Lithium ion secondary battery, positive electrode 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 positive electrode 34 that has a positive electrode mixture layer 34b and a positive electrode collector 34a; a separator 33; and a negative electrode 32 that has a negative electrode mixture layer 32b and a negative electrode collector 32a. The separator 33 is disposed between the positive electrode 34 and the negative electrode 32. A positive-electrode-side carbon coating layer 34p is disposed between the positive electrode mixture layer 34b and the positive electrode collector 34a. An end portion of the positive electrode mixture layer 34b is disposed at the same position as an end portion of the positive-electrode-side carbon coating layer 34p or at a position inward of the end portion of the positive-electrode-side carbon coating layer 34p. As a result, the internal resistance of the lithium ion secondary battery as a whole can be reduced.

Description

Lithium-ion secondary battery, positive electrode for lithium-ion secondary battery, and method for manufacturing a lithium-ion secondary battery

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

[0002] In the field of lithium-ion secondary batteries, improvements have been made not only to enhance the performance of active materials, but also to add new layers to the electrode group.

[0003] Patent Document 1 discloses a method for manufacturing a positive electrode for an energy storage device, comprising the step of forming a positive electrode active material layer by coating a slurry for forming a positive electrode active material layer containing an aqueous solvent onto a carbon coating layer of a metal foil having a carbon coating layer containing carbon and an aqueous binder. Patent Document 2 discloses a slurry for forming a positive electrode active material layer, wherein the positive electrode active material is a polyanionic compound having an olivine-type structure, LiFePO ４ It is disclosed that, in a plan view from the stacking direction, the positive electrode active material layer is formed in the central part of the carbon coat layer, and the peripheral part of the carbon coat layer is an uncoated positive electrode area where the positive electrode active material layer is not provided.

[0004] Japanese Patent Publication No. 2022-81306

[0005] In recent years, lithium iron phosphate (LFP) batteries have been increasingly developed in the fields of electric vehicles and stationary storage batteries, due to their safety, cost reduction, and long lifespan. LFP batteries offer advantages such as high safety, inexpensive cathode materials, and long lifespan. However, lithium iron phosphate-based lithium-containing composite oxides, which are the cathode active materials for LFP batteries, have a particular challenge: they have high insulation properties (internal resistance) and low electronic conductivity and lithium-ion transport characteristics. It is desirable to improve the electronic conductivity and lithium-ion transport characteristics of cathode active materials other than lithium iron phosphate-based lithium-containing composite oxides.

[0006] Patent Document 1 does not provide a means to solve the above-mentioned problems.

[0007] This disclosure aims to reduce the overall internal resistance of lithium-ion secondary batteries.

[0008] The lithium-ion secondary battery of this disclosure comprises a positive electrode having a positive electrode mixture layer and a positive electrode current collector, a separator, and a negative electrode having a negative electrode mixture layer and a negative electrode current collector, wherein the separator is disposed between the positive electrode and the negative electrode. A positive electrode side carbon coating layer is disposed between the positive electrode mixture layer and the positive electrode current collector. The end of the positive electrode mixture layer is located at the same position as the end of the positive electrode side carbon coating layer, or at a position inside the end of the positive electrode side carbon coating layer.

[0009] According to this disclosure, the overall internal resistance of the lithium-ion secondary battery can be reduced.

[0010] 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.

[0011] 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).

[0012] This specification primarily describes wound electrode groups manufactured by stacking and winding sheet-shaped positive electrodes, separators, and negative electrodes. However, the lithium-ion secondary battery according to this disclosure is not limited thereto and may also include a laminated electrode group manufactured by stacking multiple sets of sheet-shaped positive electrodes, separators, and negative electrodes without winding them.

[0013] Figure 1 is an external perspective view of a lithium-ion secondary battery according to an embodiment.

[0014] Figure 2 is an exploded perspective view of a lithium-ion secondary battery according to this embodiment.

[0015] 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.

[0016] 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.

[0017] 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.

[0018] 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.

[0019] The battery cover 6 is further provided with a positive electrode through-hole 46 and a negative electrode through-hole 26.

[0020] 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.

[0021] 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.

[0022] The positive external terminal 14 and the negative external terminal 12 each have a welded joint to which a busbar or the like is welded. The welded joint has a rectangular block shape that protrudes 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 at a predetermined height and is approximately parallel to the battery cover 6.

[0023] The positive electrode current collector plate 44 has a rectangular plate-shaped positive electrode current collector plate base portion 41 facing the lower surface of the battery cover 6, and a positive electrode side connection end portion 42 extending from the side end of the positive electrode current collector plate base portion 41 toward the bottom surface 1d 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 plate base portion 21 facing the lower surface of the battery cover 6, and a negative electrode side connection end portion 22 extending from the side end of the negative electrode current collector plate base portion 21 toward the bottom surface 1d along the wide side surface 1b of the battery can 1. The positive electrode current collector plate base portion 41 and the negative electrode current collector plate base portion 21 have a positive electrode side opening hole 43 and a negative electrode side opening hole 23, respectively.

[0024] A positive electrode connection portion 14a and a negative electrode connection portion 12a are provided so as to protrude from the lower surfaces of the positive electrode external terminal 14 and the negative electrode external terminal 12, respectively. The positive electrode connection portion 14a and the negative electrode connection portion 12a are formed integrally with the positive electrode external terminal 14 and the negative electrode external terminal 12, respectively.

[0025] 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 plate 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 plate base 21. The positive electrode connection portion 14a penetrates the battery cover 6 and the positive electrode current collector plate 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 plate base 41. The positive electrode external terminal 14 and the positive electrode current collector plate 44 are electrically connected via the positive electrode connection portion 14a and fixed to the battery cover 6. Similarly, the negative electrode connection portion 12a penetrates the battery cover 6 and the negative electrode current collector plate 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 plate base 21. The negative electrode external terminal 12 and the negative electrode current collector plate 24 are electrically connected via the negative electrode connection portion 12a and are fixed to the battery cover 6.

[0026] The positive external terminal 14 is electrically connected to the winding group 3 (winding body), which will be described later, via the positive connection part 14a and the positive current collector plate 44. Similarly, the negative external terminal 12 is electrically connected to the winding group 3 via the negative connection part 12a and the negative current collector plate 24. When the lithium-ion secondary battery 100 is being charged, electricity is supplied to the winding group 3 from an external power source via the positive external terminal 14, the positive connection part 14a, and the positive current collector plate 44, as well as the negative external terminal 12, the negative connection part 12a, and the negative current collector plate 24. When the lithium-ion secondary battery 100 is being discharged, electricity is supplied from the winding group 3 to an external load via the positive external terminal 14, the positive connection part 14a, and the positive current collector plate 44, as well as the negative external terminal 12, the negative connection part 12a, and the negative current collector plate 24.

[0027] 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, gaskets 5 are provided between the positive electrode external terminal 14 and the negative electrode external terminal 12 and the battery cover 6, and insulating plates 7 are provided between the positive electrode current collector plate 44 and the negative electrode current collector plate 24 and the battery cover 6. Examples of materials for the insulating plates 7 and gaskets 5 include insulating resin materials such as polybutylene terephthalate, polyphenylene sulfide, and perfluoroalkoxy fluororesin.

[0028] A wound group 3 and an electrolytic solution are housed inside a battery can 1. Before being housed in the battery can 1, the wound group 3 is subjected to hot pressing, thereby increasing the energy density of the lithium ion secondary battery 100. The electrolytic solution is injected into the battery can 1 through a liquid injection port 9. Here, hot pressing refers to a treatment of applying pressure to the wide side surfaces of the wound group 3 while heating the same.

[0029] FIG. 3 is an exploded perspective view showing a partially developed state of a wound group according to an embodiment.

[0030] As shown in the figure, the wound group 3 includes a negative electrode 32, a positive electrode 34, and two separators 33, 35. The separator 35, the negative electrode 32, the separator 33, and the positive electrode 34 are stacked in this order and wound into a flat shape. The separator 35 is positioned at the outermost periphery of the wound group 3, and the negative electrode 32 is positioned inside the separator 35. The two separators 33, 35 electrically insulate the positive electrode 34 from the negative electrode 32.

[0031] As shown in the figure, the wound group 3 has a pair of opposing end faces 3a, 3b perpendicular to a winding axis, and a side face 3c positioned between the pair of end faces 3a, 3b. The side face 3c includes a pair of opposing curved portions having semicircular cross-sections, and a flat portion (wide side face) continuously formed between the pair of curved portions. The wound group 3 is arranged inside the battery can 1 such that the flat portion of the side face 3c and the wide side face 1b (FIG. 2) of the battery can 1 are substantially parallel to each other.

[0032] Hereinafter, the positive electrode, the negative electrode, and the electrolytic solution of the lithium ion secondary battery will be described. Note that the "positive electrode mixture layer" corresponds to the "positive electrode active material layer" according to the present disclosure, and the "negative electrode mixture layer" corresponds to the "negative electrode active material layer" according to the present disclosure.

[0033] <Positive Electrode> As shown in FIG. 3, the positive electrode 34 includes a positive electrode current collector 34a (positive electrode base material), 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. Note that the positive electrode mixture layer 34b is preferably provided on both sides of the positive electrode current collector 34a.

[0034] 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.

[0035] An insulating layer 34i is placed between the positive electrode mixture layer 34b and the exposed portion 34c of the positive electrode current collector.

[0036] The positive electrode mixture layer 34b contains a positive electrode active material. The positive electrode active material contains, for example, a ternary material comprising 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.

[0037] 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).

[0038] Li １＋Ｘ M Ａ O ２ …(1) (In the equation, X satisfies -0.15 ≤ X ≤ 0.15, M Ａ (wherein represents a group of elements 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 above general composition formula (1) has high thermal stability and stability at high potential states, and by applying this oxide, the safety and various battery characteristics of lithium-ion secondary batteries can be improved.

[0039] Furthermore, a positive electrode active material containing iron and phosphorus may be used. In this case, olivine-based lithium iron phosphate (LiFePO) ４ Non-oxide materials such as ) are preferred.

[0040] For example, an iron phosphate-based lithium-containing composite oxide represented by the following general composition formula (2) may also be used.

[0041] Li Ｙ Fe １－Ｚ M Ｂ Ｚ PO ４ ...(2) (In the formula, Y and Z satisfy 0.90 ≤ Y ≤ 1.1 and 0 ≤ Z ≤ 0.10, and M Ｂ represents an element group including at least one selected from the group consisting of Mn, Ni and Co.) It is preferable that the surface of particles of the iron phosphate-based lithium-containing composite oxide is coated with a carbon-based material such as graphite.

[0042] Note that the lithium-containing composite oxide may contain additional elements not shown in the above general composition formulas (1) and (2).

[0043] The positive electrode mixture layer 34b contains at least one of a conductive auxiliary agent (conductive agent) and a binder (adhesive). And it is preferable that both of these are contained.

[0044] The conductive auxiliary agent is not particularly limited, but for example, a carbon-based material can be used. The carbon-based material may be crystalline carbon, amorphous carbon, carbon nanotubes, or a mixture thereof. Examples of crystalline carbon include artificial graphite, natural graphite (e.g., flake graphite), or a mixture thereof. Examples of amorphous carbon include carbon black (e.g., acetylene black, Ketjen black, channel black, furnace black, lamp black or thermal black, or a mixture thereof). Carbon nanotubes may be single-walled, multi-walled, or a mixture 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 70 μm to 120 μm from the viewpoint of producing a high-quality film while thinning it to accommodate high power output.

[0048] <Negative Electrode> The negative electrode 32 shown in Figure 3 comprises a negative electrode current collector 32a (negative electrode base material) 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 as the negative electrode active material. Preferably, the negative electrode active material further contains silicon-based material particles (silicon-based particles (Si-based material)). The negative electrode mixture layer 32b may also contain materials other than graphite particles and silicon-based particles (Si-based particles) as the negative electrode active material.

[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 shall contain low-crystalline carbon.

[0053] It is preferable that the graphite particles have amorphous carbon nanoparticles 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 nanoparticles are supported. Furthermore, the graphite particles may also contain carbon nanotubes.

[0054] In this specification, graphite, graphene, carbon nanotubes, carbon nanofibers, fullerenes, etc., are collectively referred to as "graphite." Graphite is included in carbon-based materials.

[0055] Silicon-based materials include silicon (Si) and silicon oxide (SiO2). ｘExamples 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".

[0056] 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 ２ This 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.

[0057] 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.

[0058] 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.

[0059] 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.

[0060] 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.

[0061] 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 / μm２ 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).

[0062] 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.

[0063] 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.

[0064] 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.

[0065] 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.

[0066] 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.

[0067] 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.

[0068] 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.

[0069] 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.

[0070] 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).

[0071] 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.

[0072] The negative electrode mixture layer 32b may further contain a dispersant. Carboxymethylcellulose (CMC) can be used as the dispersant.

[0073] The negative electrode 32 can be formed, for example, as follows.

[0074] 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, and more preferably 30 μm to 65 μm, from the viewpoint of producing a high-quality film while minimizing the thickness to accommodate higher power output.

[0075] <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.

[0076] 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.

[0077] 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.

[0078] 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.

[0079] 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.

[0080] <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.

[0081] The electrolyte salt, additives, and non-aqueous solvents according to the embodiment will be described in detail below.

[0082] 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.

[0083] 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.

[0084] 2. Additives The additives preferably contain lithium borate, difluorophosphate, and vinylene carbonate compounds, and may also contain fluorosulfonates and methoxysulfonates.

[0085] Specific types of lithium borate include lithium bis(oxalate)borate (LiBOB) and lithium difluoro(oxolato)borate (LiDFOB), with LiBOB being particularly preferred.

[0086] 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.

[0087] The countercation of the difluorophosphate is not particularly limited, but examples include lithium, sodium, potassium, etc.

[0088] Specific types of difluorophosphates include lithium difluorophosphate (LiPO4). ２ F ２ Examples include sodium difluorophosphate and potassium difluorophosphate, with lithium difluorophosphate being particularly preferred.

[0089] 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.

[0090] 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).

[0091] A single vinylene carbonate compound may be used alone, or two or more compounds may be used in any ratio. Among them, vinylene carbonate (C３ H ２ O ３ ) is particularly preferable because it yields excellent results.

[0092] 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.

[0093] 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.

[0094] 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).

[0095] 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.

[0096] <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.

[0097] 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.

[0098] 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.

[0099] Figure 4 is a schematic cross-sectional view showing the stacked state of the positive electrode, separator, and negative electrode according to the embodiment.

[0100] In this figure, the negative electrode current collector 32a, negative electrode mixture layer 32b, separator 33, positive electrode mixture layer 34b, positive electrode side carbon coating layer 34p, positive electrode current collector 34a, positive electrode side carbon coating layer 34p, and positive electrode mixture layer 34b are stacked in this order. The insulating layer 34i is positioned to be in contact with the positive electrode side carbon coating layer 34p, similar to the positive electrode mixture layer 34b.

[0101] The positive electrode carbon coating layer 34p comprises a carbon-based material and a binder. The carbon-based material preferably comprises at least one of carbon black, graphite, and carbon nanotubes. The carbon-based material preferably has an average particle size of 100 to 2000 nm. The positive electrode carbon coating layer 34p preferably has a thickness of 5 μm or less.

[0102] The resin constituting the insulating layer 34i is preferably a thermoplastic resin that does not degrade due to the electrolyte. Furthermore, since the winding process of the wound body includes a heating and drying process in a vacuum, the insulating resin is preferably one that has a softening temperature higher than the heating temperature (e.g., 120°C). Examples of such resins include polyester resins, polyolefin resins, and ethylene copolymer resins.

[0103] Furthermore, it is desirable that the insulating layer 34i contains organic particles. Suitable organic particles include acrylic resin particles, PVdF, etc. These organic particles have the property of absorbing the electrolyte and swelling.

[0104] The insulating layer 34i is thinner than the positive electrode mixture layer 34b. Furthermore, it is desirable to set the thickness of the insulating layer 34i such that, when the organic particles contained in the insulating layer 34i absorb the electrolyte and expand, it becomes approximately the same thickness as the positive electrode mixture layer 34b. This is to prevent the positive electrode mixture layer 34b from separating from the separator 33 if the insulating layer 34i expands to become thicker than the positive electrode mixture layer 34b.

[0105] In this embodiment, a key feature is the difference in width (horizontal dimension in the figure) between the positive electrode current collector 34a, the positive electrode carbon coating layer 34p, the insulating layer 34i, the negative electrode mixture layer 32b, the negative electrode current collector 32a, and the positive electrode mixture layer 34b.

[0106] As shown in this figure, the right end of the positive electrode current collector 34a is designated as the reference end (0), the right end of the positive electrode carbon coating layer 34p and the insulating layer 34i is designated as end (1), the right end of the positive electrode mixture layer 34b is designated as end (2), and the right end of the negative electrode current collector 32a and the negative electrode mixture layer 32b is designated as end (3).

[0107] The region from end (0), which is the right end of the positive electrode current collector 34a, to end (1) is exposed and not covered by the positive electrode side carbon coating layer 34p (exposed portion 34c of the positive electrode current collector in Figure 3). The positive electrode side carbon coating layer 34p is wider than the positive electrode mixture layer 34b and is in contact with the entire lower surface of the positive electrode mixture layer 34b in the figure. The insulating layer 34i covers the portion where the positive electrode side carbon coating layer 34p is not in contact with the positive electrode mixture layer 34b (region from end (1) to end (2)).

[0108] The negative electrode mixture layer 32b is wider than the positive electrode mixture layer 34b (width from end (2) to end (3)). This configuration is essential for the insertion and removal of Li ions.

[0109] Furthermore, the negative electrode mixture layer 32b may have a smaller area than the positive electrode carbon coating layer 34p.

[0110] It is desirable that the negative electrode mixture layer 32b has a larger area than the positive electrode mixture layer 34b.

[0111] The insulating layer 34i may be arranged so as to partially overlap the negative electrode mixture layer 32b when projected toward the negative electrode mixture layer 32b in the stacking direction. This suppresses short circuits and provides a highly reliable battery.

[0112] The positive electrode carbon coating layer 34p is preferably thinner than the positive electrode mixture layer 34b. Furthermore, the thickness of the positive electrode carbon coating layer 34p is preferably between 0.1% and 20% of the thickness of the positive electrode mixture layer 34b. In this case, the upper limit is set from the viewpoint of manufacturability, and the lower limit from the viewpoint of film stability. A more desirable range is between 0.1% and 2%. Low internal resistance and high battery characteristics can be obtained.

[0113] The end of the insulating layer 34i may be located inward (towards the left in the diagram) from the end of the positive electrode side carbon coating layer 34p. This can improve the adhesion of the insulating layer.

[0114] The end of the positive electrode carbon coating layer 34p may be located inward (towards the left in the figure) from the end of the insulating layer 34i. This can improve the reliability of the insulating properties.

[0115] When the positive electrode mixture layer 34b contains a lithium-containing composite oxide based on iron phosphate, the insulating properties (electrical resistance) of the positive electrode mixture layer 34b increase. Therefore, by widening the positive electrode side carbon coating layer 34p, the entire positive electrode mixture layer 34b can be made electrically conductive to the positive electrode current collector 34a. This reduces the overall internal resistance of the battery.

[0116] By placing an insulating layer 34i on the positive electrode side carbon coating layer 34p in the region from end (1) to end (2), it is possible to suppress the entry of foreign matter between the positive electrode and the negative electrode. Furthermore, it is possible to prevent current from flowing directly from the positive electrode mixture layer 34b to the negative electrode side via the positive electrode side carbon coating layer 34p. In addition, even if a defect occurs in the separator 33, the insulating layer 34i can prevent contact between the negative electrode mixture layer 32b and the positive electrode side carbon coating layer 34p or the positive electrode current collector 34a.

[0117] The end of the insulating layer 34i may be slightly inward (towards the left in the figure) or outward (towards the right in the figure) from the end of the positive electrode side carbon coating layer 34p, but from a cost perspective, it is desirable for it to coincide with the end (1).

[0118] A method for manufacturing a lithium-ion secondary battery includes the steps of forming a positive electrode side carbon coating layer 34p on the surface of a positive electrode current collector 34a, forming a positive electrode mixture layer 34b on the surface of the positive electrode side carbon coating layer 34p, and drying the formed positive electrode side carbon coating layer 34p and positive electrode mixture layer 34b.

[0119] Those skilled in the art can use the above description to make the most of this disclosure. The claims and 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 embodiments described above can be made without departing from the basic principles of this disclosure. In other words, various modifications and improvements to the embodiments specifically disclosed above are within the scope of this disclosure.

[0120] 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 injection port, 10: Gas discharge valve, 11: Liquid injection 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, 33: Separator, 34: Positive electrode, 34a: Positive electrode current collector, 34b: Positive electrode mixture layer, 34c: Positive electrode current collector exposed portion, 34i: Insulating layer, 34p: Positive electrode side carbon coating layer, 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 positive electrode having a positive electrode mixture layer and a positive electrode current collector, a separator, and a negative electrode having a negative electrode mixture layer and a negative electrode current collector, wherein the separator is disposed between the positive electrode and the negative electrode, and a positive electrode side carbon coating layer is disposed between the positive electrode mixture layer and the positive electrode current collector, and the end of the positive electrode mixture layer is located at the same position as the end of the positive electrode side carbon coating layer, or at a position inward from the end of the positive electrode side carbon coating layer.

2. The lithium-ion secondary battery according to claim 1, wherein an insulating layer is disposed between the separator and the positive electrode side carbon coating layer, and the insulating layer is disposed such that it overlaps a portion of the positive electrode side carbon coating layer when projected toward the positive electrode side carbon coating layer in the stacking direction.

3. The lithium-ion secondary battery according to claim 1, wherein the positive electrode mixture layer contains an iron phosphate-based lithium-containing composite oxide as the positive electrode active material.

4. The lithium-ion secondary battery according to claim 1, wherein the negative electrode mixture layer has a smaller area than the positive electrode carbon coating layer.

5. The lithium-ion secondary battery according to claim 1, wherein the negative electrode mixture layer has a larger area than the positive electrode mixture layer.

6. The lithium-ion secondary battery according to claim 2, wherein the insulating layer is arranged to overlap a portion of the negative electrode mixture layer when projected toward the negative electrode mixture layer in the stacking direction.

7. The lithium-ion secondary battery according to claim 1, wherein the positive electrode carbon coating layer is thinner than the positive electrode mixture layer.

8. The lithium-ion secondary battery according to claim 2, wherein the end of the insulating layer is located inward from the end of the positive electrode side carbon coating layer.

9. The lithium-ion secondary battery according to claim 2, wherein the end of the positive electrode carbon coating layer is located inward from the end of the insulating layer.

10. A positive electrode for a lithium-ion secondary battery, comprising a positive electrode mixture layer and a positive electrode current collector, wherein a positive electrode side carbon coating layer is disposed between the positive electrode mixture layer and the positive electrode current collector, and the end of the positive electrode mixture layer is located at the same position as the end of the positive electrode side carbon coating layer, or at a position inward from the end of the positive electrode side carbon coating layer.

11. The positive electrode for a lithium-ion secondary battery according to claim 10, further comprising an insulating layer disposed opposite to the positive electrode side carbon coating layer, wherein the insulating layer is disposed in a region of the positive electrode side carbon coating layer other than the portion covered by the positive electrode mixture layer.

12. A method for manufacturing a lithium-ion secondary battery comprising: a positive electrode having a positive electrode mixture layer and a positive electrode current collector; a separator; and a negative electrode having a negative electrode mixture layer and a negative electrode current collector, wherein the separator is disposed between the positive electrode and the negative electrode; a positive electrode side carbon coat layer is disposed between the positive electrode mixture layer and the positive electrode current collector; and the end of the positive electrode mixture layer is located at the same position as the end of the positive electrode side carbon coat layer, or at a position inward from the end of the positive electrode side carbon coat layer, the method comprising: forming the positive electrode side carbon coat layer on the surface of the positive electrode current collector; forming the positive electrode mixture layer on the surface of the positive electrode side carbon coat layer; and drying the formed positive electrode side carbon coat layer and the positive electrode mixture layer.

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