Method for manufacturing positive electrode for secondary battery

Abstract

To provide a manufacturing method for a safer and lower-cost lithium ion secondary battery.SOLUTION: A manufacturing method for a positive electrode for a secondary battery includes the steps of: manufacturing a slurry by mixing graphene oxide, binder, and a positive electrode active material using a solvent containing water; applying the slurry on a positive electrode current collector; and reducing the graphene oxide by at least one of chemical reduction and thermal reduction. As a reducer of the chemical reduction, ascorbic acid can be used.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to an article, a method, or a manufacturing method. Alternatively, the present invention relates to a process, a machine, a manufacture, or a composition of matter. In particular, one aspect of the present invention relates to a semiconductor device, a display device, a light-emitting device, a secondary battery, a power storage device, a memory device, a driving method thereof, or a manufacturing method thereof. In particular, one aspect of the present invention relates to a secondary battery, a power storage device, and a method for manufacturing the same.

[0002] In this specification, the secondary battery or the power storage device refers to an element and a device having a power storage function in general.

Background Art

[0003] In recent years, various power storage devices such as lithium ion secondary batteries, lithium ion capacitors, air batteries, and all-solid-state batteries have been actively developed. In particular, lithium ion secondary batteries with high output and high capacity have rapidly expanded their demand along with the development of the semiconductor industry and have become indispensable in modern information societies as a source of rechargeable energy.

[0004] For example, in applications such as large household secondary batteries and vehicle secondary batteries, those using lithium iron phosphate (LiFePO4, abbreviated as LFP) as a positive electrode active material of a lithium ion battery have already been commercialized (Non-Patent Document 1).

[0005] On the other hand, graphene has attracted great attention in recent years due to its excellent conductivity and the like, and production methods on a large scale and the like are being explored. As shown in Non-Patent Document 2, a compound obtained by reducing graphene oxide (GO) may be called Reduced GO (RGO), and its physical properties have been attracting attention. For example, as in Non-Patent Document 3, there are studies characterizing the physical properties of GO using a scanning electron microscope (SEM), X-ray diffraction (XRD), Raman spectroscopy, and the like. In addition, as shown in Patent Document 1, there is also an example of using GO in a secondary battery.

Prior Art Documents

Patent Document

[0006]

Patent Document 1

Non-Patent Document

[0007]

Non-Patent Document 1

Non-Patent Document 2

Non-Patent Document 3

Non-Patent Document 4

Summary of the Invention

Problems to be Solved by the Invention

[0008] Lithium-ion batteries using LFP as the positive electrode active material are promising in terms of safety and cost. Therefore, it is expected that lithium-ion batteries using LFP will be produced in larger quantities in the future.

[0009] In the electrode manufacturing process of lithium-ion batteries, including those using LFP, a large amount of organic solvents are used as solvents for the slurry obtained by mixing the active material, conductive material, binder, etc., solvents for wet mixing, etc. All of these organic solvents evaporate in subsequent processes.

[0010] One of the commonly used organic solvents for this application is N-methylpyrrolidone (NMP), an aprotic polar solvent. However, NMP is harmful to health as it causes skin irritation and has the potential for reproductive toxicity. Therefore, it must be recovered in the manufacturing factory and not released into the environment. This process increases the manufacturing cost of lithium-ion batteries.

[0011] Therefore, if a slurry can be prepared using water as the solvent in the manufacturing process, lithium-ion batteries can be manufactured more safely and inexpensively.

[0012] One aspect of the present invention aims to provide a method for manufacturing a positive electrode for a lithium-ion secondary battery using water as the solvent in the manufacturing process. Or one aspect of the present invention aims to provide a method for manufacturing a safer positive electrode for a lithium-ion secondary battery. Or one aspect of the present invention aims to provide a method for manufacturing a lithium-ion secondary battery with a lower cost. Or one aspect of the present invention aims to provide a secondary battery with good cycle characteristics. Or one aspect of the present invention aims to provide a secondary battery with good rate characteristics. Or one aspect of the present invention aims to provide a secondary battery with a higher capacity. Or one aspect of the present invention aims to provide a safer secondary battery. Or one aspect of the present invention aims to provide a novel power storage device.

[0013] Note that the description of these problems does not preclude the existence of other problems. Note that one aspect of the present invention does not necessarily need to solve all of these problems. Note that it is possible to extract other problems from the descriptions in the specification, drawings, and claims.

Means for Solving the Problems

[0014] One aspect of the present invention is a method for producing a positive electrode for a secondary battery, the method including: mixing graphene oxide, a binder, and a positive electrode active material using a solvent containing water to prepare a slurry; applying the slurry to a positive electrode current collector; and reducing the graphene oxide, where the step of reducing the graphene oxide includes at least one of chemical reduction or thermal reduction.

[0015] Another aspect of the present invention is a method for producing a positive electrode for a secondary battery, the method including: mixing graphene oxide, a binder, and a positive electrode active material using a solvent containing water to prepare a slurry; applying the slurry to a positive electrode current collector; and reducing the graphene oxide, where the step of reducing the graphene oxide includes chemical reduction and thermal reduction.

[0016] Also, in the above, the chemical reduction is a step of immersing in a reducing agent solution, and the thermal reduction is preferably a step of heating at 125°C or higher and 200°C or lower for 1 hour or longer and 20 hours or shorter.

[0017] Also, in the above, the binder preferably contains a polysaccharide. Also, it preferably contains starch as the polysaccharide.

[0018] Also, in the above, the reducing agent solution is preferably an ascorbic acid solution.

[0019] Another aspect of the present invention is a secondary battery including a positive electrode, a negative electrode, a separator, and an electrolytic solution, where the positive electrode includes a positive electrode active material, a conductive material, a binder, and a positive electrode current collector, the positive electrode active material is lithium iron phosphate, and the conductive material is reduced graphene oxide.

[0020] Also in the above, the reduced graphene oxide has carbon, oxygen, has a sheet-like shape, has a two-dimensional structure formed of carbon six-membered rings, and preferably has a portion where the concentration of carbon in the reduced graphene oxide is greater than 80 atomic% and the concentration of oxygen is 2 atomic% or more and 15 atomic% or less.

[0021] Also in the above, the reduced graphene oxide preferably has an intensity ratio G / D of the G band and the D band in the Raman spectrum of 1 or more.

Advantages of the Invention

[0022] According to one aspect of the present invention, a method for manufacturing a positive electrode for a lithium-ion secondary battery using water as a solvent in the manufacturing process can be provided. Or one aspect of the present invention can provide a method for manufacturing a safer positive electrode for a lithium-ion secondary battery. Or one aspect of the present invention can provide a method for manufacturing a lithium-ion secondary battery with more cost reduction. Or one aspect of the present invention can provide a secondary battery with good cycle characteristics. Or one aspect of the present invention can provide a secondary battery with good rate characteristics. Or one aspect of the present invention can provide a secondary battery with a higher capacity. Or one aspect of the present invention can provide a safer secondary battery. Or one aspect of the present invention can provide a novel power storage device.

[0023] Note that the description of these effects does not prevent the existence of other effects. Note that one aspect of the present invention does not necessarily have to have all of these effects. Note that other effects will naturally become clear from the descriptions in the specification, drawings, claims, etc., and it is possible to extract these other effects from the descriptions in the specification, drawings, claims, etc.

Brief Description of the Drawings

[0024]

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DETAILED DESCRIPTION OF THE INVENTION

[0025] Embodiments of the present invention will be described in detail below with reference to the drawings. However, the present invention is not limited to these descriptions, and it is easily understood by those skilled in the art that the form and details can be variously changed. Therefore, the present invention should not be construed as being limited to the description of the embodiments shown below.

[0026] In each of the drawings described in this specification, the size, thickness, etc. of each element such as a film, layer, substrate, region, etc. may be exaggerated for the sake of clarity of the description. Therefore, each component is not necessarily limited to its size, nor is it limited to the relative size between each component.

[0027] In this specification, etc., the ordinal numbers such as first, second, etc. are used for convenience and do not indicate the order of steps or the order of lamination. Therefore, for example, "first" can be appropriately replaced with "second" or "third" etc. for explanation. Also, the ordinal numbers described in this specification, etc. may not match the ordinal numbers used to identify an aspect of the present invention.

[0028] In the configuration of the present invention described in this specification, etc., the same reference numerals are commonly used between different drawings for the same part or parts having the same function, and the repeated description thereof is omitted. Also, when referring to parts having the same function, the hatch pattern may be the same and may not be particularly labeled.

[0029] In the present specification and the like, both the positive electrode and the negative electrode for the power storage device may be collectively referred to as an electrode. In this case, the electrode shall refer to at least one of the positive electrode and the negative electrode.

[0030] In the present specification and the like, as an example of a secondary battery using the positive electrode and the positive electrode active material of one aspect of the present invention, a case where lithium metal is used for the counter electrode may be shown, but the secondary battery of one aspect of the present invention is not limited thereto. Other materials may be used for the negative electrode, such as graphite, lithium titanate, and the like. The preferable properties of the positive electrode of one aspect of the present invention are not affected by the material of the negative electrode.

[0031] (Embodiment 1) In the present embodiment, an example of a method for manufacturing a positive electrode for a secondary battery according to one aspect of the present invention will be described with reference to FIGS. 1 and 2.

[0032] <Step S11> First, as Step S11, a positive electrode active material, a conductive material, a binder, and a current collector, which are materials for the positive electrode, are prepared. A solvent for mixing is also prepared.

[0033] 〔Positive Electrode Active Material〕 As the positive electrode active material, a composite oxide having an olivine-type crystal structure, a layered rock salt-type crystal structure, or a spinel-type crystal structure can be used. For example, LFP, lithium manganese phosphate (LiMnPO4), lithium iron oxide (LiFeO2), lithium cobalt oxide (LiCoO2), lithium nickel oxide (LiNiO2), lithium manganese oxide (LiMn2O4), V2O5, Cr2O5, MnO2 and other compounds can be mentioned. Further, lithium cobalt oxide in which a part of cobalt is substituted with manganese, lithium cobalt oxide in which a part of cobalt is substituted with nickel, lithium nickel-cobalt-manganese oxide, lithium nickel-cobalt-aluminum oxide, etc. may be used. Mixtures of these may also be used. Additives such as magnesium and halogens including fluorine may be added to the positive electrode active material.

[0034] In particular, LFP is highly safe, has excellent cycle characteristics, has a wide plateau, and has iron that is inexpensive compared to cobalt, which is advantageous for cost reduction and is preferable.

[0035] Lithium cobaltate has advantages such as a large capacity, being more stable in the air compared to lithium nickelate, and being thermally stable compared to lithium nickelate, and is preferable.

[0036] In addition, it is preferable to mix a lithium-containing material having a spinel-type crystal structure containing manganese such as LiMn2O4 with lithium nickelate (LiNiO2, LiNi 1-x M x O2 (0 < x < 1) (M = Co, Al, etc.)). By adopting such a configuration, the characteristics of the secondary battery can be improved.

[0037] In addition, as the positive electrode active material, the composition formula Li a Mn b M c O dIt is possible to use a lithium manganese composite oxide that can be represented by [formula]. Here, the element M is preferably a metal element selected from elements other than lithium and manganese, or silicon or phosphorus, and more preferably nickel. When measuring the entire particle of the lithium manganese composite oxide, it is preferable to satisfy 0 < a / (b + c) < 2, c > 0, and 0.26 ≤ (b + c) / d < 0.5 during discharge. The composition of metals, silicon, phosphorus, etc. in the entire particle of the lithium manganese composite oxide can be measured using, for example, ICP-MS (Inductively Coupled Plasma Mass Spectrometer). Also, the oxygen composition of the entire particle of the lithium manganese composite oxide can be measured using, for example, EDX (Energy Dispersive X-ray Analysis). Further, it can be determined by using valence evaluation of melting gas analysis and XAFS (X-ray Absorption Fine Structure) analysis in combination with ICP-MS analysis. The lithium manganese composite oxide refers to an oxide containing at least lithium and manganese, and may contain at least one element selected from the group consisting of chromium, cobalt, aluminum, nickel, iron, magnesium, molybdenum, zinc, indium, gallium, copper, titanium, niobium, silicon, and phosphorus.

[0038] 〔Conductive Material〕 As the conductive material, for example, natural graphite, artificial graphite such as mesocarbon microbeads, carbon fiber, etc. can be used. As the carbon fiber, for example, carbon fibers such as mesophase pitch-based carbon fiber and isotropic pitch-based carbon fiber can be used. Also, as the carbon fiber, carbon nanofibers, carbon nanotubes, etc. can be used. Carbon nanotubes can be produced by, for example, vapor growth method. Further, as the conductive material, for example, carbon materials such as carbon black (such as acetylene black (AB)), graphite (graphite) particles, graphene, fullerene, etc. can be used. Also, for example, metal powders, metal fibers, conductive ceramic materials, etc. such as copper, nickel, aluminum, silver, gold can be used.

[0039] Further, it is particularly preferable to use graphene and graphene compounds as the conductive material. In particular, as the first material, it is preferable to use GO, which is then reduced through the reduction process described later to obtain reduced GO.

[0040] In this specification and the like, graphene compounds include multilayer graphene, multi-graphene, GO, multilayer GO, multi-GO, reduced GO, reduced multilayer GO, reduced multi-GO, and the like. Graphene compounds refer to those having carbon, having a flat or sheet-like shape, and having a two-dimensional structure formed by carbon six-membered rings. It is preferably in a bent shape. It may also be referred to as a carbon sheet. It preferably has a functional group. Also, the graphene compound may be rolled up to be like a carbon nanofiber.

[0041] In this specification and the like, GO refers to those having carbon and oxygen, having a sheet-like shape, and having a functional group, particularly an epoxy group, a carboxyl group, or a hydroxy group.

[0042] In this specification and the like, reduced GO refers to those having carbon and oxygen, having a sheet-like shape, and having a two-dimensional structure formed by carbon six-membered rings. It may also be referred to as a carbon sheet. Reduced GO can function even with a single sheet, but multiple sheets may be stacked. Reduced GO preferably has a portion where the carbon concentration is greater than 80 atomic% and the oxygen concentration is 2 atomic% or more and 15 atomic% or less. By setting such carbon and oxygen concentrations, it can function as a highly conductive material even in small amounts. Also, reduced GO preferably has an intensity ratio G / D of the G band and the D band in the Raman spectrum of 1 or more. Reduced GO with such an intensity ratio can function as a highly conductive material even in small amounts.

[0043] In this embodiment, GO is prepared as the material of the conductive material and is reduced in a later process. The conductive material of the completed positive electrode is reduced GO.

[0044] Graphene compounds may have excellent electrical properties such as high conductivity, and excellent physical properties such as high flexibility and high mechanical strength. Also, graphene compounds have a sheet-like shape. Graphene compounds may have a curved surface, enabling surface contact with low contact resistance. Also, even when thin, they may have very high conductivity and can efficiently form conductive paths in the active material layer in a small amount. Therefore, by using a graphene compound as a conductive material, the contact area between the active material and the conductive material can be increased. It is preferable that the graphene compound adheres to at least a part of the active material particles. Also, it is preferable that the graphene compound overlaps at least a part of the active material particles. Also, it is preferable that the shape of the graphene compound coincides with at least a part of the shape of the active material particles. The shape of the active material particles refers to, for example, the unevenness of a single active material particle or the unevenness formed by a plurality of active material particles. Also, it is preferable that the graphene compound surrounds at least a part of the active material particles. Also, the graphene compound may have holes.

[0045] When using active material particles with a small particle size, for example, active material particles of 1 μm or less, the specific surface area of the active material particles is large, and more conductive paths connecting the active material particles are required. In such a case, it is preferable to use a graphene compound that can efficiently form conductive paths even in a small amount.

[0046] Due to having the above-mentioned properties, it is particularly effective to use a graphene compound as a conductive material in secondary batteries that require rapid charging and rapid discharging. For example, secondary batteries for two-wheeled or four-wheeled vehicles, secondary batteries for drones, etc. may require rapid charging and rapid discharging characteristics. Also, rapid charging characteristics may be required in mobile electronic devices, etc. Rapid charging and rapid discharging may also be referred to as high-rate charging and high-rate discharging. For example, it refers to charging and discharging at 1C, 2C, or 5C or more.

[0047] 〔Binder〕 As the binder, it is preferable to use, for example, a water-soluble polymer. As the water-soluble polymer, for example, a polysaccharide can be used. As the polysaccharide, cellulose derivatives such as starch, carboxymethyl cellulose (CMC), methyl cellulose, ethyl cellulose, hydroxypropyl cellulose, diacetyl cellulose, and regenerated cellulose can be used. Further, it is more preferable to use these water-soluble polymers in combination with the rubber material described below.

[0048] Also, as the binder, rubber materials such as styrene-butadiene rubber (SBR), styrene-isoprene-styrene rubber, acrylonitrile-butadiene rubber, butadiene rubber, and ethylene-propylene-diene copolymer can be used. Also, as the binder, fluororubber can be used.

[0049] Alternatively, as the binder, it is preferable to use materials such as polystyrene, methyl polyacrylate, polymethyl methacrylate (PMMA), sodium polyacrylate, polyvinyl alcohol (PVA), polyethylene oxide (PEO), polypropylene oxide, polyimide, polyvinyl chloride, polytetrafluoroethylene, polyethylene, polypropylene, polyisobutylene, polyethylene terephthalate, nylon, polyvinylidene fluoride (PVDF), polyacrylonitrile (PAN), ethylene propylene diene polymer, polyvinyl acetate, and nitrocellulose.

[0050] The binder may be used in combination of a plurality of the above.

[0051] For example, a material with particularly excellent viscosity adjustment effect may be used in combination with other materials. For example, rubber materials and the like are excellent in adhesive force and elastic force, but may be difficult to adjust viscosity when mixed with a solvent. In such cases, for example, it is preferable to mix with a material having a particularly excellent viscosity adjustment effect. As the material having a particularly excellent viscosity adjustment effect, for example, a water-soluble polymer may be used. Further, as the water-soluble polymer having a particularly excellent viscosity adjustment effect, the above-mentioned polysaccharides such as carboxymethyl cellulose (CMC), methyl cellulose, ethyl cellulose, hydroxypropyl cellulose and diacetyl cellulose, regenerated cellulose and the like, and starches can be used. In this specification and the like, starch refers to a polymer in which α-glucose is polymerized. Since a higher degree of polymerization can function as a better binder, the degree of polymerization is preferably 500 or more, more preferably 1000 or more. Whether or not there is gelatinization is not a concern. Also, the branching ratio and the type of plant used as the raw material are not a concern. Further, it may have impurities such as monosaccharides and disaccharides such as glucose and maltose, other polysaccharides such as cellulose, phosphoric acid, and amino acids.

[0052] In addition, cellulose derivatives such as carboxymethyl cellulose can be made into salts such as sodium salts and ammonium salts of carboxymethyl cellulose, so that the solubility increases and it is easier to exhibit the effect as a viscosity modifier. By increasing the solubility, the dispersibility with the active material and other components can also be enhanced when preparing the electrode slurry. In this specification, cellulose and cellulose derivatives used as the electrode binder include their salts.

[0053] Water-soluble polymers can stabilize viscosity by dissolving in water and can stably disperse active materials and other materials combined as binders, such as styrene-butadiene rubber, in an aqueous solution. In addition, due to having functional groups, it is expected to be easily and stably adsorbed on the surface of the active material. Also, cellulose derivatives such as carboxymethyl cellulose have many materials with functional groups such as hydroxyl groups and carboxyl groups. Due to having functional groups, it is expected that the polymers interact with each other and exist widely covering the surface of the active material.

[0054] When a binder that covers or contacts the surface of the active material forms a film, it is also expected to play a role as a passive film and suppress the decomposition of the electrolyte. Here, a passive film is a film that has no electrical conductivity or has extremely low electrical conductivity. For example, when a passive film is formed on the surface of the active material, the decomposition of the electrolyte can be suppressed at the battery reaction potential. Also, it is more desirable that the passive film suppresses electrical conductivity while allowing lithium ions to conduct.

[0055] When using polysaccharides such as starch as a binder, it is preferable that at least a part of it is reduced through a reduction process described later. Therefore, the completed positive electrode preferably has a reduced polysaccharide as a binder. The reduced polysaccharide has improved conductivity and can form a better conductive path together with the conductive material in the positive electrode active material layer.

[0056] Also, the combination of using a polysaccharide as a binder and GO as a conductive material is particularly effective. Dehydration condensation occurs between the functional groups of the polysaccharide or the reduced polysaccharide and the functional groups of GO or the reduced GO, resulting in a covalent bond, and in some cases, it may function as a better binder and conductive material even in a small amount.

[0057] [Current collector] As the current collector, materials with high conductivity such as metals like stainless steel, gold, platinum, aluminum, titanium, and alloys thereof can be used. Also, the material used for the positive electrode current collector preferably does not dissolve at the potential of the positive electrode. Further, an aluminum alloy added with elements for improving heat resistance such as silicon, titanium, neodymium, scandium, molybdenum, etc. can be used. Also, it may be formed of a metal element that reacts with silicon to form a silicide. Examples of the metal element that reacts with silicon to form a silicide include zirconium, titanium, hafnium, vanadium, niobium, tantalum, chromium, molybdenum, tungsten, cobalt, nickel, etc. The current collector can appropriately use shapes such as foil, plate, sheet, net, punching metal, expanded metal, etc. The current collector preferably has a thickness of 5 μm or more and 30 μm or less.

[0058] 〔Solvent〕 The solvent for mixing preferably has polarity. Examples of the polar solvent include water, NMP (N-methylpyrrolidone), methanol, ethanol, acetone, DMF (N,N-dimethylformamide), etc. In particular, water is preferable because it has strong polarity and less burden on the environment and the human body. Further, water can be mixed with other materials to be used as the solvent for mixing. The water contained in the solvent is preferably 10% by volume or more, more preferably 50% by volume or more, and even more preferably 90% by volume or more.

[0059] <Step S12> Next, the binder, conductive material, and positive electrode active material are mixed. The order of mixing is not limited. For example, as shown in FIG. 1, first, the binder and the solvent are mixed (Step S12a), then the conductive material is mixed (Step S12b), and then the positive electrode active material can be mixed (Step S12c). In Step S12c, it is preferable to add a solvent for viscosity adjustment.

[0060] Alternatively, as shown in FIG. 2, first, the positive electrode active material and the solvent are mixed (step S12d), then the conductive material is mixed (step S12e), and then the binder can be mixed (step S12f). In step S12f, it is preferable to add a solvent for viscosity adjustment.

[0061] <Step S13> As described above, a mixture of the binder, the conductive material, and the positive electrode active material together with the solvent is made into a slurry (step S13).

[0062] <Step S14> Next, as step S14, the slurry is coated on the current collector. For example, a doctor blade can be used for coating. The loading amount can be adjusted by adjusting the blade gap during coating.

[0063] <Step S15> Next, as step S15, the coated slurry is dried to form an electrode layer. Also, if necessary, the shape may be processed, such as punching out the current collector and the electrode layer.

[0064] <Step S16> Next, as step S16, a reduction treatment is performed on the electrode layer. As the reduction method, at least one of chemical reduction or thermal reduction can be applied.

[0065] 〔Chemical Reduction〕 Chemical reduction refers to treatment with a reducing agent. As the reducing agent, organic acids such as ascorbic acid, hydrogen, sulfur dioxide, sulfurous acid, sodium sulfite, sodium bisulfite, ammonium sulfite, or phosphorous acid can be used.

[0066] When using ascorbic acid as a reducing agent, first dissolve ascorbic acid in a solvent to prepare a reducing agent solution (ascorbic acid solution). As the solvent, water, a mixture of water and NMP, ethanol, a mixture of water and ethanol, etc. can be used. Then immerse the current collector and the electrode layer prepared in step S15 in the solution. This treatment can be carried out, for example, for 30 minutes or more and 10 hours or less, and about 1 hour is preferred. Also, heating is preferable as it can shorten the time of chemical reduction. For example, it can be heated to a temperature of room temperature or higher and 100 °C or lower, and about 60 °C is preferred.

[0067] 〔Thermal reduction〕 Thermal reduction refers to the treatment of heating the current collector and the electrode layer prepared in step S15. Heating is preferably carried out under reduced pressure. For heating, for example, a glass tube oven can be used. The glass tube oven can be heated under a reduced pressure of about 1 kPa.

[0068] The optimal heating temperature and heating time vary depending on the materials of the conductive material and the binder. For example, when GO is used as the conductive material and PVDF is used as the binder, it is preferably at a temperature such that GO is sufficiently reduced and has no adverse effect on PVDF. Specifically, 125 °C or higher and 200 °C or lower are preferred. If it is 100 °C or lower, there is a risk that the reduction of GO will not proceed sufficiently. On the other hand, if it is 250 °C or higher, it has an adverse effect on PVDF and there is a risk that the slurry will easily peel off from the current collector. The heating time is preferably 1 hour or more and 20 hours or less. If the heating time is less than 1 hour, there is a risk that GO will not be sufficiently reduced. On the other hand, if the heating time exceeds 20 hours, the productivity will decrease.

[0069] When GO is used as the conductive material and starch is used as the binder, it is preferable to heat at a higher temperature than when PVDF is used as the binder. Specifically, it is preferably heated at 200 °C or higher and 300 °C or lower. In order to sufficiently reduce and carbonize starch (see Non-Patent Document 4), it is preferably heated at 200 °C or higher. On the other hand, if the temperature is too high, the cost may increase, such as the need for a special heating device, so 300 °C or lower is preferable. The heating time is preferably 1 hour or more and 20 hours or less. If the heating time is less than 1 hour, GO may not be sufficiently reduced. On the other hand, if the heating time exceeds 20 hours, the productivity will decrease.

[0070] At least one of chemical reduction and thermal reduction can be applied to the reduction treatment, but it is more preferable to perform both chemical reduction and thermal reduction. In this case, thermal reduction may be performed after chemical reduction, or chemical reduction may be performed after thermal reduction. For example, chemical reduction can be performed as step S16a shown in FIG. 2, and thermal reduction can be performed as step S16b.

[0071] In chemical reduction and thermal reduction, the functional groups that are easily reduced are different. Chemical reduction has a great effect of reducing the carbonyl group (C=O) and carboxyl group (-COOH) of GO by proton addition with a reducing agent. On the other hand, thermal reduction has a great effect of reducing the hydroxy group (-OH) in GO by dehydration. Therefore, by performing both chemical reduction and thermal reduction, reduction can be performed more efficiently, and the conductivity of the reduced GO can be increased.

[0072] <Step S17> Next, as shown in step S17 of FIG. 2, the material subjected to the reduction treatment may be pressed. For example, a calendar roll can be used for pressing. By pressing, the density of the positive electrode active material layer can be improved.

[0073] <Step S18> The product thus produced is used as the positive electrode of one aspect of the present invention (step S18).

[0074] By preparing a slurry using water as a solvent as described above, the positive electrode can be manufactured more safely and inexpensively than in the prior art. In addition, since it has graphene and graphene compounds as conductive materials, a positive electrode with high rate characteristics can be manufactured.

[0075] This embodiment can be used in combination with other embodiments.

[0076] (Embodiment 2) In this embodiment, an example of a secondary battery according to one aspect of the present invention will be described with reference to FIGS. 3 to 6.

[0077] <Configuration Example 1 of Secondary Battery> Hereinafter, a secondary battery in which a positive electrode, a negative electrode, and an electrolytic solution are wrapped in an exterior body will be described as an example.

[0078] [Positive Electrode] As the positive electrode, the positive electrode described in the previous embodiment is used. Hereinafter, as an example, a cross-sectional configuration example in the case where graphene and graphene compounds are used as conductive materials in the active material layer 200 will be described.

[0079] FIG. 3(A) shows a longitudinal sectional view of the active material layer 200. The active material layer 200 includes granular positive electrode active material 100, graphene and graphene compounds 201 as conductive materials, and a binder (not shown). Here, the graphene and graphene compounds 201 preferably have a sheet-like or flat-plate shape. In addition, the graphene and graphene compounds 201 may be a plurality of multi-layer graphenes, or (and) a plurality of graphenes may be partially overlapped to form a sheet-like or flat-plate shape.

[0080] In the longitudinal section of the active material layer 200, as shown in FIG. 3(A), sheet-like graphene and graphene compound 201 are dispersed substantially uniformly inside the active material layer 200. In FIGS. 3(A) and 3(B), graphene and graphene compound 201 are schematically represented by thick lines, but actually they are thin films having a thickness of a single layer or multiple layers of carbon molecules. Since a plurality of graphene and graphene compound 201 are formed so as to partially cover a plurality of granular positive electrode active materials 100 or stick on the surfaces of a plurality of granular positive electrode active materials 100, they are in surface contact with each other.

[0081] Here, by bonding a plurality of graphene and graphene compounds to each other, a network-like graphene compound sheet (hereinafter referred to as a graphene compound net or a graphene net) can be formed. When the active material is covered with the graphene net, the graphene net can also function as a binder that binds the active materials to each other. Therefore, the amount of the binder can be reduced or it can be not used, so that the ratio of the active material in the electrode volume and the electrode weight can be improved. That is, the capacity of the secondary battery can be increased.

[0082] Here, it is preferable to use GO as the graphene and graphene compound 201, mix it with the active material to form a layer that becomes the active material layer 200, and then reduce it. By using GO, which has extremely high dispersibility in a polar solvent, for the formation of the graphene and graphene compound 201, the graphene and graphene compound 201 can be dispersed substantially uniformly inside the active material layer 200. Since the solvent is volatilized and removed from the dispersion medium containing uniformly dispersed GO and GO is reduced, the graphene and graphene compound 201 remaining in the active material layer 200 partially overlap and are dispersed to the extent of being in surface contact with each other, so that a three-dimensional conductive path can be formed. The reduction of GO may be performed, for example, by heat treatment or by reducing agent treatment, but it is more preferable to perform both heat treatment and reducing agent treatment.

[0083] Therefore, unlike granular conductive materials such as AB that make point contact with the active material, graphene and graphene compound 201 enable surface contact with low contact resistance. Thus, the electrical conductivity between the granular positive electrode active material 100 and graphene and graphene compound 201 can be improved with a smaller amount compared to ordinary conductive materials. Therefore, the ratio in the active material layer 200 of the positive electrode active material 100 can be increased. Thereby, the discharge capacity of the secondary battery can be increased.

[0084] Also, by using a spray dryer in advance, the entire surface of the active material can be covered to form a coating of graphene and graphene compound as a conductive material, and a conductive path can also be formed between the active materials with graphene and graphene compound.

[0085] [Negative electrode] The negative electrode has a negative electrode active material layer and a negative electrode current collector. Also, the negative electrode active material layer may have a conductive material and a binder.

[0086] [Negative electrode active material] As the negative electrode active material, for example, an alloy-based material, a carbon-based material, or the like can be used.

[0087] As the negative electrode active material, an element capable of performing a charge-discharge reaction by an alloying / dealloying reaction with lithium can be used. For example, a material containing at least one of silicon, tin, gallium, aluminum, germanium, lead, antimony, bismuth, silver, zinc, cadmium, indium, etc. can be used. Such elements have a larger capacity compared to carbon, and in particular, silicon has a high theoretical capacity of 4200 mAh / g. Therefore, it is preferable to use silicon as the negative electrode active material. Also, compounds containing these elements may be used. For example, there are SiO, Mg2Si, Mg2Ge, SnO, SnO2, Mg2Sn, SnS2, V2Sn3, FeSn2, CoSn2, Ni3Sn2, Cu6Sn5, Ag3Sn, Ag3Sb, Ni2MnSb, CeSb3, LaSn3, La3Co2Sn7, CoSb3, InSb, SbSn, etc. Here, an element capable of performing a charge-discharge reaction by an alloying / dealloying reaction with lithium, and a compound containing the element, etc. may be referred to as an alloy-based material.

[0088] In this specification, etc., SiO refers to, for example, silicon monoxide. Alternatively, SiO can also be expressed as SiO y where y preferably has a value near 1. For example, y is preferably 0.2 or more and 1.5 or less, and more preferably 0.3 or more and 1.2 or less.

[0089] As the carbon-based material, graphite, graphitizable carbon (soft carbon), non-graphitizable carbon (hard carbon), carbon nanotubes, graphene, carbon black, etc. may be used.

[0090] Examples of graphite include artificial graphite and natural graphite. Examples of artificial graphite include mesocarbon microbeads (MCMB), coke-based artificial graphite, pitch-based artificial graphite, etc. Here, spherical graphite having a spherical shape can be used as the artificial graphite. For example, MCMB may have a spherical shape, which is preferable. Also, it is relatively easy to reduce the surface area of MCMB, which may be preferable. Examples of natural graphite include flake graphite, spheroidized natural graphite, etc.

[0091] Graphite exhibits a potential as low as that of metallic lithium (0.05 V or more and 0.3 V or less vs. Li / Li + ) when lithium ions are inserted into graphite (when forming a lithium-graphite intercalation compound). As a result, lithium-ion secondary batteries can exhibit a high operating voltage. Furthermore, graphite has advantages such as a relatively high capacity per unit volume, relatively small volume expansion, low cost, and high safety compared to metallic lithium, making it preferable.

[0092] In addition, oxides such as titanium dioxide (TiO2), lithium titanate (Li4Ti5O 12 ), lithium-graphite intercalation compound (Li x C6), niobium pentoxide (Nb2O5), tungsten oxide (WO2), and molybdenum oxide (MoO2) can be used as the negative electrode active material.

[0093] In addition, Li 3-x M x N (M = Co, Ni, Cu) having an Li3N-type structure, which is a complex nitride of lithium and a transition metal, can be used. For example, Li 2.6 Co 0.4 N3 exhibits a large charge-discharge capacity (900 mAh / g, 1890 mAh / cm 3 ) and is preferable.

[0094] When using a complex nitride of lithium and a transition metal, since the negative electrode active material contains lithium ions, it can be preferably combined with materials such as V2O5 and Cr3O8 that do not contain lithium ions as the positive electrode active material. Even when using a material containing lithium ions as the positive electrode active material, by previously desorbing the lithium ions contained in the positive electrode active material, a complex nitride of lithium and a transition metal can be used as the negative electrode active material.

[0095] In addition, a material in which a conversion reaction occurs can also be used as the negative electrode active material. For example, transition metal oxides that do not form an alloy with lithium, such as cobalt oxide (CoO), nickel oxide (NiO), and iron oxide (FeO), may be used as the negative electrode active material. As materials in which a conversion reaction occurs, further, oxides such as Fe2O3, CuO, Cu2O, RuO2, and Cr2O3, CoS 0.89 Conversion reactions also occur with sulfides such as NiS and CuS, nitrides such as Zn3N2, Cu3N, and Ge3N4, phosphides such as NiP2, FeP2, and CoP3, and fluorides such as FeF3 and BiF3.

[0096] As the conductive material and binder that the negative electrode active material layer can have, the same materials as the conductive material and binder that the positive electrode active material layer can have can be used.

[0097] [Negative electrode current collector] For the negative electrode current collector, the same materials as those for the positive electrode current collector can be used. It is preferable to use a material that does not alloy with carrier ions such as lithium for the negative electrode current collector.

[0098] [Electrolyte solution] The electrolyte solution has a solvent and an electrolyte. As the solvent of the electrolyte solution, an aprotic organic solvent is preferable. For example, ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate, chloroethylene carbonate, vinylene carbonate, γ-butyrolactone, γ-valerolactone, dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), methyl formate, methyl acetate, ethyl acetate, methyl propionate, ethyl propionate, propyl propionate, methyl butyrate, 1,3-dioxane, 1,4-dioxane, dimethoxyethane (DME), dimethyl sulfoxide, diethyl ether, methyl diglyme, acetonitrile, benzonitrile, tetrahydrofuran, sulfolane, sultone, etc., one kind, or two or more of these can be used in any combination and ratio.

[0099] In addition, by using one or more ionic liquids (room temperature molten salts) that are flame retardant and have low volatility as the solvent of the electrolytic solution, even if the internal temperature rises due to internal short circuit or overcharging of the secondary battery, rupture or ignition of the secondary battery can be prevented. An ionic liquid consists of a cation and an anion, and includes an organic cation and an anion. Examples of the organic cation used in the electrolytic solution include aliphatic onium cations such as quaternary ammonium cations, tertiary sulfonium cations, and quaternary phosphonium cations, and aromatic cations such as imidazolium cations and pyridinium cations. Examples of the anion used in the electrolytic solution include monovalent amide-based anions, monovalent methide-based anions, fluorosulfonic acid anions, perfluoroalkylsulfonic acid anions, tetrafluoroborate anions, perfluoroalkylborate anions, hexafluorophosphate anions, or perfluoroalkylphosphate anions, etc.

[0100] In addition, as the electrolyte dissolved in the above solvent, for example, lithium salts such as LiPF6, LiClO4, LiAsF6, LiBF4, LiAlCl4, LiSCN, LiBr, LiI, Li2SO4, Li2B 10 Cl 10 、Li2B 12 Cl 12 、LiCF3SO3, LiC4F9SO3, LiC(CF3SO2)3, LiC(C2F5SO2)3, LiN(CF3SO2)2, LiN(C4F9SO2)(CF3SO2), LiN(C2F5SO2)2, etc. can be used singly or in any combination and ratio of two or more of these.

[0101] It is preferable to use a highly purified electrolytic solution with a low content of particulate dust and elements other than the constituent elements of the electrolytic solution (hereinafter also simply referred to as "impurities") for the secondary battery. Specifically, it is preferable that the weight ratio of impurities to the electrolytic solution is 1% or less, preferably 0.1% or less, more preferably 0.01% or less.

[0102] In addition, additives such as vinylene carbonate, propane sultone (PS), tert-butylbenzene (TBB), fluoroethylene carbonate (FEC), lithium bis(oxalate) borate (LiBOB), and dinitrile compounds such as succinonitrile and adiponitrile may be added to the electrolytic solution. The concentration of the additive material may be, for example, 0.1 wt% or more and 5 wt% or less based on the entire solvent.

[0103] Alternatively, a polymer gel electrolyte obtained by swelling a polymer with an electrolytic solution may be used.

[0104] By using a polymer gel electrolyte, the safety against liquid leakage and the like is enhanced. In addition, the secondary battery can be made thinner and lighter.

[0105] As the polymer to be gelled, silicone gel, acrylic gel, acrylonitrile gel, polyethylene oxide-based gel, polypropylene oxide-based gel, gel of fluorine-based polymer, etc. can be used.

[0106] As the polymer, for example, a polymer having a polyalkylene oxide structure such as PEO, PVDF, polyacrylonitrile, etc., and copolymers containing them can be used. For example, PVDF-HFP, which is a copolymer of PVDF and hexafluoropropylene (HFP), can be used. Further, the formed polymer may have a porous shape.

[0107] Alternatively, instead of the electrolytic solution, a solid electrolyte having an inorganic material such as a sulfide-based or oxide-based material, or a solid electrolyte having a polymer material such as a PEO-based material can be used. When using a solid electrolyte, it is not necessary to install a separator or a spacer. In addition, since the entire battery can be solidified, the risk of liquid leakage is eliminated and the safety is dramatically improved.

[0108] 〔Separator〕 The secondary battery preferably has a separator. Examples of the separator include those formed of paper, non-woven fabric, glass fiber, ceramics, or synthetic fibers using nylon (polyamide), vinylon (polyvinyl alcohol-based fiber), polyester, acrylic, polyolefin, polyurethane, etc. The separator is preferably processed into an envelope shape and arranged to wrap either the positive electrode or the negative electrode.

[0109] The separator may have a multilayer structure. For example, an organic material film such as polypropylene or polyethylene can be coated with a ceramic-based material, a fluorine-based material, a polyamide-based material, or a mixture thereof. As the ceramic-based material, for example, aluminum oxide particles, silicon oxide particles, etc. can be used. As the fluorine-based material, for example, PVDF, polytetrafluoroethylene, etc. can be used. As the polyamide-based material, for example, nylon, aramid (meta-aramid, para-aramid), etc. can be used.

[0110] Coating with a ceramic-based material improves oxidation resistance, thus suppressing deterioration of the separator during high-voltage charge and discharge and improving the reliability of the secondary battery. Also, coating with a fluorine-based material makes it easier for the separator and the electrode to adhere, improving the output characteristics. Coating with a polyamide-based material, especially aramid, improves heat resistance, thus improving the safety of the secondary battery.

[0111] For example, a mixed material of aluminum oxide and aramid may be coated on both sides of a polypropylene film. Also, a mixed material of aluminum oxide and aramid may be coated on the surface of the polypropylene film that contacts the positive electrode, and a fluorine-based material may be coated on the surface that contacts the negative electrode.

[0112] When using a separator with a multilayer structure, the safety of the secondary battery can be maintained even if the overall thickness of the separator is thin, so the capacity per unit volume of the secondary battery can be increased.

[0113] 〔Outer casing〕 As the outer casing of the secondary battery, for example, a metal material such as aluminum or a resin material can be used. Also, a film-shaped outer casing can be used. As the film, for example, on a film made of a material such as polyethylene, polypropylene, polycarbonate, ionomer, polyamide, etc., a metal thin film with excellent flexibility such as aluminum, stainless steel, copper, nickel, etc. is provided, and further, an insulating synthetic resin film such as a polyamide-based resin or a polyester-based resin is provided as the outer surface of the outer casing on the metal thin film. A three-layer structure film can be used.

[0114] <Configuration Example 2 of Secondary Battery> Hereinafter, as an example of the configuration of the secondary battery, the configuration of the secondary battery using a solid electrolyte layer will be described.

[0115] As shown in Fig. 4(A), the secondary battery 400 according to one aspect of the present invention has a positive electrode 410, a solid electrolyte layer 420, and a negative electrode 430.

[0116] The positive electrode 410 has a positive electrode current collector 413 and a positive electrode active material layer 414. The positive electrode active material layer 414 has a positive electrode active material 411 and a solid electrolyte 421. Also, the positive electrode active material layer 414 may have a conductive material and a binder.

[0117] The solid electrolyte layer 420 has a solid electrolyte 421. The solid electrolyte layer 420 is located between the positive electrode 410 and the negative electrode 430 and is a region that does not have either the positive electrode active material 411 or the negative electrode active material 431.

[0118] The negative electrode 430 has a negative electrode current collector 433 and a negative electrode active material layer 434. The negative electrode active material layer 434 has a negative electrode active material 431 and a solid electrolyte 421. Also, the negative electrode active material layer 434 may have a conductive material and a binder. When metallic lithium is used for the negative electrode 430, as shown in Fig. 4(B), the negative electrode 430 that does not have the solid electrolyte 421 can be used. When metallic lithium is used for the negative electrode 430, the energy density of the secondary battery 400 can be improved, which is preferable.

[0119] As the solid electrolyte 421 included in the solid electrolyte layer 420, for example, a sulfide-based solid electrolyte, an oxide-based solid electrolyte, a halide-based solid electrolyte, or the like can be used.

[0120] Among sulfide-based solid electrolytes, there are thioborosilicon-based (Li 10 GeP2S 12 、Li 3.25 Ge 0.25 P 0.75 S4, etc.), sulfide glasses (70Li2S·30P2S5, 30Li2S·26B2S3·44LiI, 63Li2S·36SiS2·1Li3PO4, 57Li2S·38SiS2·5Li4SiO4, 50Li2S·50GeS2, etc.), and sulfide crystallized glasses (Li7P3S 11 、Li 3.25 P 0.95 S4, etc.). Sulfide-based solid electrolytes have advantages such as having materials with high conductivity, being synthesizable at low temperatures, and being relatively soft, so the conductive path is likely to be maintained even after charge and discharge.

[0121] Among oxide-based solid electrolytes, there are materials having a perovskite-type crystal structure (La 2 / 3-z Li 3z TiO3(0<z<2 / 3), etc.), materials having a NASICON-type crystal structure (Li 1+A Al A Ti 2-A (PO4)3(0<A<1), etc.), materials having a garnet-type crystal structure (Li7La3Zr2O 12 , etc.), materials having a LISICON-type crystal structure (Li 14 ZnGe4O 16 , etc.), oxide glasses (Li3PO4-Li4SiO4, 50Li4SiO4·50Li3BO3, etc.), and oxide crystallized glasses (Li 1.07 Al 0.69 Ti 1.46 (PO4)3, Li 1.5 Al 0.5 Ge 1.5 (PO4)3, etc.). Oxide-based solid electrolytes have the advantage of being stable in the atmosphere.

[0122] Halide-based solid electrolytes include LiAlCl4, Li3InBr6, LiF, LiCl, LiBr, LiI, etc. Also, composite materials in which these halide-based solid electrolytes are filled in the pores of porous aluminum oxide or porous silica can be used as solid electrolytes.

[0123] Also, different solid electrolytes may be mixed and used.

[0124] Among them, Li 1+B Al B Ti 2-B (PO4)3 (0 < B < 1) (hereinafter referred to as LATP) contains aluminum and titanium, which are elements that the positive electrode active material used in the secondary battery 400 of one aspect of the present invention may have. Therefore, a synergistic effect can be expected for improving the cycle characteristics, which is preferable. Also, an improvement in productivity due to the reduction of processes can be expected. In this specification, etc., the NASICON-type crystal structure refers to a compound represented by M2(XO4)3 (M: transition metal, X: S, P, As, Mo, W, etc.), which has a structure in which MO6 octahedra and XO4 tetrahedra share vertices and are three-dimensionally arranged.

[0125] 〔Shape of the exterior body and the secondary battery〕 For the exterior body of the secondary battery 400 of one aspect of the present invention, those of various materials and shapes can be used, but it is preferable to have a function of pressing the positive electrode, the solid electrolyte layer, and the negative electrode.

[0126] For example, FIGS. 5(A) to 5(C) are an example of a cell for evaluating the materials of an all-solid-state battery.

[0127] FIG. 5(A) is a schematic cross-sectional view of an evaluation cell. The evaluation cell includes a lower member 761, an upper member 762, and fixing screws and wing nuts 764 for fixing them. By rotating the pressing screw 763, the electrode plate 753 is pressed to fix the evaluation material. An insulator 766 is provided between the lower member 761 and the upper member 762, both of which are made of stainless steel. An O-ring 765 for sealing is provided between the upper member 762 and the pressing screw 763.

[0128] The evaluation material is placed on the electrode plate 751, surrounded by an insulating tube 752 around it, and is in a state of being pressed by the electrode plate 753 from above. A perspective view of the periphery of this evaluation material enlarged is FIG. 5(B).

[0129] As an example of the evaluation material, a stack of a positive electrode 750a, a solid electrolyte layer 750b, and a negative electrode 750c is shown, and a cross-sectional view is shown in FIG. 5(C). The same reference numerals are used for the same parts in FIGS. 5(A) to 5(C).

[0130] The electrode plate 751 and the lower member 761 that are electrically connected to the positive electrode 750a can be said to correspond to the positive electrode terminal. The electrode plate 753 and the upper member 762 that are electrically connected to the negative electrode 750c can be said to correspond to the negative electrode terminal. Electrical resistance and the like can be measured while applying pressure to the evaluation material via the electrode plate 751 and the electrode plate 753.

[0131] In addition, for the exterior body of the secondary battery according to one aspect of the present invention, it is preferable to use a package having excellent airtightness. For example, a ceramic package or a resin package can be used. Also, when sealing the exterior body, it is preferably performed in an atmosphere where the outside air is blocked and sealed, for example, inside a glove box.

[0132] FIG. 6(A) shows a perspective view of a secondary battery according to one aspect of the present invention having an exterior body and a shape different from those in FIG. 5. The secondary battery in FIG. 6(A) has external electrodes 771 and 772 and is sealed with an exterior body having a plurality of package members.

[0133] An example of a cross-section cut along the dashed line in FIG. 6(A) is shown in FIG. 6(B). The laminate having the positive electrode 750a, the solid electrolyte layer 750b, and the negative electrode 750c has a structure that is surrounded and sealed by a package member 770a provided with an electrode layer 773a on a flat plate, a frame-shaped package member 770b, and a package member 770c provided with an electrode layer 773b on a flat plate. Insulating materials, such as resin materials or ceramics, can be used for the package members 770a, 770b, and 770c.

[0134] The external electrode 771 is electrically connected to the positive electrode 750a via the electrode layer 773a and functions as a positive electrode terminal. Further, the external electrode 772 is electrically connected to the negative electrode 750c via the electrode layer 773b and functions as a negative electrode terminal.

[0135] This embodiment can be used in appropriate combination with other embodiments.

[0136] (Embodiment 3) In this embodiment, an example of the shape of the secondary battery having the positive electrode described in the previous embodiment will be described. The materials used for the secondary battery described in this embodiment can refer to the description of the previous embodiment.

[0137] <Coin-type secondary battery> First, an example of a coin-type secondary battery will be described. FIG. 7(A) is an external view of a coin-type (single-layer flat-type) secondary battery, and FIG. 7(B) is a cross-sectional view thereof.

[0138] The coin-type secondary battery 300 has a positive electrode can 301 that also serves as a positive electrode terminal and a negative electrode can 302 that also serves as a negative electrode terminal, which are insulated and sealed by a gasket 303 made of polypropylene or the like. The positive electrode 304 is formed by a positive electrode current collector 305 and a positive electrode active material layer 306 provided in contact therewith. Further, the negative electrode 307 is formed by a negative electrode current collector 308 and a negative electrode active material layer 309 provided in contact therewith.

[0139] Note that for the positive electrode 304 and the negative electrode 307 used in the coin-type secondary battery 300, the active material layer may be formed only on one side.

[0140] For the positive electrode can 301 and the negative electrode can 302, metals such as nickel, aluminum, and titanium that are corrosion-resistant to the electrolyte, or alloys thereof or alloys of these and other metals (such as stainless steel, etc.) can be used. Also, in order to prevent corrosion by the electrolyte, it is preferable to coat with nickel or aluminum, etc. The positive electrode can 301 is electrically connected to the positive electrode 304, and the negative electrode can 302 is electrically connected to the negative electrode 307, respectively.

[0141] These negative electrode 307, positive electrode 304, and separator 310 are impregnated with the electrolyte, and as shown in Fig. 7(B), with the positive electrode can 301 facing down, the positive electrode 304, separator 310, negative electrode 307, and negative electrode can 302 are laminated in this order, and the positive electrode can 301 and the negative electrode can 302 are crimped via the gasket 303 to manufacture the coin-type secondary battery 300.

[0142] By using the positive electrode described in the previous embodiment for the positive electrode 304, an inexpensive coin-type secondary battery 300 with excellent rate characteristics can be obtained.

[0143] Here, the flow of current during charging of the secondary battery will be described with reference to FIG. 7(C). When a secondary battery using lithium is regarded as a single closed circuit, the movement of lithium ions and the flow of current are in the same direction. Note that in a secondary battery using lithium, the anode (positive electrode) and cathode (negative electrode) are reversed during charging and discharging, and the oxidation reaction and reduction reaction are also reversed. Therefore, the electrode with a higher reaction potential is called the positive electrode, and the electrode with a lower reaction potential is called the negative electrode. Thus, in this specification, whether during charging, discharging, when applying a reverse pulse current, or when applying a charging current, the positive electrode is referred to as the "positive electrode" or the "+ electrode (plus electrode)", and the negative electrode is referred to as the "negative electrode" or the "- electrode (minus electrode)". Using terms such as anode (positive electrode) and cathode (negative electrode) related to oxidation and reduction reactions would be reversed during charging and discharging, which may cause confusion. Therefore, the terms anode (positive electrode) and cathode (negative electrode) will not be used in this specification. If the terms anode (positive electrode) and cathode (negative electrode) are used, it is necessary to specify whether it is during charging or discharging, and also indicate which one corresponds to the positive electrode (plus electrode) and the negative electrode (minus electrode).

[0144] A charger is connected to the two terminals shown in FIG. 7(C), and the secondary battery 300 is charged. As the charging of the secondary battery 300 progresses, the potential difference between the electrodes increases.

[0145] <Cylindrical secondary battery> Next, an example of a cylindrical secondary battery will be described with reference to FIG. 8. An external view of the cylindrical secondary battery 600 is shown in FIG. 8(A). FIG. 8(B) is a diagram schematically showing a cross-section of the cylindrical secondary battery 600. As shown in FIG. 8(B), the cylindrical secondary battery 600 has a positive electrode cap (battery lid) 601 on the upper surface and a battery can (outer can) 602 on the side surface and the bottom surface. The positive electrode cap 601 and the battery can (outer can) 602 are insulated by a gasket (insulating packing) 610.

[0146] Inside the hollow cylindrical battery can 602, a battery element is provided in which a strip-shaped positive electrode 604 and a negative electrode 606 are wound with a separator 605 interposed therebetween. Although not shown, the battery element is wound around a center pin. One end of the battery can 602 is closed and the other end is open. For the battery can 602, metals such as nickel, aluminum, and titanium that are corrosion-resistant to the electrolyte, or alloys thereof or alloys of these with other metals (for example, stainless steel, etc.) can be used. Further, in order to prevent corrosion by the electrolyte, it is preferable to coat the battery can 602 with nickel, aluminum, or the like. Inside the battery can 602, the battery element around which the positive electrode, negative electrode, and separator are wound is sandwiched by a pair of opposing insulating plates 608 and 609. Also, a non-aqueous electrolyte (not shown) is injected into the interior of the battery can 602 in which the battery element is provided. As the non-aqueous electrolyte, the same one as that used for a coin-type secondary battery can be used.

[0147] Since the positive and negative electrodes used in the cylindrical storage battery are wound, it is preferable to form the active material on both sides of the current collector. A positive electrode terminal (positive current collector lead) 603 is connected to the positive electrode 604, and a negative electrode terminal (negative current collector lead) 607 is connected to the negative electrode 606. Both the positive electrode terminal 603 and the negative electrode terminal 607 can use a metal material such as aluminum. The positive electrode terminal 603 is resistance-welded to the safety valve mechanism 612, and the negative electrode terminal 607 is resistance-welded to the bottom of the battery can 602, respectively. The safety valve mechanism 612 is electrically connected to the positive electrode cap 601 via a PTC element (Positive Temperature Coefficient) 611. The safety valve mechanism 612 disconnects the electrical connection between the positive electrode cap 601 and the positive electrode 604 when the internal pressure of the battery rises beyond a predetermined threshold value. Also, the PTC element 611 is a thermal resistance element whose resistance increases when the temperature rises, and it limits the current amount due to the increase in resistance to prevent abnormal heat generation. For the PTC element, barium titanate (BaTiO3)-based semiconductor ceramics or the like can be used.

[0148] Alternatively, as shown in Fig. 8(C), a module 615 may be configured by sandwiching a plurality of secondary batteries 600 between a conductive plate 613 and a conductive plate 614. The plurality of secondary batteries 600 may be connected in parallel, in series, or in parallel first and then in series. By configuring a module 615 having a plurality of secondary batteries 600, a large amount of power can be extracted.

[0149] Fig. 8(D) is a top view of the module 615. The conductive plate 613 is shown by a dotted line for clarity. As shown in Fig. 8(D), the module 615 may have a conducting wire 616 that electrically connects a plurality of secondary batteries 600. A conductive plate can be superimposed and provided on the conducting wire 616. Further, a temperature control device 617 may be provided between the plurality of secondary batteries 600. When the secondary battery 600 is overheated, it can be cooled by the temperature control device 617, and when the secondary battery 600 is too cold, it can be heated by the temperature control device 617. Therefore, the performance of the module 615 is less likely to be affected by the outside air temperature. The heat medium of the temperature control device 617 preferably has insulation and incombustibility.

[0150] By using the positive electrode described in the previous embodiment for the positive electrode 604, a cylindrical secondary battery 600 that is inexpensive and has excellent rate characteristics can be obtained.

[0151] <Structural Example of Secondary Battery> Another structural example of the secondary battery will be described with reference to Figs. 9 to 13.

[0152] Figs. 9(A) and 9(B) are views showing the external appearance of a battery pack. The battery pack has a secondary battery 913 and a circuit board 900. The secondary battery 913 is connected to an antenna 914 via the circuit board 900. A label 910 is attached to the secondary battery 913. Further, as shown in Fig. 9(B), the secondary battery 913 is connected to a terminal 951 and a terminal 952. The circuit board 900 is fixed with a seal 915.

[0153] The circuit board 900 has a terminal 911 and a circuit 912. The terminal 911 is connected to a terminal 951, a terminal 952, an antenna 914, and the circuit 912. Note that a plurality of terminals 911 may be provided, and each of the plurality of terminals 911 may be used as a control signal input terminal, a power supply terminal, or the like.

[0154] The circuit 912 may be provided on the back surface of the circuit board 900. Note that the antenna 914 is not limited to a coil shape, and may be, for example, linear or plate-shaped. Also, antennas such as a planar antenna, an aperture antenna, a traveling wave antenna, an EH antenna, a magnetic field antenna, and a dielectric antenna may be used. Alternatively, the antenna 914 may be a flat conductor. This flat conductor can function as one of the conductors for electric field coupling. That is, the antenna 914 may function as one of the two conductors of the capacitor. Thereby, power can be exchanged not only by an electromagnetic field and a magnetic field but also by an electric field.

[0155] The battery pack has a layer 916 between the antenna 914 and the secondary battery 913. The layer 916 has a function of shielding, for example, the electromagnetic field generated by the secondary battery 913. As the layer 916, for example, a magnetic material can be used.

[0156] Note that the structure of the battery pack is not limited to that shown in FIG. 9.

[0157] For example, as shown in FIGS. 10(A1) and 10(A2), antennas may be provided on each of a pair of opposing surfaces of the secondary battery 913 shown in FIGS. 9(A) and 9(B). FIG. 10(A1) is an external view showing one of the pair of surfaces, and FIG. 10(A2) is an external view showing the other of the pair of surfaces. Note that for the same parts as the secondary battery shown in FIGS. 9(A) and 9(B), the description of the secondary battery shown in FIGS. 9(A) and 9(B) can be appropriately incorporated.

[0158] As shown in Fig. 10(A1), an antenna 914 is provided with a layer 916 interposed between one of a pair of surfaces of the secondary battery 913, and as shown in Fig. 10(A2), an antenna 918 is provided with a layer 917 interposed between the other of the pair of surfaces of the secondary battery 913. The layer 917 has a function of being able to shield, for example, an electromagnetic field by the secondary battery 913. As the layer 917, for example, a magnetic material can be used.

[0159] By adopting the above structure, the sizes of both the antenna 914 and the antenna 918 can be increased. The antenna 918 has a function of being able to perform data communication with an external device, for example. For the antenna 918, an antenna having a shape applicable to the antenna 914 can be applied, for example. As a communication method between the secondary battery and another device via the antenna 918, a response method that can be used between the secondary battery and another device, such as NFC (Near Field Communication), can be applied.

[0160] Alternatively, as shown in Fig. 10(B1), a display device 920 may be provided on the secondary battery 913 shown in Figs. 9(A) and 9(B). The display device 920 is electrically connected to the terminal 911. Note that a label 910 does not have to be provided at the portion where the display device 920 is provided. For the same portions as the secondary battery shown in Figs. 9(A) and 9(B), the description of the secondary battery shown in Figs. 9(A) and 9(B) can be appropriately cited.

[0161] The display device 920 may display, for example, an image indicating whether charging is in progress, an image indicating the power storage amount, or the like. As the display device 920, for example, electronic paper, a liquid crystal display device, an electroluminescence (also referred to as EL) display device, or the like can be used. For example, by using electronic paper, the power consumption of the display device 920 can be reduced.

[0162] Alternatively, as shown in FIG. 10(B2), a sensor 921 may be provided in the secondary battery 913 shown in FIGS. 9(A) and 9(B). The sensor 921 is electrically connected to the terminal 911 via the terminal 922. For the same parts as the secondary battery shown in FIGS. 9(A) and 9(B), the description of the secondary battery shown in FIGS. 9(A) and 9(B) can be appropriately incorporated by reference.

[0163] The sensor 921 may have a function of measuring, for example, displacement, position, speed, acceleration, angular velocity, rotation speed, distance, light, liquid, magnetism, temperature, chemical substances, sound, time, hardness, electric field, current, voltage, power, radiation, flow rate, humidity, gradient, vibration, odor, or infrared rays. By providing the sensor 921, for example, data (such as temperature) indicating the environment where the secondary battery is placed can be detected and stored in the memory in the circuit 912.

[0164] Furthermore, structural examples of the secondary battery 913 will be described with reference to FIGS. 11 and 12.

[0165] The secondary battery 913 shown in FIG. 11(A) has a wound body 950 in which a terminal 951 and a terminal 952 are provided inside a housing 930. The wound body 950 is impregnated with an electrolytic solution inside the housing 930. The terminal 952 is in contact with the housing 930, and the terminal 951 is not in contact with the housing 930 by using an insulating material or the like. In FIG. 11(A), for the sake of convenience, the housing 930 is shown separately, but in reality, the wound body 950 is covered by the housing 930, and the terminals 951 and 952 extend outside the housing 930. As the housing 930, a metal material (such as aluminum) or a resin material can be used.

[0166] As shown in FIG. 11(B), the housing 930 shown in FIG. 11(A) may be formed of a plurality of materials. For example, the secondary battery 913 shown in FIG. 11(B) has a housing 930a and a housing 930b bonded together, and a wound body 950 is provided in the region surrounded by the housing 930a and the housing 930b.

[0167] As the housing 930a, an insulating material such as an organic resin can be used. In particular, by using a material such as an organic resin on the surface where the antenna is formed, shielding of the electric field by the secondary battery 913 can be suppressed. If the shielding of the electric field by the housing 930a is small, an antenna such as the antenna 914 may be provided inside the housing 930a. As the housing 930b, for example, a metal material can be used.

[0168] Furthermore, the structure of the wound body 950 is shown in FIG. 12. The wound body 950 includes a negative electrode 931, a positive electrode 932, and a separator 933. The wound body 950 is a wound body in which the negative electrode 931 and the positive electrode 932 overlap and are laminated with the separator 933 interposed therebetween, and the laminated sheet is wound. Note that a plurality of laminations of the negative electrode 931, the positive electrode 932, and the separator 933 may be further stacked.

[0169] The negative electrode 931 is connected to the terminal 911 shown in FIG. 9 via one of the terminals 951 and 952. The positive electrode 932 is connected to the terminal 911 shown in FIG. 9 via the other of the terminals 951 and 952.

[0170] By using the positive electrode described in the previous embodiment for the positive electrode 932, an inexpensive secondary battery 913 with excellent rate characteristics can be obtained.

[0171] <Laminated secondary battery> Next, an example of a laminated secondary battery will be described with reference to FIGS. 13 to 17. If the laminated secondary battery has a flexible configuration, it can be mounted on an electronic device having at least a part of a flexible portion, and the secondary battery can also be bent in accordance with the deformation of the electronic device.

[0172] With reference to FIG. 13, a laminated secondary battery 980 will be described. The laminated secondary battery 980 has a wound body 993 shown in FIG. 13(A). The wound body 993 includes a negative electrode 994, a positive electrode 995, and a separator 996. Similar to the wound body 950 described with reference to FIG. 12, the wound body 993 is formed by laminating the negative electrode 994 and the positive electrode 995 with the separator 996 interposed therebetween, and then winding the laminated sheet.

[0173] Note that the number of laminations of the stack composed of the negative electrode 994, the positive electrode 995, and the separator 996 may be appropriately designed according to the required capacity and the element volume. The negative electrode 994 is connected to a negative electrode current collector (not shown) via one of the lead electrodes 997 and 998, and the positive electrode 995 is connected to a positive electrode current collector (not shown) via the other of the lead electrodes 997 and 998.

[0174] As shown in FIG. 13(B), the secondary battery 980 can be manufactured by housing the above-described wound body 993 in a space formed by thermocompression bonding or the like a film 981 serving as an exterior body and a film 982 having a recess. The wound body 993 has lead electrodes 997 and 998 and is impregnated with an electrolytic solution inside the film 981 and the film 982 having a recess.

[0175] For the film 981 and the film 982 having a recess, a metal material such as aluminum or a resin material can be used. If a resin material is used as the material for the film 981 and the film 982 having a recess, the film 981 and the film 982 having a recess can be deformed when an external force is applied, and a flexible storage battery can be manufactured.

[0176] Also, although FIGS. 13(B) and 13(C) show an example using two films, a space may be formed by bending a single film, and the above-described wound body 993 may be housed in the space.

[0177] By using the positive electrode described in the previous embodiment for the positive electrode 995, a secondary battery 980 that is inexpensive and has excellent rate characteristics can be obtained.

[0178] In FIG. 13, an example of the secondary battery 980 having a wound body in a space formed by a film serving as an exterior body has been described. However, for example, as shown in FIG. 14, a secondary battery having a plurality of strip-shaped positive electrodes, separators, and negative electrodes in a space formed by a film serving as an exterior body may also be used.

[0179] The laminated secondary battery 500 shown in FIG. 14(A) includes a positive electrode 503 having a positive electrode current collector 501 and a positive electrode active material layer 502, a negative electrode 506 having a negative electrode current collector 504 and a negative electrode active material layer 505, a separator 507, an electrolytic solution 508, and an exterior body 509. A separator 507 is installed between the positive electrode 503 and the negative electrode 506 provided in the exterior body 509. Further, the interior of the exterior body 509 is filled with the electrolytic solution 508. As the electrolytic solution 508, the electrolytic solution shown in the previous embodiment can be used.

[0180] In the laminated secondary battery 500 shown in FIG. 14(A), the positive electrode current collector 501 and the negative electrode current collector 504 also serve as terminals for obtaining electrical contact with the outside. Therefore, a part of the positive electrode current collector 501 and the negative electrode current collector 504 may be arranged to be exposed outside the exterior body 509. Alternatively, the positive electrode current collector 501 and the negative electrode current collector 504 may not be exposed outside the exterior body 509, and a lead electrode may be ultrasonically bonded to the lead electrode and the positive electrode current collector 501 or the negative electrode current collector 504 so that the lead electrode is exposed outside.

[0181] In the laminated secondary battery 500, for the exterior body 509, a laminated film having a three-layer structure can be used, in which a metal thin film with excellent flexibility such as aluminum, stainless steel, copper, nickel, etc. is provided on a film made of a material such as polyethylene, polypropylene, polycarbonate, ionomer, polyamide, etc., and an insulating synthetic resin film such as a polyamide-based resin or a polyester-based resin is provided as the outer surface of the exterior body on the metal thin film.

[0182] Further, an example of the cross-sectional structure of the laminated secondary battery 500 is shown in FIG. 14(B). In FIG. 14(A), for simplicity, an example composed of two current collectors is shown, but actually, as shown in FIG. 14(B), it is composed of a plurality of electrode layers.

[0183] In FIG. 14(B), as an example, the number of electrode layers is 16. Note that even when the number of electrode layers is 16, the secondary battery 500 has flexibility. FIG. 14(B) shows a structure of a total of 16 layers, with 8 layers of the negative electrode current collector 504 and 8 layers of the positive electrode current collector 501. Note that FIG. 14(B) shows the cross-section of the extraction part of the negative electrode, and 8 layers of the negative electrode current collector 504 are ultrasonically bonded. Of course, the number of electrode layers is not limited to 16, and it may be more or less. When the number of electrode layers is large, a secondary battery with a larger capacity can be obtained. Also, when the number of electrode layers is small, it can be made thinner and a secondary battery with excellent flexibility can be obtained.

[0184] Here, examples of the external views of the laminated secondary battery 500 are shown in FIGS. 15 and 16. FIGS. 15 and 16 have a positive electrode 503, a negative electrode 506, a separator 507, an exterior body 509, a positive electrode lead electrode 510, and a negative electrode lead electrode 511.

[0185] FIG. 17(A) shows the external views of the positive electrode 503 and the negative electrode 506. The positive electrode 503 has a positive electrode current collector 501, and the positive electrode active material layer 502 is formed on the surface of the positive electrode current collector 501. Also, the positive electrode 503 has a region where a part of the positive electrode current collector 501 is exposed (hereinafter referred to as a tab region). The negative electrode 506 has a negative electrode current collector 504, and the negative electrode active material layer 505 is formed on the surface of the negative electrode current collector 504. Also, the negative electrode 506 has a region where a part of the negative electrode current collector 504 is exposed, that is, a tab region. The area and shape of the tab regions of the positive electrode and the negative electrode are not limited to the example shown in FIG. 17(A).

[0186] <Manufacturing method of laminated secondary battery> Here, an example of the manufacturing method of the laminated secondary battery whose external view is shown in FIG. 15 will be described with reference to FIGS. 17(B) and (C).

[0187] First, the negative electrode 506, the separator 507, and the positive electrode 503 are laminated. FIG. 17(B) shows the laminated negative electrode 506, separator 507, and positive electrode 503. Here, an example is shown in which five sets of negative electrodes and four sets of positive electrodes are used. Next, the tabs of the positive electrode 503 are joined together, and the positive electrode lead electrode 510 is joined to the tab region of the outermost positive electrode. For the joining, for example, ultrasonic welding or the like may be used. Similarly, the tabs of the negative electrode 506 are joined together, and the negative electrode lead electrode 511 is joined to the tab region of the outermost negative electrode.

[0188] Next, the negative electrode 506, the separator 507, and the positive electrode 503 are arranged on the exterior body 509.

[0189] Next, as shown in FIG. 17(C), the exterior body 509 is bent at the portion indicated by the broken line. Then, the outer peripheral portion of the exterior body 509 is joined. For the joining, for example, thermocompression bonding or the like may be used. At this time, a region that is not joined (hereinafter referred to as an inlet) is provided in a part (or one side) of the exterior body 509 so that the electrolytic solution 508 can be introduced later.

[0190] Next, the electrolytic solution 508 (not shown) is introduced into the interior of the exterior body 509 through the inlet provided in the exterior body 509. The introduction of the electrolytic solution 508 is preferably performed under a reduced-pressure atmosphere or an inert atmosphere. And finally, the inlet is joined. In this way, the laminated secondary battery 500 can be manufactured.

[0191] By using the positive electrode described in the previous embodiment for the positive electrode 503, a secondary battery 500 that is inexpensive and has excellent rate characteristics can be obtained.

[0192] This embodiment can be used in appropriate combination with other embodiments.

[0193] (Embodiment 4) In this embodiment, an example of mounting a secondary battery, which is one aspect of the present invention, on an electronic device will be described.

[0194] First, an example of mounting the secondary battery described in the previous embodiment on an electronic device is shown in FIGS. 18(A) to 18(F). Examples of electronic devices to which the secondary battery described in the previous embodiment is applied include, for example, a television device (also referred to as a TV or a television receiver), a monitor for a computer, a digital camera, a digital video camera, a digital photo frame, a mobile phone (also referred to as a cellular phone or a mobile phone device), a portable game machine, a portable information terminal, a portable battery, an audio playback device, and a large game machine such as a pachinko machine.

[0195] FIG. 18(A) shows an example of a mobile phone. The mobile phone 7400 includes, in addition to a display unit 7402 incorporated in a housing 7401, operation buttons 7403, an external connection port 7404, a speaker 7405, a microphone 7406, and the like. The mobile phone 7400 has a secondary battery 7407. By using the secondary battery of one aspect of the present invention for the secondary battery 7407, an inexpensive and high-performance mobile phone can be provided.

[0196] FIG. 18(B) shows an example of a wristwatch-type portable information terminal. The portable information terminal 7200 includes a housing 7201, a display unit 7202, a band 7203, a buckle 7204, operation buttons 7205, input / output terminals 7206, and the like.

[0197] The portable information terminal 7200 can execute various applications such as mobile phone calls, e-mails, text viewing and creation, music playback, Internet communication, and computer games.

[0198] The display surface of the display unit 7202 is provided in a curved shape, and display can be performed along the curved display surface. Further, the display unit 7202 includes a touch sensor and can be operated by touching the screen with a finger or a stylus. For example, an application can be launched by touching an icon 7207 displayed on the display unit 7202.

[0199] In addition to time setting, the operation button 7205 can have various functions such as power on / off operations, wireless communication on / off operations, execution and cancellation of the manner mode, execution and cancellation of the power saving mode, etc. For example, the functions of the operation button 7205 can also be freely set by the operating system incorporated in the portable information terminal 7200.

[0200] Also, the portable information terminal 7200 is capable of performing communication-standardized short-range wireless communication. For example, it can communicate hands-free by mutually communicating with a wireless communication-capable headset.

[0201] The portable information terminal 7200 is also provided with an input / output terminal 7206 and can directly exchange data with other information terminals via a connector. Charging can also be performed via the input / output terminal 7206. Note that the charging operation may also be performed by wireless power supply without going through the input / output terminal 7206.

[0202] The display unit 7202 of the portable information terminal 7200 has a secondary battery according to one aspect of the present invention. By using the secondary battery according to one aspect of the present invention, an inexpensive and high-performance portable information terminal can be provided. For example, the secondary battery 7104 shown in Fig. 18(D) can be incorporated in a curved state inside the housing 7201 or in a state where it can be curved inside the band 7203.

[0203] The portable information terminal 7200 preferably has a sensor. As the sensor, for example, it is preferable to mount a human body sensor such as a fingerprint sensor, a pulse sensor, a body temperature sensor, or a touch sensor, a pressure sensor, an acceleration sensor, etc.

[0204] FIG. 18(C) shows an example of a bangle-type display device. The portable display device 7100 includes a housing 7101, a display unit 7102, operation buttons 7103, and a secondary battery 7104. Further, FIG. 18(D) shows the state of the secondary battery 7104 bent. When the secondary battery 7104 is bent and worn on the user's arm, the housing deforms and the curvature of part or all of the secondary battery 7104 changes. Note that the degree of bending at an arbitrary point on the curve, represented by the value of the radius of the corresponding circle, is called the radius of curvature, and the reciprocal of the radius of curvature is called the curvature. Specifically, part or all of the main surface of the housing or the secondary battery 7104 changes within a range where the radius of curvature is 40 mm or more and 150 mm or less. If the radius of curvature of the main surface of the secondary battery 7104 is in the range of 40 mm or more and 150 mm or less, high reliability can be maintained. By using the secondary battery according to one aspect of the present invention for the secondary battery 7104, an inexpensive and high-performance portable display device can be provided.

[0205] FIG. 18(E) shows an example of a bracelet-type display device. The display device 7300 has a display unit 7304 and has a secondary battery according to one aspect of the present invention. Further, the display device 7300 may be provided with a touch sensor in the display unit 7304 and may also function as a portable information terminal.

[0206] The display surface of the display unit 7304 is curved, and display can be performed along the curved display surface. Further, the display device 7300 can change the display status by means of short-range wireless communication conforming to a communication standard or the like.

[0207] Further, the display device 7300 is provided with input / output terminals and can directly exchange data with other information terminals via a connector. Charging can also be performed via the input / output terminals. Note that the charging operation may be performed by wireless power supply without passing through the input / output terminals.

[0208] By using the secondary battery according to one aspect of the present invention as the secondary battery of the display device 7300, an inexpensive and high-performance display device can be provided.

[0209] FIG. 18(F) shows an example of a mobile battery. The mobile battery 7350 has a secondary battery and a plurality of terminals 7351. Charging of other electronic devices is possible via the terminals 7351. By using the secondary battery of one aspect of the present invention as the secondary battery included in the mobile battery 7350, an inexpensive and high-performance mobile battery 7350 can be obtained.

[0210] Also, an example of mounting a secondary battery with good cycle characteristics shown in the previous embodiment on an electronic device will be described with reference to FIGS. 18(G), 19, and 20.

[0211] By using the secondary battery of one aspect of the present invention as the secondary battery of an electronic device, a lightweight and long-life product can be provided. For example, examples of the electronic device include an electric toothbrush, an electric shaver, and an electric beauty device. As the secondary battery for these products, considering the ease of use by the user, a secondary battery having a stick shape, being small, lightweight, and having a large capacity is desired.

[0212] FIG. 18(G) is a perspective view of a device also called a tobacco-containing smoking device (electronic cigarette). In FIG. 18(G), the electronic cigarette 7500 includes an atomizer 7501 including a heating element, a secondary battery 7504 that supplies power to the atomizer, and a cartridge 7502 including a liquid supply bottle, a sensor, and the like. To enhance safety, a protection circuit for preventing overcharging and over-discharging of the secondary battery 7504 may be electrically connected to the secondary battery 7504. The secondary battery 7504 shown in FIG. 18(G) has external terminals so that it can be connected to a charging device. Since the secondary battery 7504 becomes the tip portion when held, it is desirable that the total length is short and the weight is light. Since the secondary battery of one aspect of the present invention is inexpensive and has good rate characteristics, an inexpensive electronic cigarette 7500 with high heating performance can be provided.

[0213] Next, FIGS. 19(A) and 19(B) show an example of a foldable tablet terminal. The tablet terminal 9600 shown in FIGS. 19(A) and 19(B) includes a housing 9630a, a housing 9630b, a movable part 9640 connecting the housing 9630a and the housing 9630b, a display unit 9631 including a display unit 9631a and a display unit 9631b, switches 9625 to 9627, a fastener 9629, and an operation switch 9628. By using a flexible panel for the display unit 9631, a tablet terminal with a wider display unit can be achieved. FIG. 19(A) shows the tablet terminal 9600 in an open state, and FIG. 19(B) shows the tablet terminal 9600 in a closed state.

[0214] Also, the tablet terminal 9600 has a power storage body 9635 inside the housing 9630a and the housing 9630b. The power storage body 9635 is provided across the housing 9630a and the housing 9630b through the movable part 9640.

[0215] The display unit 9631 can have all or part of its area as a touch panel area, and data input can be performed by touching an image, character, input form, etc. including an icon displayed in the area. For example, keyboard buttons can be displayed on the entire surface of the display unit 9631a on the housing 9630a side, and information such as characters and images can be displayed and used on the display unit 9631b on the housing 9630b side.

[0216] Alternatively, a keyboard can be displayed on the display unit 9631b on the housing 9630b side, and information such as characters and images can be displayed and used on the display unit 9631a on the housing 9630a side. Also, a keyboard display switching button for the touch panel can be displayed on the display unit 9631, and the keyboard can be displayed on the display unit 9631 by touching the button with a finger or a stylus.

[0217] Moreover, touch input can be simultaneously performed on the touch panel area of the display unit 9631a on the housing 9630a side and the touch panel area of the display unit 9631b on the housing 9630b side.

[0218] In addition, the switches 9625 to 9627 may be interfaces not only for operating the tablet terminal 9600 but also for switching various functions. For example, at least one of the switches 9625 to 9627 may function as a switch for turning on and off the power of the tablet terminal 9600. Also, for example, at least one of the switches 9625 to 9627 may have a function of switching the display orientation such as vertical display or horizontal display, or a function of switching between black and white display and color display. Also, for example, at least one of the switches 9625 to 9627 may have a function of adjusting the brightness of the display unit 9631. Further, the brightness of the display unit 9631 can be optimized according to the amount of external light detected by the optical sensor built in the tablet terminal 9600 during use. Note that the tablet terminal may incorporate not only an optical sensor but also other detection devices such as sensors for detecting inclination such as a gyro and an acceleration sensor.

[0219] Also, in FIG. 19(A), an example is shown where the display areas of the display unit 9631a on the housing 9630a side and the display unit 9631b on the housing 9630b side are substantially the same, but the respective display areas of the display unit 9631a and the display unit 9631b are not particularly limited, and the size of one may be different from that of the other, and the display quality may also be different. For example, one may be a display panel that can perform a higher-definition display than the other.

[0220] FIG. 19(B) shows a state where the tablet terminal 9600 is closed in a two-fold manner, and the tablet terminal 9600 has a charge / discharge control circuit 9634 including a housing 9630, a solar cell 9633, and a DCDC converter 9636. Also, as the power storage body 9635, a power storage body according to one aspect of the present invention is used.

[0221] As described above, since the tablet terminal 9600 can be folded in half, the housing 9630a and the housing 9630b can be folded so as to overlap each other when not in use. By folding, the display unit 9631 can be protected, thereby enhancing the durability of the tablet terminal 9600. In addition, since the power storage body 9635 using the secondary battery according to one aspect of the present invention is inexpensive and has good rate characteristics, an inexpensive and high-performance tablet terminal 9600 can be provided.

[0222] In addition, the tablet terminal 9600 shown in FIGS. 19(A) and 19(B) can also have functions such as a function of displaying various information (still images, moving images, text images, etc.), a function of displaying a calendar, date, or time on the display unit, a touch input function of touch input operation or editing the information displayed on the display unit, a function of controlling processing by various software (programs), and the like.

[0223] Power can be supplied to the touch panel, the display unit, the video signal processing unit, etc. by the solar cell 9633 mounted on the surface of the tablet terminal 9600. Note that the solar cell 9633 can be provided on one or both sides of the housing 9630, and can be configured to efficiently charge the power storage body 9635. When a lithium ion battery is used as the power storage body 9635, there are advantages such as miniaturization.

[0224] In addition, the configuration and operation of the charge / discharge control circuit 9634 shown in FIG. 19(B) will be described with reference to the block diagram in FIG. 19(C). FIG. 19(C) shows the solar cell 9633, the power storage body 9635, the DCDC converter 9636, the converter 9637, the switches SW1 to SW3, and the display unit 9631. The power storage body 9635, the DCDC converter 9636, the converter 9637, and the switches SW1 to SW3 correspond to the locations of the charge / discharge control circuit 9634 shown in FIG. 19(B).

[0225] First, an example of the operation when power is generated by the solar cell 9633 using external light will be described. The power generated by the solar cell is stepped up or down by the DCDC converter 9636 to a voltage for charging the power storage body 9635. When the power from the solar cell 9633 is used for the operation of the display unit 9631, the switch SW1 is turned on, and the converter 9637 steps up or down the voltage to the voltage required for the display unit 9631. When the display is not performed on the display unit 9631, SW1 may be turned off and SW2 may be turned on to charge the power storage body 9635.

[0226] Note that the solar cell 9633 is shown as an example of the power generation means, but it is not particularly limited, and a configuration may be adopted in which the power storage body 9635 is charged by other power generation means such as a piezoelectric element (piezo element) or a thermoelectric conversion element (Peltier element). For example, a contactless power transmission module that wirelessly (non-contact) transmits and receives power for charging, or a configuration that combines other charging means may be adopted.

[0227] FIG. 20 shows an example of another electronic device. In FIG. 20, the display device 8000 is an example of an electronic device using the secondary battery 8004 according to one aspect of the present invention. Specifically, the display device 8000 corresponds to a display device for receiving TV broadcasts, and includes a housing 8001, a display unit 8002, a speaker unit 8003, a secondary battery 8004, and the like. The secondary battery 8004 according to one aspect of the present invention is provided inside the housing 8001. The display device 8000 can receive power supply from a commercial power source or use the power stored in the secondary battery 8004. Therefore, even when power supply from the commercial power source cannot be received due to a power outage or the like, the display device 8000 can be used by using the secondary battery 8004 according to one aspect of the present invention as an uninterruptible power supply.

[0228] The display unit 8002 can use a semiconductor display device such as a liquid crystal display device, a light-emitting device having a light-emitting element such as an organic EL element in each pixel, an electrophoretic display device, a DMD (Digital Micromirror Device), a PDP (Plasma Display Panel), or an FED (Field Emission Display).

[0229] In addition, the display device includes all display devices for information display, such as those for personal computers and advertising displays, in addition to those for receiving TV broadcasts.

[0230] In FIG. 20, the installed lighting device 8100 is an example of an electronic device using the secondary battery 8103 according to one aspect of the present invention. Specifically, the lighting device 8100 includes a housing 8101, a light source 8102, a secondary battery 8103, and the like. In FIG. 20, the case where the secondary battery 8103 is provided inside the ceiling 8104 where the housing 8101 and the light source 8102 are installed is illustrated, but the secondary battery 8103 may be provided inside the housing 8101. The lighting device 8100 can receive power supply from a commercial power source or use the power stored in the secondary battery 8103. Therefore, even when power supply from the commercial power source cannot be received due to a power outage or the like, the secondary battery 8103 according to one aspect of the present invention can be used as an uninterruptible power supply, making it possible to use the lighting device 8100.

[0231] In addition, although FIG. 20 illustrates the installed lighting device 8100 provided on the ceiling 8104, the secondary battery according to one aspect of the present invention can be used for installed lighting devices provided on other than the ceiling 8104, such as the side wall 8105, the floor 8106, the window 8107, etc., or for desktop lighting devices.

[0232] Further, as the light source 8102, an artificial light source that artificially obtains light using power can be used. Specifically, an incandescent lamp, a discharge lamp such as a fluorescent lamp, and a light-emitting element such as an LED or an organic EL element can be cited as an example of the above artificial light source.

[0233] In FIG. 20, an air conditioner having an indoor unit 8200 and an outdoor unit 8204 is an example of an electronic device using a secondary battery 8203 according to an aspect of the present invention. Specifically, the indoor unit 8200 includes a housing 8201, an air outlet 8202, a secondary battery 8203, and the like. In FIG. 20, the case where the secondary battery 8203 is provided in the indoor unit 8200 is illustrated, but the secondary battery 8203 may be provided in the outdoor unit 8204. Alternatively, the secondary battery 8203 may be provided in both the indoor unit 8200 and the outdoor unit 8204. The air conditioner can receive power supply from a commercial power source or use the power stored in the secondary battery 8203. In particular, when the secondary battery 8203 is provided in both the indoor unit 8200 and the outdoor unit 8204, even when power supply from the commercial power source cannot be received due to a power outage or the like, by using the secondary battery 8203 according to an aspect of the present invention as an uninterruptible power supply, the air conditioner can be used.

[0234] Note that in FIG. 20, a separate type air conditioner composed of an indoor unit and an outdoor unit is illustrated, but a secondary battery according to an aspect of the present invention can also be used in an integrated type air conditioner having the functions of the indoor unit and the outdoor unit in one housing.

[0235] In FIG. 20, an electric refrigerator-freezer 8300 is an example of an electronic device using a secondary battery 8304 according to an aspect of the present invention. Specifically, the electric refrigerator-freezer 8300 includes a housing 8301, a refrigerator door 8302, a freezer door 8303, a secondary battery 8304, and the like. In FIG. 20, the secondary battery 8304 is provided inside the housing 8301. The electric refrigerator-freezer 8300 can receive power supply from a commercial power source or use the power stored in the secondary battery 8304. Therefore, even when power supply from the commercial power source cannot be received due to a power outage or the like, by using the secondary battery 8304 according to an aspect of the present invention as an uninterruptible power supply, the electric refrigerator-freezer 8300 can be used.

[0236] Among the electronic devices described above, high-frequency heating devices such as microwave ovens and electronic devices such as electric rice cookers require high power in a short period of time. Therefore, by using the secondary battery according to one aspect of the present invention as an auxiliary power source for supplementing power that cannot be covered by the commercial power supply, it is possible to prevent the breaker of the commercial power supply from tripping when the electronic device is in use.

[0237] In addition, during the time when the electronic device is not in use, particularly during the time when the ratio of the actually used power amount to the total power amount that can be supplied by the commercial power supply source (referred to as the power usage rate) is low, by storing power in the secondary battery, it is possible to suppress an increase in the power usage rate outside the above time period. For example, in the case of the electric refrigerator 8300, at night when the temperature is low and the doors 8302 for the refrigerator compartment and 8303 for the freezer compartment are not opened or closed, power is stored in the secondary battery 8304. Then, during the day when the temperature rises and the doors 8302 for the refrigerator compartment and 8303 for the freezer compartment are opened or closed, by using the secondary battery 8304 as an auxiliary power source, the power usage rate during the day can be kept low.

[0238] According to one aspect of the present invention, the cycle characteristics of the secondary battery are improved, and the reliability can be enhanced. Also, according to one aspect of the present invention, a high-capacity secondary battery can be obtained. Therefore, the characteristics of the secondary battery can be improved, and thus the secondary battery itself can be made smaller and lighter. Therefore, by mounting the secondary battery, which is one aspect of the present invention, on the electronic device described in this embodiment, a more inexpensive and high-performance electronic device can be achieved.

[0239] This embodiment can be implemented in appropriate combination with other embodiments.

[0240] (Embodiment 5) In this embodiment, an example of an electronic device using the secondary battery described in the previous embodiment will be described with reference to FIGS. 21 to 22.

[0241] FIG. 21(A) shows an example of a wearable device. The wearable device uses a secondary battery as a power source. Also, when the user uses it in daily life or outdoors, a wearable device that can perform not only wired charging with an exposed connector part to enhance splash-proof performance, water resistance performance, or dust-proof performance but also wireless charging is desired.

[0242] For example, a secondary battery according to one aspect of the present invention can be mounted on a glasses-type device 4000 as shown in FIG. 21(A). The glasses-type device 4000 has a frame 4000a and a display unit 4000b. By mounting the secondary battery on the temple part of the frame 4000a having a curvature, a lightweight glasses-type device 4000 with good weight balance and a long continuous use time can be obtained. By providing a secondary battery according to one aspect of the present invention, an inexpensive and high-performance glasses-type device 4000 can be realized.

[0243] Also, a secondary battery according to one aspect of the present invention can be mounted on a headset-type device 4001. The headset-type device 4001 has at least a microphone part 4001a, a flexible pipe 4001b, and an earphone part 4001c. The secondary battery can be provided inside the flexible pipe 4001b or inside the earphone part 4001c. By providing a secondary battery according to one aspect of the present invention, an inexpensive and high-performance headset-type device 4001 can be realized.

[0244] Also, a secondary battery according to one aspect of the present invention can be mounted on a device 4002 that can be directly attached to the body. The secondary battery 4002b can be provided inside the thin housing 4002a of the device 4002. By providing a secondary battery according to one aspect of the present invention, an inexpensive and high-performance device 4002 can be realized.

[0245] Furthermore, a secondary battery according to one aspect of the present invention can be mounted on a device 4003 that can be attached to clothing. A secondary battery 4003b can be provided inside a thin housing 4003a of the device 4003. By providing a secondary battery according to one aspect of the present invention, an inexpensive and high-performance device 4003 can be realized.

[0246] Also, a secondary battery according to one aspect of the present invention can be mounted on a belt-type device 4006. The belt-type device 4006 has a belt portion 4006a and a wireless power supply and reception portion 4006b, and a secondary battery can be mounted inside the belt portion 4006a. By providing a secondary battery according to one aspect of the present invention, an inexpensive and high-performance belt-type device 4006 can be realized.

[0247] Also, a secondary battery according to one aspect of the present invention can be mounted on a wristwatch-type device 4005. The wristwatch-type device 4005 has a display portion 4005a and a belt portion 4005b, and a secondary battery can be provided in the display portion 4005a or the belt portion 4005b. By providing a secondary battery according to one aspect of the present invention, an inexpensive and high-performance wristwatch-type device 4005 can be realized.

[0248] The display portion 4005a can display not only the time but also various information such as incoming mails and phone calls.

[0249] Also, since the wristwatch-type device 4005 is a wearable device of a type that is directly wound around the wrist, a sensor for measuring the user's pulse, blood pressure, etc. may be mounted. Data regarding the user's exercise amount and health can be accumulated to manage the health.

[0250] Fig. 21(B) shows a perspective view of the wristwatch-type device 4005 removed from the wrist.

[0251] Further, the side view is shown in Fig. 21(C). Fig. 21(C) shows a state in which the secondary battery 913 is built inside. The secondary battery 913 is the secondary battery shown in Embodiment 3. The secondary battery 913 is provided at a position overlapping the display unit 4005a, and is small and lightweight.

[0252] Fig. 22(A) shows an example of a cleaning robot. The cleaning robot 6300 has a display unit 6302 arranged on the upper surface of the housing 6301, a plurality of cameras 6303 arranged on the side surface, a brush 6304, an operation button 6305, various sensors, and the like. Although not shown, the cleaning robot 6300 is provided with tires, a suction port, and the like. The cleaning robot 6300 can move automatically, detect dust 6310, and suck dust from the suction port provided on the lower surface.

[0253] For example, the cleaning robot 6300 can analyze the image captured by the camera 6303 and determine the presence or absence of obstacles such as walls, furniture, or steps. Further, when an object likely to be entangled with the brush 6304 such as wiring is detected by image analysis, the rotation of the brush 6304 can be stopped. The cleaning robot 6300 includes a secondary battery 6306 according to an aspect of the present invention and a semiconductor device or electronic components inside. By using the secondary battery according to an aspect of the present invention in the cleaning robot 6300, the cleaning robot 6300 can be made an inexpensive and high-performance electronic device.

[0254] Fig. 22(B) shows an example of a robot. The robot 6400 shown in Fig. 22(B) includes a secondary battery 6409, an illuminance sensor 6401, a microphone 6402, an upper camera 6403, a speaker 6404, a display unit 6405, a lower camera 6406, an obstacle sensor 6407, a moving mechanism 6408, an arithmetic unit, and the like.

[0255] The microphone 6402 has a function of detecting the user's voice and environmental sound, etc. Further, the speaker 6404 has a function of emitting sound. The robot 6400 can communicate with the user by using the microphone 6402 and the speaker 6404.

[0256] The display unit 6405 has a function of displaying various information. The robot 6400 can display the information desired by the user on the display unit 6405. The display unit 6405 may be equipped with a touch panel. Further, the display unit 6405 may be a removable information terminal, and by installing it at a fixed position of the robot 6400, charging and data transfer are enabled.

[0257] The upper camera 6403 and the lower camera 6406 have a function of imaging the surroundings of the robot 6400. Further, the obstacle sensor 6407 can detect the presence or absence of an obstacle in the traveling direction when the robot 6400 moves forward using the moving mechanism 6408. The robot 6400 can recognize the surrounding environment and move safely using the upper camera 6403, the lower camera 6406, and the obstacle sensor 6407.

[0258] The robot 6400 includes a secondary battery 6409 according to an aspect of the present invention and a semiconductor device or electronic components inside. By using the secondary battery according to an aspect of the present invention in the robot 6400, the robot 6400 can be made into an inexpensive, high-performance, and highly reliable electronic device.

[0259] Figure 22(C) shows an example of an aircraft. The aircraft 6500 shown in Figure 22(C) has a propeller 6501, a camera 6502, a secondary battery 6503, etc., and has a function of flying autonomously.

[0260] For example, the image data captured by the camera 6502 is stored in the electronic component 6504. The electronic component 6504 can analyze the image data and detect the presence or absence of obstacles when moving. Also, the remaining battery level can be estimated from the change in the storage capacity of the secondary battery 6503 by the electronic component 6504. The flying object 6500 includes a secondary battery 6503 according to one aspect of the present invention inside thereof. The secondary battery according to one aspect of the present invention has good rate characteristics and high output. Therefore, by using it in the flying object 6500, the flying object 6500 with high acceleration performance and the like can be achieved.

[0261] This embodiment can be implemented in appropriate combination with other embodiments.

[0262] (Embodiment 6) In this embodiment, an example of mounting a secondary battery, which is one aspect of the present invention, on a vehicle is shown.

[0263] When a secondary battery is mounted on a vehicle, next-generation clean energy vehicles such as a hybrid vehicle (HV), an electric vehicle (EV), or a plug-in hybrid vehicle (PEV) can be realized.

[0264] In FIG. 23, a vehicle using a secondary battery, which is one aspect of the present invention, is illustrated. The automobile 8400 shown in FIG. 23(A) is an electric vehicle that uses an electric motor as a power source for running. Or, it is a hybrid vehicle that can appropriately select and use an electric motor and an engine as a power source for running. By using one aspect of the present invention, a vehicle with a long cruising range can be realized. Also, the automobile 8400 has a secondary battery. For the secondary battery, the modules of the secondary battery shown in FIGS. 8(C) and 8(D) may be arranged with respect to the floor portion inside the vehicle. Also, a battery pack combining a plurality of secondary batteries shown in FIG. 11 may be installed with respect to the floor portion inside the vehicle. The secondary battery can not only drive the electric motor 8406 but also supply power to a light-emitting device such as a headlight 8401 or a room light (not shown).

[0265] In addition, the secondary battery can supply power to display devices such as speedometers and tachometers of the automobile 8400. Further, the secondary battery can supply power to semiconductor devices such as the navigation system of the automobile 8400.

[0266] The automobile 8500 shown in FIG. 23(B) can be charged by receiving power supply from an external charging facility by a plug-in method, a non-contact power supply method, etc. to the secondary battery of the automobile 8500. FIG. 23(B) shows a state in which charging is being performed from a ground-mounted charging device 8021 to secondary batteries 8024 and 8025 mounted on the automobile 8500 via a cable 8022. At the time of charging, the charging method, the standard of the connector, etc. may be appropriately performed in a predetermined method such as CHAdeMO (registered trademark) or Combo. The charging device 8021 may be a charging station provided in a commercial facility or may be a household power source. For example, by plug-in technology, the secondary batteries 8024 and 8025 mounted on the automobile 8500 can be charged by external power supply. Charging can be performed by converting AC power into DC power via a conversion device such as an AC-DC converter.

[0267] Also, although not shown, a power receiving device can be mounted on the vehicle, and power can be supplied non-contact from a power transmission device on the ground for charging. In the case of this non-contact power supply method, by incorporating a power transmission device into a road or an outer wall, charging can be performed not only while the vehicle is stopped but also while it is running. Further, power transmission and reception may be performed between vehicles using this non-contact power supply method. Furthermore, a solar cell may be provided on the exterior of the vehicle to charge the secondary battery when the vehicle is stopped or running. For such non-contact power supply, an electromagnetic induction method or a magnetic field resonance method can be used.

[0268] Also, FIG. 23(C) is an example of a two-wheeled vehicle using a secondary battery according to an aspect of the present invention. The scooter 8600 shown in FIG. 23(C) includes a secondary battery 8602, a side mirror 8601, and a direction indicator light 8603. The secondary battery 8602 can supply electricity to the direction indicator light 8603.

[0269] In addition, for the scooter 8600 shown in Fig. 23(C), the secondary battery 8602 can be stored in the under-seat storage 8604. Even if the under-seat storage 8604 is small, the secondary battery 8602 can be stored in the under-seat storage 8604. The secondary battery 8602 is removable. During charging, the secondary battery 8602 can be carried indoors for charging and then stored before driving.

[0270] According to one aspect of the present invention, the productivity of a secondary battery with good rate characteristics and high output can be improved. Therefore, by using the secondary battery according to one aspect of the present invention in a vehicle, the vehicle can have improved acceleration performance and the like. In addition, the secondary battery mounted on the vehicle can also be used as a power supply other than the vehicle. In this case, for example, it is possible to avoid using a commercial power supply during the peak of power demand. Avoiding the use of a commercial power supply during the peak of power demand can contribute to energy conservation and reduction of carbon dioxide emissions. In addition, if the cycle characteristics are good, the secondary battery can be used for a long time, so the amount of use of rare metals such as cobalt can be reduced.

[0271] This embodiment can be implemented in appropriate combination with other embodiments.

Example

[0272] In this example, the reduction method of GO was investigated. For this purpose, a GO film for basic evaluation was prepared, which was reduced by several methods and various analyses were performed.

[0273] <Fabrication of GO film> As GO, the one prepared using potassium permanganate and sulfuric acid in the oxidation process by the Modified Hummers method was used. 600 mL of water was added to 200 mL of a dispersion obtained by dispersing 3 wt% of GO in water, and stirring was carried out with a stirrer at 600 rpm for 12 hours to prepare a dispersion A.

[0274] Next, a graphene compound sheet (GO film) was formed by the spray drying method using a GO dispersion as a raw material. Here, the GO film was formed on the wall surface of the chamber of the spray drying apparatus. Details will be described below.

[0275] As the spray drying apparatus, a Mini Spray Dryer B-290 manufactured by BUCHI was used. The inlet temperature was set to 160 °C. It is considered that the vicinity of the nozzle was heated to a temperature of 100 °C or higher. Dispersion A was supplied to the nozzle of the spray drying apparatus at a rate of about 65 mL / min. Dispersion A was supplied into the chamber as a spray together with nitrogen gas having a flow rate of 12 L / min from the nozzle.

[0276] Part of Dispersion A supplied into the chamber as a spray was recovered in the recovery container as GO powder, and part of it was formed into a GO film on the inner wall of the cylindrical chamber.

[0277] Next, the GO film was peeled off from the inner wall of the chamber. A GO film as shown by the arrow in Fig. 24 was obtained. The GO film has a plurality of GO overlapping each other. The average value of the thickness of the GO film was 8.6 μm. This GO film before reduction was designated as Sample 1 (comparative example).

[0278] <Chemical reduction> Next, the GO film was reduced by a chemical method. Ascorbic acid was used as the reducing agent. A 0.078 mol / L L-ascorbic acid solution was prepared, and the GO film was immersed therein. Then, the reaction was carried out at 60 °C for 1 hour (h). This chemically reduced product was designated as Sample 2.

[0279] <Thermal reduction> Next, the GO film was reduced by a thermal method. A glass tube oven was used for heating. The heating temperatures were 100 °C, 120 °C, 150 °C, 170 °C, 200 °C, or 250 °C. The heating time was 10 hours. However, for the heating temperature of 170 °C, samples with a heating time of 1 hour were also prepared. Note that the heating time included the time during temperature rise. The heating rate was about 11 °C / min. All were heated under reduced pressure (about 1 kPa). These thermally reduced samples were designated as Sample 3 to Sample 9.

[0280] Also, after chemical reduction by treating with an ascorbic acid solution at 60 °C for 1 hour, the sample that was thermally reduced at 170 °C for 10 hours was designated as Sample 10.

[0281] The preparation conditions of Sample 1 to Sample 10 are shown in Table 1.

[0282]

Table 1

[0283] <Raman Spectroscopy> Samples 1, 7, and 10 prepared as described above were analyzed using Raman spectroscopy. The laser wavelength was 532 nm, the wavelength dispersion D = 0.6, the pinhole was 100 μm in diameter, and the central wave number of the spectrometer was 2000 cm -1 , the diffraction grating was 150 - 500 nm, the exposure time was 10 seconds, and the number of integrations was 5 times. The samples were fixed to a glass plate with double-sided tape for measurement.

[0284] The Raman spectrum is shown in Fig. 25. The G band (peak derived from sp 2 hybrid orbitals) occurs around 1590 cm -1 . The D band (peak derived from sp 3 hybrid orbitals) occurs around 1350 cm -1 .

[0285] As shown in Fig. 25, in Sample 7 that was only thermally reduced, compared with Sample 1 that was not reduced, the intensity of the G band peak became slightly stronger, and the D band peak broadened. Therefore, sp2 It was suggested that both the hybrid orbitals and the defects increased.

[0286] On the other hand, in Sample 10 where both chemical reduction and thermal reduction were performed, the intensity of the G-band peak became stronger compared to Sample 1. Therefore, it was suggested that the sp 2 orbitals increased.

[0287] The intensity ratio G / D of the G-band and the D-band was 0.936 for Sample 1, 1.06 for Sample 5, and 1.63 for Sample 10.

[0288] <FT-IR and XRD (Reduction Temperature)> Next, Fourier transform infrared spectroscopy (FT-IR) and XRD were used to analyze and compare Sample 1 of the comparative example with Samples 3, 4, 5, 6, and 9 with different thermal reduction temperatures. FT-IR was measured by the total reflection measurement method (ATR method).

[0289] The results of FT-IR are shown in Fig. 26. In FT-IR, the absorption derived from the hydroxy group (O-H) occurs at a wavenumber of 3000 cm -1 or higher and 3600 cm -1 or lower. The absorption derived from the carbonyl group (C=O) occurs at a wavenumber near 1720 cm -1 . The absorption derived from the carbon double bond (C=C) occurs at a wavenumber near 1640 cm -1 . The absorption derived from the bond between carbon and oxygen (C-O) occurs at a wavenumber near 1050 cm -1 . These are shown in gray in Fig. 26.

[0290] As shown in Fig. 26, as the reduction temperature increases, the absorption derived from O-H occurring at a wavenumber of 3000 cm -1 or higher and 3600 cm -1 or lower, and the absorption derived from C-O occurring near a wavenumber of 1050 cm -1 decrease. Therefore, it is suggested that the hydroxy group has detached. Also, from the increase in the absorption derived from C=C occurring near a wavenumber of 1640 cm -1 , an increase in the carbon double bond (C=C) is suggested.

[0291] The results of XRD are shown in Fig. 27. XRD was powder XRD using CuKα1 line and measured in air. The electrode was attached to a silicon non-reflecting plate with grease to maintain flatness. The broad peak at about 2θ = 19° in XRD is the background. Also, graphite has a peak derived from the interlayer distance at around 2θ = 25°.

[0292] As shown in Fig. 27, Sample 1 without thermal reduction and Sample 3 reduced at 100 °C had peaks at about 2θ = 10° to 12°. On the other hand, Samples 4, 5, 6, and 9 thermally reduced at 125 °C or higher had peaks near 2θ = 24°. This is close to the peak derived from the interlayer distance of graphite.

[0293] From these results, it became clear that in the case of thermal reduction, the reduction progresses as the temperature is increased. Also, it became clear that when thermal reduction is carried out at 125 °C or higher, the reduction progresses significantly.

[0294] <Sheet resistance> Next, the surface resistivity of Samples 1, 2, 6, and 10 was measured. The measurement was performed by the four-probe method. The results are shown in Fig. 28.

[0295] As shown in Fig. 28, compared with Sample 1 without reduction treatment, the conductivity of Samples 2, 6, and 10 that had undergone some reduction treatment was improved. Compared with Sample 2 that had undergone chemical reduction, Sample 6 that had undergone thermal reduction had a lower surface resistivity. Therefore, it was suggested that thermal reduction contributes more to reducing the resistance than chemical reduction using ascorbic acid.

[0296] <XRD (Reduction method)> Next, sample 1, and samples 2, 6, and 10 with different reduction methods were analyzed and compared using XRD in the same manner as in Fig. 27. The results are shown in Fig. 29. The broad peak at about 2θ = 19° indicated by an asterisk in Fig. 29 is the background. Also, the peak near 2θ = 25° indicated by a dotted line in Fig. 29 is the peak derived from the interlayer distance of graphite.

[0297] Table 2 shows the results of calculating the distance between carbon sheets of each sample from the peak position of the (002) plane-derived peak in the XRD spectrum shown in Fig. 29 using Bragg's equation. The surface resistivity measured above is also shown.

[0298]

Table 2

[0299] From Fig. 29 and Table 2, it was found that sample 10, which was subjected to both chemical reduction and thermal reduction, had the lowest resistivity and the shortest interlayer distance. Also, sample 6, which was thermally reduced, had a lower resistivity and a shorter interlayer distance than sample 2, which was only chemically reduced. From these results, it became clear that the lower the distance between carbon sheets, the more the resistance is reduced.

[0300] <FT-IR (Reduction Method)> Next, sample 1, and samples 2, 6, and 10 with different reduction methods were analyzed and compared using FT-IR in the same manner as in Fig. 26. The results are shown in Fig. 30. The absorption of each functional group is shown in gray in the figure in the same manner as in Fig. 26.

[0301] As shown in Fig. 30, it was revealed that sample 10, which was subjected to both chemical reduction and thermal reduction, showed absorption near a wave number of 1640 cm -1 and that C=C was generated. Also, samples 6 and 10, which were thermally reduced, showed absorption near a wave number of 1050 cm -1 and near a wave number of 3000 cm -1 and above 3600 cm -1The following absorption has decreased, and it has been revealed that the hydroxy group (-OH) has detached from carbon. Also, in Samples 2 and 10 that underwent chemical reduction, the absorption near the wavenumber 1720 cm -1 has decreased, and it has been revealed that the carbonyl group (C=O) and the carboxy group (-COOH) have been reduced.

[0302] <xps> Next, without using the GO film and leaving it in powder form, samples that were subjected to the same reduction treatment as Sample 1, Sample 2, Sample 8, and Sample 10 were analyzed using XPS. The results are shown in Table 3.

[0303]

Table 3

[0304] As shown in Table 3, in Sample 2 where chemical reduction was performed, since the ratio of C-O decreased, it became clear that the epoxy group or carboxyl group was reduced. Although C-O may also be considered to be derived from the hydroxyl group, considering the results of FT-IR, it is considered that Sample 2 was not sufficiently reduced. Also, the carbon double bond (C=C) increased compared to Sample 7 which was only thermally reduced.

[0305] From the above analysis, it became clear that chemical reduction has a great effect of reducing the carbonyl group (C=O) and carboxyl group (-COOH) in GO by proton addition with a reducing agent. Also, it became clear that thermal reduction has a great effect of reducing the hydroxyl group (-OH) in GO by dehydration. As reduction of GO proceeds, the interlayer distance shrinks. As a result, the conductivity increases.

[0306] Thus, it became clear that by performing both chemical reduction and thermal reduction on GO, reduction can be carried out more efficiently and the conductivity increases. On the other hand, chemical reduction can perform the reduction treatment at a lower temperature compared to thermal reduction. Therefore, chemical reduction is effective when the heat resistance of the material used for the positive electrode is low, etc.

Example

[0307] In this example, a secondary battery using chemically reduced or thermally reduced GO as a conductive material was fabricated and its characteristics were evaluated.

[0308] <Fabrication of Secondary Battery> For evaluation, a coin-type secondary battery of CR2032 type (diameter 20 mm, height 3.2 mm) was fabricated.

[0309] LFP was used as the positive electrode active material of the secondary battery. GO (manufactured by Nippon Kayaku Materials Co., Ltd., using the Modified Hummers method in the oxidation process) was used as the conductive material. This is reduced in a later process. PVDF was used as the binder. The positive electrode active material, conductive material, and binder were mixed so that the ratio was 94.2:0.8:5 (wt%), and a slurry was prepared. NMP was used as the solvent. The slurry was coated on the current collector and dried. An aluminum foil with a carbon undercoat was used as the current collector.

[0310] Next, GO in the positive electrode active material layer was reduced by chemical reduction or thermal reduction.

[0311] L-ascorbic acid was used as the reducing agent for chemical reduction. The solvent was water:NMP = 1:9 (volume ratio), and a 0.078 mol / L L-ascorbic acid solution was prepared. The current collector coated with the positive electrode active material layer was immersed in the ascorbic acid solution and reacted at 60°C for 1 hour, and the product was designated as Sample 11. This is the same reduction condition as Sample 2 of Example 1.

[0312] Also, the product obtained by performing thermal reduction at a heating temperature of 170°C and a heating time of 10 hours was designated as Sample 12. This is the same reduction condition as Sample 6.

[0313] Also, after performing chemical reduction by immersing in a 0.078 mol / L L-ascorbic acid solution and reacting at 60°C for 1 hour, thermal reduction was performed at a heating temperature of 170°C and a heating time of 10 hours, and the product was designated as Sample 13. This is the same reduction condition as Sample 10.

[0314] After each reduction treatment, it was pressed at a linear pressure of 210 kN / m to obtain a positive electrode.

[0315] As comparative examples, samples 14, 15, and 16 were prepared as follows: For sample 14, AB was used as the conductive material, and the ratio of the positive electrode active material: conductive material: binder was 94.2:0.8:5 (by weight). For sample 15, AB was used as the conductive material, and the ratio was 85:10:5 (by weight). For sample 16, graphene (manufactured by Graphene Supermarket, grade A-12) was used as the conductive material, and the ratio was 85:10:5 (by weight). This graphene has not undergone an oxidation process. Samples 14 to 16 were prepared in the same manner as samples 11 to 13 except for the material of the conductive material and the mixing ratio of the positive electrode active material layer.

[0316] Lithium metal was used as the counter electrode.

[0317] As the electrolyte in the electrolytic solution, 1 mol / L lithium hexafluorophosphate (LiPF6) was used, and as the electrolytic solution, a mixture of ethylene carbonate (EC) and diethyl carbonate (DEC) with a volume ratio of EC:DEC = 3:7 was used.

[0318] Polypropylene with a thickness of 25 μm was used as the separator.

[0319] The positive electrode can and the negative electrode can were made of stainless steel (SUS).

[0320] The preparation conditions of samples 11 to 16 are shown in Table 4.

[0321]

Table 4

[0322] <sem> The positive electrode of Sample 13 prepared above was observed with an electron microscope. The surface SEM image is shown in Fig. 31, and the cross-sectional SEM image is shown in Fig. 32(A). Fig. 32(B) shows a tracing with a black line of a part of the reduced GO that is observed as white in Fig. 32(A) for better visibility.

[0323] From Fig. 31, it can be seen that the reduced GO covers a plurality of positive electrode active material particles. From Fig. 32(A) and Fig. 32(B), it can be seen that the reduced GO is not aggregated and is well dispersed in the positive electrode active material layer. Also, it can be said that the reduced GO is distributed in a network pattern or forms a net-like structure. Since it is a net-like structure formed by the reduced GO, it can also be called a graphene net.

[0324] <Battery characteristics> Next, charge-discharge tests were performed on Samples 11 to 16. Charging was carried out by CCCV (0.2C, 4.3V, termination current 0.02C), and discharging was carried out by CC (0.2C, termination voltage 2.0V), and the measurements were made at 25°C. In this example, 1C was taken as 170 mA / g.

[0325] The first discharge curves of Samples 11 to 13 using reduced GO as the conductive material are shown in Fig. 33(A). The first discharge capacities were 55 mAh / g for Sample 11, 156 mAh / g for Sample 12, and 158 mAh / g for Sample 13. In particular, Sample 13 using GO subjected to both chemical reduction and thermal reduction had good discharge characteristics with a wide plateau. It became clear that GO had a lower resistance by combining the reduction methods.

[0326] Next, a cycle test was performed on Sample 13 while changing the discharge rate. Discharging was carried out at 0.2C for cycles 1 to 10, 0.5C for cycles 11 to 20, 1C for cycles 21 to 30, and 0.2C for cycles 31 and 32. Other conditions were the same as those in the above charge-discharge test.

[0327] The discharge capacity of Sample 13 is shown in Fig. 33(B). It was clarified that Sample 13 using GO that had undergone both chemical reduction and thermal reduction exhibited good battery characteristics even when the discharge rate increased.

[0328] Next, the first charge-discharge curves of Samples 13 to 16 are shown in Figs. 34(A) and 34(B). Fig. 34(A) shows the discharge capacity per weight of the active material, and Fig. 34(B) shows the discharge capacity per volume of the positive electrode active material layer.

[0329] Sample 13, which used 0.8 wt% of reduced GO as the conductive material, showed good discharge characteristics even though the amount of the conductive material was much less than that of Sample 15. It was suggested that a sufficient conductive path could be formed within the positive electrode active material layer with 0.8 wt% of reduced GO. The discharge capacity of Sample 13 was 158 mAh / g per weight of the active material and 304 mAh / cm 3 for the positive electrode active material layer volume.

[0330] On the other hand, the discharge capacity of Sample 14, which used 0.8 wt% of AB as the conductive material, was 1 mAh / g or less and 1 mAh / cm 3 for the positive electrode active material layer volume, and it hardly functioned as a battery. It was suggested that a conductive path could not be formed with 0.8 wt% of AB.

[0331] Sample 15, which used 10 wt% of AB as the conductive material, also showed relatively good discharge characteristics, suggesting that a conductive path was formed within the positive electrode active material layer. However, the discharge capacity of Sample 15 was 139 mAh / g per weight of the active material and 201 mAh / cm 3 for the positive electrode active material layer volume. In particular, the discharge capacity per volume was less than 2 / 3 of that of Sample 14. It became clear that when 10 wt% of AB was used as the conductive material, the volume of AB increased, resulting in a decrease in the discharge capacity per weight of the active material and per volume of the positive electrode active material layer.

[0332] Sample 16, which used 10 wt% graphene as the conductive material, also showed good discharge characteristics comparable to those of Sample 14, but it did not reach the level of Sample 13 that used reduced GO. It was shown that reduced GO was better dispersed in a smaller amount than graphene and could efficiently form conductive paths.

[0333] Next, the rate characteristics of Sample 13 and Sample 15 were compared. The discharge rate was changed to 0.2C, 0.5C, 1C, 2C, and 5C for measurement. Other conditions were the same as the charge-discharge test described above.

[0334] The discharge curve of Sample 13 is shown in Fig. 35(A), and the discharge curve of Sample 15 is shown in Fig. 35(B). The discharge capacity of Sample 13 was 158 mAh / g at 0.2C, 153 mAh / g at 0.5C, 149 mAh / g at 1C, 143 mAh / g at 2C, and 130 mAh / g at 5C. On the other hand, the discharge capacity of Sample 15 was 139 mAh / g at 0.2C, 127 mAh / g at 0.5C, 118 mAh / g at 1C, 107 mAh / g at 2C, and 92 mAh / g at 5C.

[0335] Thus, Sample 13 using 0.8 wt% of reduced GO showed better rate characteristics than Sample 15 using 10 wt% of AB. Since reduced GO can make surface contact with the positive electrode active material particles, this property was considered to contribute to the reduction of the internal resistance of the secondary battery.

[0336] As shown in the above examples, it became clear that a secondary battery using reduced GO as the conductive material has good battery characteristics, such as high energy density and high rate characteristics, with a small amount of conductive material.

Examples

[0337] In this example, a secondary battery having reduced GO as the conductive material, a secondary battery having graphene, and a secondary battery having AB were fabricated, and their characteristics were compared.

[0338] <Fabrication of Secondary Battery> For evaluation, a coin-type secondary battery of the CR2032 type (diameter 20 mm, height 3.2 mm) was fabricated.

[0339] For the positive electrode active material of the secondary battery, LFP or lithium nickel cobalt manganese oxide (NCM) was used. As LFP, a material synthesized by a solid-phase method using lithium carbonate, ammonium dihydrogen phosphate, and iron(II) oxalate dihydrate as raw materials was used. As NCM, a material with Ni:Co:Mn = 5:2:3 (atomic ratio) (manufactured by MTI Corporation) was used. This may be denoted as NCM523. Samples 17 to 19 used LFP, and samples 20 to 22 used NCM523.

[0340] As the conductive material, GO or AB was used. GO is reduced in a later process. Samples 17, 20, and 22 used GO. Sample 18 used graphene (manufactured by Graphene Supermarket, grade A-12). Samples 19 and 21 used AB.

[0341] PVDF was used as the binder. The blending ratio of the binder in the sum of the positive electrode active material, conductive material, and binder was 5 wt%.

[0342] The positive electrode active material, conductive material, and binder were mixed to prepare a slurry. NMP was used as the solvent. The slurry was coated on a current collector and dried. As the current collector, an aluminum foil having a carbon undercoat was used.

[0343] Next, samples 17, 20, and 22 using GO as the conductive material were subjected to chemical reduction and thermal reduction.

[0344] L-ascorbic acid was used as the reducing agent for chemical reduction. The solvent was water:NMP = 1:9 (volume ratio), and a 0.078 mol / L L-ascorbic acid solution was prepared. The current collector coated with the positive electrode active material layer was immersed in the ascorbic acid solution and reacted at 60 °C for 1 hour.

[0345] Next, thermal reduction was performed at a heating temperature of 170 °C and a heating time of 10 hours.

[0346] After the reduction treatment, it was pressed at a linear pressure of 210 kN / m to obtain a positive electrode.

[0347] Lithium metal was used as the counter electrode.

[0348] As the electrolyte in the electrolytic solution, 1 mol / L lithium hexafluorophosphate (LiPF6) was used. For the electrolytic solution, a mixture of ethylene carbonate (EC) and diethyl carbonate (DEC) with EC:DEC = 3:7 (volume ratio) was used, to which 2 wt% vinylene carbonate (VC) was added.

[0349] Polypropylene with a thickness of 25 μm was used as the separator.

[0350] Stainless steel (SUS) was used for the positive electrode can and the negative electrode can.

[0351] The production conditions of Samples 17 to 22 are shown in Table 5.

[0352]

Table 5

[0353] <Battery characteristics> Charge-discharge tests were conducted on Samples 17 to 22.

[0354] Fig. 36(A) shows the charge rate characteristics of Samples 17 to 19 at 25°C. Charging was performed at CC (0.2C, 0.5C, 1C, 2C or 5C, cut-off voltage 4.3V), and discharging was performed at CC (0.2C, cut-off voltage 2.0V). In this example, 1C was defined as 170 mA / g. Fig. 36(B) shows the discharge rate characteristics of Samples 17 to 19 at 25°C. Charging was performed at CCCV (0.2C, 4.3V, cut-off current 0.02C), and discharging was performed at CC (0.2C, 1C, 2C, 5C or 10C, cut-off voltage 2.0V).

[0355] The CC charge capacities of Sample 17 using GO reduced as a conductive material were 154.24 mAh / g at 0.2C, 148.77 mAh / g at 0.5C, 143.99 mAh / g at 1C, 138.33 mAh / g at 2C, and 127.92 mAh / g at 5C. On the other hand, the CC charge capacities of Sample 18 using graphene as a conductive material were 115.4 mAh / g at 0.2C, 101.0 mAh / g at 0.5C, 89.35 mAh / g at 1C, 78.30 mAh / g at 2C, and 60.88 mAh / g at 5C. Also, the CC charge capacities of Sample 19 using AB as a conductive material were 120.59 mAh / g at 0.2C, 107.79 mAh / g at 0.5C, 99.18 mAh / g at 1C, 89.76 mAh / g at 2C, and 76.17 mAh / g at 5C.

[0356] Fig. 37(A) shows the charge rate characteristics of Sample 17 and Sample 19 at 0°C. Fig. 37(B) shows the discharge rate characteristics of Sample 17 and Sample 19 at 0°C. The charge-discharge conditions were the same as those in Fig. 36(A) and Fig. 36(B).

[0357] As shown in Table 5, Fig. 36(A) and Fig. 36(B), Fig. 37(A) and Fig. 37(B), Sample 17 using GO reduced as a conductive material showed extremely good high-rate characteristics, even though the weight ratio of the conductive material was much less than that of Sample 18 using graphene and Sample 19 using AB. At 25°C and under the conditions of 0.5C to 5C, it showed a charge capacity more than twice that of Sample 19.

[0358] Thus, in the secondary battery using LFP as the positive electrode active material, it was revealed that by using GO reduced as a conductive material, the high-speed charging and high-power discharge characteristics were improved. This is considered to be because the reduced GO forms a good conductive path in the positive electrode active material layer. Also, it was revealed that using GO reduced rather than graphene improved the high-power discharge characteristics. This is considered to be because the dispersibility of GO reduced is extremely high compared to graphene.

[0359] Figure 38(A) shows the charge rate characteristics of Sample 20 and Sample 21 at 25°C. Figure 38(B) shows the discharge rate characteristics of Sample 20 and Sample 21 at 25°C. The charge and discharge conditions were the same as those in Figure 36.

[0360] As shown in Figure 38(A), even when the positive electrode active material is NCM523, Sample 20 using reduced GO as the conductive material showed good characteristics especially in high-rate charging, despite the low weight ratio of the conductive material. This is presumably because the reduced GO forms a good conductive path in the positive electrode active material layer.

[0361] Figure 39(A) shows the cycle characteristics of Sample 21 and Sample 22 at 25°C. Figure 39(B) shows the cycle characteristics of Sample 21 and Sample 22 at 45°C. Figure 39(C) shows the cycle characteristics of Sample 21 and Sample 22 at 60°C. Charging was CC (0.2C, end voltage 4.3V), and discharging was CC (0.2C, end voltage 2.0V).

[0362] As shown in Figures 39(A) to 39(C), especially at 60°C, the capacity of Sample 21 using AB as the conductive material decreased significantly as the cycles progressed, while Sample 22 using reduced GO as the conductive material showed extremely good cycle characteristics. Sample 22 also showed better cycle characteristics than Sample 21 at other temperatures.

[0363] Figure 40(A) shows the charge rate characteristics of Sample 21 and Sample 22 at 0°C. Charging was CCCV (0.5C, 1C, 2C, 5C or 10C, 4.3V, end current 0.02C), and discharging was CC (0.2C, end voltage 2.0V).

[0364] Figure 40(B) shows the discharge rate characteristics of Sample 21 and Sample 22 at 0°C. Charging was CCCV (0.2C, 4.3V, end current 0.02C), and discharging was CC (0.5C, 1C, 2C, 5C or 10C, end voltage 2.0V).

[0365] Figure 41(A) shows the charge curves at 1C at 0°C for Sample 21 and Sample 22. Figure 41(B) shows the charge characteristics at 0.2C at 0°C for Sample 21 and Sample 22.

[0366] As shown in FIGS. 40(A) and 41(A), at 0°C, Sample 21 using AB ended charging immediately due to voltage drop and had an extremely low charge capacity. On the other hand, Sample 22 using reduced GO showed good charge capacity even at 2C.

[0367] Also, as shown in FIGS. 40(B) and 41(B), Sample 22 showed better characteristics in terms of the discharge rate.

[0368] Thus, it was shown that the secondary battery using reduced GO had good charge rate and discharge rate characteristics even at a low temperature of 0°C.

Example

[0369] In this example, a secondary battery having a positive electrode with reduced GO as a conductive material and reduced polysaccharide as a binder was fabricated, and its characteristics were evaluated.

[0370] <Fabrication of positive electrode> Potato starch was used as the polysaccharide for the binder. Similar to Steps S11 and S12a in FIG. 1, the binder and the solvent were mixed.

[0371] 1 g of starch was dispersed in 9 g of water and mixed while heating to 100°C in a SUS container. This was made into a 10 wt% starch paste prepared by Method 1. This was used for Sample 23 and Sample 24.

[0372] Also, 1 g of starch and 9 g of water were weighed and placed in a container for stirring, covered, and heated in a hot water bath at 80°C for 5 minutes. This was made into a 10 wt% starch paste prepared by Method 2. This was used for Samples 25 to 30.

[0373] Next, in the same manner as in step S12b, 10 wt% starch paste and a conductive material were mixed. As the conductive material, powdered GO (manufactured by Graphenea) or amine-modified GO (manufactured by Graphenea) was used. GO was used for Samples 23, 24, 26 to 28. Amine-modified GO was used for Sample 25. Also, samples using AB as a comparative example were prepared as Samples 29 and 30. They were stirred at 2000 rpm for 3 minutes using a planetary mixer (Avataro Rentaro, manufactured by THINKY).

[0374] Next, in the same manner as in step S12c, a cathode active material was mixed into the mixture of 10 wt% starch paste and GO. As the cathode active material, LFP without carbon coating was used. Specifically, commercially available LFP (manufactured by ATR) or LFP synthesized by the solid-phase method was used. As the LFP synthesized by the solid-phase method, that synthesized by the solid-phase method using lithium carbonate, ammonium dihydrogen phosphate, and iron(II) oxalate dihydrate as raw materials was used. Stirring at 2000 rpm for 3 minutes was repeated 5 times using a planetary mixer. Water was appropriately added for viscosity adjustment. This was made into a slurry (step S13).

[0375] Next, the slurry was coated on the current collector (step S14). As the current collector, aluminum foil having a carbon undercoat was used.

[0376] After coating, the slurry was dried at 80°C (step S15). Then, the current collector and the slurry were punched out for a coin-type secondary battery of CR2032 type (diameter 20 mm, height 3.2 mm).

[0377] Next, reduction treatment was performed (step S16). As the reduction treatment, only thermal reduction or thermal reduction after chemical reduction was performed. For the heating of thermal reduction, a glass tube oven was used and it was performed for 10 hours under reduced pressure (about 1 kPa). The heating time included the time during temperature rise. The rate of temperature rise was about 11°C / min. The heating temperature was 170°C, 200°C, 250°C, or 300°C.

[0378] Ascorbic acid was used as the reducing agent for chemical reduction. Ethanol was used as the solvent. A 0.078 mol / L ethanol solution of L-ascorbic acid was prepared, and the current collector and the slurry were immersed therein. Then, the reaction was carried out at 60 °C for 1 hour.

[0379] Sample 27 was subjected to thermal reduction after chemical reduction. The heating time for thermal reduction was 170 °C. Samples 23 to 26 and samples 28 to 30 were only subjected to thermal reduction. The heating temperatures were 300 °C for samples 23 and 24, 250 °C for sample 25, 200 °C for sample 26, 250 °C for samples 28 and 29, and 200 °C for sample 30.

[0380] The above-mentioned reduced product was used as the positive electrode (step S18).

[0381] The preparation conditions of samples 23 to 30 are shown in Table 6. The mixing amount of starch as the binder is indicated by the weight of starch contained in the starch paste.

[0382]

Table 6

[0383] <Fabrication of Secondary Battery> Using the positive electrodes of the above-mentioned samples 23 to 30, a coin-type secondary battery of CR2032 type (diameter 20 mm, height 3.2 mm) was fabricated.

[0384] Lithium metal was used as the counter electrode.

[0385] As the electrolyte of the electrolytic solution, 1 mol / L lithium hexafluorophosphate (LiPF6) was used, and as the electrolytic solution, a mixture of ethylene carbonate (EC) and diethyl carbonate (DEC) with EC:DEC = 3:7 (volume ratio) was used.

[0386] A 25-μm-thick polypropylene was used as the separator.

[0387] For the positive electrode can and the negative electrode can, those made of stainless steel (SUS) were used.

[0388] <sem> The positive electrode of Sample 23 prepared above was observed with an electron microscope. The surface SEM images are shown in FIGS. 42(A) and 42(B). It can be seen from FIG. 42(A) that the reduced GO is well dispersed in the positive electrode active material layer. Also, it can be seen from FIG. 42(B) that the reduced GO covers a plurality of positive electrode active material particles.

[0389] <Battery characteristics and charge-discharge cycle characteristics> Next, charge-discharge tests were conducted on Samples 23 to 30. Charging was performed in CCCV mode (0.5C, 4.3V, termination current 0.05C), and discharging was performed in CC mode (0.5C, termination voltage 2.5V), and the measurements were taken at 25°C. In this example, 1C was defined as 170 mA / g.

[0390] The charge-discharge curves of Samples 23 for cycles 1 to 50 are shown in FIG. 43(A), and the discharge energy retention rate as the charge-discharge cycle characteristic is shown in FIG. 43(B). It was revealed that the positive electrode using starch as a binder and GO as a conductive material and thermally reduced at 300°C can be sufficiently charged and discharged.

[0391] The charge-discharge cycle characteristics of Samples 23 and 24 are shown in FIG. 44. Sample 23 using GO as a conductive material could be charged and discharged. However, almost no discharge capacity was obtained for Sample 24 using amine-modified GO as a conductive material. It is considered that this is because amine-modified GO has worse dispersibility in starch paste than GO, and a sufficient conductive path could not be formed even after reduction.

[0392] The charge-discharge cycle characteristics of Samples 25 to 27 are shown in Fig. 45. Sample 25, which was thermally reduced at 250°C, showed better cycle characteristics than Sample 26, which was thermally reduced at 200°C. Sample 27, which was thermally reduced at 170°C after chemical reduction, was also fully charge-dischargeable. On the other hand, thermal reduction at a high temperature such as 250°C or higher may reduce the strength of the positive electrode active material layer depending on the binder and the material of the positive electrode active material. Therefore, thermal reduction at a relatively low temperature of 200°C or lower may also be effective depending on the binder and the material of the positive electrode active material. In addition, the strength of the positive electrode active material layer may be increased by leaving a part of the conductive material that is not carbonized.

[0393] The charge-discharge curves of Sample 28 for 1 to 8 cycles are shown in Fig. 46(A), and the discharge energy retention rate as the charge-discharge cycle characteristic is shown in Fig. 46(B). The loading amount of Sample 28 was 4.57 mg / cm 2 Here, the loading amount refers to the weight of the positive electrode active material per unit area. In this example, etc., it was calculated by the weight of the positive electrode active material layer after reduction / the mixing ratio of the positive electrode active material.

[0394] The charge-discharge curves of Sample 29 for 1 to 28 cycles are shown in Fig. 47(A), and the discharge energy retention rate as the charge-discharge cycle characteristic is shown in Fig. 47(B). The loading amount of Sample 29 was 1.64 mg / cm 2 It was.

[0395] The charge-discharge curves of Sample 30 for 1 to 31 cycles are shown in Fig. 48(A), and the discharge energy retention rate as the charge-discharge cycle characteristic is shown in Fig. 48(B). The loading amount of Sample 30 was 1.65 mg / cm 2 It was.

[0396] Samples 28 to 30 were all fully charge-dischargeable. However, for the positive electrode using AB as the conductive material and starch as the binder, after coating the slurry, the positive electrode active material layer was very likely to peel off from the current collector, and it was difficult to fabricate the positive electrode with a loading amount similar to that of Sample 28. Therefore, Samples 29 and 30 were able to be fabricated and charge-discharged only by significantly reducing the loading amount.

[0397] On the other hand, Sample 28 using GO as the conductive material formed sufficient conductive paths even in small amounts, and the strength of the positive electrode active material layer was also good.

[0398] The initial discharge capacities of Sample 28 and Sample 29 are shown in Fig. 49. The discharge capacity per unit volume was 198.0 mAh / cm³ for Sample 28 using GO as the conductive material 3 and 158.6 mAh / cm³ for Sample 29 using AB. 3 As shown in Fig. 49, Sample 28 had a wide plateau, a large discharge capacity, and exhibited good discharge characteristics.

[0399] Thus, the secondary battery using GO as the conductive material and starch as the binder was superior to the secondary battery using AB as the conductive material in terms of the strength of the positive electrode active material layer, discharge characteristics, etc.

Description of Reference Numerals

[0400] 100 Positive electrode active material 200 Active material layer 201 Graphene and graphene compounds< / sem> < / sem> < / xps>

Claims

1. A method for manufacturing a positive electrode for a secondary battery, comprising: a step of mixing graphene oxide, a binder having starch, and a positive electrode active material using a solvent containing water to prepare a slurry; a step of coating the slurry on a positive electrode current collector; a step of reducing the graphene oxide; and the step of reducing the graphene oxide includes performing thermal reduction after chemical reduction, the method for manufacturing a positive electrode for a secondary battery.

2. In Claim 1, the chemical reduction is a step of immersing in a reducing agent solution, and the thermal reduction is a step of heating at 125°C or higher and 200°C or lower for 1 hour or longer and 20 hours or shorter, the method for manufacturing a positive electrode for a secondary battery.

3. In Claim 2, the reducing agent solution is an ascorbic acid solution, the method for manufacturing a positive electrode for a secondary battery.

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