Lithium-ion battery

Abstract

To provide a lithium-ion battery that exhibits excellent charge / discharge efficiency and repeatability even if gas such as hydrogen is generated inside due to aging change or other factors.SOLUTION: A lithium-ion battery 20 includes: a negative electrode active material layer 13 having one surface in contact with a negative electrode current collector 11 and containing negative electrode active material particles 17 whose surfaces are covered with a gel polymer 18 swollen by an electrolyte 19; a positive electrode active material layer 14 having one surface in contact with a positive electrode current collector 12 and containing positive electrode active material particles 16 whose surfaces are covered with the gel polymer 18; and a separator 25 which is sandwiched between the negative electrode active material layer 13 and the positive electrode active material layer 14 and whose surface is covered with the gel polymer 18.SELECTED DRAWING: Figure 3

Description

[Technical field]

[0001] The present invention relates to a lithium ion battery having a separator as a component. [Background technology]

[0002] Lithium-ion batteries are used as power sources in a wide range of fields, including as a power source for electric vehicles (EVs) and hybrid electric vehicles (HEVs), as well as for small electronic devices such as laptop computers and mobile phones.

[0003] The separator, a component of lithium-ion batteries, has both ionic conductivity and electrical insulation. This separator is placed between the positive and negative electrodes, electrically insulating them to prevent internal short circuits while allowing lithium ions to pass through the electrolyte. The role of the separator is becoming increasingly important in order for lithium-ion batteries to be charged and discharged efficiently and to repeatedly operate stably.

[0004] Furthermore, in order to improve various performances of lithium ion batteries, prior art documents have been disclosed that devise innovative separators or use gel polymers as electrolytes (eg, Patent Documents 1 to 4). [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Publication No. 9-293518 [Patent Document 2] Patent No. 4891470 [Patent Document 3] Japanese Patent Application Publication No. 10-289732 [Patent Document 4] Patent No. 7038760 Summary of the Invention [Problem to be solved by the invention]

[0006] It is known that gases such as hydrogen are generated inside lithium-ion batteries due to initial charging, aging, etc. In the area where this gas is in direct contact with the separator, the separator becomes dry and does not contain electrolyte. This inhibits ion conduction in the separator and causes lithium to precipitate, reducing the efficiency and repetitive stability of the charge / discharge cycle in the lithium-ion battery.

[0007] According to the above-mentioned prior art documents, although the separator is devised and a gel polymer is used for the electrolyte, the above-mentioned problems are not yet solved.

[0008] The present invention aims to solve these problems, and to provide a lithium-ion battery that has excellent charge / discharge efficiency and repeated charge / discharge stability even if gas such as hydrogen is generated inside due to aging or other reasons. [Means for solving the problem]

[0009] The lithium ion battery according to the present invention comprises: a negative electrode active material layer having one side in contact with a negative electrode current collector and including negative electrode active material particles having a surface covered with a gel polymer swollen with an electrolyte; a positive electrode active material layer having one side in contact with a positive electrode current collector and including positive electrode active material particles having a surface covered with the gel polymer; and a separator sandwiched on both sides by the negative electrode active material layer and the positive electrode active material layer and having a surface covered with the gel polymer. Effect of the Invention

[0010] The present invention provides a lithium ion battery that has excellent charge / discharge efficiency and repeated charge / discharge stability even if gas such as hydrogen is generated inside due to aging or the like. [Brief description of the drawings]

[0011] [Figure 1] 1 is a cross-sectional view of a lithium-ion battery according to an embodiment of the present invention. [Diagram 2] 1 is a cross-sectional view of a battery cell constituting a lithium-ion battery according to an embodiment. [Diagram 3] FIG. 2A is an enlarged cross-sectional view of a battery cell in an embodiment, FIG. 2B is an enlarged cross-sectional view of a separator interface portion after deterioration over time in an embodiment, and FIG. 2C is an enlarged cross-sectional view of a separator interface portion after deterioration over time in a comparative example. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0012] Hereinafter, an embodiment of the present invention will be described with reference to the drawings. Fig. 1 is a cross-sectional view of a lithium ion battery 20 according to an embodiment of the present invention. Thus, the lithium ion battery 20 is configured by housing a stack of a plurality of battery cells 10 (101, 102, 103, 104) in an exterior body 27. A negative electrode current collector (negative electrode current collector 11 in Fig. 2) of the battery cell 101 located at one end of the stack is connected to a negative electrode current collector. A positive electrode current collector (positive electrode current collector 12 in Fig. 2) of the battery cell 104 located at the other end of the stack is connected to a positive electrode current collector.

[0013] 2 is a cross-sectional view of a battery cell 10 constituting a lithium-ion battery 20 according to an embodiment. The battery cell 10 is formed by stacking a negative electrode current collector 11, a negative electrode active material layer 13, a separator 25, a positive electrode active material layer 14, and a positive electrode current collector 12 in this order, with a sealing member 26 disposed in an annular shape so as to surround the outer periphery of the battery cell.

[0014] Fig. 3(A) is an enlarged cross-sectional view of a battery cell 10 (indicated by the dashed line in Fig. 2) in this embodiment. Here, the negative electrode active material layer 13 has one side in contact with the negative electrode current collector 11 and contains negative electrode active material particles 17 whose surfaces are covered with gel polymer 18 swollen with the electrolyte 19. The positive electrode active material layer 14 has one side in contact with the positive electrode current collector 12 and contains positive electrode active material particles 16 whose surfaces are covered with gel polymer 18 swollen with the electrolyte 19. The separator 25 is sandwiched on both sides by the negative electrode active material layer 13 and the positive electrode active material layer 14 and has a surface covered with gel polymer 18.

[0015] In addition to the case where the gel polymer 18 covering the surface of adjacent positive electrode active material particles 16 or negative electrode active material particles 17 is in close contact with each other, the electrolyte 19 is dispersed and interposed between the particles. In some cases, the electrolyte 19 causes the gel polymer 18 to swell and the dispersed phase of the electrolyte 19 disappears. In other words, the electrolyte 19 is not dispersed and interposed between the particles, and the electrolyte 19 is entirely a continuous phase of the gel polymer 18. In addition, it is not essential that the surfaces of the negative electrode active material particles 17 and the positive electrode active material particles 16 are covered with the gel polymer 18, and the particles may be directly covered with the electrolyte 19.

[0016] The gel polymer 18 is a matrix polymer (host polymer) made of an ion-conductive polymer into which an electrolyte solution 19 is injected. Examples of ion-conductive polymers include polyethylene oxide (PEO), polypropylene oxide (PPO), polyethylene glycol (PEG), polyacrylonitrile (PAN), polyvinylidene fluoride-hexafluoropropylene (PVdF-HEP), polymethyl methacrylate (PMMA), and copolymers thereof.

[0017] The electrolytic solution 19 is a nonaqueous solvent in which an electrolyte is dissolved. As the nonaqueous solvent, a nonaqueous solvent used in a known nonaqueous electrolytic solution can be used. For example, carbonates such as ethylene carbonate (EC), propylene carbonate (PC), dimethyl carbonate (DMC), diethyl carbonate (DEC), and ethyl methyl carbonate, lactone compounds, chain carboxylate esters, cyclic or chain ethers, phosphate esters, nitrile compounds, amide compounds, sulfones, sulfolane, and mixtures thereof can be mentioned.

[0018] The electrolyte may be any electrolyte used in a known non-aqueous electrolyte. For example, lithium salts of inorganic acids such as LiPF6, LiBF4, LiSbF6, LiAsF6, and LiClO4, and lithium salts of organic acids such as LiN(CF3SO2)2, LiN(C2F5SO2)2, and LiC(CF3SO2)3 are included. Among these electrolytes, LiPF6 is preferred from the viewpoint of battery output and charge / discharge cycle characteristics.

[0019] In addition to the electrolyte 19, the gel polymer 18 may contain one or more selected from a conductive assistant, an adhesive resin, and a known solution-drying type electrode binder (also called a binder). The conductive assistant is not particularly limited, but examples thereof include metals [nickel, aluminum, stainless steel (SUS), silver, copper, titanium, etc.], carbon [graphite and carbon black (acetylene black, ketjen black, furnace black, channel black, thermal lamp black, etc.), carbon nanofibers, carbon nanotubes, etc.]. In addition, one selected from these conductive assistants may be used alone, or two or more may be used in combination. In addition, alloys or metal oxides of these may be used. By blending these conductive assistants, good electronic conductivity is imparted to the gel polymer 18.

[0020] From the viewpoint of electrical stability, the conductive assistant is preferably aluminum, stainless steel, carbon, silver, copper, titanium, or a mixture thereof, and more preferably silver, aluminum, or stainless steel. The conductive assistant may be a particle-based ceramic material or a resin material coated with a conductive material (a metal among the conductive assistant materials described above) by plating or the like.

[0021] The negative electrode active material particles 17 are, for example, carbon-based materials [graphite, non-graphitizable carbon, amorphous carbon, resin baked bodies (e.g., phenolic resin, furan resin, etc. baked and carbonized), cokes (e.g., pitch coke, needle coke, petroleum coke, etc.), and carbon fibers, etc.], silicon-based materials [silicon, silicon oxide (SiOx), silicon-carbon composites (carbon particles whose surfaces are coated with silicon and / or silicon carbide, silicon particles or silicon oxide particles whose surfaces are coated with carbon and / or silicon carbide, and silicon carbide, etc.), and silicon alloys (silicon-aluminum Examples of the conductive materials include particles of conductive materials (e.g., aluminum alloy, silicon-lithium alloy, silicon-nickel alloy, silicon-iron alloy, silicon-titanium alloy, silicon-manganese alloy, silicon-copper alloy, silicon-tin alloy, etc.), conductive polymers (e.g., polyacetylene, polypyrrole, etc.), metals (tin, aluminum, zirconium, titanium, etc.), metal oxides (titanium oxide, lithium-titanium oxide, etc.), metal alloys (e.g., lithium-tin alloy, lithium-aluminum alloy, lithium-aluminum-manganese alloy, etc.), and mixtures of these with carbon-based materials.

[0022] The positive electrode active material particles 16 are not particularly limited as long as they are used in lithium ion batteries. For example, a composite oxide of lithium and a transition metal {composite oxides containing one type of transition metal (LiCoO2, LiNiO2, LiAlMnO4, LiMnO2, LiMn2O4, etc.), composite oxides containing two types of transition metal elements (for example, LiFeMnO4, LiNi 1-x Co x O2, LiMn 1-y Co y O2, LiNi 1 / 3 Co 1 / 3 Al 1 / 3 O2 and LiNi 0.8 Co 0.15 Al 0.05 O2) and composite oxides containing three or more metal elements (e.g., LiMaM'bM''cO2 (where M, M', and M'' are different transition metal elements and satisfy a+b+c=1), LiNi 1 / 3 Mn 1 / 3 Co 1 / 3O2, etc.), lithium-containing transition metal phosphates (e.g., LiFePO4, LiCoPO4, LiMnPO4, and LiNiPO4), transition metal oxides (e.g., MnO2 and V2O5), transition metal sulfides (e.g., MoS2 and TiS2), and conductive polymers (e.g., polyaniline, polypyrrole, polythiophene, polyacetylene, poly-p-phenylene, and polyvinylcarbazole) are examples of particles. The lithium-containing transition metal phosphate may be one in which a part of the transition metal site is replaced with another transition metal. In addition, one type selected from these compounds may be used alone, or two or more types may be used in combination.

[0023] The separator 25 has a function of retaining the electrolyte 19 to ensure lithium ion conductivity between the negative electrode current collector 11 and the positive electrode current collector 12, and a function as a partition between the negative electrode active material layer 13 and the positive electrode active material layer 14. As the separator 25, for example, a known separator such as a porous separator made of a polymer or fiber that absorbs and retains the electrolyte 19, or a nonwoven fabric separator can be used.

[0024] A conductive filler (not shown) is dispersed in a resin (not shown) constituting a continuous phase of the negative electrode current collector 11. The conductive filler (not shown) used in the negative electrode current collector 11 is required to have the property of neither occluding lithium ions nor causing a volume change, and specific examples of the conductive filler include metal elements such as platinum, gold, silver, copper, nickel, and titanium.

[0025] The positive electrode current collector 12 has a conductive filler (not shown) such as graphite dispersed in a resin (not shown) constituting a continuous phase. The positive electrode current collector 12 does not occlude lithium ions and does not expand in volume, unlike the negative electrode current collector 11. Examples of the conductive filler (not shown) used in the positive electrode current collector 12 include copper, aluminum, titanium, stainless steel, nickel, baked carbon, conductive polymers, and conductive glass.

[0026] The resin (not shown) forming the continuous phase in the negative electrode current collector 11 and the resin (not shown) forming the continuous phase in the positive electrode current collector 12 may be a polymer material that is either conductive or non-conductive. Examples of conductive resins include polyaniline, polypyrrole, polythiophene, polyacetylene, polyparaphenylene, polyphenylenevinylene, and polyoxadiazole.

[0027] Examples of non-conductive resins include aliphatic polyolefins [polyvinylidene fluoride (PVdF), polytetrafluoroethylene (PTFE), polyethylene (PE), polypropylene (PP), polymethylpentene (PMP), polyisobutylene, polybutadiene, polymethylpentene (PMP), and copolymers thereof, etc.], alicyclic polyolefins [polycycloolefin (PCO), etc.], polyester resins [polyethylene terephthalate (PET), etc.], polyethernitrile (PEN), synthetic rubbers [styrene butadiene rubber (SBR), etc.], acrylic resins [polyacrylonitrile (PAN), polymethyl acrylate (PMA), polymethyl methacrylate (PMMA), etc.], crosslinked or non-crosslinked epoxy resins, silicone resins, and mixtures thereof.

[0028] In addition, it is preferable that the negative electrode current collector 11 and the positive electrode current collector 12 adjacent to each other in the stacked battery cells 10 are integrated into a bipolar electrode.

[0029] 3B is an enlarged cross-sectional view of the separator interface portion after aging in the embodiment. When the gel polymer 18 covers the surface of the separator 25 in this manner, the negative electrode active material particles 17 (positive electrode active material particles 16) are firmly fixed by the separator 25 and are difficult to move. As a result, even if gas 15 is generated, it is difficult for it to grow large, it is dispersed, the surface tension does not increase, and ion transmission is not hindered.

[0030] 3(C) is an enlarged cross-sectional view of the separator interface portion after aging in the comparative example. In this way, when the gel polymer 18 does not cover the surface of the separator 25, the generated gas 15 starts to accumulate at the interface of the separator 25, and accumulates and grows. As a result, the gas 15 peels off the negative electrode active material particles 17 (positive electrode active material particles 16) from the separator 25 due to surface tension, grows larger, dries the separator 25, deteriorates ion conductivity, and makes it easier for lithium to precipitate.

[0031] In this embodiment, the surface of the separator 25 is configured to be covered with the same gel polymer 18 as the negative electrode active material particles 17 and the positive electrode active material particles 16. Since the gel polymer 18 contains a large amount of electrolyte, drying of the separator 25 is prevented and ion conductivity is ensured. Furthermore, since the surfaces of the negative electrode active material particles 17 and the positive electrode active material particles 16 are bonded to the gel polymer 18, good ion conductivity is maintained.

[0032] 3, the gel polymer 18 covering the surface of the separator 25 is illustrated as having a uniform thickness, but the thickness may vary depending on the surface position. Alternatively, there may be surface positions of the separator 25 that are not covered with the gel polymer 18. It is preferable that the gel polymer 18 thickly covers the surface positions of the separator 25 where gas 15 is likely to be generated, and when there are surface positions of the separator 25 where gas 15 is unlikely to be generated, the gel polymer 18 may or may not cover them.

[0033] Furthermore, at least one of the negative electrode current collector 11 and the positive electrode current collector 12 (both in FIG. 3(A)) may be covered with a gel polymer 18. In this case, as in the case of the separator 25, the current collectors 11 and 12 are prevented from drying due to the gas 15, lithium deposition is prevented, and good electron transfer properties are maintained.

[0034] According to the lithium ion battery 20 described above, the gel polymer 18 that covers the surfaces of the positive electrode active material particles 16 and the negative electrode active material particles 17 also covers the surface of the separator 25, so that the battery has excellent charge / discharge efficiency and repeated stability even if gases such as hydrogen are generated inside due to aging or the like.

[0035] Although some embodiments of the present invention have been described, these embodiments are presented as examples and are not intended to limit the scope of the invention. These embodiments can be implemented in various other forms, and various omissions, substitutions, modifications, and combinations can be made without departing from the spirit of the invention. These embodiments and their modifications are included in the scope of the invention and its equivalents described in the claims, as well as in the scope and spirit of the invention. [Explanation of symbols]

[0036] 10...battery cell, 11...negative electrode current collector, 12...positive electrode current collector, 13...negative electrode active material layer, 14...positive electrode active material layer, 15...air bubbles, 16...positive electrode active material particles, 17...negative electrode active material particles, 18...gel polymer, 19...electrolyte, 20...lithium ion battery, 21...negative electrode lead wire, 22...positive electrode lead wire, 25...separator, 26...sealing member, 27...exterior body.

Claims

1. a negative electrode active material layer including negative electrode active material particles having one side in contact with a negative electrode current collector and a surface covered with a gel polymer swollen with an electrolyte; a positive electrode active material layer having one side in contact with a positive electrode current collector and including positive electrode active material particles the surface of which is covered with the gel polymer; a separator sandwiched on both sides by the negative electrode active material layer and the positive electrode active material layer and having a surface covered with the gel polymer.

2. 2. The lithium ion battery according to claim 1, A lithium ion battery in which the electrolyte forms a single dispersed phase.

3. The lithium ion battery according to claim 1 or 2, A lithium ion battery in which the thickness of the gel polymer varies depending on the surface position of the separator.

4. 4. The lithium ion battery according to claim 3, A lithium ion battery in which the gel polymer is not covered by a surface portion of the separator.

5. The lithium ion battery according to claim 1 or 2, A lithium ion battery, wherein at least one of the negative electrode current collector and the positive electrode current collector has a surface covered with the gel polymer.

6. The lithium ion battery according to claim 1 or 2, A lithium ion battery, wherein the gel polymer contains a conductive additive.

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