Lithium ion secondary battery, negative electrode for lithium ion secondary battery, wound body for lithium ion secondary battery, and method for manufacturing lithium ion secondary battery

Abstract

A lithium ion secondary battery 100 according to the present disclosure comprises: a negative electrode that has a negative electrode mixture layer 32b; a separator 33; and a positive electrode that has a positive electrode mixture layer 34b. The separator 33 is disposed between the negative electrode and the positive electrode. The negative electrode mixture layer 32b contains a carbon-based material and a silicon-based material. A coating layer (32p, 34p) is disposed between the negative electrode mixture layer 32b and the separator 33 and / or between the separator 33 and the positive electrode mixture layer 34b. Thus, provided is a highly reliable lithium ion secondary battery in which, when a wound group of the lithium ion secondary battery is hot-pressed, damage to the separator by the silicon-based material used as a negative electrode active material can be suppressed.

Description

Lithium-ion secondary battery, negative electrode for lithium-ion secondary battery, winding body for lithium-ion secondary battery, method for manufacturing a lithium-ion secondary battery

[0001] This disclosure relates to a lithium-ion secondary battery, a negative electrode for a lithium-ion secondary battery, a wound body for a lithium-ion secondary battery, and a method for manufacturing a lithium-ion secondary battery.

[0002] In the automotive industry, fuel efficiency and environmental regulations are being strengthened in various countries and regions. To comply with these regulations, the development of electric vehicles (EVs) powered by batteries and fuel cell vehicles (fuel cell vehicles) that do not emit carbon dioxide is attracting attention. However, electric vehicles have problems such as insufficient charging infrastructure and long charging times compared to refueling, while fuel cell vehicles have problems such as the enormous cost required to develop hydrogen station infrastructure and the high cost of fuel cells. For this reason, xEVs (EVs (Electric Vehicles), PHEVs (Plug-in Hybrid Electric Vehicles), HEVs (Hybrid Electric Vehicles), etc.) that use both internal combustion engines and batteries as power sources and have low carbon dioxide emissions are becoming a strong candidate to meet fuel efficiency and environmental regulations.

[0003] xEVs typically use lithium-ion rechargeable batteries.

[0004] Patent Document 1 discloses a laminated battery comprising a plurality of laminates and an electrolyte, wherein the laminated structure positioned vertically upward has a first separator, and the laminated structure positioned vertically downward has a second separator, and the permeability coefficient of the electrolyte in the first separator is smaller than that of the second separator, and the air permeability of the first separator is larger than that of the second separator. Patent Document 1 also discloses that the negative electrode active material may include silicon, silicon oxide, silicon-based alloys, etc.

[0005] Japanese Patent Publication No. 2023-135289

[0006] In lithium-ion secondary batteries, to improve input / output characteristics, the negative electrode mixture layer is made multilayered, and SiC is added to the graphite in the upper layer of the multilayered negative electrode mixture layer that is on the positive electrode mixture layer side.

[0007] In the manufacturing process of such lithium-ion secondary batteries, the electrode coils are compressed (pressed) in the thickness direction before being placed inside the battery container. This reduces the thickness of the coils to less than or equal to the internal dimensions of the battery container, making them easier to insert into the battery container.

[0008] However, silicon carbide (SiC) contained in the upper layer of the negative electrode mixture is a sharper and harder particle than graphite. Therefore, in the manufacturing process of lithium-ion secondary batteries, when the coil is compressed in the thickness direction, the negative electrode, separator, and positive electrode are compressed in a direction that brings them closer together. This requires suppressing physical and electrical effects such as damage to the separator or positive electrode caused by the SiC contained in the upper layer of the negative electrode mixture.

[0009] Patent Document 1 discloses the use of silicon-containing compounds (silicon-based materials) as the negative electrode active material. However, it does not disclose any means to suppress separator damage or short circuits between the positive and negative electrodes when silicon-based materials are used.

[0010] This disclosure aims to provide a highly reliable lithium-ion secondary battery by suppressing damage to the separator caused by silicon-based materials used as the negative electrode active material when applying heat pressing to the winding group of the lithium-ion secondary battery.

[0011] The lithium-ion secondary battery of this disclosure comprises a negative electrode having a negative electrode mixture layer, a separator, and a positive electrode having a positive electrode mixture layer. The separator is disposed between the negative electrode and the positive electrode. The negative electrode mixture layer includes a carbon-based material and a silicon-based material. A coating layer is disposed between the negative electrode mixture layer and the separator, and between the separator and the positive electrode mixture layer, at least one of these.

[0012] According to this disclosure, when applying heat pressing to the windings of a lithium-ion secondary battery, it is possible to suppress damage to the separator by the silicon-based material used as the negative electrode active material, thereby providing a highly reliable lithium-ion secondary battery.

[0013] This is an external perspective view of a lithium-ion secondary battery according to an embodiment. This is an exploded perspective view of a lithium-ion secondary battery according to an embodiment. This is an exploded perspective view showing a part of the winding group according to an embodiment unfolded. This is a schematic cross-sectional view showing the stacked state of the positive electrode, separator, and negative electrode according to an embodiment. This is a schematic cross-sectional view showing the stacked state of the positive electrode, separator, and negative electrode according to an embodiment. This is a schematic cross-sectional view showing the stacked state of the positive electrode, separator, and negative electrode according to an embodiment.

[0014] Embodiments will be described below with reference to the drawings as appropriate. The following description provides specific examples of the contents of this disclosure, and this disclosure is not limited to these descriptions. Various changes and modifications are possible by those skilled in the art within the scope of the technical ideas disclosed herein. In addition, in all the drawings used to illustrate this disclosure, components having the same function are denoted by the same reference numerals, and repeated descriptions may be omitted. Also, the dimensional ratios in the drawings may differ from the actual ratios for the sake of explanation, and some components may be omitted from the drawings. Furthermore, in this application, numerical ranges represented by the symbol "~" include the numerical values ​​written before and after the symbol "~" as the lower limit and upper limit, respectively. In addition, in the following description, "particle diameter" is used to mean the average particle diameter. The particle diameter is calculated based on the area of ​​each particle measured in an image such as a scanning electron microscope (SEM).

[0015] Figure 1 is an external perspective view of a lithium-ion secondary battery according to an embodiment.

[0016] Figure 2 is an exploded perspective view of a lithium-ion secondary battery according to this embodiment.

[0017] The lithium-ion secondary battery 100 shown in Figures 1 and 2 comprises a battery case 1 and a battery cover 6. As shown in Figure 2, the battery case 1 has a rectangular bottom surface 1d, sides including a pair of relatively large opposing wide sides 1b and a pair of relatively small opposing narrow sides 1c rising from the bottom surface 1d, and an opening 1a that opens upward at the upper ends of the wide sides 1b and narrow sides 1c. Upward refers to the Z direction as shown in Figures 1 and 2.

[0018] The opening 1a of the battery can 1 is sealed by the battery cover 6. The battery cover 6 is a roughly rectangular flat plate and is welded to the battery can 1 so as to close the opening 1a of the battery can 1, thereby sealing the battery can 1.

[0019] A gas release valve 10 is integrally provided on the battery cover 6. When the pressure inside the battery case 1 rises, the gas release valve 10 opens, releasing gas from inside the battery case 1 and reducing the pressure inside the battery case 1. This ensures the safety of the lithium-ion secondary battery 100.

[0020] The battery cover 6 has a liquid injection port 9 for injecting electrolyte into the battery case 1. After the electrolyte is injected into the battery case 1, the liquid injection port 9 is sealed by a liquid injection plug 11. The liquid injection plug 11 is joined to the battery cover 6 by laser welding to seal the liquid injection port 9 and seal the lithium-ion secondary battery 100.

[0021] The battery cover 6 is further provided with a positive electrode through-hole 46 and a negative electrode through-hole 26.

[0022] A positive external terminal 14 and a negative external terminal 12 are provided above the battery cover 6. A positive current collector plate 44 and a negative current collector plate 24 are provided below the battery cover 6 and inside the battery can 1.

[0023] Examples of materials for forming the positive electrode external terminal 14 and the positive electrode current collector plate 44 include aluminum alloy, while examples of materials for forming the negative electrode external terminal 12 and the negative electrode current collector plate 24 include copper alloy.

[0024] The positive electrode external terminal 14 and the negative electrode external terminal 12 each have a welded joint to which a bus bar or the like is welded and joined. The welded joint has a rectangular parallelepiped block shape protruding upward from the battery cover 6. The lower surface of the welded joint faces the surface of the battery cover 6, and the upper surface of the welded joint is located at a predetermined height and is substantially parallel to the battery cover 6.

[0025] The positive electrode current collector plate has a rectangular plate-shaped positive electrode current collector base 41 facing the lower surface of the battery cover 6, and a positive electrode side connection end 42 extending from the side end of the positive electrode current collector base 41 toward the bottom surface 1d side along the wide side surface 1b of the battery can 1. Similarly, the negative electrode current collector plate 24 has a rectangular plate-shaped negative electrode current collector base 21 facing the lower surface of the battery cover 6, and a negative electrode side connection end 22 extending from the side end of the negative electrode current collector base 21 toward the bottom surface 1d side along the wide side surface 1b of the battery can 1. A positive electrode side opening hole 43 and a negative electrode side opening hole 23 are respectively formed in the positive electrode current collector base 41 and the negative electrode current collector base 21.

[0026] A positive electrode connection portion 14a and a negative electrode connection portion 12a are provided so as to respectively protrude from the lower surfaces of the positive electrode external terminal 14 and the negative electrode external terminal 12. The positive electrode connection portion 14a and the negative electrode connection portion 12a are each formed integrally with the positive electrode external terminal 14 and the negative electrode external terminal 12, respectively.

[0027] The positive electrode connection portion 14a has a cylindrical shape that can be inserted into the positive electrode side through hole 46 of the battery cover 6 and the positive electrode side opening hole 43 of the positive electrode current collector base 41. Similarly, the negative electrode connection portion 12a has a cylindrical shape that can be inserted into the negative electrode side through hole 26 of the battery cover 6 and the negative electrode side opening hole 23 of the negative electrode current collector base 21. The positive electrode connection portion 14a penetrates the battery cover 6 and the positive electrode current collector base 41 through the positive electrode side through hole 46 of the battery cover 6 and the positive electrode side opening hole 43 of the positive electrode current collector base 41. The positive electrode external terminal 14 and the positive electrode current collector plate 44 are electrically connected to each other via the positive electrode connection portion 14a and are fixed to the battery cover 6. Similarly, the negative electrode connection portion 12a penetrates the battery cover 6 and the negative electrode current collector base 21 through the negative electrode side through hole 26 of the battery cover 6 and the negative electrode side opening hole 23 of the negative electrode current collector base 21. The negative electrode external terminal 12 and the negative electrode current collector plate 24 are electrically connected to each other via the negative electrode connection portion 12a and are fixed to the battery cover 6.

[0028] The positive electrode external terminal 14 is electrically connected to a winding group 3 (wound body) described later via a positive electrode connection portion 14a and a positive electrode current collector plate 44. Similarly, the negative electrode external terminal 12 is electrically connected to the winding group 3 via a negative electrode connection portion 12a and a negative electrode current collector plate 24. When charging the lithium ion secondary battery 100, electricity is supplied from an external power source to the winding group 3 via the positive electrode external terminal 14, the positive electrode connection portion 14a, the positive electrode current collector plate 44, as well as the negative electrode external terminal 12, the negative electrode connection portion 12a, and the negative electrode current collector plate 24. When discharging the lithium ion secondary battery 100, electricity is supplied from the winding group 3 to an external load via the positive electrode external terminal 14, the positive electrode connection portion 14a, the positive electrode current collector plate 44, as well as the negative electrode external terminal 12, the negative electrode connection portion 12a, and the negative electrode current collector plate 24.

[0029] In order to electrically insulate the positive electrode current collector plate 44, the negative electrode current collector plate 24, the positive electrode external terminal 14, and the negative electrode external terminal 12 from the battery cover 6 respectively, a gasket 5 is provided between each of the positive electrode external terminal 14, the negative electrode external terminal 12 and the battery cover 6, and an insulating plate 7 is provided between each of the positive electrode current collector plate 44, the negative electrode current collector plate 24 and the battery cover 6. Examples of materials for the insulating plate 7 and the gasket 5 include insulating resin materials such as polybutylene terephthalate, polyphenylene sulfide, and perfluoroalkoxy fluororesin.

[0030] The winding group 3 and an electrolyte solution are housed in the battery can 1. The winding group 3 is subjected to hot pressing before being housed in the battery can 1, thereby improving the energy density of the lithium ion secondary battery 100. The electrolyte solution is injected into the battery can 1 through a liquid injection port 9. Here, the hot pressing refers to a process of pressing and heating the wide side surface of the winding group 3.

[0031] Fig. 3 is an exploded perspective view showing an expanded state of a part of the winding group according to the embodiment.

[0032] As shown in the figure, the winding group 3 has a negative electrode 32, a positive electrode 34, and two separators 33 and 35. The separators 35, negative electrode 32, separator 33, and positive electrode 34 are stacked in this order and wound in a flattened shape. Separator 35 is located on the outermost periphery of the winding group 3, and the negative electrode 32 is located inside it. The two separators 33 and 35 electrically insulate the positive electrode 34 and the negative electrode 32.

[0033] As shown in this figure, the winding group 3 has a pair of opposing end faces 3a and 3b perpendicular to the winding axis, and a side surface 3c located between the pair of end faces 3a and 3b. The side surface 3c has a pair of opposing curved portions with a semicircular cross-section, and a flat portion (wide side surface) continuously formed between these curved portions. The winding group 3 is arranged inside the battery can 1 such that the flat portion of the side surface 3c and the wide side surface 1b (Figure 2) of the battery can 1 are substantially parallel.

[0034] The following describes the positive electrode, negative electrode, and electrolyte of a lithium-ion secondary battery. Note that the "positive electrode mixture layer" corresponds to the "positive electrode active material layer" as defined in this disclosure, and the "negative electrode mixture layer" corresponds to the "negative electrode active material layer" as defined in this disclosure.

[0035] <Positive Electrode> As shown in Figure 3, the positive electrode 34 has a positive electrode current collector 34a and a positive electrode mixture layer 34b (positive electrode active material layer) provided on at least one side of the positive electrode current collector 34a. It is preferable that the positive electrode mixture layer 34b be provided on both sides of the positive electrode current collector 34a.

[0036] The positive electrode current collector 34a is formed from any material that has high conductivity and does not alloy with lithium ions. The positive electrode current collector 34a may have a plate-like (sheet-like) shape. For example, a positive electrode foil can be used as the positive electrode current collector 34a, and specifically, a metal foil such as aluminum foil can be used. At one end of the positive electrode current collector 34a in the width direction, a positive electrode current collector exposed portion 34c is provided, which is the portion not covered by the positive electrode mixture layer 34b. The positive electrode current collector exposed portion 34c is provided on and near the end face 3a of the winding group 3. The positive electrode current collector exposed portion 34c faces and is electrically connected to the positive electrode side connection end 42 (Figure 2) of the positive electrode current collector plate 44.

[0037] The positive electrode mixture layer 34b contains a positive electrode active material. The positive electrode active material contains, for example, a ternary material including at least nickel, cobalt and manganese, and specifically, a composite oxide of nickel, cobalt and manganese can be used. Here, the positive electrode active material may contain aluminum instead of manganese.

[0038] As the positive electrode active material, it is preferable to use a ternary lithium-containing composite oxide represented by the following general composition formula (1).

[0039] Li １＋Ｘ M Ａ O ２     ...(1) (wherein X satisfies -0.15≦X≦0.15, M Ａ represents an element group including at least one selected from the group consisting of Mn and Al, Ni, and Co.) The ternary lithium-containing composite oxide represented by the general composition formula (1) has high thermal stability and high stability in a high potential state, and by applying the oxide, the safety and various battery characteristics of a lithium ion secondary battery can be improved.

[0040] Further, as the positive electrode active material, one containing iron and phosphorus may be used. In this case, olivine-type lithium iron phosphate (LiFePO ４ ) non-oxides such as the above are desirable.

[0041] For example, an iron phosphate-based lithium-containing composite oxide represented by the following general composition formula (2) may be used.

[0042] Li Ｙ Fe １－Ｚ M Ｂ Ｚ PO ４     ...(2) (wherein Y and Z satisfy 0.90≦Y≦1.1 and 0≦Z≦0.10, M Ｂ represents an element group including at least one selected from the group consisting of Mn, Ni and Co.) Note that the lithium-containing composite oxide may contain additional elements not described in the general composition formulas (1) and (2) above.

[0043] The positive electrode mixture layer 34b contains at least one of a conductive additive and a binder (adhesive). Preferably, it contains both.

[0044] The conductive additive (conductive agent) is not particularly limited, but for example, carbon-based materials can be used. The carbon-based material may be crystalline carbon, amorphous carbon, or a mixture thereof. Examples of crystalline carbon include artificial graphite, natural graphite (e.g., flake graphite), carbon nanotubes, or mixtures thereof. Examples of amorphous carbon include carbon black (e.g., acetylene black, Ketjen black, channel black, furnace black, lamp black, or thermal black, or mixtures thereof).

[0045] The binder is not particularly limited, but examples include polyvinylidene fluoride (PVdF), polytetrafluoroethylene (PTFE), polyethylene, polystyrene, polybutadiene, polyacrylonitrile, polyvinyl fluoride, polypropylene fluoride, polychloroprene fluoride, butyl rubber, nitrile rubber, styrene-butadiene rubber (SBR), polysulfur rubber, nitrocellulose, cyanoethylcellulose, various latexes, acrylic resins, polyamide-imide or polyimide, or mixtures thereof.

[0046] The positive electrode 34 can be formed, for example, as follows.

[0047] A positive electrode active material, and optionally at least one of a conductive additive and a binder, are dispersed in a solvent (e.g., N-methyl-2-pyrrolidone (NMP), water) to prepare a paste-like or slurry-like positive electrode mixture composition. This positive electrode mixture composition is applied to the surface (one or both sides) of the positive electrode current collector 34a, dried, and calendered as necessary to form a positive electrode mixture layer 34b. This yields the positive electrode 34. However, the positive electrode is not limited to that formed by the above method and may be formed by other methods. The thickness of the positive electrode active material layer is preferably 30 μm to 150 μm from the viewpoint of producing a high-quality film while thinning it to accommodate high power output. It is even more preferably 30 μm to 65 μm.

[0048] <Negative Electrode> The negative electrode 32 shown in Figure 3 comprises a negative electrode current collector 32a and a negative electrode mixture layer 32b (negative electrode active material layer) provided on at least one side of the negative electrode current collector 32a. It is preferable that the negative electrode mixture layer 32b be provided on both sides of the negative electrode current collector 32a.

[0049] The negative electrode current collector 32a is formed from any material that has high conductivity and does not alloy with lithium ions. At one end of the negative electrode current collector 32a in the width direction, there is a negative electrode current collector exposed portion 32c, which is the portion not covered by the negative electrode mixture layer 32b. The negative electrode current collector exposed portion 32c is provided on and near the end face 3b of the winding group 3. The negative electrode current collector exposed portion 32c faces the negative electrode side connection end 22 (Figure 2) of the negative electrode current collector plate 24 and is electrically connected to it.

[0050] Preferably, the portion of the negative electrode 32 coated with the negative electrode mixture layer 32b is wider in the width direction than the portion of the positive electrode 34 coated with the positive electrode mixture layer 34b, so that the portion coated with the positive electrode mixture layer 34b is sandwiched between the portions coated with the negative electrode mixture layer 32b. Preferably, the exposed positive electrode current collector portion 34c and the exposed negative electrode current collector portion 32c are bundled together at their planar portions and connected by welding or the like. The separators 33 and 35 are wider in the width direction than the portion coated with the negative electrode mixture layer 32b, but since they are wound around the exposed current collectors at the ends of the exposed positive electrode current collector portion 34c and the exposed negative electrode current collector portion 32c, they do not pose an obstacle when bundling and welding.

[0051] The negative electrode mixture layer 32b contains graphite particles and silicon-based material particles (silicon-based particles (Si-based material)) as negative electrode active material. The negative electrode active material is not particularly limited as long as it contains graphite particles and silicon-based particles (Si-based particles).

[0052] Furthermore, the negative electrode active material layer may contain amorphous carbon. Amorphous carbon is, for example, acetylene black or carbon black. The amorphous carbon should contain low-crystalline carbon. In addition, the negative electrode active material layer may contain carbon nanotubes.

[0053] It is preferable that the graphite particles have amorphous carbon fine particles supported on them. In particular, it is preferable that the graphite particles contain graphite particles (A) and graphite particles (B) on which the amorphous carbon fine particles are supported.

[0054] Silicon-based materials include silicon (Si) and silicon oxide (SiO2). ｘ Examples include silicon carbide (SiC), silicon oxycarbide (SiOC), etc. Furthermore, silicon-based particles may be a mixture of silicon-based materials and other materials. Examples of silicon-based particles include silicon oxide (Li-SiO2), which is silicon pre-doped with lithium. ｘ (0 < x < 2) is preferably used. Note that the silicon-based particles may be silicon (Si) oxides as well as silicon compounds containing other anions, etc. In this specification, such oxides and silicon compounds are collectively referred to as "Si-containing compounds".

[0055] The negative electrode mixture layer 32b contains, in addition to the negative electrode active material, at least one of a negative electrode additive and a binder. Preferably, both of these are included. In one embodiment, the negative electrode mixture layer 32b contains a negative electrode active material comprising graphite particles (A) and graphite particles (B) on which amorphous carbon fine particles are supported, wherein the number of amorphous carbon fine particles per unit area of ​​the graphite particles (B) is 0.4 particles / μm ２This concludes the description. The negative electrode mixture layer 32b may be formed solely from graphite particles (A) without containing graphite particles (B) on which amorphous carbon fine particles are supported. In another embodiment, the negative electrode mixture layer 32b comprises a negative electrode active material and a binder that holds the negative electrode active material, wherein the negative electrode active material contains graphite particles and amorphous carbon fine particles. The negative electrode mixture layer 32b may further contain a negative electrode additive containing copper oxide.

[0056] The graphite particles are not particularly limited, but examples include natural graphite particles and artificial graphite particles, with natural graphite particles being preferred. Examples of natural graphite include flake graphite, lump graphite, and earthy graphite. The graphite particles constituting graphite particles (A) and graphite particles constituting graphite particles (B) supported with amorphous carbon fine particles may be of the same type or of different types.

[0057] Regarding graphite, graphite coated with amorphous carbon may be used. Coating with amorphous carbon prevents reaction with excess electrolyte. Examples of amorphous carbon include pitch. That is, the graphite particles constituting graphite particles (A) and graphite particles (B) on which amorphous carbon fine particles are supported are preferably graphite particles coated with amorphous carbon, and more preferably pitch-coated graphite particles. Furthermore, the graphite particles constituting graphite particles (A) and graphite particles (B) on which amorphous carbon fine particles are supported are particularly preferably natural graphite particles coated with amorphous carbon.

[0058] Pitch-coated natural graphite (hereinafter also referred to as "pitch-coated natural graphite") is harder than unpitch-coated natural graphite. Pitch-coated natural graphite may consist of multiple layers, with resin coatings applied to the edges of the layers along the lamination direction. The resin may, for example, contain conductive carbon. The graphite particles may be easily graphitizable carbon or difficult-to-graphitize carbon. Easily graphitizable carbon corresponds to soft carbon. Difficult-to-graphitize carbon corresponds to hard carbon.

[0059] The supported amorphous carbon nanoparticles that constitute the graphite particles (B) are not particularly limited, but examples include carbon blacks such as acetylene black, Ketjen black, channel, furnace black, lamp black, and thermal black. Note that supported amorphous carbon nanoparticles refer to amorphous carbon particles scattered on the surface of the graphite particles, and are different from amorphous carbon coating the graphite particles as described above. Amorphous carbon coating the graphite particles refers to amorphous carbon covering all or part of the surface of the graphite particles.

[0060] Graphite particles (B) supported with amorphous carbon nanoparticles have a number of amorphous carbon nanoparticles of 0.4 / μm² per unit area. ２ The above is preferable, and preferably the number of amorphous carbon particles per unit area is 0.4 particles / μm ２ ~2.4 pieces / μm ２ More preferably, the number of amorphous carbon particles per unit area is 0.8 particles / μm ２ ~2.0 pieces / μm ２ Furthermore, if the graphite particles support a small amount of amorphous carbon fine particles, that is, if the number of amorphous carbon fine particles per unit area is 0.4 particles / μm ２ Graphite particles supported with amorphous carbon nanoparticles below a certain number do not fall under the category of graphite particles (B). The number of amorphous carbon nanoparticles per unit area is 0.4 particles / μm. ２ Graphite particles supporting amorphous carbon fine particles with a size less than 350 mm may be used as graphite particles (A).

[0061] The average particle size of the graphite particles (A) is preferably 4 μm or more and 20 μm or less, more preferably 4 μm or more and 12 μm or less, and particularly preferably 5 μm or more and 10 μm or less.

[0062] The average particle size of the graphite particles (B) supported with amorphous carbon fine particles is preferably 4 μm or more and 20 μm or less, more preferably 4 μm or more and 12 μm or less, and particularly preferably 5 μm or more and 10 μm or less. The average particle size of the graphite particles (A) and the average particle size of the graphite particles (B) supported with amorphous carbon fine particles may be the same or different.

[0063] In graphite particles (B) supported with amorphous carbon fine particles, the average particle size of the supported amorphous carbon fine particles is preferably smaller than the average particle size of graphite particles (A) and the average particle size of the graphite particles constituting graphite particles (B) supported with amorphous carbon fine particles. The average particle size of the amorphous carbon fine particles is preferably 0.05 μm or more and 0.5 μm or less, and more preferably 0.1 μm or more and 0.4 μm or less.

[0064] As a negative electrode active material containing graphite particles (A) and graphite particles (B) on which amorphous carbon fine particles are supported, it is preferable that, for example, graphite particles (A) without amorphous carbon fine particles and graphite particles (B) on which amorphous carbon fine particles are supported (amorphous carbon fine particles are supported on the graphite particles) are present in the negative electrode active material layer in a mixed state.

[0065] When the negative electrode active material includes graphite particles (A) and graphite particles (B) on which amorphous carbon fine particles are supported, the mass ratio (graphite particles (A)) / (graphite particles (B) on which amorphous carbon fine particles are supported) is preferably 0.25 or more and 5 or less, and more preferably 0.5 or more and 2 or less.

[0066] Furthermore, as the negative electrode active material, for example, a material may be used in which a graphite material is mixed with carbon black such as acetylene black, Ketjen black, channel black, furnace black, lamp black, or thermal black as a conductive additive, or a composite material may be used in which these conductive additives are mixed with a graphite material and then coated with amorphous carbon. The shape of the negative electrode active material is not particularly limited and may be spherical, flaky, fibrous, or a pulverized form thereof.

[0067] As a result of diligent research, the inventors have found that by using a negative electrode active material containing graphite particles (A) and graphite particles (B) on which amorphous carbon fine particles are supported, the lithium-ion secondary battery can reduce internal resistance over a wide range from low SOC to high SOC while maintaining battery capacity, thereby achieving high output and good storage characteristics.

[0068] Furthermore, as a result of diligent research by the inventors, it has been found that by having a negative electrode active material containing graphite particles and amorphous carbon fine particles, and a negative electrode active material layer having a negative electrode active material and a binder that holds the negative electrode active material, the lithium-ion secondary battery can reduce internal resistance over a wide range from low SOC to high SOC while maintaining battery capacity, thereby achieving high output and good storage characteristics.

[0069] In the negative electrode mixture layer 32b shown in Figure 3, the negative electrode active material and the negative electrode additive may exist as separate particles that are not compounded with each other. This allows the negative electrode active materials to be electrically connected well without being hindered by the negative electrode additive which has high electrical resistance, thereby suppressing an increase in the internal resistance of the lithium-ion secondary battery. The negative electrode additive may have an average particle size smaller than the average particle size of the negative electrode active material, and may have an average particle size of 1 μm to 10 μm. This allows the negative electrode active materials to be electrically connected well without being hindered by the negative electrode additive which has high electrical resistance. This suppresses an increase in the internal resistance of the lithium-ion secondary battery. The average particle sizes of the negative electrode active material (e.g., graphite particles, amorphous carbon fine particles, etc.) and the negative electrode additive are determined based on observation images from a scanning electron microscope (SEM).

[0070] As the binder for the negative electrode mixture layer 32b, the same material as the material exemplified for the binder of the positive electrode mixture layer 34b can be used.

[0071] The negative electrode mixture layer 32b may further contain a dispersant. Carboxymethylcellulose (CMC) can be used as the dispersant.

[0072] The negative electrode 32 can be formed, for example, as follows.

[0073] First, a negative electrode active material, a negative electrode additive, a binder, and optionally a dispersant are prepared. The negative electrode active material and negative electrode additive may be in particulate form. The negative electrode active material and negative electrode additive may be separate particles that are not compounded with each other. The negative electrode active material, negative electrode additive, binder, and optionally a dispersant are dispersed in a solvent (e.g., N-methyl-2-pyrrolidone (NMP), water) to prepare a paste-like or slurry-like negative electrode mixture composition. This negative electrode mixture composition is applied to the surface (one or both sides) of the negative electrode current collector 32a, dried, and calendered as necessary to form a negative electrode mixture layer 32b. This yields the negative electrode 32. However, the negative electrode 32 is not limited to that formed by the above method, and may be formed by other methods. The thickness of the negative electrode active material layer is preferably 30 μm to 150 μm from the viewpoint of producing a high-quality film while thinning it to accommodate high power output. Furthermore, it is even more desirable that the particle size be between 30 μm and 65 μm.

[0074] <Separator> Separators 33 and 35 have an insulating function to prevent short circuits between the positive electrode 34 and the negative electrode 32, and a function to retain the electrolyte. By defining the configuration and physical properties of the separator as follows, it is possible to improve the output characteristics.

[0075] 1. A porous sheet made of structural resin has a structure in which three layers of film made of one resin material are laminated. The one resin material may be a mixture of two or more polymer compounds. From the viewpoint of ionic conductivity, the number of layers of the resin film is preferably three. However, the number of layers of the resin film may be three or more.

[0076] 2. The morphological resin film may be of any form as long as it can perform the function of a separator. Specifically, examples include porous resin films, woven fabrics, nonwoven fabrics, etc., but a porous resin film is preferred.

[0077] 3. The constituent components of the porous sheet made of resin are preferably made of hydrocarbon resins such as polyolefins. Specifically, these include, but are not limited to, polyethylene (PE), polypropylene (PP), polyimide, and aramid. Among these, polyolefin films are preferred as constituent components of the porous sheet made of resin, and PE and PP are particularly preferred. In the embodiments of this disclosure, it is preferable that the separator has a three-layer structure of PP / PE / PP, with the first layer being PE as the central layer and the second and third layers being PP as the outer layers.

[0078] 4. The thickness separator has a first layer that is thicker than the second and third layers, or a first layer that is thinner than the second and third layers, and it is desirable that the thickness of the separator is thinner than the thickness of the positive electrode active material layer and the negative electrode active material layer. From the viewpoint of increasing power output in the low-temperature region, the thickness of the separator as a whole of the three layers is preferably 12 μm to 20 μm, and more preferably 14 μm to 18 μm. The thickness of the first layer is preferably 3 μm to 9 μm, and more preferably 3 μm to 6 μm. The thicknesses of the second and third layers are preferably 3 μm to 9 μm, and more preferably 3 μm to 6 μm, respectively.

[0079] <Electrolyte> The electrolyte (non-aqueous electrolyte) of a lithium-ion secondary battery contains at least a non-aqueous solvent, an electrolyte salt, and an additive. By including an additive along with the non-aqueous solvent and electrolyte salt in the electrolyte, the protective film called SEI (Solid Electrolyte Interface) formed on the surface of the negative electrode active material is strengthened, and side reactions occurring at the interface between the electrolyte and the negative electrode active material are suppressed even under high-temperature storage conditions.

[0080] The electrolyte salt, additives, and non-aqueous solvents according to the embodiment will be described in detail below.

[0081] 1. In an electrolyte solution, at least one type of electrolyte salt is dissolved in a non-aqueous solvent. The electrolyte salt is a lithium salt, and as the lithium salt, for example, a lithium salt containing fluorine (fluorinated lithium salt) is preferred. Specifically, the fluorinated lithium salt is lithium hexafluoride phosphate (LiPF). ６ ), lithium tetrafluoroborate (LiBF ４ ) and other inorganic anionic salts, lithium trifluoromethanesulfonate (LiCF ３ SO ３ These include organic anionic salts such as ) and especially LiPF ６ It is preferable.

[0082] The lithium salt may be used alone or in combination of two or more types in any ratio. The amount of lithium salt contained in the electrolyte is arbitrary as long as it does not significantly impair the effects of this disclosure, but for example, it is 0.01% by mass or more, preferably 0.1% by mass or more. Alternatively, the amount of lithium salt contained in the electrolyte may be 30% by mass or less, preferably 20% by mass or less.

[0083] 2. Additives The additives preferably contain lithium borate, difluorophosphate, and vinylene carbonate compounds, and may also contain fluorosulfonates and methoxysulfonates.

[0084] Specific types of lithium borate include lithium bis(oxalate)borate (LiBOB) and lithium difluoro(oxolato)borate (LiDFOB), with LiBOB being particularly preferred.

[0085] Lithium borate may be used alone or in combination of two or more types in any ratio. The lithium borate content in the electrolyte is, for example, 0.001% by mass or more, preferably 0.1% by mass or more, and more preferably 0.4% by mass or more. Alternatively, the lithium borate content in the electrolyte is, for example, 3% by mass or less, preferably 2% by mass or less, and more preferably 1% by mass or less. Within this range, it is easy to achieve the effect of improving the high-temperature storage characteristics of lithium-ion secondary batteries and to avoid an increase in negative electrode resistance due to excessive addition.

[0086] The countercation of the difluorophosphate is not particularly limited, but examples include lithium, sodium, potassium, etc.

[0087] Specific types of difluorophosphates include lithium difluorophosphate (LiPO4). ２ F ２ Examples include sodium difluorophosphate and potassium difluorophosphate, with lithium difluorophosphate being particularly preferred.

[0088] Difluorophosphates may be used individually or in combination of two or more in any ratio. The content of difluorophosphates in the electrolyte is, for example, 0.001% by mass or more, preferably 0.1% by mass or more, and more preferably 0.5% by mass or more. Alternatively, the content of difluorophosphates in the electrolyte is, for example, 3% by mass or less, preferably 2% by mass or less, and more preferably 1% by mass or less.

[0089] Specific types of vinylene carbonate compounds include, for example, vinylene carbonate (1,3-dioxol-2-one), methylvinylene carbonate (4-methyl-1,3-dioxol-2-one), and ethylvinylene carbonate (4-ethyl-1,3-dioxol-2-one).

[0090] A single vinylene carbonate compound may be used alone, or two or more compounds may be used in any ratio. In particular, vinylene carbonate (C３ H ２ O ３ ) is particularly preferable because it yields excellent results.

[0091] The content of vinylene carbonate compound in the electrolyte is, for example, 0.001% by mass or more, preferably 0.1% by mass or more, and more preferably 0.4% by mass or more. Alternatively, the content of vinylene carbonate compound in the electrolyte is, for example, 3% by mass or less, preferably 2% by mass or less, and more preferably 1% by mass or less. Within this range, it is easy to exhibit the effect of improving the high-temperature storage characteristics of lithium-ion secondary batteries and to avoid swelling of the battery due to increased gas generation.

[0092] 3. The non-aqueous solvent electrolyte contains at least one type of non-aqueous solvent. Specific examples of non-aqueous solvents include cyclic carbonates, fluorine-containing cyclic carbonates, linear carbonates, and fluorine-containing linear carbonates.

[0093] Specific examples of cyclic carbonates include ethylene carbonate (EC), propylene carbonate (PC), and butylene carbonate (BC). Specific examples of fluorine-containing cyclic carbonates include fluoroethylene carbonate (FEC). Specific examples of chain carbonates include dimethyl carbonate (DMC), diethyl carbonate (DEC), and ethyl methyl carbonate (EMC).

[0094] The non-aqueous solvent may be used alone or in combination of two or more types in any ratio. From the viewpoint of balancing the dielectric constant and viscosity of the electrolyte, the non-aqueous solvent is preferably one that contains at least a cyclic carbonate and a linear carbonate. Specifically, the non-aqueous solvent is preferably one that contains at least one cyclic carbonate solvent and at least one linear carbonate solvent. In this case, the volume ratio of cyclic carbonate to linear carbonate contained in the non-aqueous solvent is preferably 10:90 to 50:50. Furthermore, the non-aqueous solvent is preferably one that contains ethylene carbonate as the cyclic carbonate and ethyl methyl carbonate and dimethyl carbonate as the linear carbonates.

[0095] <Other> If necessary, an axis (not shown) may be placed at the innermost circumference of the winding group 3. As the axis, a resin sheet or the like with higher bending rigidity than any of the positive electrode current collector, negative electrode current collector, or separators 33 and 35 can be used.

[0096] An insulating protective film (not shown) may optionally be wrapped around the winding group 3. The insulating protective film is not particularly limited, but for example, it consists of a single sheet or multiple film members made of a synthetic resin such as PP (polypropylene), and has a length that allows it to be wrapped around the winding group 3 with the winding center in a direction parallel to the flattened plane and perpendicular to the winding axis.

[0097] Next, an electrode laminate including a positive electrode, a separator, and a negative electrode according to an embodiment of this disclosure will be described with reference to the drawings.

[0098] Figure 4 is a schematic cross-sectional view showing the stacked state of the positive electrode, separator, and negative electrode according to the embodiment.

[0099] In this figure, the positive electrode mixture layer 34b, separator 33, negative electrode coating layer 32p, negative electrode mixture layer 32b, and negative electrode current collector 32a are stacked in this order. Note that the positive electrode current collector is omitted in this figure.

[0100] The negative electrode mixture layer 32b includes a carbon-based material such as graphite and a silicon-based material.

[0101] The negative electrode coating layer 32p preferably contains carbon-based material particles and does not contain silicon-based material. The negative electrode coating layer 32p is preferably porous. Furthermore, the negative electrode coating layer 32p preferably has a higher porosity than the negative electrode mixture layer 32b. This reduces the resistance to electrolyte movement and allows for high output characteristics.

[0102] In this specification, graphite, graphene, carbon nanotubes, carbon nanofibers, fullerenes, etc., are collectively referred to as "graphite." Graphite is included in carbon-based materials.

[0103] Figure 5 is a schematic cross-sectional view showing the stacked state of the positive electrode, separator, and negative electrode according to the embodiment.

[0104] In this figure, the positive electrode mixture layer 34b, the positive electrode side coating layer 34p, the separator 33, the negative electrode mixture layer 32b, and the negative electrode current collector 32a are stacked in this order. Note that the positive electrode current collector is omitted in this figure.

[0105] The positive electrode coating layer 34p contains carbon-based material particles but does not contain silicon-based material. It is desirable that the positive electrode coating layer 34p be porous.

[0106] Figure 6 is a schematic cross-sectional view showing the stacked state of the positive electrode, separator, and negative electrode according to the embodiment.

[0107] In this figure, the positive electrode mixture layer 34b, the positive electrode side coating layer 34p, the separator 33, the negative electrode side coating layer 32p, the negative electrode mixture layer 32b, and the negative electrode current collector 32a are stacked in this order. Note that the positive electrode current collector is omitted in this figure. The configuration of the positive electrode side coating layer 34p and the negative electrode side coating layer 32p is the same as described above.

[0108] In this specification, the negative electrode side coating layer 32p and the positive electrode side coating layer 34p are collectively referred to as the "coating layer".

[0109] As shown in Figures 4-6, the coating layer is installed on either the positive electrode side, the negative electrode side, or both.

[0110] The coating layer may contain alumina in addition to carbon-based materials.

[0111] In this specification, dibusite, bayerite, nordstrandite, boehmite, diaspore, todite, alumina gel (amorphous alumina hydrate), etc., are collectively referred to as "alumina." Alumina is further classified into α, γ, θ, κ, δ, η, χ, ρ, etc., based on differences in crystal structure, unit cell dimensions, etc.

[0112] The particle size of the silicon-based material is preferably 3 to 10 μm, and more preferably 4 to 8 μm.

[0113] The particle size of the carbon-based material is preferably 5 to 20 μm, and more preferably 7 to 10 μm.

[0114] When the coating layer is made of a carbon-based material, the thickness of the coating layer is preferably 10 to 60 μm, and more preferably 10 to 20 μm. This is because carbon-based materials, especially graphite, have low hardness, so a relatively large thickness is desirable. This point is particularly important when the coating layer is porous.

[0115] On the other hand, when the coating layer contains 50% or more by volume of alumina, the thickness of the coating layer is preferably 3 to 10 μm. This is because alumina is harder than carbon-based materials, so an effect can be obtained even with a relatively small thickness. However, if the thickness of the alumina coating layer is less than 3 μm, coating the layer becomes difficult. Also, if the thickness of the alumina coating layer exceeds 10 μm, it is undesirable because the cell capacity decreases.

[0116] If the coating layer is porous, the resistance to electrolyte movement can be reduced. Furthermore, damage to separators and other components due to the expansion and contraction of the active material can be prevented.

[0117] The ratio of the thickness of the coating layer to the negative electrode mixture layer should preferably be 1:9 to 3:7. This ensures the energy density of the battery.

[0118] The particle size of the silicon-based material should preferably be smaller than the particle size of the carbon-based material in the coating layer. This prevents the silicon-based material particles from reaching the separator.

[0119] It is desirable that the separator and the coating layer be joined by an adhesive. This ensures good adhesion between the separator and the positive or negative electrode. For example, an adhesive layer may be formed on the surface of the main surface of the separator.

[0120] If the negative electrode has a coating layer, it consists of a substrate made of metal foil (copper foil), a negative electrode mixture layer, and a coating layer, which are laminated in this order.

[0121] The present disclosure will be described in detail below using examples. However, the present disclosure is not limited to these examples.

[0122] (Hot pressing) Li as the positive electrode active material １．０ Ni ０．８０ Co ０．１０ Mn ０．０５ O ２ We prepared a powder, acetylene black as a conductive additive, and polyvinylidene fluoride (PVdF) as a binder.

[0123] The positive electrode active material, conductive agent, and binder were mixed in a weight ratio of 94:3:3. N-methyl-2-pyrrolidone (NMP) was added to the resulting mixture to adjust the viscosity and obtain a positive electrode slurry.

[0124] A 15 μm thick aluminum foil was prepared as the positive electrode current collector. Uncoated areas were left on both sides of the positive electrode current collector to serve as welded areas (exposed areas of the positive electrode current collector), and a layer of positive electrode slurry was formed by coating with positive electrode slurry using a slot die coating method. Next, the positive electrode slurry layer was dried and pressed to form a positive electrode mixture layer, and the positive electrode was manufactured. The thickness of the mixture layer of the positive electrode was 55 μm on each side.

[0125] Pitch-coated natural graphite and SiO were prepared as the negative electrode active material, carbon nanotubes as a conductive additive, styrene-butadiene rubber (SBR) as a binder, and carboxymethylcellulose (CMC) as a dispersant.

[0126] The negative electrode active material, conductive additive, binder, and dispersant were mixed in a weight ratio of 96.9:0.1:1:1. Deionized water was added to the resulting mixture to adjust the viscosity and obtain a negative electrode slurry. Two types of negative electrode slurry (negative electrode slurry A and negative electrode slurry B) were prepared with different compositions of negative electrode active material. The negative electrode active material of negative electrode slurry A was 100 wt% pitch-coated natural graphite (average particle size 10 μm). The negative electrode active material of negative electrode slurry B was pitch-coated natural graphite (average particle size 15 μm):SiO (average particle size 8 μm) in a weight ratio of 75:25.

[0127] Uncoated areas (exposed negative electrode current collector areas) were left on both sides of a 10 μm thick copper foil. Two layers of negative electrode slurry were simultaneously applied using a slot die coating method (when the current collector side was the lower layer and the area above it was the upper layer, negative electrode slurry A was applied to the upper layer and negative electrode slurry B to the lower layer). Next, the negative electrode slurry layers were dried and pressed to form a negative electrode mixture layer, and a negative electrode was fabricated. The thickness of the mixture layer of the obtained negative electrode was 20 μm on the upper layer and 30 μm on the lower layer on the other side.

[0128] A separator was placed between the fabricated positive electrode and negative electrode, and the material was wound to create a winding group. The resulting winding group was held under a load of 2 tons for 20 minutes while the surface of a hot press was heated to 100°C. As a result, the negative electrode slurry A became a layer in direct contact with the separator and functioned as a coating layer.

[0129] (Insulation Test) An insulation test was performed between terminals of the hot-pressed winding group using an insulation resistance tester at a test voltage of 300V.

[0130] In the above examples, SiO is used as the silicon-based material, but SiC can be used similarly, and high insulating properties can be obtained by using the same SiC particle size, coating layer thickness, etc. as described above.

[0131] Table 1 shows the bonding conditions, presence or absence of a coating layer, presence or absence and thickness of a graphite layer, and the results of the insulation test for the examples and comparative examples.

[0132] This table shows the case where the separator is bonded to the positive and / or negative electrode, where there is a coating layer on the surface of the positive or negative electrode, and where a graphite layer of 20 μm or 60 μm thickness is placed on the surface of the negative electrode.

[0133] As shown in this table, in the example, no short circuits occurred in the five insulation tests, indicating that the coating layer or the graphite layer on the negative electrode surface is effective. In contrast, in the comparative example where neither the coating layer nor the graphite layer on the negative electrode surface was installed, short circuits occurred in all five insulation tests, and insulation could not be maintained against hot pressing.

[0134]

[0135] The above description can be used by those skilled in the art to make the most of this disclosure. The embodiments disclosed herein are merely descriptive and illustrative and should be construed as not limiting the scope of this disclosure in any way. With the help of this disclosure, modifications to the details of the above embodiments can be made without departing from the basic principles of this disclosure. In other words, various modifications and improvements to the embodiments specifically disclosed in the above specification are within the scope of this disclosure.

[0136] 1: Battery can, 1a: Opening, 1b: Wide side, 1c: Narrow side, 1d: Bottom, 3: Winding group, 5: Gasket, 6: Battery cover, 7: Insulating plate, 9: Liquid filling port, 10: Gas discharge valve, 11: Liquid filling plug, 12: Negative electrode external terminal, 12a: Negative electrode connection part, 14: Positive electrode external terminal, 14a: Positive electrode connection part, 21: Negative electrode current collector plate base, 22: Negative electrode side connection end, 23: Negative electrode side opening hole, 24: Negative electrode current collector plate, 26: Negative electrode side through hole, 32: Negative electrode, 32a : Negative electrode current collector, 32b: Negative electrode mixture layer, 32c: Negative electrode current collector exposed portion, 32p: Negative electrode side coating layer, 34p: Positive electrode side coating layer, 33: Separator, 34: Positive electrode, 34a: Positive electrode current collector, 34b: Positive electrode mixture layer, 34c: Positive electrode current collector exposed portion, 35: Separator, 41: Positive electrode current collector plate base, 42: Positive electrode side connection end, 43: Positive electrode side opening hole, 44: Positive electrode current collector plate, 46: Positive electrode side through hole, 100: Lithium-ion secondary battery.

Claims

1. A lithium-ion secondary battery comprising: a negative electrode having a negative electrode mixture layer; a separator; and a positive electrode having a positive electrode mixture layer, wherein the separator is disposed between the negative electrode and the positive electrode; the negative electrode mixture layer comprises a carbon-based material and a silicon-based material; and a coating layer is disposed between the negative electrode mixture layer and the separator, and between the separator and the positive electrode mixture layer.

2. The lithium-ion secondary battery according to claim 1, wherein the coating layer comprises a carbon-based material.

3. The lithium-ion secondary battery according to claim 2, wherein the coating layer further comprises alumina.

4. The lithium-ion secondary battery according to claim 1, wherein the coating layer comprises a carbon-based material, and the thickness ratio of the coating layer to the negative electrode mixture layer is 1:9 to 3:

7.

5. The lithium-ion secondary battery according to claim 1, wherein the coating layer comprises a carbon-based material, and the particle size of the silicon-based material is smaller than the particle size of the carbon-based material in the coating layer.

6. The lithium-ion secondary battery according to claim 1, wherein the coating layer has a higher porosity than the negative electrode mixture layer.

7. The lithium-ion secondary battery according to claim 1, wherein the separator and the coating layer are joined together by an adhesive.

8. A negative electrode for a lithium-ion secondary battery, comprising a substrate formed of metal foil, a negative electrode mixture layer, and a coating layer, wherein the substrate, the negative electrode mixture layer, and the coating layer are laminated in this order, and the negative electrode mixture layer contains a carbon-based material and a silicon-based material.

9. A winding for a lithium-ion secondary battery, comprising a negative electrode having a negative electrode mixture layer, a separator, and a positive electrode having a positive electrode mixture layer, wherein the separator is disposed between the negative electrode and the positive electrode, the negative electrode mixture layer comprises a carbon-based material and a silicon-based material, and a coating layer is disposed between the negative electrode mixture layer and the separator, and between the separator and the positive electrode mixture layer.

10. A method for manufacturing a lithium-ion secondary battery comprising a negative electrode having a negative electrode mixture layer, a separator, and a positive electrode having a positive electrode mixture layer, wherein the separator is disposed between the negative electrode and the positive electrode, the negative electrode mixture layer comprises a carbon-based material and a silicon-based material, and a coating layer is disposed between the negative electrode mixture layer and the separator and between the separator and the positive electrode mixture layer, the method comprising a hot press step of pressurizing and heating the broad side surface of a wound body including the negative electrode, the separator, and the positive electrode.

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