Lithium-ion secondary battery

Abstract

This lithium-ion secondary battery comprises: a positive electrode having a positive electrode active material layer; a separator; and a negative electrode having a negative electrode active material layer, wherein the separator is disposed between the positive electrode and the negative electrode. The lithium-ion secondary battery has a rated capacity of 3 Ah to 7 Ah. The separator has a first layer containing polyethylene as a main component, and a second layer and a third layer containing polypropylene as a main component. The first layer is disposed between the second layer and the third layer, the ratio R defined by the following expression 1 is 2.0 to 6.0, and the thickness of the positive electrode active material layer is 30 µm to 65 µm.　(Expression 1): R = (the total thickness of the positive electrode active material layer) / (the thickness of the entire separator) (In the expression, the unit of thickness is [µm]).　Due to said feature, the output characteristics of the lithium-ion secondary battery can be improved not only under a rated temperature condition but also in a low-temperature region.

Description

Lithium-ion secondary battery

[0001] The present disclosure relates to lithium-ion secondary batteries.

[0002] In the automotive industry, fuel economy and environmental regulations are being strengthened in each country and region. To comply with these regulations, attention is being paid to the development of battery-powered electric vehicles and fuel cell vehicles (FCVs) that do not emit carbon dioxide. However, electric vehicles face issues such as insufficient charging infrastructure and the longer charging time required compared to refueling. Fuel cell vehicles face issues such as the significant cost of establishing hydrogen station infrastructure and the high cost of fuel cells. Therefore, plug-in hybrid electric vehicles (PHEVs) and hybrid electric vehicles (HEVs), which are powered by both an internal combustion engine and a battery and emit low carbon dioxide, are becoming leading candidates for complying with fuel economy and environmental regulations.

[0003] PHEVs and HEVs typically use lithium ion secondary batteries.

[0004] Patent Document 1 discloses that the thickness of each active material layer of a lithium ion secondary battery is preferably about 2 to 1500 μm, and that the thickness of a separator including a resin film having a three-layer structure made of polyolefin is 11 to 25 μm.

[0005] Patent Document 2 discloses a polyolefin battery separator membrane for secondary lithium batteries or rechargeable lithium batteries, in which the thickness of the microporous membrane is less than 25 μm, as a separator for lithium ion secondary batteries.

[0006] Patent Document 3 discloses a separator having a predetermined air permeability, the separator comprising a polyolefin microporous film having a predetermined thickness and an inorganic particle layer containing inorganic particles formed on one surface of the polyolefin microporous film.

[0007] Republished Patent Publication No. 2018 / 029832 Special Publication No. 2018-516441 Japanese Patent Application Laid-Open No. 2020-145123

[0008] In recent years, customer requirements for HEVs have shifted toward lower temperatures, resulting in an increase in current values. Therefore, there is a demand for improved output characteristics, not only under the temperature conditions (hereinafter referred to as "rated temperature conditions") (e.g., 25°C) when measuring rated characteristics, but also in low-temperature regions (e.g., -10°C). To achieve improved output characteristics, it may be necessary to suppress increases in cell resistance. Factors that increase cell resistance include those attributable to materials (electrode active material, separator, electrolyte) and components (e.g., positive electrode aluminum foil). Regarding materials, cell resistance is significantly affected by the specifications of the electrode active material and separator, in particular, so it may be desirable to standardize these physical properties.

[0009] The technology described in Patent Document 1 is intended to increase capacity, and is not intended to improve output characteristics. Furthermore, the technologies described in Patent Documents 2 and 3 do not describe the specifications of the electrode active material layer, and therefore are not intended to improve output characteristics, and technology for realizing the improvement of output characteristics is desired.

[0010] The present disclosure has been made in view of such problems, and has as its main object to improve the output characteristics of a lithium ion secondary battery not only under rated temperature conditions but also in low temperature ranges.

[0011] In order to solve the above problems, the lithium ion secondary battery of the present disclosure includes a positive electrode having a positive electrode active material layer, a separator, and a negative electrode having a negative electrode active material layer, with the separator being disposed between the positive electrode and the negative electrode, the lithium ion secondary battery having a rated capacity of 3 Ah or more and 7 Ah or less, the separator having a first layer mainly composed of polyethylene and second and third layers mainly composed of polypropylene, the first layer being disposed between the second and third layers, a ratio R defined by the following (Equation 1) being 2.0 or more and 6.0 or less, and the thickness of the positive electrode active material layer being 30 μm or more and 65 μm or less.

[0012] R = (total thickness of positive electrode active material layer) / (total thickness of separator) (Equation 1) (wherein the unit of thickness is [μm].)

[0013] According to the present disclosure, it is possible to improve the output characteristics of a lithium ion secondary battery not only under rated temperature conditions but also in low temperature ranges.

[0014] It is an external perspective view of the lithium ion secondary battery according to the embodiment. It is an exploded perspective view of the lithium ion secondary battery according to the embodiment. It is an exploded perspective view showing a state in which a part of the wound pack according to the embodiment is developed. It is a schematic cross-sectional view showing a separator according to the embodiment.

[0015] Hereinafter, embodiments will be described with reference to the drawings as appropriate. The following description illustrates specific examples of the contents of the present disclosure, and the present disclosure is not limited to these descriptions. Various changes and modifications can be made by those skilled in the art within the scope of the technical ideas disclosed herein. Furthermore, in all drawings used to explain the present disclosure, components having the same function are designated by the same reference numerals, and repeated description thereof may be omitted. Furthermore, for the sake of convenience, the dimensional ratios of the drawings may differ from the actual ratios, and some components may be omitted from the drawings. Furthermore, in this application, a numerical range expressed using the symbol "to" includes the numerical values ​​before and after the symbol "to" as the lower and upper limits, respectively.

[0016] FIG. 1 is a perspective view of the appearance of a lithium ion secondary battery according to an embodiment.

[0017] FIG. 2 is an exploded perspective view of the lithium ion secondary battery according to the embodiment.

[0018] 1 and 2 includes a battery can 1 and a battery lid 6. As shown in Fig. 2, the battery can 1 has a rectangular bottom surface 1d, side surfaces including a pair of opposing wide side surfaces 1b having relatively large areas and a pair of opposing narrow side surfaces 1c having relatively small areas, rising from the bottom surface 1d, and an opening 1a that opens upward at the upper ends of the wide side surfaces 1b and the narrow side surfaces 1c. Note that "upward" refers to the Z direction shown in Figs. 1 and 2.

[0019] The opening 1a of the battery can 1 is sealed by a battery lid 6. The battery lid 6 has a substantially rectangular flat plate shape and is welded to close the opening 1a of the battery can 1, thereby sealing the battery can 1.

[0020] A gas release valve 10 is integrally provided on the battery lid 6. When the pressure inside the battery can 1 increases, the gas release valve 10 opens, allowing gas to be released from inside the battery can 1, thereby reducing the pressure inside the battery can 1. This ensures the safety of the lithium-ion secondary battery 100.

[0021] A liquid filling port 9 for filling the electrolyte into the battery can 1 is formed in the battery lid 6. The liquid filling port 9 is sealed by a liquid filling plug 11 after the electrolyte is filled into the battery can 1. The liquid filling plug 11 is joined to the battery lid 6 by laser welding to seal the liquid filling port 9 and hermetically seal the lithium ion secondary battery 100.

[0022] The battery cover 6 is further provided with a positive electrode side through-hole 46 and a negative electrode side through-hole 26 .

[0023] A positive electrode external terminal 14 and a negative electrode external terminal 12 are provided above the battery lid 6. A positive electrode current collector plate 44 and a negative electrode current collector plate 24 are provided below the battery lid 6 and inside the battery can 1.

[0024] The positive external terminal 14 and the positive current collector plate 44 may be made of, for example, an aluminum alloy, and the negative external terminal 12 and the negative current collector plate 24 may be made of, for example, a copper alloy.

[0025] 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. The welded joint has a rectangular block shape that protrudes upward from the battery lid 6. The lower surface of the welded joint faces the surface of the battery lid 6, and the upper surface of the welded joint is located at a predetermined height and is approximately parallel to the battery lid 6.

[0026] The positive current collector 44 has a rectangular plate-shaped positive current collector base 41 facing the underside of the battery lid 6, and a positive electrode side connection end 42 extending from a side end of the positive current collector base 41 along the wide side surface 1 b of the battery can 1 toward the bottom surface 1 d. Similarly, the negative current collector 24 has a rectangular plate-shaped negative current collector base 21 facing the underside of the battery lid 6, and a negative electrode side connection end 22 extending from a side end of the negative current collector base 21 along the wide side surface 1 b of the battery can 1 toward the bottom surface 1 d. A positive electrode side opening hole 43 and a negative electrode side opening hole 23 are formed in the positive current collector base 41 and the negative current collector base 21, respectively.

[0027] A positive electrode connecting portion 14a and a negative electrode connecting 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 connecting portion 14a and the negative electrode connecting portion 12a are formed integrally with the positive electrode external terminal 14 and the negative electrode external terminal 12, respectively.

[0028] 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 lid 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 lid 6 and the negative electrode side opening hole 23 of the negative electrode current collector base 21. The positive electrode connection portion 14a passes through the positive electrode side through-hole 46 of the battery lid 6 and the positive electrode side opening hole 43 of the positive electrode current collector base 41, penetrating the battery lid 6 and the positive electrode current collector base 41. The positive electrode external terminal 14 and the positive electrode current collector 44 are electrically connected and fixed to the battery lid 6 via the positive electrode connection portion 14a. Similarly, the negative electrode connection portion 12a passes through the negative electrode side through-hole 26 of the battery lid 6 and the negative electrode side opening hole 23 of the negative electrode current collector base 21, penetrating the battery lid 6 and the negative electrode current collector base 21. The negative electrode external terminal 12 and the negative electrode current collector plate 24 are electrically connected via a negative electrode connecting portion 12 a and are fixed to the battery lid 6 .

[0029] The positive electrode external terminal 14 is electrically connected to the wound pack 3 (described later) via the positive electrode connection portion 14 a and the positive electrode current collector plate 44. Similarly, the negative electrode external terminal 12 is electrically connected to the wound pack 3 via the negative electrode connection portion 12 a and the negative electrode current collector plate 24. When the lithium ion secondary battery 100 is being charged, electricity is supplied from an external power source to the wound pack 3 via the positive electrode external terminal 14, the positive electrode connection portion 14 a, and the positive electrode current collector plate 44, as well as the negative electrode external terminal 12, the negative electrode connection portion 12 a, and the negative electrode current collector plate 24. When the lithium ion secondary battery 100 is being discharged, electricity is supplied from the wound pack 3 to an external load via the positive electrode external terminal 14, the positive electrode connection portion 14 a, and the positive electrode current collector plate 44, as well as the negative electrode external terminal 12, the negative electrode connection portion 12 a, and the negative electrode current collector plate 24.

[0030] In order to electrically insulate the positive electrode current collector 44, the negative electrode current collector 24, the positive electrode external terminal 14, and the negative electrode external terminal 12 from the battery lid 6, a gasket 5 is provided between each of the positive electrode external terminal 14 and the negative electrode external terminal 12 and the battery lid 6, and an insulating plate 7 is provided between each of the positive electrode current collector 44 and the negative electrode current collector 24 and the battery lid 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.

[0031] The battery can 1 contains an electrolyte solution EL and a wound group 3. The electrolyte solution is injected into the battery can 1 through an injection port 9.

[0032] FIG. 3 is an exploded perspective view showing a state in which a part of the wound group according to the embodiment is developed.

[0033] As shown in the figure, the wound pack 3 has a negative electrode 32, a positive electrode 34, and two separators 33 and 35. The separator 35, the negative electrode 32, the separator 33, and the positive electrode 34 are stacked in this order and wound in a flat shape. The separator 35 is located at the outermost periphery of the wound pack 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.

[0034] FIG. 4 is a schematic cross-sectional view showing a separator according to an embodiment.

[0035] As shown in this figure, the separator 33 has a three-layer structure in which a central polyethylene layer 33a (first layer) is sandwiched between outer polypropylene layers 33b (second layer) and 33c (third layer).

[0036] Although not shown, the separator 35 also has a three-layer structure similar to the separator 33 .

[0037] 3, the wound group 3 has a pair of opposing end faces 3a, 3b perpendicular to the winding axis and a side face 3c located between the pair of end faces 3a, 3b. The side face 3c has a pair of curved portions that face each other and have a semicircular cross section, and a flat portion that is formed continuously between the pair of curved portions. The wound group 3 is placed in the battery can 1 so that the flat portion of the side face 3c and the wide side face 1b (FIG. 2) of the battery can 1 are approximately parallel.

[0038] The positive electrode, negative electrode, and electrolyte of the lithium-ion secondary battery will be described below. 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.

[0039] 3, the positive electrode 34 includes a positive electrode current collector 34a and a positive electrode mixture layer 34b (positive electrode active material layer) provided on at least one surface of the positive electrode current collector 34a. The positive electrode mixture layer 34b is preferably provided on both surfaces of the positive electrode current collector 34a.

[0040] The positive electrode current collector 34a is formed from any material that is highly conductive and does not alloy with lithium ions. The positive electrode current collector 34a may have a plate (sheet) shape. For example, a positive electrode foil, specifically, a metal foil such as aluminum foil, can be used as the positive electrode current collector 34a. A positive electrode current collector exposed portion 34c, which is a portion not covered with the positive electrode mixture layer 34b, is provided at one end of the positive electrode current collector 34a in the width direction. 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 ( FIG. 2 ) of the positive electrode current collector plate 44.

[0041] The positive electrode mixture layer 34b contains a positive electrode active material. The positive electrode active material contains, for example, a ternary material containing at least nickel, cobalt, and manganese. Specifically, a composite oxide of nickel, cobalt, and manganese can be used. Here, the positive electrode active material may contain aluminum instead of manganese.

[0042] 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).

[0043] Li １＋Ｘ M Ａ O ２ ... (1) (wherein X satisfies -0.15≦X≦0.15, and 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) above has high thermal stability and stability in a high potential state, and by using this oxide, the safety and various battery characteristics of a lithium ion secondary battery can be improved.

[0044] The positive electrode mixture layer 34b contains at least one of a conductive additive and a binder, and preferably contains both of them.

[0045] The conductive additive is not particularly limited, and may be, for example, a carbon-based material. 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), or a mixture thereof. Examples of amorphous carbon include carbon black (e.g., acetylene black, ketjen black, channel black, furnace black, lamp black, thermal black, or a mixture thereof).

[0046] The binder is not particularly limited, but examples thereof include polyvinylidene fluoride (PVdF), polytetrafluoroethylene (PTFE), polyethylene, polystyrene, polybutadiene, polyacrylonitrile, polyvinyl fluoride, polypropylene fluoride, polychloroprene fluoride, butyl rubber, nitrile rubber, styrene butadiene rubber (SBR), polysulfide rubber, nitrocellulose, cyanoethyl cellulose, various latexes or acrylic resins, or mixtures thereof.

[0047] The positive electrode 34 can be formed, for example, as follows.

[0048] 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 or slurry positive electrode mixture composition. This positive electrode mixture composition is applied to the surface (one or both sides) of a positive electrode current collector 34a, dried, and optionally subjected to a calendar treatment to form a positive electrode mixture layer 34b. This results in a positive electrode 34. However, the positive electrode is not limited to one formed by the above method, and may be formed by other methods. The thickness of the positive electrode active material layer is set to 30 μm or more and 65 μm or less from the viewpoint of producing a high-quality film while reducing the thickness in anticipation of higher output.

[0049] 3 includes a negative electrode current collector 32 a and a negative electrode mixture layer 32 b (negative electrode active material layer) provided on at least one surface of the negative electrode current collector 32 a. Note that the negative electrode mixture layer 32 b is preferably provided on both surfaces of the negative electrode current collector 32 a.

[0050] The negative electrode current collector 32a is formed from any material that is highly conductive and does not alloy with lithium ions. An exposed portion 32c of the negative electrode current collector 32a, which is a portion not covered with the negative electrode mixture layer 32b, is provided at one end of the negative electrode current collector 32a in the width direction. The exposed portion 32c of the negative electrode current collector 32a is provided on and near the end face 3b of the wound group 3. The exposed portion 32c of the negative electrode current collector 32a faces the negative electrode connection end 22 (FIG. 2) of the negative electrode current collector plate 24 and is electrically connected thereto.

[0051] The portion of the negative electrode 32 coated with the negative electrode mixture layer 32b is preferably wider in the width direction than the portion of the positive electrode 34 coated with the positive electrode mixture layer 34b, such that the portion coated with the positive electrode mixture layer 34b is sandwiched between the portions coated with the negative electrode mixture layer 32b. The positive electrode current collector exposed portion 34c and the negative electrode current collector exposed portion 32c are preferably bundled together at their flat portions and connected by welding or the like. Note that although the separators 33 and 35 are wider in the width direction than the portion coated with the negative electrode mixture layer 32b, they are wound around the ends of the positive electrode current collector exposed portion 34c and the negative electrode current collector exposed portion 32c at positions where the current collectors are exposed, so this does not hinder bundling and welding.

[0052] The negative electrode mixture layer 32b includes a negative electrode active material containing graphite particles and amorphous carbon fine particles. The negative electrode active material is not particularly limited as long as it contains graphite particles and amorphous carbon fine particles, but the amorphous carbon fine particles are preferably supported on the graphite particles, and particularly, the graphite particles are preferably a mixture of graphite particles (A) and graphite particles (B) supporting the amorphous carbon fine particles.

[0053] The negative electrode mixture layer 32b contains at least one of a negative electrode additive and a binder in addition to the negative electrode active material. Preferably, both of these are contained. In one aspect of the embodiment, the negative electrode mixture layer 32b contains a negative electrode active material containing graphite particles (A) and graphite particles (B) carrying amorphous carbon fine particles, and the graphite particles (B) carrying amorphous carbon fine particles have a density of 0.4 amorphous carbon fine particles per unit area / μm. ２ That's all. The negative electrode mixture layer 32b may be formed only from graphite particles (A) without including graphite particles (B) carrying amorphous carbon microparticles. In another aspect of the embodiment, the negative electrode mixture layer 32b has a negative electrode active material and a binder that holds the negative electrode active material, and the negative electrode active material contains graphite particles and amorphous carbon microparticles. The negative electrode mixture layer 32b may further include a negative electrode additive containing copper oxide.

[0054] The graphite particles are not particularly limited, but examples thereof include natural graphite particles and artificial graphite particles, with natural graphite particles being preferred. Examples of natural graphite include flake graphite, lump graphite, and amorphous graphite. The graphite particles (A) and the graphite particles constituting the graphite particles (B) carrying amorphous carbon fine particles may be the same type of graphite particles or different types of graphite particles.

[0055] Examples of negative electrode active materials other than graphite include carbon-based materials such as non-graphitizable carbon (hard carbon) and easily graphitizable carbon (soft carbon).

[0056] Regarding graphite, graphite whose surface is coated with amorphous carbon may be used. Coating with amorphous carbon can prevent reaction with excess electrolyte. Examples of amorphous carbon include pitch. That is, the graphite particles constituting the graphite particles (A) and the graphite particles (B) carrying amorphous carbon microparticles are preferably graphite particles coated with amorphous carbon, more preferably pitch-coated graphite particles. Furthermore, the graphite particles constituting the graphite particles (A) and the graphite particles (B) carrying amorphous carbon microparticles are particularly preferably natural graphite particles coated with amorphous carbon.

[0057] The supported amorphous carbon fine particles constituting the graphite particles (B) carrying amorphous carbon fine particles are not particularly limited, and examples thereof include carbon black such as acetylene black, ketjen black, channel black, furnace black, lamp black, and thermal black. The supported amorphous carbon fine particles refer to amorphous carbon particles scattered on the surface of graphite particles, and are different from the amorphous carbon coating graphite particles described above. The amorphous carbon coating graphite particles refers to amorphous carbon covering the entire or part of the surface of graphite particles.

[0058] The graphite particles (B) carrying amorphous carbon particles have an amorphous carbon particle count of 0.4 particles / μm per unit area. ２ or more, and preferably the number of amorphous carbon particles per unit area is 0.4 / μm ２ ~2.4 pieces / μm ２ and more preferably, the number of amorphous carbon particles per unit area is 0.8 / μm ２ ~2.0 pieces / μm ２ In addition, when the graphite particles support a small amount of amorphous carbon fine particles, that is, when the number of amorphous carbon fine particles per unit area is 0.4 particles / μm ２ Graphite particles carrying amorphous carbon particles with a particle size of less than 0.4 particles / μm do not fall under the category of graphite particles (B). ２ Graphite particles carrying amorphous carbon fine particles at a molecular weight of less than 1000 may be used as the graphite particles (A).

[0059] The average particle size of the graphite particles (A) is preferably 4 μm or more and 12 μm or less, and more preferably 5 μm or more and 10 μm or less.

[0060] The average particle size of the graphite particles (B) carrying amorphous carbon fine particles is preferably 4 μm or more and 12 μm or less, more 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) carrying amorphous carbon fine particles may be the same or different.

[0061] The average particle size of the supported amorphous carbon fine particles constituting the graphite particles (B) on which amorphous carbon fine particles are supported is preferably smaller than the average particle size of the graphite particles (A) and the average particle size of the graphite particles constituting the graphite particles (B) on which amorphous carbon fine particles are supported. 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.

[0062] As the negative electrode active material containing graphite particles (A) and graphite particles (B) carrying amorphous carbon fine particles, for example, a negative electrode active material in which graphite particles (A) not carrying amorphous carbon fine particles and graphite particles (B) carrying amorphous carbon fine particles (amorphous carbon fine particles carried by graphite particles) are present in a mixed state in the negative electrode active material layer is preferred.

[0063] When the negative electrode active material contains graphite particles (A) and graphite particles (B) carrying amorphous carbon fine particles, the mass ratio thereof (graphite particles (A)) / (graphite particles (B) carrying amorphous carbon fine particles) is preferably 0.25 or more and 5 or less, more preferably 0.5 or more and 2 or less.

[0064] The negative electrode active material may be, for example, a material obtained by mixing a graphite material with carbon black such as acetylene black, ketjen black, channel black, furnace black, lamp black, or thermal black as a conductive additive, a composite material obtained by mixing such a conductive additive with a graphite material and then coating the graphite material with amorphous carbon, or a material obtained by mixing a graphite material with non-graphitizable carbon (hard carbon), easily graphitizable carbon (soft carbon), or the like. The shape of the negative electrode active material is not particularly limited, and may be, for example, spherical, flaky, fibrous, or a pulverized form of these.

[0065] As a result of extensive research, the present inventors have found that by using a negative electrode active material containing graphite particles (A) and graphite particles (B) carrying amorphous carbon fine particles, a lithium ion secondary battery can reduce internal resistance over a wide range from a low SOC region to a high SOC region while ensuring battery capacity, and therefore has high output and good storage characteristics.

[0066] Furthermore, as a result of extensive research, the present inventors have found that by configuring the negative electrode active material to contain graphite particles and amorphous carbon fine particles, and the negative electrode active material layer to have the 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 a low SOC region to a high SOC region while ensuring battery capacity, and therefore has high output and good storage characteristics.

[0067] The negative electrode active material layer preferably contains copper oxide as a negative electrode additive. By configuring the negative electrode active material layer to contain 0.5% by mass or more and 15% by mass or less of copper oxide, based on 100% by mass of the total of the negative electrode active material and copper oxide, the lithium ion secondary battery can further ensure battery capacity while reducing internal resistance over a wide range from low SOC to high SOC. This allows the lithium ion secondary battery to have high output and good storage characteristics.

[0068] Copper oxide is copper(I) oxide (Cu ２Copper oxide may be copper(II) oxide (CuO) or copper(II) oxide (CuO), or a mixture thereof. ２ The negative electrode additive may be at least one copper oxide selected from O and CuO. The negative electrode additive may be in a particulate form. The shape of the particulate negative electrode additive is not particularly limited, and may be, for example, spherical, scaly, fibrous, or a pulverized form thereof. The particulate negative electrode additive may be copper(I) oxide (Cu ２ The particles may be particles containing copper(II) oxide (CuO) or copper(II) oxide (CuO) or a mixture thereof, and may be substantially copper(I) oxide (Cu ２ The negative electrode additive may be particles of copper(II) oxide (CuO), copper(II) oxide (CuO), or a mixture thereof. These various particulate negative electrode additives may be used alone or in combination.

[0069] In the negative electrode mixture layer 32b shown in FIG. 3, the negative electrode active material and the negative electrode additive may exist as separate particles that are not composited with each other. This allows the negative electrode active materials to be electrically connected to each other without being hindered by the negative electrode additive having 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 to each other without being hindered by the negative electrode additive having high electrical resistance. This allows an increase in the internal resistance of the lithium-ion secondary battery to be suppressed. 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 images observed with a scanning electron microscope (SEM).

[0070] The binder for the negative electrode mixture layer 32b may be the same as the materials exemplified as the binder for the positive electrode mixture layer 34b.

[0071] The negative electrode mixture layer 32b may further contain a dispersant, such as carboxymethyl cellulose (CMC).

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

[0073] First, a negative electrode active material, a negative electrode additive containing copper oxide, a binder, and optionally a dispersant are prepared. The negative electrode active material and the negative electrode additive may be in particulate form. The negative electrode active material and the negative electrode additive may be separate particles that are not composited with each other. The negative electrode active material, the negative electrode additive, the binder, and optionally the dispersant are dispersed in a solvent (e.g., N-methyl-2-pyrrolidone (NMP) or water) to prepare a paste or slurry 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 optionally subjected to a calendar treatment to form the negative electrode mixture layer 32b. This results in the negative electrode 32. However, the negative electrode 32 is not limited to one formed by the above method and may be formed by other methods. The thickness of the negative electrode active material layer is set to 30 μm or more and 65 μm or less from the viewpoint of producing a high-quality film while reducing the thickness in anticipation of high output.

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

[0075] 1. Structure The resin porous sheet has a structure in which three layers of films made of one resin material are laminated. One resin material may be a mixture of two or more types of polymer compounds. From the viewpoint of ion conductivity, the number of layers of the resin film is preferably three. Note that the number of layers of the resin film may be three or more.

[0076] 2. Form The resin film may be in any form as long as it can function as a separator. Specific examples include porous resin films, woven fabrics, and nonwoven fabrics, but a porous resin film is preferred.

[0077] 3. Constituent Components The constituent components of the resin porous sheet are preferably those made of hydrocarbon resins such as polyolefins. Specific examples include, but are not limited to, polyethylene (PE), polypropylene (PP), polyimide, and aramid. Among these, the constituent components of the resin porous sheet are preferably polyolefin films, with PE and PP being particularly preferred. In an embodiment according to the present disclosure, a separator having a three-layer structure of PP / PE / PP is preferred, with the first central layer being made of PE and the second and third outer layers being made of PP.

[0078] 4. Thickness The 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 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 achieving high output in the low temperature range, the thickness of the separator is preferably 12 μm to 20 μm, and more preferably 14 μm to 18 μm, as the total thickness of the three layers. The thickness of the first layer is preferably 3 μm to 9 μm, and more preferably 3 μm to 6 μm. The thickness of the second and third layers is preferably 3 μm to 9 μm, and more preferably 3 μm to 6 μm, respectively.

[0079] 5. Ratio of Thickness of Positive Electrode Active Material Layer to Thickness of Separator From the viewpoint of achieving high output, it is preferable to define the ratio R of the thickness of the electrode active material layer to the thickness of the separator. The ratio R is defined by the following (Equation 1), where the unit of thickness is [μm].

[0080] R = (total thickness of positive electrode active material layer) / (total thickness of separator) (Equation 1) Because the thickness of the negative electrode active material layer fluctuates significantly after injection, it is particularly preferable to define the thickness of the positive electrode active material layer, which has less thickness fluctuation. Therefore, in the present disclosure, a ratio range of the total thickness of the positive electrode active material layer to the total thickness of the separator is defined. When R is calculated based on a total thickness range of 30 μm to 65 μm of the positive electrode active material layer and a total thickness range of 12 μm to 20 μm of the separator, an R of 2.0 or more and 6.0 or less can improve output characteristics. An R of 2.3 or more and 5.5 or less is more preferable because it reduces the direct current resistance (DCR) of the lithium ion secondary battery and improves output characteristics. Furthermore, an R of 2.7 or more and 5.0 or less is particularly preferable because it further reduces the DCR of the lithium ion secondary battery and further improves output characteristics.

[0081] 6. Air Permeability The air permeability of the separator may be in the range of 100 sec / 100 cc to 300 sec / 100 cc, and preferably in the range of 120 sec / 100 cc to 250 sec / 100 cc. If the air permeability is less than 100 sec / 100 cc, the mechanical strength of the separator may be reduced. On the other hand, if the air permeability exceeds 300 sec / 100 cc, the DCR of the lithium-ion secondary battery may increase, resulting in a reduction in output characteristics.

[0082] 7. Tortuosity (Tortuosity) The tortuosity T of a separator is a value obtained by dividing the pore path length by the thickness of the porous membrane, and is defined by the following (Equation 2) by measuring the membrane resistance in a state where the separator is immersed in an electrolyte.

[0083] T = {(R ｍ ε) / (ρ t) ０．５ ...(Formula 2) In the formula, R ｍ is the membrane resistance of the separator, ε is the porosity, ρ is the specific resistance of the electrolyte, and t is the thickness of the separator.

[0084] The flexion degree of the separator may be in the range of 1.0 to 3.0, and is particularly preferably in the range of 1.5 to 2.5. If the flexion degree is less than 1.0, the separator is more likely to break, and if it exceeds 3.0, the DCR of the lithium ion secondary battery increases.

[0085] <Electrolyte> The electrolyte (nonaqueous electrolyte) of a lithium ion secondary battery contains at least a nonaqueous solvent, an electrolyte salt, and an additive. When the electrolyte contains an additive in addition to the nonaqueous solvent and the electrolyte salt, a protective film called a solid electrolyte interface (SEI) formed on the surface of the negative electrode active material of the negative electrode is strengthened, and side reactions occurring at the interface between the electrolyte and the negative electrode active material are suppressed even in a high-temperature storage environment.

[0086] The electrolyte salt, additives, and non-aqueous solvent according to the embodiment will be specifically described below.

[0087] 1. Electrolyte Salt In the electrolyte solution, at least one type of electrolyte salt is dissolved in a non-aqueous solvent. The electrolyte salt is a lithium salt, and the lithium salt is preferably, for example, a lithium salt containing fluorine (fluorine-containing lithium salt). Specifically, the fluorine-containing lithium salt is lithium hexafluorophosphate (LiPF ６ ), lithium tetrafluoroborate (LiBF ４ ), inorganic anion salts such as lithium trifluoromethanesulfonate (LiCF ３ SO ３ ), and in particular, organic anion salts such as LiPF ６ is preferred.

[0088] The lithium salt may be used alone or in any combination of two or more kinds in any ratio. The content of the lithium salt contained in the electrolyte solution is arbitrary as long as it does not significantly impair the effects of the present disclosure, but is, for example, 0.01% by mass or more, preferably 0.1% by mass or more. The content of the lithium salt contained in the electrolyte solution is, for example, 30% by mass or less, preferably 20% by mass or less.

[0089] 2. Additives The additives preferably contain lithium borate, difluorophosphate, and vinylene carbonate compounds, and may also contain fluorosulfonate and methoxysulfonate.

[0090] Specific examples of lithium borates include lithium bis(oxalato)borate (LiBOB) and lithium difluoro(oxolato)borate (LiDFOB), with LiBOB being particularly preferred.

[0091] The lithium borate may be used alone or in any combination of two or more kinds at any ratio. The content of the lithium borate contained in the electrolyte solution 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. The content of the lithium borate contained in the electrolyte solution is, for example, 3% by mass or less, preferably 2% by mass or less, and more preferably 1% by mass or less. Within such a range, the effect of improving the high-temperature storage characteristics of the lithium ion secondary battery can be easily exhibited, and an increase in negative electrode resistance due to excessive addition can be avoided.

[0092] The counter cation of the difluorophosphate is not particularly limited, but examples thereof include lithium, sodium, potassium, and the like.

[0093] Specific types of difluorophosphates include lithium difluorophosphate (LiPO ２ F ２ ), sodium difluorophosphate, potassium difluorophosphate, etc., and lithium difluorophosphate is particularly preferred.

[0094] The difluorophosphate may be used singly or in any combination of two or more kinds at any ratio. The content of the difluorophosphate contained in the electrolytic solution is, for example, 0.001% by mass or more, preferably 0.1% by mass or more, more preferably 0.5% by mass or more. The content of the difluorophosphate contained in the electrolytic solution is, for example, 3% by mass or less, preferably 2% by mass or less, more preferably 1% by mass or less.

[0095] Specific examples of vinylene carbonate compounds include vinylene carbonate (1,3-dioxol-2-one), methyl vinylene carbonate (4-methyl-1,3-dioxol-2-one), and ethyl vinylene carbonate (4-ethyl-1,3-dioxol-2-one).

[0096] The vinylene carbonate compound may be used alone or in combination of two or more kinds in any ratio. ３ H ２ O ３ ) is particularly preferred because it provides excellent effects.

[0097] The content of the vinylene carbonate compound contained in the electrolyte solution 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. The content of the vinylene carbonate compound contained in the electrolyte solution 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, the effect of improving the high-temperature storage characteristics of the lithium-ion secondary battery can be easily achieved, and battery swelling due to increased gas generation can be avoided.

[0098] 3. Nonaqueous Solvent The electrolyte solution contains at least one nonaqueous solvent. Specific examples of the nonaqueous solvent include cyclic carbonates, fluorine-containing cyclic carbonates, chain carbonates, and fluorine-containing chain carbonates.

[0099] Specific examples of cyclic carbonates include ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), etc. Specific examples of fluorine-containing cyclic carbonates include fluoroethylene carbonate (FEC), etc. Specific examples of chain carbonates include dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), etc.

[0100] The nonaqueous solvent may be used alone or in any combination of two or more types in any ratio. From the viewpoint of the balance between the dielectric constant and viscosity of the electrolyte, the nonaqueous solvent preferably contains at least a cyclic carbonate and a chain carbonate. Specifically, the nonaqueous solvent preferably contains at least one cyclic carbonate solvent and at least one chain carbonate solvent. In this case, the volume ratio of the cyclic carbonate to the chain carbonate contained in the nonaqueous solvent is preferably 10:90 to 50:50. Furthermore, the nonaqueous solvent preferably contains ethylene carbonate as the cyclic carbonate and ethyl methyl carbonate and dimethyl carbonate as the chain carbonate.

[0101] <Others> If necessary, a core (not shown) may be disposed on the innermost periphery of the wound pack 3. As the core, a resin sheet or the like having higher bending rigidity than any of the positive electrode current collector, the negative electrode current collector, and the separators 33 and 35 can be used.

[0102] An insulating protective film (not shown) may be optionally wrapped around the wound group 3. The insulating protective film is not particularly limited, but may be, for example, a single sheet or a plurality of film members made of a synthetic resin such as PP (polypropylene), and has a length that allows it to be wound around the winding center in a direction parallel to the flat surface of the wound group 3 and perpendicular to the winding axis direction.

[0103] The present disclosure will be specifically described below using examples, but the present disclosure is not limited to these examples.

[0104] [Example 1] First, Li was used as a positive electrode active material. １．０ Ni ０．３３ Co ０．３３ Mn ０．３３ O ２ Powder, acetylene black as a conductive additive, and polyvinylidene fluoride (PVdF) as a binder were prepared.

[0105] Next, the positive electrode active material, the conductive additive, and the binder were mixed in a mass ratio of 90:5:5. N-methyl-2-pyrrolidone (NMP) was added to the obtained mixture to adjust the viscosity, thereby obtaining a positive electrode slurry (a slurry-like positive electrode mixture composition).

[0106] Next, a 15 μm thick aluminum foil was prepared as the positive electrode current collector 34a. Next, leaving uncoated areas on both sides of the positive electrode current collector 34a to serve as welds (positive electrode current collector exposed portions 34c), the positive electrode slurry was simultaneously applied in two layers using a slot die coating method to form a positive electrode slurry layer. The positive electrode slurry layer was then dried and pressed to form a positive electrode mixture layer 34b, producing the positive electrode 34 shown in FIG. 3 . The total thickness of the positive electrode active material layer was 63 μm.

[0107] Next, as negative electrode active materials, natural graphite particles coated with amorphous carbon (negative electrode active material A), and natural graphite particles coated with amorphous carbon and carrying amorphous carbon fine particles (negative electrode active material B), styrene butadiene rubber (SBR) as a binder, and carboxymethyl cellulose (CMC) as a dispersant were prepared.

[0108] Next, negative electrode active material A, negative electrode active material B, binder, and dispersant were mixed so that the mass ratio of the total of negative electrode active material A and negative electrode active material B:binder:dispersant was 98:1:1. The contents (total 98 mass%) of negative electrode active material A and negative electrode active material B and the specifications are shown in Table 1 below. Ion-exchanged water was added to the obtained mixture to adjust the viscosity, and a negative electrode slurry (slurry-like negative electrode mixture composition) was obtained.

[0109] Next, a 10 μm-thick copper foil was prepared as the negative electrode current collector 32a. Next, an uncoated portion to serve as the weld (negative electrode current collector exposed portion 32c) was left on both sides of the negative electrode current collector 32a, and the negative electrode slurry was simultaneously applied in two layers by slot die coating. The negative electrode slurry layer was then dried and pressed to form the negative electrode mixture layer 32b, thereby producing the negative electrode 32 shown in FIG. 3 .

[0110] Next, a three-layer separator consisting of PP / PE / PP layers with a thickness of 18 μm was prepared as a separator, i.e., R=3.75.

[0111] Next, separators 33 and 35 were sandwiched between the prepared positive electrode 34 and negative electrode 32 to prepare a wound pack 3 having the configuration shown in Fig. 3. The positive electrode side connection end 42 and the negative electrode side connection end 22 of the positive electrode current collector 44 and the negative electrode current collector 24 connected to the battery lid 6 were welded to the uncoated portions of the wound pack 3 (positive electrode current collector exposed portion 34c, negative electrode current collector exposed portion 32c), the wound pack 3 was covered with an insulating protective film (not shown), the wound pack 3 was sealed in a battery can 1, and the battery lid 6 and the battery can 1 were welded together (see Figs. 1 and 2).

[0112] Next, a non-aqueous solvent containing a cyclic carbonate (ethylene carbonate (EC)) and a chain carbonate (ethyl methyl carbonate (EMC) and dimethyl carbonate (DMC)) with a volume ratio of 30:70 was prepared. Next, an electrolyte salt and an additive were added to this non-aqueous solvent to prepare an electrolyte solution. Next, the prepared electrolyte solution was poured into the battery can 1 through the pouring port 9 of the battery lid 6, and the pouring port 9 was then sealed with the pouring plug 11, thereby producing a lithium-ion secondary battery 100.

[0113] Examples 2 to 4 and Comparative Examples 1 and 2 A lithium ion secondary battery 100 was fabricated in the same manner as in Example 1, except that the thickness of the positive electrode and the specifications of the separator were changed as shown in Table 1 below.

[0114] [Evaluation] The lithium ion secondary batteries prepared in Examples 1 to 4 and Comparative Examples 1 and 2 were initialized by charging and discharging, and then the output characteristics were evaluated according to the following procedure. The evaluation results are shown in Table 1 below.

[0115] <Evaluation of Output Characteristics> A lithium-ion secondary battery placed in a thermostatic chamber at 25°C was subjected to constant-current discharge (CC discharge) at a discharge current of 1C until the battery voltage reached 2.8V. Then, the lithium-ion secondary battery placed in the thermostatic chamber at 25°C was subjected to CC-CV charging at a charging current of 1C for 1.5 hours until the battery's state of charge (SOC) reached 50%. After a 30-minute pause, the battery was discharged at a current of 150A for 10 seconds, and the voltage drop due to discharge was measured. The DCR at 50% SOC was calculated from the measured voltage drop. The DCR (relative value) at 50% SOC of the lithium-ion secondary battery of Example 1 was normalized to 100 for the lithium-ion secondary battery of Example 1. Table 1 below shows the DCR at 50% SOC of the lithium-ion secondary batteries of each Example and Comparative Example. Here, DCR is an index of output characteristics, with a smaller value indicating better output characteristics.

[0116]

[0117] <Discussion> Table 1 shows the evaluation results of the positive electrode thickness A, separator thickness B, ratio R (= A / B), and output characteristic DCR of the lithium ion secondary batteries of each Example and Comparative Example. As shown in Table 1, in Examples 1 to 4, the DCR was small, suggesting improved output characteristics. On the other hand, in Comparative Examples 1 and 2, the DCR increased significantly, suggesting poor output characteristics.

[0118] Therefore, by employing the positive electrode and separator of the present disclosure in a lithium ion secondary battery, it is possible to improve the output characteristics.

[0119] Those skilled in the art can use the above description to make the most of the present disclosure. The claims and embodiments disclosed herein are merely descriptive and exemplary and should not be construed as limiting the scope of the present disclosure in any way. With the aid of this disclosure, changes can be made to the details of the above embodiments without departing from the basic principles of the present disclosure. In other words, various modifications and improvements of the embodiments specifically disclosed in the above specification are within the scope of the present disclosure.

[0120] 1: battery can, 1a: opening, 1b: wide side surface, 1c: narrow side surface, 1d: bottom surface, 3: winding group, 5: gasket, 6: battery lid, 7: insulating plate, 9: liquid filling port, 10: gas exhaust 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, 33: separator, 33a: polyethylene layer, 33b, 33c: polypropylene layers, 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 base, 42: positive electrode side connection end portion, 43: positive electrode side opening hole, 44: positive electrode current collector, 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 active material layer, a separator, and a negative electrode having a negative electrode active material layer, the separator being disposed between the positive electrode and the negative electrode, wherein the lithium ion secondary battery has a rated capacity of 3 Ah or more and 7 Ah or less, the separator having a first layer primarily composed of polyethylene and second and third layers primarily composed of polypropylene, the first layer being disposed between the second and third layers, the ratio R defined by the following (Equation 1) being 2.0 or more and 6.0 or less, and the thickness of the positive electrode active material layer being 30 μm or more and 65 μm or less: R = (total thickness of the positive electrode active material layer) / (total thickness of the separator) ... (Equation 1) (wherein the unit of thickness is μm).

2. The lithium ion secondary battery according to claim 1, wherein the thickness of the separator is 12 μm or more and 20 μm or less.

3. The lithium ion secondary battery according to claim 1, wherein the separator has an air permeability of 100 sec / 100 cc or more and 300 sec / 100 cc or less.

4. The lithium ion secondary battery according to claim 1, wherein the separator has a degree of curvature of 1.0 or more and 3.0 or less.

5. The lithium ion secondary battery according to claim 1, wherein the ratio R is 2.3 or more and 5.5 or less.

6. The lithium ion secondary battery according to claim 1, wherein the ratio R is 2.7 or more and 5.0 or less.

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