt refers to, for example, a salt formed by reacting a carbonyl group such as an epoxy group or a carboxyl group or a hydroxyl group bonded to a six-membered ring or polycyclic ring composed of carbon with ammonia, amine, alkali metal, etc. Therefore, in this specification, "graphene oxide" includes "graphene oxide salt". In addition, graphene oxide and graphene oxide salt include one sheet or a laminate formed by laminating 2 to 100 layers of the one sheet, and the laminate can also be referred to as multilayer graphene oxide and multilayer graphene oxide salt. One embodiment of the present invention is a method for generating graphene, in which a layer containing graphene oxide is formed on a first conductive layer, and a potential capable of causing a reduction reaction of graphene oxide is provided to the first conductive layer in an electrolytic solution in which the first conductive layer as a working electrode and the second conductive layer as a counter electrode are immersed to generate graphene. Specifically, the potential provided to the first conductive layer is a potential capable of causing a reduction reaction of graphene oxide, which is 1.6 V or more and 2.4 V or less (based on the oxidation-reduction potential of lithium), and graphene is generated by reducing the graphene oxide. In addition, hereinafter, "based on the oxidation-reduction potential of lithium" is sometimes described as "vs.Li / Li + ". In addition, one embodiment of the present invention is a method for generating graphene, in which a layer containing graphene oxide is formed on a first conductive layer, and the potential of the first conductive layer is scanned in an electrolytic solution in which the first conductive layer as a working electrode and the second conductive layer as a counter electrode are immersed in such a manner as to at least include a potential capable of causing a reduction reaction of graphene oxide, and graphene is generated by reducing the graphene oxide. Specifically, as described above, the potential is scanned in such a manner as to at least include a potential capable of reducing graphene oxide, that is, 1.4 V or more and 2.6 V or less (vs.Li / Li + ), preferably 1.6 V or more and 2.4 V or less (vs.Li / Li + ). And the potential of the first conductive layer can be scanned periodically in such a manner as to include this range. By performing periodic scanning, graphene oxide can be sufficiently reduced. An energy storage device can be manufactured by using the above method. One embodiment of the present invention is a method for manufacturing an energy storage device including at least a positive electrode, a negative electrode, an electrolyte, and a separator, wherein at least one or both of the positive electrode and the negative electrode form an active material layer containing at least an active material and graphene oxide on a current collector, and a potential capable of causing a reduction reaction of the graphene oxide is provided to the current collector to form graphene. Specifically, in one or both of the positive electrode and the negative electrode, the potential provided to the current collector is set to be 1.4 V or more and 2.6 V or less (vs. Li / Li + ), preferably 1.6 V or more and 2.4 V or less (vs. Li / Li + ) to reduce the graphene oxide to generate graphene. In addition, one embodiment of the present invention is a method for manufacturing an electrode and an energy storage device using the electrode, wherein an active material layer containing at least an active material and graphene oxide is formed on a current collector, and the potential of the current collector is scanned in a manner including at least a potential capable of causing a reduction reaction of the graphene oxide, and graphene is generated by reducing the graphene oxide. Specifically, as described above, in a range including at least a potential capable of reducing the graphene oxide, that is, 1.4 V or more and 2.6 V or less (vs. Li / Li + ), preferably 1.6 V or more and 2.4 V or less (vs. Li / Li + ), the potential of the current collector is scanned. At this time, graphene is formed on the surface of the active material or within the active material layer. In addition, the potential of the current collector can also be scanned periodically in a manner including this range. By periodically scanning the potential of the current collector, for example, the graphene oxide within the active material layer can also be sufficiently reduced. In addition, in the graphene manufactured according to the above method for manufacturing graphene, the composition of carbon atoms measured by X-ray photoelectron spectroscopy (XPS) is 80% or more and 90% or less, and the composition of oxygen atoms is 10% or more and 20% or less. And, among the carbon atoms measured by XPS for the graphene, the carbon atoms forming sp 2 bonds are 50% or more and 80% or less, preferably the carbon atoms are 60% or more and 70% or less or 70% or more and 80% or less, that is, preferably 60% or more and 80% or less. In addition, one embodiment of the present invention further includes an energy storage device including graphene in one or both of the positive electrode and the negative electrode. Compared with the case of generating graphene by heat treatment, according to one aspect of the present invention, the proportion of carbon-carbon bonds as double bonds of sp 2 bonds can be increased, whereby graphene with improved conductivity and a method for manufacturing the same can be provided. In addition, an energy storage device with improved charge / discharge capacity, reliability, and durability per unit electrode weight and a method for manufacturing the same can be provided. S111: Step S112: Step S121: Step S122: Step S123: Step S124: Step S125: Step S126: Step S127: Step 113: Container 114: Electrolyte 115: Conductive layer 116: Counter electrode 201: Negative current collector 203: Negative active material layer 205: Negative electrode 211: Negative active material 213: Graphene 221: Negative active material 221a: Common part 221b: Protrusion 223: Graphene 307: Positive current collector 309: Positive active material layer 311: Positive electrode 321: Positive active material 323: Graphene 400: Lithium secondary battery 401: Positive current collector 403: Positive active material layer 405: Positive electrode 407: Negative current collector 409: Negative active material layer 411: Negative electrode 413: Separator 415: Electrolyte 417: External terminal 419: External terminal 421: Gasket 501_O: Dashed line 501_R: Dashed line 502: Dashed line 502_R: Dashed line 503_O: Dashed line 503_R: Dashed line 511_O: Curve 511_R: Curve 512_O: Curve 512_R: Curve 531_O: Curve 531_R: Curve 532_O: Curve 532_R: Curve 533_O: Curve 533_R: Curve 5000: Display device 5001: Housing 5002: Display unit 5003: Speaker unit 5004: Energy storage device 5100: Lighting equipment 5101: Housing 5102: Light source 5103: Energy storage device 5104: Ceiling 5105: Wall 5106: Floor 5107: Window 5200: Indoor unit 5201: Housing 5202: Air outlet 5203: Energy storage device 5204: Outdoor unit 5300: Electric refrigerator-freezer 5301: Housing 5302: Door for refrigerator compartment 5303: Freezer door 5304: Energy storage device 9630: Housing 9631: Display unit 9631a: Display unit 9631b: Display unit 9632a: Area of touch screen 9632b: Area of touch screen 9033: Clip 9034: Display mode switch 9035: Power switch 9036: Power saving mode switch 9038: Operation switch 9639: Keyboard display switch button 9633: Solar cell 9634: Charge and discharge control circuit 9635: Battery 9636: DC-DC converter 9637: Operation key 9638: Converter In the diagrams: Figures 1A and 1B are diagrams showing a method for manufacturing graphene and the apparatus used according to one embodiment of the present invention; Figure 2 is a diagram showing a method for manufacturing graphene oxide according to one embodiment of the present invention; Figure 3 is a diagram showing a method for manufacturing graphene oxide according to one embodiment of the present invention; Figures 4A to 4C are diagrams showing a positive electrode according to one embodiment of the present invention; Figures 5A to 5D are diagrams showing a negative electrode according to one embodiment of the present invention; Figure 6 is a diagram showing an energy storage device according to one embodiment of the present invention; Figure 7 is a diagram showing an electrical device; Figures 8A to 8C are diagrams showing an electrical device; Figure 9 is a diagram showing the results of cyclic voltammetry measurement; Figure 10 is a diagram showing the results of cyclic voltammetry measurement; Figure 11 is a diagram showing the results of cyclic voltammetry measurement; Figure 12 is a diagram showing the analysis results of the surface element composition using XPS; Figure 13 is a diagram showing the analysis results of the atomic bonding state using XPS; Figure 14 is a diagram showing a conventional method for manufacturing graphene; Figures 15A and 15B are diagrams showing the results of cyclic voltammetry measurement; Figures 16A and 16B are diagrams showing the results of cyclic voltammetry measurement. An example of an embodiment and an example of the present invention will be described below with reference to the drawings. However, the present invention is not limited to the following description, and those of ordinary skill in the art can easily understand the fact that the mode and details of the present invention can be changed into various forms without departing from its gist and scope. Therefore, the present invention should not be construed as being limited to the description of the embodiments and examples shown below. In addition, when referring to the drawings in the description, the same reference numerals may sometimes be used in different drawings to represent the same parts. In addition, the same hatching is sometimes used to represent the same parts without particularly attaching reference numerals. Embodiment 1 In this embodiment, a method for manufacturing graphene according to one aspect of the present invention will be described with reference to FIGS. 1A and 1B. FIG. 1A is a diagram illustrating the process of graphene, and FIG. 1B is a schematic diagram of an apparatus for manufacturing graphene. In a method for manufacturing graphene according to one aspect of the present invention, when generating graphene, instead of reducing graphene oxide by heat treatment, electrical energy is used to electrochemically reduce graphene oxide. 〈Step S111〉 As step S111 shown in FIG. 1A, a layer containing graphene oxide is formed on the surface of the conductive layer. For example, a dispersion containing graphene oxide is coated on the conductive layer. As the dispersion containing graphene oxide, commercially available products or a dispersion obtained by dispersing graphene oxide manufactured by the method described in FIG. 14 or the like into a solvent can be used. In addition, a dispersion obtained by dispersing graphene oxide (graphene oxide salt) manufactured by the method described below into a solvent can also be used. The conductive layer may be made of a material having conductivity. For example, metal materials such as aluminum (Al), copper (Cu), nickel (Ni), or titanium (Ti), and alloy materials composed of multiple materials among the above metal materials. As the alloy material, for example, Al-Ni alloy, Al-Cu alloy, etc. can be cited. The conductive layer can be appropriately in the form of a foil, a plate, a net, etc., and a formation formed of the above metal material or the above alloy material formed on another substrate can be peeled off, and the peeled product can be used as the conductive layer. As a method for coating a dispersion containing graphene oxide on the conductive layer, coating methods, spin coating methods, dip coating methods, spray coating methods, etc. can be cited. In addition, multiple of the above methods can also be combined. For example, by coating a dispersion containing graphene oxide on the conductive layer using the dip coating method and then rotating the conductive layer in the same manner as the spin coating method, the thickness uniformity of the coated dispersion containing graphene oxide can be improved. After applying the dispersion containing graphene oxide on the conductive layer, the solvent in the dispersion is removed. For example, the solvent in the dispersion containing graphene oxide applied on the conductive layer can be removed by performing vacuum drying for a certain period of time. In addition, the time for performing vacuum drying varies depending on the amount of the applied dispersion. Further, the vacuum drying can be performed while heating the conductive layer at a temperature that does not reduce graphene oxide. For example, when the thickness of graphene oxide after step S111 is about 10 μm, it is preferably performed by heating the conductive layer at a temperature above room temperature and below 100 °C for about 1 hour of vacuum drying, and then performing vacuum drying at room temperature for about 1 hour. 〈Step S112〉 Next, graphene is formed by reducing graphene oxide provided on the conductive layer. In this step, the electrical energy electrochemical reduction of graphene oxide is used as described above. Generally speaking, in this step, a closed circuit is formed by using the conductive layer provided with graphene oxide formed in step S111, and a potential capable of causing a reduction reaction of the graphene oxide or a potential capable of reducing the graphene oxide is provided to the conductive layer to reduce the graphene oxide to graphene. In addition, in this specification, the potential capable of causing a reduction reaction of graphene oxide or the potential capable of reducing the graphene oxide is also referred to as the reduction potential. A specific method for reducing graphene oxide will be described with reference to FIG. 1B. The container 113 is filled with the electrolyte 114, and then the conductive layer 115 provided with graphene oxide and the counter electrode 116 are inserted and immersed in the electrolyte. In this step, by using the conductive layer 115 provided with graphene oxide formed in step S111 as the working electrode, and at least using the counter electrode 116 and the electrolyte 114 to form an electrochemical cell (open circuit), the graphene oxide is reduced to graphene by providing the reduction potential of the graphene oxide to the above-mentioned conductive layer 115 (working electrode). As the electrolyte 114, an aprotic organic solution such as ethylene carbonate or diethyl carbonate can be used. In addition, the provided reduction potential refers to: the reduction potential with respect to the counter electrode 116; or a reference electrode is provided to the electrochemical cell, and the reduction potential with respect to the reference electrode. For example, when using lithium metal as the counter electrode 116 and the reference electrode, the provided reduction potential is the reduction potential based on the redox potential of lithium metal (vs. Li / Li + ). By this step, a reduction current flows through the electrochemical cell (closed circuit) when the graphene oxide is reduced. Therefore, the reduction of graphene oxide can be confirmed by sequentially confirming the above reduction current, and the state where the reduction current is lower than the fixed value (corresponding to the state where the peak of the reduction current disappears) is regarded as the state where the graphene oxide is reduced (the end state of the reduction reaction). In addition, in this step, as the potential of the conductive layer 115, it can be fixed at the reduction potential of graphene oxide, or the reduction potential of graphene oxide can be scanned, and this scan can also be repeated periodically like cyclic voltammetry. Note that although there is no limit on the scanning speed of the potential of the conductive layer 115, it is preferably 0.005 mV / s or more and 1 mV / s or less. In addition, when scanning the potential of the conductive layer 115, it can be scanned from the high potential side to the low potential side, or from the low potential side to the high potential side. Although the value of the reduction potential of graphene oxide varies slightly depending on the structure of graphene oxide (presence or absence of functional groups, formation of graphene oxide salts, etc.) and the potential control method (scanning speed, etc.), it is approximately 2.0 V (vs. Li / Li + ) or so. Specifically, the potential of the above-mentioned conductive layer 115 can be controlled to be 1.4 V or more and 2.6 V or less (vs. Li / Li + ), preferably 1.6 V or more and 2.4 V or less (vs. Li / Li + ). In addition, the reduction potential of graphene oxide will be described in detail in the following examples. Graphene can be generated on the conductive layer 115 by the above steps. In addition, in the graphene produced by using the method for producing graphene according to one embodiment of the present invention, the composition of carbon atoms measured by XPS is 80% or more and 90% or less, and the composition of oxygen atoms is 10% or more and 20% or less. Among these carbon atoms, the carbon atoms forming sp 2 bonds are 50% or more and 80% or less. Preferably, the carbon atoms are 60% or more and 70% or less or 70% or more and 80% or less, that is, preferably 60% or more and 80% or less. In addition, as a method for reducing graphene oxide, in addition to the electrochemical reduction method using electric energy, there is also a reduction method (also called thermal reduction) in which oxygen atoms in graphene oxide are removed in the form of carbon dioxide by heat treatment. The graphene according to one embodiment of the present invention is at least different from the graphene produced by thermal reduction in the following: The graphene according to one embodiment of the present invention electrochemically reduces graphene oxide by using electric energy, and as sp 2The ratio of the carbon-carbon double bonds of the key double bonds is higher than that of graphene produced by thermal reduction. Therefore, compared with graphene produced by thermal reduction, the graphene according to one aspect of the present invention has more π electrons that do not locally exist at a specific position but widely promote the bonding between carbons. Therefore, it can be said that the conductivity is increased compared with graphene produced by thermal reduction. In the method illustrated in FIG. 14 as an example of the method for manufacturing graphene oxide that can be used in step S111, the washing process of graphene oxide in step S103 requires a large amount of water. And although the acid in graphite oxide can be removed by repeating step S103, the less the acid content, the more difficult it is to separate the graphite oxide as a precipitate from the acid in the supernatant, resulting in a decrease in the yield of graphite oxide. Eventually, the yield of graphene will decrease. Here, a method for manufacturing graphene oxide different from the method illustrated in FIG. 14 in step S111 will be described. FIG. 2 is a diagram illustrating the process of graphene oxide (or graphene oxide salt). 〈Oxidation treatment of graphite〉 As shown in step S121, graphite is oxidized by using an oxidizing agent to form graphite oxide. The following are used as oxidizing agents: sulfuric acid, nitric acid, and potassium chlorate; sulfuric acid and potassium permanganate; or potassium chlorate and fuming nitric acid. Here, graphite, sulfuric acid, and potassium permanganate are mixed to oxidize the graphite, and water is added thereto to form a mixed solution 1 containing graphite oxide. Then, in order to remove the remaining oxidizing agent, hydrogen peroxide and water may be added to the mixed solution 1. Unreacted potassium permanganate can be reduced by hydrogen peroxide to react with sulfuric acid to form manganese sulfate. Since manganese sulfate is soluble in water, it can be separated from the water-insoluble graphite oxide. 〈Recovery of graphite oxide〉 Next, as shown in step S122, graphite oxide is recovered from the mixed solution 1. By performing one or more of filtration, centrifugation, etc. on the mixed solution 1, a precipitate 1 containing graphite oxide is recovered from the mixed solution 1. Note that the precipitate 1 sometimes contains unreacted graphite. 〈Washing of graphite oxide〉 Next, as shown in step S123, metal ions and sulfate ions in the precipitate 1 containing graphite oxide are removed by using an acidic solution. Here, the metal ions from the oxidizing agent in the graphite oxide can be dissolved in the acidic solution to remove the metal ions and sulfate ions in the graphite oxide. In this way, by washing the graphite oxide with an acidic solution, the yields of graphene oxide and graphene oxide salt can be increased. Therefore, the method for manufacturing graphene oxide shown in FIG. 2 can increase the productivity of graphene oxide and the productivity of graphene. Typical examples of the acidic solution include hydrochloric acid, dilute sulfuric acid, nitric acid, etc. Note that when using an acid with high volatility, typically hydrochloric acid, for this treatment, the residual acidic solution can be easily removed in the subsequent drying step, so it is preferred. As a method for removing metal ions and sulfate ions from the precipitate 1, examples include: after mixing the precipitate 1 and the acidic solution, performing any one or more of filtration, centrifugation, dialysis, etc.; placing the precipitate 1 on filter paper and rinsing the precipitate 1 with the acidic solution; and so on. Here, the precipitate 1 is placed on filter paper, and the metal ions and sulfate ions in the precipitate 1 are rinsed off with the acidic solution to recover the precipitate 2 containing graphene oxide. Note that the precipitate 2 may contain unreacted graphite. 〈Production of graphene oxide〉 Next, as shown in step S124, the precipitate 2 and water are mixed to form a mixed solution 2 in which the precipitate 2 is dispersed. Then, the graphene oxide contained in the mixed solution 2 is exfoliated to form graphene oxide. As a method for exfoliating graphene oxide to form graphene oxide, examples include applying ultrasonic waves and mechanical stirring. In addition, the mixed solution in which graphene oxide is dispersed is referred to as a mixed solution 3. Note that in the graphene oxide formed by this step, the six-membered rings composed of carbon expand in the planar direction, and in a part of them, polycyclic rings such as seven-membered rings, eight-membered rings, nine-membered rings, and ten-membered rings are formed. In addition, the polycyclic ring refers to a cyclic carbon skeleton in which a part of the carbon bonds of the six-membered ring composed of carbon are cut off and the cut carbon bonds are joined in a manner that increases the number of carbon atoms. The region surrounded by the carbon constituting the polycyclic ring becomes a pore. In addition, a part of the carbon constituting the six-membered ring or polycyclic ring is bonded to a carbonyl group such as an epoxy group or a carboxyl group, or a hydroxyl group. Note that multilayer graphene oxide may be dispersed instead of graphene oxide. 〈Recovery of graphene oxide〉 Next, as shown in step S125, by performing one or more of filtration, centrifugation, etc. on the mixed solution 3, the mixed solution 3 is separated into a mixed solution containing graphene oxide and a precipitate 3 containing graphite, and the mixed solution containing graphene oxide is recovered. In addition, the mixed solution containing graphene oxide is referred to as a mixed solution 4. In particular, since graphene oxide having a carbonyl group undergoes hydrogen ionization in a polar mixed solution, graphene oxide is ionized and graphene oxide is more easily dispersed. The mixed solution 4 produced by the above steps can be used as the dispersion used in step S111 shown in FIG. 1A. Since the mixed solution 4 may contain a large amount of impurities, in order to improve the purity of the graphene obtained by the method for manufacturing graphene according to one embodiment of the present invention, it is preferable to purify the graphene oxide in the mixed solution 4 produced by step S125. Therefore, it is preferable to perform step S126 and step S127 after step S125. Step S126 and step S127 will be described below. 〈Formation of graphene oxide salt〉 As shown in step S126, after mixing an alkaline solution with the mixed solution 4 to form a graphene oxide salt, an organic solvent is added to form a mixed solution 5 in which the graphene oxide salt precipitates as precipitate 4. As a typical example of the alkaline solution, it is preferable to use a mixed solution containing an alkali that can undergo a neutralization reaction with graphene oxide and does not remove the oxygen bonded to carbon in graphene oxide due to reduction, such as an aqueous sodium hydroxide solution, an aqueous potassium hydroxide solution, an aqueous ammonia solution, a methylamine solution, an ethanolamine solution, a dimethylamine solution, or a trimethylamine solution. The organic solvent is used to precipitate the graphene oxide salt. Therefore, as the organic solvent, typical ones are acetone, methanol, and ethanol. 〈Recovery of graphene oxide salt〉 Next, as shown in step S127, by performing one or more of filtration, centrifugation, etc. on the mixed solution 5, the mixed solution 5 is separated into a solvent and precipitate 4 containing the graphene oxide salt, and precipitate 4 containing the graphene oxide salt is recovered. Next, precipitate 4 is dried to obtain the graphene oxide salt. By using the solution obtained by dispersing the graphene oxide salt manufactured by the above steps in a solvent as the dispersion liquid in step S111 shown in FIG. 1A, the purity of the graphene obtained by the method for manufacturing graphene according to one embodiment of the present invention can be improved. In addition, in the steps after step S123 in FIG. 2, graphite oxide salt (step S134) can be formed without forming graphene oxide, and graphene oxide salt can be formed after recovering the graphite oxide salt (step S135) (refer to FIG. 3). In step S134, after mixing precipitate 2 obtained by step S123 and water, an alkaline solution is mixed to form graphite oxide salt, and then an organic solvent is added to form a mixed solution in which the graphite oxide salt precipitates. The alkaline solution and the organic solvent can respectively use the alkaline solution and the organic solvent used in step S126. In step S135, one or more of filtration, centrifugation, etc. are performed on the mixed solution in which the graphite oxide salt precipitates obtained by step S134 to separate the organic solvent and the precipitate containing the graphite oxide salt, and the precipitate containing the graphite oxide salt is recovered. The other steps of the method for manufacturing the graphene oxide salt shown in FIG. 3 are the same as the respective steps shown in FIG. 2. As described above, compared with the method of generating graphene by heat treatment, using the present embodiment can increase the proportion of carbon-carbon bonds with double bonds as sp 2 bonds, and thus graphene with improved conductivity can be manufactured. In addition, the present embodiment can be implemented by appropriately combining with the structures described in other embodiments and examples. Embodiment 2 In the present embodiment, an energy storage device according to one aspect of the present invention will be described. Specifically, an energy storage device having an electrode manufactured by the method for manufacturing graphene described in Embodiment 1 will be described. In addition, in the present embodiment, the case where the energy storage device according to one aspect of the present invention is assumed to be a lithium secondary battery will be described. First, the positive electrode 311 will be described. FIG. 4A is a cross-sectional view of the positive electrode 311. In the positive electrode 311, a positive electrode active material layer 309 is formed on the positive electrode current collector 307. In addition, the positive electrode active material layer 309 contains at least a positive electrode active material 321 and graphene 323 (not shown), and may further contain a binder, a conductive agent, and the like. In addition, the active material refers to a material related to the introduction and detachment of ions (hereinafter referred to as carrier ions) that become carriers in the energy storage device. Thus, the active material is distinguished from the active material layer. As the positive electrode current collector 307, a material with high conductivity such as platinum, aluminum, copper, titanium, stainless steel, etc. can be used. In addition, the positive electrode current collector 307 can be appropriately in the form of a foil, a plate, a net, etc. As the material of the positive electrode active material 321 contained in the positive electrode active material layer 309, materials such as LiFeO 2 , LiCoO 2 , LiNiO 2 , LiMn 2 O 4 and other lithium compounds, V 2 O 5 , Cr 2 O 5 , MnO 2 etc. Alternatively, as the positive electrode active material 321, a lithium-containing phosphate having an olivine structure (general formula LiMPO 4 (M is one or more of Fe(II), Mn(II), Co(II), Ni(II)). As a typical example of the general formula LiMPO 4 can be cited LiFePO 4 , LiNiPO 4 , LiCoPO 4 , LiMnPO 4 , LiFe a Ni b PO 4 , LiFe a Co b PO 4 , LiFe a Mn b PO 4 , LiNi a Co b PO 4 , LiNi a Mn b PO 4 (a + b is 1 or less, and 0 < a < 1, 0 < b < 1), LiFe c Ni d Co e PO 4 , LiFe c Ni d Mn e PO 4 , LiNi c Co d Mn e PO 4 (where c + d + e is less than or equal to 1, and 0 < c < 1, 0 < d < 1, 0 < e < 1), LiFe f Ni g Co h Mn i PO 4 (where f + g + h + i is less than or equal to 1, and 0 < f < 1, 0 < g < 1, 0 < h < 1, 0 < i < 1), etc., lithium compounds. Alternatively, as the positive electrode active material 321, a general formula Li 2 MSiO 4 (M is one or more of Fe(II), Mn(II), Co(II), Ni(II)), etc., lithium-containing silicates. As the general formula Li 2 MSiO 4 Typical examples of can include Li 2 FeSiO 4 , Li 2 NiSiO 4 , Li 2 CoSiO 4 , Li 2 MnSiO 4 , Li 2 Fe k Ni l SiO 4 , Li 2 Fe k Co l SiO 4 、 Li 2 Fe k Mn l SiO 4 、 Li 2 Ni k Co l SiO 4 、 Li 2 Ni k Mn l SiO 4 (k + l is less than or equal to 1, and 0 < k < 1, 0 < l < 1), Li 2 Fe m Ni n Co q SiO 4 、 Li 2 Fe m Ni n Mn q SiO 4 、Li 2 Ni m Co n Mn q SiO 4 (m + n + q is less than or equal to 1, and 0 < m < 1, 0 < n < 1, 0 < q < 1), Li 2 Fe r Ni s Co t Mn u SiO 4 Lithium compounds such as (r + s + t + u is 1 or less, and 0 < r < 1, 0 < s < 1, 0 < t < 1, 0 < u < 1). In addition, when alkali metal ions, alkaline earth metal ions, beryllium ions or magnesium ions other than lithium ions are used as carrier ions, the positive electrode active material 321 may contain a compound in which lithium in the lithium compound is replaced with an alkali metal (for example, sodium, potassium, etc.), an alkaline earth metal (for example, calcium, strontium, barium, etc.), beryllium or magnesium of the same kind as the carrier ion. As shown in FIG. 4B which is a plan view of a part of the positive electrode active material layer 309, the positive electrode active material layer 309 is composed of particulate positive electrode active materials 321 capable of occluding and releasing carrier ions and graphene 323 covering a plurality of positive electrode active materials 321 and having the positive electrode active materials 321 therein. In addition, when looking down on the positive electrode active material layer 309, the surfaces of the plurality of positive electrode active materials 321 are covered by different graphene 323. In addition, the positive electrode active materials 321 may be exposed in a part of the positive electrode active material layer 309. The particle diameter of the positive electrode active material 321 is preferably 20 nm or more and 100 nm or less. In addition, since electrons (and carrier ions) move within the positive electrode active material layer 309, it is preferable that the particle diameter of the positive electrode active material 321 is as small as possible in order to increase the surface area of the positive electrode active material 321 and shorten the movement distance of electrons (and carrier ions). In addition, although sufficient characteristics can be obtained even if the surface of the positive electrode active material 321 is not covered with a carbon film, when a positive electrode active material covered with a carbon film and graphene are used together, current flows between the positive electrode active materials 321 by hopping conduction, so it is preferable that the positive electrode active material is covered with a carbon film. FIG. 4C is a cross-sectional view of a part of the positive electrode active material layer 309 of FIG. 4B. FIG. 4C shows the positive electrode active material 321 and the graphene 323 covering the positive electrode active material 321 when looking down on the positive electrode active material layer 309. In the cross-sectional view, linear graphene 323 is observed. The same graphene or multiple graphenes overlap with a plurality of positive electrode active materials 321, or a plurality of positive electrode active materials 321 are surrounded by the same graphene or multiple graphenes. In addition, the graphene 323 is in a bag shape, and sometimes a plurality of positive electrode active materials 321 are included therein. In addition, the graphene 323 has a locally open portion, and sometimes the positive electrode active material 321 is exposed in this area. Regarding the thickness of the positive electrode active material layer 309, a desired thickness of 20 μm or more and 100 μm or less is selected. In addition, preferably, the thickness of the positive electrode active material layer 309 is appropriately adjusted so that cracks and peeling do not occur. In addition, the positive electrode active material layer 309 may also include acetylene black particles having a volume 0.1 times or more and 10 times or less the volume of graphene, or known conductive agents such as carbon particles diffused in one dimension (carbon nanofibers, etc.), and known binders such as polyvinylidene fluoride (PVDF). As the positive electrode active material, for example, there is a material that expands according to the occlusion volume of carrier ions. By using the positive electrode active material of this material, the positive electrode active material layer becomes fragile due to charge and discharge, resulting in damage to a part of the positive electrode active material layer and reducing the reliability (such as cycle characteristics, etc.) of the energy storage device. However, since the periphery of the positive electrode active material 321 of the positive electrode of the energy storage device according to one embodiment of the present invention is covered with graphene 323, even if the volume of the positive electrode active material 321 expands due to charge and discharge, powdering of the positive electrode active material 321 and damage to the positive electrode active material layer 309 can be prevented by the graphene 323. That is to say, the graphene 323 contained in the positive electrode of the energy storage device according to one embodiment of the present invention has the following function: even if the volume of the positive electrode active material 321 expands and contracts during charge and discharge, the adhesion between the positive electrode active materials 321 can be maintained. Therefore, the durability of the energy storage device can be improved by using the positive electrode 311. That is to say, when forming the positive electrode active material layer 309, it is not necessary to use a binder, so the amount of the positive electrode active material in the positive electrode active material layer of a fixed weight can be increased. Therefore, the charge and discharge capacity per unit electrode weight can be increased. In addition, since the graphene 323 has conductivity and contacts a plurality of positive electrode active materials 321, it can also be used as a conductive agent. That is, when forming the positive electrode active material layer 309, it is not necessary to use a conductive agent, so the amount of the positive electrode active material in the positive electrode active material layer of a fixed weight can be increased. Therefore, the charge and discharge capacity per unit electrode weight can be increased. In addition, the graphene 323 is the graphene according to one embodiment of the present invention. That is, the graphene 323 is the graphene electrochemically reduced using electric energy as described in Embodiment 1, and its conductivity is improved compared with the graphene reduced by heat treatment. Since an efficient and sufficient conduction path (conduction path of carrier ions) is formed in the positive electrode active material layer 309, the positive electrode active material layer 309 and the positive electrode 311 have good conductivity. Therefore, the energy storage device having the positive electrode 311 can efficiently utilize the capacity of the positive electrode active material 321 equivalent to the theoretical capacity, and thus the discharge capacity can be sufficiently improved. Next, a method for manufacturing the positive electrode 311 will be described. A slurry containing particulate positive electrode active material 321 and graphene oxide is formed. Specifically, the particulate positive electrode active material 321 and a dispersion liquid containing graphene oxide are kneaded to form a slurry. In addition, the dispersion liquid containing graphene oxide can be manufactured by the method described in Embodiment 1. Next, after applying the above slurry onto the positive electrode current collector 307, it is dried for a certain period of time to remove the solvent in the slurry applied onto the positive electrode current collector 307. For detailed description, reference can be appropriately made to Embodiment 1. In addition, at this time, pressure forming can be performed as needed. Then, in the same manner as the method for manufacturing graphene shown in Embodiment 1, graphene 323 is generated by electrochemically reducing graphene oxide using electric energy. Through the above process, a positive electrode active material layer 309 can be formed on the positive electrode current collector 307, and thus the positive electrode 311 can be manufactured. In addition, when manufacturing the positive electrode 311, since graphene oxide contains oxygen and is negatively charged in a polar solvent, as a result, graphene oxides are dispersed from each other. Therefore, the positive electrode active material 321 in the slurry is not easily aggregated, and thus an increase in the particle diameter of the positive electrode active material 321 during the manufacturing process of the positive electrode 311 can be suppressed. Therefore, an increase in the internal resistance can be suppressed, and electrons (and carrier ions) in the positive electrode active material 321 can move easily, and the conductivity of the positive electrode active material layer 309 and the conductivity of the positive electrode 311 can be improved. In addition, when manufacturing the positive electrode 311, after forming an energy storage device using a negative electrode, an electrolyte, and a separator, the process of reducing graphene oxide to generate graphene 323 can be performed. That is, after forming the energy storage device, a potential capable of causing a reduction reaction of graphene oxide can be provided to the positive electrode current collector 307. Next, the negative electrode and its manufacturing method will be described. FIG. 5A is a cross-sectional view of the negative electrode 205. In the negative electrode 205, a negative electrode active material layer 203 is formed on the negative electrode current collector 201. In addition, the negative electrode active material layer 203 contains at least the negative electrode active material 211 and graphene 213, and may further contain a binder and a conductive agent. The negative electrode current collector 201 can be made of a highly conductive material such as copper, stainless steel, iron, or nickel. In addition, the negative electrode current collector 201 can be appropriately in the form of a foil, a plate, a mesh, or the like. As the negative electrode active material layer 203, a negative electrode active material 211 capable of occluding and releasing carrier ions is used. Typical examples of the negative electrode active material 211 include lithium, aluminum, graphite, silicon, tin, and germanium. Alternatively, a compound containing one or more selected from lithium, aluminum, graphite, silicon, tin, and germanium can also be cited. In addition, the negative electrode current collector 201 may not be used, and the negative electrode active material layer 203 alone may be used as the negative electrode. As the negative electrode active material 211, germanium, silicon, lithium, and aluminum have a larger theoretical capacity than graphite. When the theoretical capacity is large, the amount of the negative electrode active material can be reduced, thereby reducing the cost and realizing miniaturization of the energy storage device. FIG. 5B shows a plan view of a part of the negative electrode active material layer 203. The negative electrode active material layer 203 is composed of particulate negative electrode active material 211 and graphene 213 that covers a plurality of the negative electrode active materials 211 and has the negative electrode active materials 211 inside thereof. When the negative electrode active material layer 203 is viewed from above, the surfaces of the plurality of negative electrode active materials 211 are covered by different graphene 213. In addition, the negative electrode active material 211 may be exposed in a part. FIG. 5C is a cross-sectional view of a part of the negative electrode active material layer 203 of FIG. 5B. FIG. 5C shows the negative electrode active material 211 and the graphene 213 that covers the negative electrode active material 211 when the negative electrode active material layer 203 is viewed from above. In the cross-sectional view, linear graphene 213 is observed. The same graphene or a plurality of graphenes overlap with a plurality of negative electrode active materials 211, or a plurality of negative electrode active materials 211 are surrounded by the same graphene or a plurality of graphenes. In addition, the graphene 213 is in a bag shape, and sometimes a plurality of negative electrode active materials 211 are included therein. In addition, the graphene 213 has a locally open portion, and sometimes the negative electrode active material 211 is exposed in this region. Regarding the thickness of the negative electrode active material layer 203, a desired thickness of 20 μm or more and 100 μm or less is selected. In addition, the negative electrode active material layer 203 may also include known conductive agents such as acetylene black particles having a volume 0.1 times or more and 10 times or less the volume of graphene or carbon particles diffused in one dimension (such as carbon nanofibers) and known binders such as polyvinylidene fluoride (PVDF). In addition, the negative electrode active material layer 203 may be pre-doped with lithium. As a method of pre-doping lithium, a lithium layer can be formed on the surface of the negative electrode active material layer 203 by a sputtering method. Alternatively, the negative electrode active material layer 203 can be pre-doped with lithium by providing a lithium foil on the surface of the negative electrode active material layer 203. Especially when graphene 323 is generated in the positive electrode active material layer 309 of the positive electrode 311 after the energy storage device is assembled, it is preferable to pre-dope the negative electrode active material layer 203 with lithium. In addition, as an example of the negative electrode active material 211, a material that expands according to the occlusion volume of the carrier ions can be cited. For this reason, the negative electrode active material layer becomes fragile due to charge and discharge, resulting in damage to a part of the negative electrode active material layer, and the reliability (such as cycle characteristics, etc.) of the energy storage device is reduced. However, since the graphene 213 covers the periphery of the negative electrode active material 211 of the negative electrode of the energy storage device according to one embodiment of the present invention, even if the negative electrode active material 211 expands in volume due to charge and discharge, the graphene 213 can prevent the pulverization of the negative electrode active material 211 and the damage of the negative electrode active material layer 203. That is to say, the graphene 213 contained in the negative electrode of the energy storage device according to one embodiment of the present invention has the following function: even if the volume of the negative electrode active material 211 expands and contracts with charge and discharge, the adhesion between the negative electrode active materials 211 can be maintained. Therefore, the durability of the energy storage device can be improved by using the negative electrode 205. That is to say, when forming the negative electrode active material layer 203, it is not necessary to use an adhesive, so the amount of the negative electrode active material in the negative electrode active material layer of a fixed weight (a certain volume) can be increased. For example, the charge and discharge capacity per unit electrode weight (electrode volume) can be increased thereby. In addition, since the graphene 213 has conductivity and contacts a plurality of negative electrode active materials 211, it can also be used as a conductive agent. That is, when forming the negative electrode active material layer 203, it is not necessary to use a conductive agent, so the amount of the negative electrode active material in the negative electrode active material layer of a fixed weight (a certain volume) can be increased. Therefore, the charge and discharge capacity per unit electrode weight (electrode volume) can be increased. In addition, the graphene 213 is the graphene according to one embodiment of the present invention. That is, the graphene 213 is the graphene electrochemically reduced by using electric energy as described in Embodiment 1, and its conductivity is improved compared with the graphene reduced by heat treatment. Since an efficient and sufficient conduction path (conduction path of carrier ions) is formed in the negative electrode active material layer 203, the negative electrode active material layer 203 and the negative electrode 205 have good conductivity. Therefore, the energy storage device having the negative electrode 205 can efficiently utilize the capacity of the negative electrode active material 211 equivalent to the theoretical capacity, and thereby the charging capacity can be fully improved. In addition, since the graphene 213 also has the function of a negative electrode active material capable of occluding and releasing carrier ions, the charging capacity of the negative electrode 205 can be improved. Next, a method for manufacturing the negative electrode active material layer 203 shown in FIGS. 5B and 5C will be described. A slurry containing particulate negative electrode active material 211 and graphene oxide is formed. Specifically, the particulate negative electrode active material 211 and a dispersion liquid containing graphene oxide are kneaded to form a slurry. In addition, the dispersion liquid containing graphene oxide can be manufactured by the method described in Embodiment 1. Next, after coating the above slurry on the negative electrode current collector 201, vacuum drying is performed for a certain period of time to remove the solvent in the slurry coated on the negative electrode current collector 201. For a detailed description, reference can be made to Embodiment 1. In addition, at this time, pressure forming can be performed as needed. Then, in the same manner as the method for manufacturing graphene shown in Embodiment 1, graphene 213 is generated by electrochemically reducing graphene oxide using electric energy. Through the above process, a negative electrode active material layer 203 can be formed on the negative electrode current collector 201, and thus the negative electrode 205 can be manufactured. In addition, when manufacturing an energy storage device including a positive electrode 311 and a negative electrode 205, when graphene is manufactured for both the positive electrode 311 and the negative electrode 205 by the method described in Embodiment 1, it is preferably to generate graphene in the positive electrode 311 or the negative electrode 205 in advance before assembling the energy storage device. This is because when the energy storage device is composed of graphene oxide provided in the positive electrode 311 and the negative electrode 205, the potential cannot be effectively provided to the positive electrode 311 and the negative electrode 205, and thus graphene oxide cannot be sufficiently reduced or it takes time to sufficiently reduce graphene oxide. In addition, when manufacturing the negative electrode 205, since graphene oxide contains oxygen and is negatively charged in a polar solvent. As a result, graphene oxides are dispersed from each other. Therefore, the negative electrode active material 211 in the slurry is not likely to aggregate, and thus an increase in the particle diameter of the negative electrode active material 211 during the manufacturing process of the negative electrode 205 can be suppressed. Therefore, an increase in the internal resistance can be suppressed, and electrons (and carrier ions) in the negative electrode active material 211 can move easily, and the conductivity of the negative electrode active material layer 203 and the conductivity of the negative electrode 205 can be improved. Next, the structure of the negative electrode shown in Fig. 5D will be described. Fig. 5D is a cross-sectional view of a negative electrode in which a negative electrode active material layer 203 is formed on a negative electrode current collector 201. The negative electrode active material layer 203 includes: a negative electrode active material 221 having a concavo-convex surface; and graphene 223 covering the surface of the negative electrode active material 221. The concavo-convex negative electrode active material 221 has a common portion 221a and convex portions 221b protruding from the common portion 221a. The convex portions 221b suitably have a shape such as a columnar shape like a cylindrical shape or a prismatic shape, or a needle-like shape like a conical shape or a pyramidal shape. In addition, the top of the convex portion may be curved. In addition, similar to the negative electrode active material 211, the negative electrode active material 221 is formed of a negative electrode active material capable of occluding and releasing carrier ions (typically lithium ions). In addition, the common portion 221a and the convex portions 221b may be made of the same material. Alternatively, the common portion 221a and the convex portions 221b may be made of different materials. In addition, as an example of the negative electrode active material, silicon can increase the volume of the ions serving as carriers to about four times due to occlusion. Therefore, since the negative electrode active material becomes fragile during charge and discharge, a part of the negative electrode active material layer is damaged, and the reliability (for example, cycle characteristics, etc.) of the energy storage device is reduced. However, when silicon is used as the negative electrode active material 221 in the negative electrode shown in FIG. 5D, since the negative electrode active material 221 is covered with graphene 223, even if the volume of the negative electrode active material 221 expands during charge and discharge, pulverization of the negative electrode active material 221 and damage to the negative electrode active material layer 203 can be prevented. In addition, when the surface of the negative electrode active material layer comes into contact with the electrolyte constituting the energy storage device, the electrolyte reacts with the negative electrode active material, and a film is formed on the surface of the negative electrode. This film is called SEI (Solid Electrolyte Interface), and it can moderate the reaction between the negative electrode and the electrolyte and maintain stability. However, when the thickness of this film is thick, it is not easy for the carrier ions to be occluded by the negative electrode, and problems such as a decrease in the conductivity of the carrier ions between the electrode and the electrolyte and consumption of the electrolyte occur. By using graphene 213 to cover the surface of the negative electrode active material layer 203, an increase in the thickness of this film can be suppressed, and thus a decrease in the charge-discharge capacity can be suppressed. Next, a method for manufacturing the negative electrode active material layer 203 shown in FIG. 5D will be described. The concavo-convex negative electrode active material 221 is provided on the negative electrode current collector 201 by using a printing method, an inkjet method, a CVD method, etc. Alternatively, after a film-like negative electrode active material is provided by using a coating method, a sputtering method, an evaporation method, etc., the concavo-convex negative electrode active material 221 is provided on the negative electrode current collector 201 by selectively removing. Alternatively, a part of the surface of a foil or sheet formed of lithium, aluminum, graphite, or silicon is removed to form the concavo-convex negative electrode current collector 201 and the negative electrode active material 221. In addition, a net formed of lithium, aluminum, graphite, or silicon can be used as the negative electrode active material and the negative electrode current collector. Next, a dispersion liquid containing graphene oxide is coated on the concave-convex negative electrode active material 221. As a method for coating the dispersion liquid containing graphene oxide, the method described in Embodiment 1 can be appropriately used. Next, the solvent in the dispersion liquid containing graphene oxide is removed as described in Embodiment 1. Then, graphene 213 is generated by electrochemically reducing graphene oxide using electric energy as described in Embodiment 1. In this way, by generating graphene using the dispersion liquid containing graphene oxide, graphene 213 with a uniform thickness can be covered on the surface of the concave-convex negative electrode active material 221. In addition, when manufacturing an energy storage device including a positive electrode 311 and the negative electrode shown in FIG. 5D, when graphene is manufactured for both the positive electrode 311 and the negative electrode by the method described in Embodiment 1, it is preferably to generate graphene in the positive electrode 311 or the negative electrode in advance before assembling the energy storage device. This is because: since graphene oxide cannot be fully reduced or it takes time to fully reduce graphene oxide, even if the energy storage device is assembled in the case where graphene oxide is provided for both the positive electrode 311 and the negative electrode, the potential cannot be efficiently provided to the positive electrode 311 and the negative electrode. In addition, a concave-convex negative electrode active material 221 (hereinafter referred to as silicon whiskers) formed of silicon can be provided on the negative electrode current collector 201 by the LPCVD method using silane, chlorosilane, fluorosilane, etc. as raw material gases. The silicon whiskers can also have an amorphous structure. When the silicon whiskers having an amorphous structure are used as the negative electrode active material layer 203, since they can withstand volume changes caused by the occlusion and release of carrier ions (for example, relieve stress caused by volume expansion), it is possible to prevent the pulverization of the silicon whiskers and the damage of the negative electrode active material layer 203 due to repeated charge and discharge. Therefore, an energy storage device with further improved cycle characteristics can be manufactured. In addition, the silicon whiskers can also have a crystal structure. In this case, a crystalline structure with excellent conductivity and excellent carrier ion mobility is widely in contact with the current collector. Therefore, the conductivity of the entire negative electrode can be further improved, and faster charge and discharge can be performed, and an energy storage device with further improved charge and discharge capacity can be manufactured. In addition, the silicon whiskers can also include a core having a crystalline region and a shell provided to cover the core and having an amorphous region. The amorphous outer shell has the following characteristics, namely, it can withstand volume changes caused by the occlusion and release of carrier ions (for example, relieve stress caused by volume expansion). In addition, the crystalline core has excellent conductivity and excellent carrier ion mobility, and the occlusion rate and release rate of carrier ions per unit mass are very fast. Therefore, by using the silicon whiskers having a core and an outer shell as the negative electrode active material layer, high-speed charge and discharge can be performed, and an energy storage device with improved charge and discharge capacity and cycle characteristics can be manufactured. Next, a method for forming an energy storage device according to one embodiment of the present invention will be described. FIG. 6 is a cross-sectional view of a lithium secondary battery 400, and its cross-sectional structure will be described below. The lithium secondary battery 400 includes: a negative electrode 411 composed of a negative electrode current collector 407 and a negative electrode active material layer 409; a positive electrode 405 composed of a positive electrode current collector 401 and a positive electrode active material layer 403; and a separator 413 sandwiched between the negative electrode 411 and the positive electrode 405. In addition, the separator 413 contains an electrolytic solution 415. Further, the negative electrode current collector 407 is connected to an external terminal 419, and the positive electrode current collector 401 is connected to an external terminal 417. The end portion of the external terminal 419 is buried in a gasket 421. That is, the external terminal 417 and the external terminal 419 are insulated by the gasket 421. As the negative electrode current collector 407 and the negative electrode active material layer 409, the above-described negative electrode current collector 201 and negative electrode active material layer 203 can be appropriately used. As the positive electrode current collector 401 and the positive electrode active material layer 403, the above-described positive electrode current collector 307 and positive electrode active material layer 309 can be appropriately used, respectively. The separator 413 uses an insulating porous body. As a typical example of the separator 413, for example, it can be formed of the following substances: paper, non-woven fabric, glass fiber, ceramic, or synthetic fibers such as nylon (polyamide), vinylon (Vinylon) (polyvinyl alcohol fiber), polyester, acrylic resin, polyolefin, and polyurethane. However, a material that does not dissolve in the electrolytic solution 415 needs to be selected. In addition, when the positive electrode having a spacer on the positive electrode active material layer is used as the positive electrode 405, the separator 413 may not be provided. A material that can transport carrier ions and contains carrier ions is used as the solute of the electrolytic solution 415. As a typical example of the solute, LiClO 4 、LiAsF 6 、LiBF 4 、LiPF 6 、Li(C 2 F 5 SO 2 ) 2 Lithium salts such as N. In addition, when the carrier ion is an alkali metal ion, alkaline earth metal ion, beryllium ion or magnesium ion other than lithium ion, a compound in which lithium in the above lithium salt is replaced with an alkali metal (e.g., sodium, potassium, etc.), alkaline earth metal (e.g., calcium, strontium, barium, etc.), beryllium or magnesium can also be used as the solute of the electrolyte 415. Furthermore, as the solvent of the electrolyte 415, a material capable of transporting the carrier ion is used. As the solvent of the electrolyte 415, it is preferably a non-protic organic solvent. As typical examples of the non-protic organic solvent, one or more of ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, γ-butyrolactone, acetonitrile, ethylene glycol dimethyl ether, tetrahydrofuran, etc. can be used. In addition, when a gelled polymer material is used as the solvent of the electrolyte 415, safety such as leakage prevention is improved. And the thinning and weight reduction of the lithium secondary battery 400 can be achieved. As typical examples of the gelled polymer material, silicone, acrylic glue, acrylonitrile glue, polyethylene oxide, polypropylene oxide, fluoropolymers, etc. can be cited. Additionally, by using one or more ionic liquids (room temperature molten salts) having flame retardancy and low volatility as the solvent of the electrolyte 415, even if the internal temperature rises due to internal short circuit, overcharging, etc. of the energy storage device, rupture or fire of the energy storage device can be prevented. In addition, as the electrolyte 415, Li 3 PO 4 and other solid electrolytes can be used. As other solid electrolytes, those for Li 3 PO 4 Li mixed with nitrogen x PO y N z (x, y, z are positive real numbers), Li 2 S-SiS 2 、Li 2 S-P 2 S 5 、Li 2 S-B 2 S 3 etc., and substances such as LiI doped in the above-mentioned solid electrolytes. In addition, when a solid electrolyte is used as the electrolyte 415, the separator 413 is not required. As the external terminals 417 and 419, metal members such as stainless steel plates and aluminum plates can be appropriately used. In the present embodiment, although a button-type lithium secondary battery is shown as the lithium secondary battery 400, various-shaped lithium secondary batteries such as a sealed-type lithium secondary battery, a cylindrical lithium secondary battery, and a square lithium secondary battery can be adopted. In addition, a structure in which a plurality of positive electrodes, a plurality of negative electrodes, and a plurality of separators are stacked and a structure in which a positive electrode, a negative electrode, and a separator are wound can also be adopted. The lithium secondary battery has a small memory effect, a high energy density, a large capacity, and a high output voltage. Therefore, miniaturization and weight reduction can be achieved. In addition, since deterioration due to repeated charge and discharge is small, the lithium secondary battery can be used for a long time, thereby reducing costs. The positive electrode 405 and the negative electrode 411 are appropriately manufactured by using the manufacturing methods of the positive electrode and the negative electrode shown in Embodiment 1 and the present embodiment. Next, the positive electrode 405, the separator 413, and the negative electrode 411 are immersed in the electrolyte 415. Next, the positive electrode 405, the separator 413, the gasket 421, the negative electrode 411, and the external terminal 419 can be sequentially stacked on the external terminal 417, and the external terminal 417 and the external terminal 419 are fitted using a "coin cell crimper" to manufacture a coin-type lithium secondary battery. In addition, a spacer and a washer can be inserted between the external terminal 417 and the positive electrode 405 or between the external terminal 419 and the negative electrode 411 to further improve the connectivity between the external terminal 417 and the positive electrode 405 and the connectivity between the external terminal 419 and the negative electrode 411. The present embodiment can be appropriately combined with other embodiments and implemented. Embodiment 3 The energy storage device according to one aspect of the present invention can be used as a power source for various electrical appliances driven by electricity. Specific examples of electrical appliances using the energy storage device according to one aspect of the present invention include: display devices; lighting equipment; desktop or notebook personal computers; image reproduction devices for reproducing still images or moving images stored in storage media such as DVDs (Digital Versatile Discs); mobile phones; portable game machines; portable information terminals; e-book readers; cameras; digital cameras; high-frequency heating devices such as microwave ovens; rice cookers; washing machines; air conditioning equipment such as air conditioners; electric refrigerators; electric freezers; electric refrigerator-freezers; freezers for DNA preservation; and dialysis devices. In addition, moving bodies propelled by an electric motor using electric power from the energy storage device are also included in the category of electrical appliances. Examples of such moving bodies include: electric vehicles; hybrid vehicles having both an internal combustion engine and an electric motor; and electric bicycles including electric assist bicycles. In addition, in the above electrical appliances, as the energy storage device (also referred to as the main power source) for supplying most of the power consumption, an energy storage device according to one aspect of the present invention can be used. Or, in the above electrical appliances, as the energy storage device (also referred to as the uninterruptible power supply) that can supply power to the electrical appliance when the power supply from the above main power source or commercial power source stops, an energy storage device according to one aspect of the present invention can be used. Or, in the above electrical appliances, as the energy storage device (also referred to as the auxiliary power source) that supplies power to the electrical appliance simultaneously with the power supply to the electrical appliance from the above main power source or commercial power source, an energy storage device according to one aspect of the present invention can be used. FIG. 7 shows the specific structure of the above electrical appliance. In FIG. 7, the display device 5000 is an example of an electrical appliance using the energy storage device 5004. Specifically, the display device 5000 corresponds to a display device for receiving television broadcasts, and has a housing 5001, a display unit 5002, a speaker unit 5003, and an energy storage device 5004. The energy storage device 5004 is provided inside the housing 5001. The energy storage device 5004 uses an energy storage device according to one aspect of the present invention. The display device 5000 can receive power supply from the commercial power source and can also use the power stored in the energy storage device 5004. Therefore, even when the power supply from the commercial power source cannot be received due to a power outage or the like, the display device 5000 can be used by using the energy storage device 5004 as an uninterruptible power supply. As the display unit 5002, semiconductor display devices such as liquid crystal display devices, light-emitting devices having light-emitting elements such as organic EL elements in each pixel, electrophoretic display devices, DMD (Digital Micromirror Device), PDP (Plasma Display Panel), and FED (Field Emission Display) can be used. In addition to the display devices for receiving television broadcasts, the display devices also include all display devices for displaying information, such as those for personal computers or advertising displays. In FIG. 7, the recessed lighting device 5100 is an example of an electrical device using the energy storage device 5103. Specifically, the lighting device 5100 includes a housing 5101, a light source 5102, and an energy storage device 5103. The energy storage device 5103 uses the energy storage device according to one aspect of the present invention. Although FIG. 7 illustrates the case where the energy storage device 5103 is disposed inside the ceiling 5104 in which the housing 5101 and the light source 5102 are recessed, the energy storage device 5103 can also be disposed inside the housing 5101. The lighting device 5100 can receive power supply from a commercial power source and can also use the power stored in the energy storage device 5103. Therefore, even when power supply from a commercial power source cannot be received due to a power outage or the like, the lighting device 5100 can be used by using the energy storage device 5103 as an uninterruptible power supply. In addition, although the recessed lighting device 5100 disposed on the ceiling 5104 is illustrated in FIG. 7, the energy storage device according to one aspect of the present invention can be used for recessed lighting devices disposed on, for example, a wall 5105, a floor 5106, or a window 5107 other than the ceiling 5104, and can also be used for table lamps. In addition, as the light source 5102, an artificial light source that artificially obtains light using electricity can be used. Specifically, as an example of the above artificial light source, incandescent bulbs, discharge lamps such as fluorescent lamps, and light-emitting elements such as LEDs or organic EL elements can be cited. In Fig. 7, an air conditioner having an indoor unit 5200 and an outdoor unit 5204 is an example of an electrical appliance using an energy storage device 5203. Specifically, the indoor unit 5200 has a housing 5201, an air outlet 5202, and an energy storage device 5203, etc. The energy storage device 5203 uses an energy storage device according to one aspect of the present invention. Although the case where the energy storage device 5203 is provided in the indoor unit 5200 is illustrated in Fig. 7, the energy storage device 5203 may also be provided in the outdoor unit 5204. Alternatively, the energy storage device 5203 may be provided in both the indoor unit 5200 and the outdoor unit 5204. The air conditioner can receive power supply from a commercial power source and can also use the power stored in the energy storage device 5203. In particular, when the energy storage device 5203 is provided in both the indoor unit 5200 and the outdoor unit 5204, even when power supply from the commercial power source cannot be received due to a power outage or the like, the air conditioner can be used by using the energy storage device 5203 as an uninterruptible power supply. In addition, although a split air conditioner composed of an indoor unit and an outdoor unit is illustrated in Fig. 7, an energy storage device according to one aspect of the present invention can also be used for an integrated air conditioner having the functions of an indoor unit and an outdoor unit in one housing. In Fig. 7, an electric refrigerator-freezer 5300 is an example of an electrical appliance using an energy storage device 5304 according to one aspect of the present invention. Specifically, the electric refrigerator-freezer 5300 has a housing 5301, a refrigerator door 5302, a freezer door 5303, and an energy storage device 5304, etc. The energy storage device 5304 uses an energy storage device according to one aspect of the present invention. In Fig. 7, the energy storage device 5304 is provided inside the housing 5301. The electric refrigerator-freezer 5300 can receive power supply from a commercial power source and can also use the power stored in the energy storage device 5304. Therefore, even when power supply from the commercial power source cannot be received due to a power outage or the like, the electric refrigerator-freezer 5300 can be utilized by using the energy storage device 5304 as an uninterruptible power supply. In addition, among the above electrical appliances, high-frequency heating devices such as microwave ovens and electrical appliances such as rice cookers require high power in a short period of time. Therefore, by using an energy storage device according to one aspect of the present invention as an auxiliary power source for assisting the power that cannot be sufficiently supplied by the commercial power source, it is possible to prevent the main switch of the commercial power source from tripping when using the electrical appliance. In addition, during time periods when electrical equipment is not in use, especially during time periods when the ratio of the actual power consumption to the total power that can be supplied by the commercial power supply source (referred to as the power utilization rate) is low, power is stored in the energy storage device, thereby suppressing an increase in the power utilization rate during time periods other than the above-mentioned time periods. For example, as the electric refrigerator-freezer 5300, power is stored in the energy storage device 5304 at night when the temperature is low and the refrigerator door 5302 or the freezer door 5303 is not opened or closed. And, during the day when the temperature is high and the refrigerator door 5302 or the freezer door 5303 is opened or closed, the energy storage device 5304 is used as an auxiliary power source, thereby suppressing the power utilization rate during the day. Next, a portable information terminal using an energy storage device according to one embodiment of the present invention will be described with reference to FIGS. 8A, 8B, and 8C. FIGS. 8A and 8B are two foldable tablet terminals. FIG. 8A shows the state when opened. The tablet terminal includes a housing 9630, a display unit 9631a, a display unit 9631b, a display mode switch 9034, a power switch 9035, a power saving mode switch 9036, a clip 9033, and an operation switch 9038. A part of the display unit 9631a can be formed as a touch screen area 9632a, whereby data can be input by touching the operation keys 9637 displayed on the touch screen. In addition, as an example, in the display unit 9631a, a structure is exemplified in which half of the area has only a display function and the other half has a touch screen function, but it is not limited thereto. A structure in which the entire area of the display unit 9631a has a touch screen function can also be adopted. For example, the entire surface of the display unit 9631a can be a touch screen for displaying keyboard buttons, and the display unit 9631b can be used as a display screen. In addition, similar to the display unit 9631a, a part of the display unit 9631b can also be a touch screen area 9632b. In addition, by touching the position where the keyboard display switch button 9639 is displayed on the touch screen using a finger or a stylus, etc., the keyboard buttons can be displayed on the display unit 9631b. In addition, touch input can be performed on the touch screen area 9632a and the touch screen area 9632b of the touch screen simultaneously. In addition, the screen mode switch 9034 can switch the display direction such as portrait display and landscape display and can perform selections such as switching between black and white display and color display. According to the amount of external light detected by the light sensor built in the tablet terminal during use, the power saving mode switch 9036 can set the brightness of the display to the most suitable brightness. In addition to the light sensor, the tablet terminal can also be built in with other detection devices such as a gyroscope and an acceleration sensor for detecting the inclination degree. In addition, FIG. 8A shows an example in which the display area of the display unit 9631b is the same as that of the display unit 9631a. However, it is not limited to this. The size of one side and the size of the other side may also be different, and their display qualities may also vary. For example, a display panel in which one of the display units can perform higher-resolution display than the other can be used. FIG. 8B shows the closed state, and the tablet terminal includes a housing 9630, a solar cell 9633, a charge and discharge control circuit 9634, a battery 9635, and a DCDC converter 9636. In addition, in FIG. 8B, a structure having a battery 9635 and a DCDC converter 9636 is shown as an example of the charge and discharge control circuit 9634, and the battery 9635 uses an energy storage device according to one aspect of the present invention. In addition, since the tablet terminal can be folded, the housing 9630 can be closed when not in use. Therefore, the display units 9631a and 9631b can be protected, and a tablet terminal having good durability and good reliability from the viewpoint of long-term use can be provided. In addition, the tablet terminal shown in FIGS. 8A and 8B may also have the following functions: displaying various types of information (static images, moving images, text images, etc.); displaying a calendar, date, time, etc. on the display unit; touch input for operating or editing the information displayed on the display unit; control processing by various software (programs), etc. By using the solar cell 9633 mounted on the surface of the tablet terminal, electric power can be supplied to the touch screen, the display unit, the video signal processing unit, etc. Note that since the solar cell 9633 can be structured to efficiently charge the battery 9635 on one or both sides of the housing 9630, it is preferable. In addition, when the battery 9635 uses an energy storage device according to one aspect of the present invention, there are advantages such as miniaturization. In addition, the structure and operation of the charge and discharge control circuit 9634 shown in FIG. 8B will be described with reference to the block diagram shown in FIG. 8C. FIG. 8C shows the solar cell 9633, the battery 9635, the DCDC converter 9636, the converter 9638, switches SW1 to SW3, and the display unit 9631. The battery 9635, the DCDC converter 9636, the converter 9638, and switches SW1 to SW3 correspond to the charge and discharge control circuit 9634 shown in FIG. 8B. First, an example of the operation when using external light to generate electricity with the solar cell 9633 will be described. The DCDC converter 9636 is used to step up or step down the electricity generated by the solar cell to make it the voltage for charging the battery 9635. Also, when operating the display unit 9631 using the electricity from the solar cell 9633, the switch SW1 is turned on, and the converter 9638 is used to step up or step down the voltage to the voltage required by the display unit 9631. Additionally, a structure can be adopted where when the display in the display unit 9631 is not being performed, SW1 is turned off and SW2 is turned on to charge the battery 9635. Note that although the solar cell 9633 is shown as an example of a power generation method, it is not limited thereto, and other power generation methods such as a piezoelectric element or a thermoelectric conversion element (Peltier element) can also be used to charge the battery 9635. For example, a wireless power transmission module capable of charging in a wireless (non-contact) manner or a combination of other charging methods can also be used for charging. In addition, of course, as long as the energy storage device described in the above embodiment is provided, it is not limited to the electrical equipment shown in FIGS. 8A to 8C. This embodiment can be implemented in appropriate combination with other embodiments. Example 1 In this example, a lithium secondary battery (referred to as lithium secondary battery 1) is manufactured using one aspect of the present invention, and this lithium secondary battery 1 is measured using cyclic voltammetry (CV). First, the structure and manufacturing method of the lithium secondary battery 1 will be described. The lithium secondary battery 1 is a coin-type lithium secondary battery. As the working electrode of the lithium secondary battery, an electrode having an active material layer containing LiFePO 4 and graphene oxide on a current collector formed of aluminum is used. Additionally, as the counter electrode and reference electrode, lithium metal is used. Also, as the separator, a polypropylene sheet is used. Additionally, as the electrolyte, a mixed solution of 1M LiPF 6 (ethylene carbonate solvent) and diethyl carbonate mixed at a volume ratio of 1:1 is used. Here, the manufacturing method of the working electrode will be described. 〈Synthesis method of LiFePO 4 〉 Lithium carbonate (Li 2 CO 3 ) ferric oxalate (Fe 2 CO 4 .2H 2 O) and ammonium dihydrogen phosphate (NH 4 H 2 PO 4 ). Next, the raw materials were pulverized and mixed for 2 hours using a wet ball mill with a rotation speed of 300 rpm (ball diameter: 3 mm, acetone used as the solvent), and then dried. Next, heating was carried out for 10 hours in a nitrogen atmosphere at 350 °C. After pre-calcining the pulverized and mixed raw materials, pulverization and mixing were carried out again for 2 hours using a wet ball mill with a rotation speed of 300 rpm (ball diameter: 3 mm, acetone used as the solvent). Then, roasting was carried out for 10 hours in a nitrogen atmosphere at 600 °C to obtain LiFePO 4 . 〈Synthesis method of graphene oxide〉 2 g of graphite and 92 ml of concentrated sulfuric acid were mixed to form mixture 1. Next, mixture A was stirred in an ice bath, and 12 g of potassium permanganate was added simultaneously to form mixture B. Next, after removing the ice bath and stirring at room temperature for 2 hours, it was left at 35 °C for 30 minutes to oxidize the graphite to obtain mixture C with graphite oxide. Next, mixture C was stirred in an ice bath, and 184 ml of water was added simultaneously to obtain mixture D. Next, mixture D was stirred in an oil bath at approximately 98 °C for 15 minutes to cause a reaction, and then while stirring, 580 ml of water and 36 ml of hydrogen peroxide water (concentration: 30 wt%) were added to mixture D to reduce the unreacted potassium permanganate, and mixture E with soluble manganese sulfate and graphite oxide was obtained. Next, after suction filtration of mixture E using a membrane filter with a pore size of 0.45 μm to obtain precipitate A, precipitate A was mixed with 3 wt% hydrochloric acid to obtain mixture F in which manganese ions, potassium ions, and sulfate ions were dissolved. Next, suction filtration of mixture F was carried out to obtain precipitate B with graphite oxide. After mixing 500 ml of water with precipitate B to obtain mixture G, ultrasonic waves with a frequency of 40 kHz were applied to mixture G for 1 hour to peel the carbon layers constituting the graphite oxide from each other to generate graphene oxide. Next, centrifuge at 4000 rpm for about 30 minutes to recover the supernatant containing graphene oxide. This supernatant is designated as mixture H. Next, ammonia water is added to mixture H and adjusted to a pH of about 11 to produce mixture I. Then, 2500 ml of acetone is added to mixture I to obtain mixture J. At this time, the graphene oxide contained in mixture H reacts with the ammonia contained in the ammonia water to form a graphene oxide salt (specifically, an ammonium salt of graphene oxide), which precipitates in mixture J. Filter mixture J, and dry the precipitate of mixture J in a vacuum at room temperature to recover the graphene oxide salt. <Manufacturing method of the active material layer> In mixing 97 wt% of LiFePO 4 and 3 wt% of the graphene oxide salt and approximately twice the total weight of LiFePO 4 and the graphene oxide salt to form a paste with NMP (N-methylpyrrolidone), the paste is applied to a current collector made of aluminum, dried by ventilation at 120 °C for 15 minutes, and then the current collector is heated at 100 °C for 1 hour of vacuum drying to form a working electrode with an active material layer on the current collector. Next, the assembly steps of the lithium secondary battery 1 will be described. First, a working electrode immersed in the electrolyte is disposed in the first battery can, a separator immersed in the electrolyte is disposed on the working electrode, and a gasket is disposed on the separator. Next, lithium metal is disposed on the separator and the gasket, and a spacer and a spring washer are provided on the lithium electrode. Next, after a second battery can is provided on the spring washer, the first battery can is crimped thereto to form the lithium secondary battery 1. Next, CV measurement is performed on the lithium secondary battery 1. The scanning speed is set to 1 mV / s. First, as the first step, the scanning potential is set to 3 V to 4 V, and as the applied potential, after scanning the potential from 3 V to 4 V, the potential from 4 V to 3 V is scanned, and this process is repeated four times. Next, as the second step, the scanning potential is set to 1.5 V to 3 V, and as the applied potential, after scanning the potential from 3 V to 1.5 V, the potential from 1.5 V to 3 V is scanned, and this process is repeated four times. Next, as the third step, the scanning potential is set to 3 V to 4 V, and as the applied potential, after scanning the potential from 3 V to 4 V, the potential from 4 V to 3 V is scanned, and this process is repeated four times. FIG. 9 shows the current-potential curve at this time. In FIG. 9, the horizontal axis represents the potential of the working electrode (vs. Li / Li +) The vertical axis represents the current generated by oxidation-reduction. Additionally, as the current value, a negative value represents the reduction current, and a positive value represents the oxidation current. In addition, the current with a peak current surrounded by the dashed line 501_R is the reduction current in the first step, and the current with a peak current surrounded by the dashed line 501_O is the oxidation current in the first step. Further, the current with a peak current surrounded by the dashed line 502_R is the reduction current during the first potential scan in the second step, and the current represented by the dashed line 502 is the reduction current during the second to fourth potential scans in the second step and the oxidation current during the first to fourth potential scans. The current with a peak current surrounded by the dashed line 503_R is the reduction current in the third step, and the current with a peak current surrounded by the dashed line 503_O is the oxidation current in the third step. It is known that by performing a potential scan of 1.5 V to 3 V between the first step and the third step, the current value of the lithium secondary battery 1 increases. That is, by providing a reduction treatment with a potential that can promote the reduction reaction of the active material layer, that is, by electrochemical reduction treatment, the resistance of the active material layer decreases, and in the third step, the current value increases. And considering that the oxidation-reduction potential of LiFePO 4 is approximately around 3.4 V, it can be considered that the reduction current near 2 V is the current generated when graphene oxide is reduced, and the reduction potential of graphene oxide is around 2 V. Next, FIG. 10 shows an enlarged view of the current-potential curve in the second step in FIG. 9. In FIG. 10, the curve 511_R represents the reduction current during the first potential scan, and the curve 511_O represents the oxidation current during the first potential scan. The curve 512_R represents the reduction current during the second to fourth potential scans, and the curve 512_O represents the oxidation current during the second to fourth potential scans. As shown in FIG. 10, the reduction current during the first potential scan exhibits a peak near a potential of 2 V. However, no peak is observed near 2 V in the potential scans after the second time. In addition, there is not much change in the oxidation current during the first to fourth potential scans. From the above results, it can be seen that by performing a scan near a reduction potential of 2 V, a reduction reaction occurs at the working electrode, but no reduction reaction occurs in the potential scans after the second time. Here, in order to confirm the reduction reaction occurring near 2 V, a comparative battery cell having an active material layer of the working electrode made only of graphene oxide is used, and CV measurement is performed on this comparative battery cell. First, the structure and manufacturing method of the comparative battery cell will be described. The reference battery cell is a coin-type battery. Except that only graphene oxide is used as the active material layer of the working electrode on the current collector formed of aluminum, the other structure is the same as that of the lithium secondary battery 1. In addition, the graphene oxide is produced by the same process as the graphene oxide used for the active material layer of the working electrode of the above lithium secondary battery 1. In addition, after mixing 50 mg of graphene oxide and 4.5 g of water to form a paste, the paste is applied to the current collector formed of aluminum and dried in a vacuum at 40 °C, thereby forming a working electrode with an active material layer on the current collector. In addition, the composition steps of the reference battery cell are the same as those of the lithium secondary battery 1. Next, CV measurement is performed on the reference battery cell. The scanning speed is set to 0.1 mV / s. The scanning potential is set to 1.5 V to 3 V, and after scanning the potential from 3 V to 1.5 V as the provided potential, the potential from 1.5 V to 3 V is scanned, and the above process is repeated three times. Fig. 11 shows the potential-current curve at this time. In Fig. 11, the horizontal axis represents the potential of the working electrode (vs. Li / Li + ), and the vertical axis represents the current generated by oxidation-reduction. In addition, the curve 531_R represents the reduction current during the first potential scan, and the curve 531_O represents the oxidation current during the first potential scan. The curve 532_R represents the reduction current during the second potential scan, and the curve 532_O represents the oxidation current during the second potential scan. The curve 533_R represents the reduction current during the third potential scan, and the curve 533_O represents the oxidation current during the third potential scan. As shown in Fig. 11, the reduction current during the first potential scan shows a peak around a potential of 2 V. From this, it can be considered that the reduction potential of graphene oxide is about 2 V. However, no peak was observed around 2 V in the potential scans after the second time. In addition, compared with the first potential scan, the oxidation currents during the second and third potential scans are higher, but the oxidation currents during the second and third potential scans do not change much. Next, Figs. 12 and 13 show the results of analyzing the surface element composition and atomic bonding state of carbon, oxygen, and other elements before and after the electrochemical reduction treatment of the working electrode of the reference battery cell using X-ray photoelectron spectroscopy (XPS). The mixed solution H containing graphene oxide described in the process of the working electrode of the lithium secondary battery 1 was placed on a substrate made of aluminum and heated in a vacuum at 40 °C for 1 hour, and this sample was designated as Sample 1. In addition, after immersing Sample 1 in the electrolyte in the lithium secondary battery 1 for one day, it was washed with diethyl carbonate and dried in a vacuum at room temperature for 3 hours, and this sample was designated as Sample 2. Note that Sample 1 and Sample 2 are samples before the electrochemical reduction treatment. In addition, a comparative battery cell was manufactured. After performing one CV measurement, the comparative battery cell was disassembled and the obtained working electrode was washed with diethyl carbonate and dried in a vacuum at room temperature for 3 hours, and this sample was designated as Sample 3. On the other hand, the following samples were used as comparative examples: a sample using a method of forming graphene by thermally reducing graphene oxide without reducing graphene oxide by electrochemical reduction treatment and a sample using graphite. The mixed solution H containing graphene oxide described in the process of the working electrode of the lithium secondary battery 1 was dried, and the obtained powdery graphene oxide was placed on indium foil, and this sample was designated as Comparative Example 1. In addition, Comparative Example 1 was heated in a vacuum at 300 °C for 10 hours to reduce graphene oxide, and the obtained graphene was placed on indium foil, and this sample was designated as Comparative Example 2. In addition, the sample in which graphite powder was placed on indium foil was designated as Comparative Example 3. Figure 12 shows the results of analyzing the surface element composition of Samples 1 to 3 and Comparative Examples 1 to 3 using X-ray photoelectron spectroscopy. By comparing Samples 1, 2, and 3 in Figure 12, it can be seen that the ratio of oxygen decreases and the ratio of carbon increases in Sample 3. The oxygen in Sample 3 obtained by electrochemical reduction treatment is 14.8 atomic %. In addition, by comparing Comparative Example 1 and Comparative Example 2, it can be seen that the oxygen ratio in Comparative Example 2 decreases. The oxygen in Comparative Example 2 obtained by thermal reduction is 13.4 atomic %. From this result, it can be seen that graphene oxide is reduced by electrochemical reduction treatment of graphene oxide. In addition, it can be seen that graphene oxide is reduced by thermal reduction. Next, Figure 13 shows the results of analyzing the atomic bonding state near the surface of Samples 1 to 3 and Comparative Examples 1 to 3 using X-ray photoelectron spectroscopy. Figure 13 shows the sp of C that is C=C in Samples 1 to 3 and Comparative Examples 1 to 3 respectively 2 bonds, sp of C such as C-C and C-H 3 bonds, C-O bonds, C=O bonds, CO 2 bonds (O=C-O bonds) and CF 2A chart for evaluating the ratio of bonds. By comparing Sample 1, Sample 2, and Sample 3, it can be seen that for C with C=C in Sample 3, the sp 2 ratio of the bond increases, while for C with C-C, C-H, etc., the sp 3 bonds, C-O bonds, C=O bonds, and CO 2 bond ratios decrease. From this result, it can be known that through electrochemical reduction treatment, sp 3 bonds, C-O bonds, C=O bonds, and CO 2 bonds react to form sp 2 bonds. The sp 2 bond in Sample 3 is 67.2%. On the other hand, by comparing Comparative Example 1 and Comparative Example 2, it can be seen that, similarly to Sample 3, the sp 2 bond increases in Comparative Example 2, but the amount of the sp 2 bond is lower than that in Sample 3. The sp 2 bond in Comparative Example 2 is 44.1%. That is, it can be known that through electrochemical reduction treatment, the ratio of the sp 2 bond becomes 50% or more and 70% or less. From the above, it can be seen that in FIGS. 11 to 13, by scanning the reduction potential near 2V, graphene oxide is reduced to generate graphene with more sp 2 bonds. In addition, it can be known that in FIGS. 10 and 12, by scanning the reduction potential near 2V, the resistance of the active material layer decreases, thereby enabling the current value of the lithium secondary battery to be increased. From the analysis results of FIGS. 11 to 13, it can be speculated that the above reduction in resistance is due to the generation of highly conductive graphene from low-conductive graphene oxide through electrochemical reduction treatment. Example 2 In this example, the reduction potential of graphene oxide measured using a measurement system that excludes the electrode resistance component will be described. For the electrode of graphene oxide manufactured by the method shown in Example 1, since graphene oxide is stacked, the resistance of the entire electrode increases. In this example, graphene oxide is sparsely attached to the electrode, and the reduction potential of graphene oxide is measured using a measurement system that excludes the resistance component generated due to the stacking of graphene oxide. Specifically, glassy carbon to be used as a working electrode and platinum to be used as a counter electrode were immersed in a graphene oxide dispersion in which graphene oxide was dispersed at a ratio of 0.0027 g / L with water as a solvent, and a voltage of 10 V was applied to the working electrode and the counter electrode for 30 seconds. Then, the glassy carbon with graphene oxide attached thereto was dried under vacuum. Here, the glassy carbon with graphene oxide attached thereto was used as graphene oxide electrode A. In addition, the graphene oxide used in this example was produced in the same manner as the graphene oxide in Example 1. Thus, by performing electrophoresis in the graphene oxide dispersion while controlling conditions, graphene oxide can be sparsely attached to the glassy carbon serving as the working electrode. Next, CV measurement was performed using graphene oxide electrode A as the working electrode, platinum as the counter electrode, and lithium as the reference electrode. In addition, in this CV measurement, a solution in which 1 M of LiPF was dissolved in a mixed solvent of EC and DEC mixed at a ratio of 1:1 was used as the electrolyte. 6 In addition, in this CV measurement, three conditions of Condition 1 to Condition 3 were adopted for the scanning speed. Condition 1, the scanning speed was 10 mV / s. Condition 2, the scanning speed was 50 mV / s. Condition 3, the scanning speed was 250 mV / s. The scanned potential range was exactly the same for Condition 1 to Condition 3, and the potential was scanned from the low potential side in a range of 1.8 V or more and 3.0 V or less compared to the dipping potential, and this potential scan was repeated three times. Figures 15A, 15B and Figure 16A show the CV measurement results for Condition 1 to Condition 3. Figure 15A is the result of Condition 1, Figure 15B is the result of Condition 2, and Figure 16A is the result of Condition 3. Figure 16B is the CV measurement result of a comparative example using only glassy carbon as the working electrode. Except for performing two potential scans, the CV measurement of the comparative example was the same as Condition 2. In addition, in Figures 15A and 15B and Figures 16A and 16B, the horizontal axis represents the potential of the working electrode (vs. Li / Li + ), and the vertical axis represents the current generated by oxidation-reduction. As can be seen from Figure 16B, in the comparative example where graphene oxide was not attached to the working electrode, no oxidation-reduction reaction occurred in the range of 1.8 V or more and 3.0 V. On the other hand, as for graphene oxide electrode A with graphene oxide attached thereto as shown in Condition 1 to Condition 3, only in the first potential scan, peaks were confirmed at positions of 2.3 V and 2.6 V as an irreversible reduction reaction, and in the second and third potential scans, no such peaks were confirmed as in the comparative example (refer to Figures 15A and 15B and Figure 16A). ​In addition, it can be seen that: among Conditions 1 to 3, the magnitude of the current flowing through the measurement system is different according to the scanning speed of the potential, but the positions of the peaks are independent of the scanning speed of the potential and are around 2.3 V and around 2.6 V under any condition. As can be seen from the above, the peaks confirmed at 2.3 V and 2.6 V are the peaks of the reduction reaction of graphene oxide. From the above, it can be confirmed that by providing a potential capable of causing a reduction reaction of graphene oxide as in one aspect of the present invention, graphene can be generated.

Claims

1. A secondary battery comprising a carbon material having a six-membered ring composed of carbon, wherein the carbon atom composition of the aforementioned carbon material is 80% or more but less than 90% when measured by X-ray photoelectron spectroscopy; the oxygen atom composition is 10% or more but less than 20% when measured by X-ray photoelectron spectroscopy; and among the bonds possessed by the aforementioned carbon atoms, the proportion of sp2 bonds formed between carbon atoms is 50% or more but less than 80% when measured by X-ray photoelectron spectroscopy.

2. A secondary battery comprising an active material and a carbon material having a six-membered ring composed of carbon, wherein the carbon atom composition of the aforementioned carbon material is 80% or more but less than 90% when measured by X-ray photoelectron spectroscopy; the oxygen atom composition is 10% or more but less than 20% when measured by X-ray photoelectron spectroscopy; and among the bonds possessed by the aforementioned carbon atoms, the proportion of sp2 bonds formed between carbon atoms is 50% or more but less than 80% when measured by X-ray photoelectron spectroscopy.

3. A secondary battery comprising a positive electrode active material and a carbon material having a six-membered ring composed of carbon, wherein the carbon atom composition of the aforementioned carbon material is 80% or more but less than 90% when measured by X-ray photoelectron spectroscopy; the oxygen atom composition is 10% or more but less than 20% when measured by X-ray photoelectron spectroscopy; and among the bonds possessed by the aforementioned carbon atoms, the proportion of sp2 bonds formed between carbon atoms is 50% or more but less than 80% when measured by X-ray photoelectron spectroscopy.

4. The secondary battery as claimed in claim 3, wherein the aforementioned positive electrode active material includes LiFeO2, LiCoO2, LiNiO2 or LiMn2O4.

5. A secondary battery comprising a negative electrode active material and a carbon material having a six-membered ring composed of carbon, wherein the negative electrode active material comprises silicon, and the carbon atom composition of the aforementioned carbon material is 80% or more but less than 90% when measured by X-ray photoelectron spectroscopy; the oxygen atom composition is 10% or more but less than 20% when measured by X-ray photoelectron spectroscopy; and among the bonds possessed by the aforementioned carbon atoms, the proportion of sp2 bonds formed between carbon atoms is 50% or more but less than 80% when measured by X-ray photoelectron spectroscopy.

6. A secondary battery comprising a carbon material having a multi-ring structure composed of carbon atoms, wherein the carbon atom composition of the aforementioned carbon material is 80% or more but less than 90% as measured by X-ray photoelectron spectroscopy; the oxygen atom composition is 10% or more but less than 20% as measured by X-ray photoelectron spectroscopy; and among the bonds possessed by the aforementioned carbon atoms, the proportion of sp2 bonds formed between carbon atoms is 50% or more but less than 80% as measured by X-ray photoelectron spectroscopy.

7. A secondary battery comprising an active material and a carbon material having a multi-ring structure composed of carbon atoms, wherein the carbon atom composition of the aforementioned carbon material is 80% or more but less than 90% when measured by X-ray photoelectron spectroscopy; the oxygen atom composition is 10% or more but less than 20% when measured by X-ray photoelectron spectroscopy; and among the bonds possessed by the aforementioned carbon atoms, the proportion of sp2 bonds formed between carbon atoms is 50% or more but less than 80% when measured by X-ray photoelectron spectroscopy.

8. A secondary battery comprising a positive electrode active material and a carbon material having a multi-ring structure composed of carbon atoms, wherein the carbon atom composition of the aforementioned carbon material is 80% or more but less than 90% when measured by X-ray photoelectron spectroscopy; the oxygen atom composition is 10% or more but less than 20% when measured by X-ray photoelectron spectroscopy; and among the bonds possessed by the aforementioned carbon atoms, the proportion of sp2 bonds formed between carbon atoms is 50% or more but less than 80% when measured by X-ray photoelectron spectroscopy.

9. The secondary battery of claim 8, wherein the aforementioned positive electrode active material comprises LiFeO2, LiCoO2, LiNiO2 or LiMn2O4.

10. A secondary battery comprising a negative electrode active material and a carbon material having a multi-ring structure composed of carbon, wherein the negative electrode active material comprises silicon, and the carbon atom composition of the aforementioned carbon material is 80% or more but less than 90% when measured by X-ray photoelectron spectroscopy; the oxygen atom composition is 10% or more but less than 20% when measured by X-ray photoelectron spectroscopy; and among the bonds possessed by the aforementioned carbon atoms, the proportion of sp2 bonds formed between carbon atoms is 50% or more but less than 80% when measured by X-ray photoelectron spectroscopy.

11. The secondary battery of any one of claims 6 to 8 and claim 10, wherein the aforementioned multi-element ring includes a seven-element ring, an eight-element ring, a nine-element ring or a ten-element ring.

12. The secondary battery as claimed in claim 2 or 7, wherein the aforementioned carbon material and the aforementioned active material are randomly dispersed.

13. The secondary battery as claimed in claim 3 or 8, wherein the aforementioned carbon material and the aforementioned positive electrode active material are randomly dispersed.

14. The secondary battery as claimed in claim 5 or 10, wherein the aforementioned carbon material and the aforementioned negative electrode active material are randomly dispersed.

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