Lithium-ion rechargeable battery

Abstract

Provided is a lithium-ion secondary battery wherein breaking and elongation of the negative electrode due to repeated charging and discharging is suppressed. The lithium-ion secondary battery according to an embodiment of the present invention comprises a positive electrode, a negative electrode, a separator that separates the positive electrode and the negative electrode from each other, and an electrolyte. The negative electrode has a negative electrode current collector, and a negative electrode mixture layer formed on a surface of the negative electrode current collector. The negative electrode mixture layer includes a carbon-based material and a silicon-based material as a negative electrode active material. The discharge capacity of the negative electrode active material is 400-750 mAh / g. The thickness of the negative electrode current collector is 4-12 μm, and the 1% offset yield strength of the negative electrode current collector is 300-700 MPa. The separator has a base material layer, a filler layer formed on a surface of the base material layer, and a resin layer formed on a surface of the base material layer or the filler layer. The porosity of the resin layer is 30-80%

Description

[Technical Field]

[0001] This disclosure relates to lithium-ion secondary batteries. [Background technology]

[0002] Lithium-ion secondary batteries are used as power sources for a wide range of devices, including electric vehicles, and there is a demand for even higher capacity and longer lifespan. To increase the capacity of the negative electrode, silicon-based materials, which have a larger capacity per unit volume than carbon-based materials such as graphite, are being considered as negative electrode active materials. However, silicon-based materials undergo large volume changes during charging and discharging, so batteries using negative electrode active materials made of carbon-based and silicon-based materials may deform due to these volume changes. Patent Document 1 discloses a technology that suppresses battery deformation by using a high-strength negative electrode current collector. [Prior art documents] [Patent Documents]

[0003] [Patent Document 1] International Publication No. 2013 / 047432 [Overview of the project] [Problems that the invention aims to solve]

[0004] However, after diligent research by the inventors, it was found that when the proportion of silicon-based material in the negative electrode active material increases, the negative electrode may fracture or elongate. The technology described in Patent Document 1 does not address the fracture or elongation of the negative electrode, and there is still room for improvement.

[0005] The purpose of this disclosure is to provide a lithium-ion secondary battery with high capacity and suppressed fracture and elongation of the negative electrode due to repeated charging and discharging. [Means for solving the problem]

[0006] A lithium-ion secondary battery according to one aspect of the present disclosure comprises a positive electrode, a negative electrode, a separator that isolates the positive electrode and the negative electrode from each other, and an electrolyte. The negative electrode has a negative electrode current collector and a negative electrode mixture layer formed on the surface of the negative electrode current collector. The negative electrode mixture layer contains a carbon-based material and a silicon-based material as negative electrode active materials. The discharge capacity of the negative electrode active materials is 400 mAh / g to 750 mAh / g. The thickness of the negative electrode current collector is 4 μm to 12 μm. The 1% yield strength of the negative electrode current collector is 300 MPa to 700 MPa. The separator has a base layer, a filler layer formed on the surface of the base layer, and a resin layer formed on the surface of the base layer or the filler layer. The porosity of the resin layer is 30% to 80%. [Effects of the Invention]

[0007] A lithium-ion secondary battery, according to one aspect of this disclosure, has high capacity and can suppress fracture and elongation of the negative electrode due to repeated charging and discharging. [Brief explanation of the drawing]

[0008] [Figure 1] This is a cross-sectional view of a lithium-ion secondary battery, which is an example of an embodiment. [Figure 2] This figure shows the relationship between strain and stress when a tensile test is performed on the negative electrode current collector. [Figure 3] This figure shows an example of a cross-section of a separator. [Modes for carrying out the invention]

[0009] As a result of the inventors' diligent research into increasing the capacity of lithium-ion secondary batteries, it was found that when the content of silicon-based materials in the negative electrode mixture layer increases, the negative electrode may break or stretch. This is presumed to be because the stress associated with the expansion and contraction of the silicon compound during charging and discharging is concentrated locally rather than dispersed. The inventors further investigated and found that even when the proportion of silicon-based materials in the negative electrode active material is increased to a negative electrode discharge capacity of 400 mAh / g to 750 mAh / g, the breakdown and stretching of the negative electrode due to repeated charging and discharging can be suppressed by using a predetermined negative electrode current collector and separator. More specifically, the above-mentioned problems with the negative electrode can be suppressed by using a negative electrode current collector with a thickness of 4 μm to 12 μm and a 1% yield strength of 300 MPa to 700 MPa, and a separator having a base layer, a filler layer, and a resin layer with a porosity of 30% to 80%.

[0010] Hereinafter, with reference to the drawings, an example of an embodiment of the negative electrode and a lithium-ion secondary battery using said negative electrode will be described in detail. It should be noted that selective combination of the multiple embodiments and modifications described below is included in this disclosure.

[0011] In the following, a cylindrical battery is given as an example in which a wound electrode body 14 is housed in a bottomed cylindrical outer casing 16. However, the battery casing is not limited to a cylindrical casing, and may be, for example, a rectangular casing (rectangular battery) or a coin-shaped casing (coin-type battery), or an outer casing made of a laminate sheet including a metal layer and a resin layer (laminated battery). Furthermore, the electrode body may be a laminated electrode body in which multiple positive electrodes and multiple negative electrodes are alternately stacked with separators in between.

[0012] Figure 1 is a schematic diagram showing a cross-section of a lithium-ion secondary battery 10, which is an example of an embodiment. As shown in Figure 1, the lithium-ion secondary battery 10 comprises a wound electrode body 14, an electrolyte, and an outer casing 16 that houses the electrode body 14 and the electrolyte. The electrode body 14 has a positive electrode 11, a negative electrode 12, and a separator 13, and has a wound structure in which the positive electrode 11 and the negative electrode 12 are wound in a spiral shape via the separator 13. The outer casing 16 is a bottomed cylindrical metal container with one side open in the axial direction, and the opening of the outer casing 16 is sealed by a sealing body 17. In the following explanation, for convenience, the side of the battery with the sealing body 17 will be considered the top, and the bottom side of the outer casing 16 will be considered the bottom.

[0013] The electrolyte includes, for example, a non-aqueous solvent and an electrolyte salt dissolved in the non-aqueous solvent. Examples of non-aqueous solvents include esters, ethers, nitriles, amides, and mixtures of two or more of these. The non-aqueous solvent may also contain halogen-substituted solvents in which at least some of the hydrogen atoms in the solvent are replaced with halogen atoms such as fluorine. Examples of non-aqueous solvents include ethylene carbonate (EC), ethyl methyl carbonate (EMC), dimethyl carbonate (DMC), and mixtures thereof. Examples of electrolyte salts include lithium salts such as LiPF6. Note that the electrolyte is not limited to a liquid electrolyte and may also be a solid electrolyte.

[0014] The positive electrode 11, negative electrode 12, and separator 13 constituting the electrode body 14 are all elongated strip-shaped bodies that are alternately stacked in the radial direction of the electrode body 14 by being wound in a spiral shape. The negative electrode 12 is formed to be slightly larger in dimensions than the positive electrode 11 in order to prevent lithium deposition. That is, the negative electrode 12 is formed to be longer in the longitudinal direction and the width direction (short direction) than the positive electrode 11. The separator 13 is formed to be at least slightly larger in dimensions than the positive electrode 11, and two separators are arranged so as to sandwich the positive electrode 11. The electrode body 14 has a positive electrode lead 20 connected to the positive electrode 11 by welding or the like, and a negative electrode lead 21 connected to the negative electrode 12 by welding or the like.

[0015] Insulating plates 18 and 19 are respectively arranged on the upper and lower sides of the electrode assembly 14. In the example shown in FIG. 1, a positive electrode lead 20 extends through the through-hole of the insulating plate 18 to the sealing body 17 side, and a negative electrode lead 21 extends through the outer side of the insulating plate 19 to the bottom side of the outer can 16. The positive electrode lead 20 is connected to the lower surface of the internal terminal plate 23 of the sealing body 17 by welding or the like, and a cap 27, which is the top plate of the sealing body 17 and electrically connected to the internal terminal plate 23, serves as the positive electrode terminal. The negative electrode lead 21 is connected to the inner surface of the bottom of the outer can 16 by welding or the like, and the outer can 16 serves as the negative electrode terminal.

[0016] As described above, the outer can 16 is a bottomed cylindrical metal container with one axial side open. A gasket 28 is provided between the outer can 16 and the sealing body 17, which ensures the airtightness inside the battery and the insulation between the outer can 16 and the sealing body 17. The outer can 16 is formed with a grooved portion 22 that supports the sealing body 17, where a part of the side surface portion protrudes inward. The grooved portion 22 is preferably formed in an annular shape along the circumferential direction of the outer can 16, and supports the sealing body 17 on the upper surface thereof. The sealing body 17 is fixed to the upper portion of the outer can 16 by the grooved portion 22 and the open end of the outer can 16 crimped onto the sealing body 17.

[0017] The sealing body 17 has a structure in which an internal terminal plate 23, a lower valve body 24, an insulating member 25, an upper valve body 26, and a cap 27 are laminated in this order from the electrode assembly 14 side. Each member constituting the sealing body 17 has, for example, a disk shape or a ring shape, and each member except the insulating member 25 is electrically connected to each other. The lower valve body 24 and the upper valve body 26 are connected at their respective central portions, and the insulating member 25 is interposed between their respective peripheral edge portions. When an abnormality occurs in the battery and the internal pressure rises, the lower valve body 24 deforms so as to push the upper valve body 26 toward the cap 27 side and breaks, thereby cutting off the current path between the lower valve body 24 and the upper valve body 26. When the internal pressure further rises, the upper valve body 26 breaks, and gas is discharged from the opening of the cap 27.

[0018] Hereinafter, the positive electrode 11, the negative electrode 12, and the separator 13 constituting the lithium ion secondary battery 10 will be described in detail, with particular emphasis on the negative electrode 12.

[0019] [Positive electrode] The positive electrode (11) comprises a positive electrode current collector and a positive electrode mixture layer formed on a surface of the positive electrode current collector. For the positive electrode current collector, a foil of a metal stable within the potential range of the positive electrode (11) such as aluminum or an aluminum alloy, or a film having the metal disposed on a surface layer thereof, or the like can be used. The positive electrode mixture layer contains a positive electrode active material, a conductive agent, and a binder, and is preferably provided on both surfaces of the positive electrode current collector excluding an exposed current collector portion which is a portion to which a positive electrode lead is connected. The thickness of the positive electrode mixture layer on one side of the positive electrode current collector is, for example, 50 μm to 150 μm. The positive electrode (11) can be produced by applying a positive electrode mixture slurry containing a positive electrode active material, a conductive agent, a binder, and the like onto a surface of the positive electrode current collector, drying the coating film, and then compressing the dried coating to form the positive electrode mixture layers on both surfaces of the positive electrode current collector.

[0020] The positive electrode active material is composed mainly of a lithium-transition metal composite oxide. Elements other than Li contained in the lithium-transition metal composite oxide include Ni, Co, Mn, Al, B, Mg, Ti, V, Cr, Fe, Cu, Zn, Ga, Sr, Zr, Nb, In, Sn, Ta, W, Si, P, and the like. One example of a suitable lithium-transition metal composite oxide is a composite oxide containing at least one of Ni, Co, and Mn. Specific examples include a lithium-transition metal composite oxide containing Ni, Co, and Mn, and a lithium-transition metal composite oxide containing Ni, Co, and Al.

[0021] Examples of the conductive agent contained in the positive electrode mixture layer include carbon-based particles such as carbon black, acetylene black, Ketjen black, carbon nanotubes, graphene, and graphite. Examples of the binder contained in the positive electrode mixture layer include fluorine resins such as polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVDF), polyacrylonitrile (PAN), polyimide resins, acrylic resins, polyolefin resins, and the like. These resins may be used in combination with a cellulose derivative such as carboxymethyl cellulose (CMC) or a salt thereof, polyethylene oxide (PEO), or the like.

[0022] [Negative electrode] The negative electrode 12 comprises a negative electrode current collector and a negative electrode mixture layer formed on the surface of the negative electrode current collector. Preferably, the negative electrode mixture layer is provided on both sides of the negative electrode current collector. The thickness of the negative electrode current collector is 4 μm to 12 μm. The thickness of the negative electrode mixture layer is, for example, 50 μm to 150 μm on one side of the negative electrode current collector.

[0023] The negative electrode current collector can be made of a metal foil that is stable in the negative electrode potential range, such as copper or a copper alloy, or a film with the metal arranged on its surface. The negative electrode current collector is preferably made of copper. Furthermore, the crystal grain size of the copper used as the negative electrode current collector is preferably 0.2 μm to 2 μm. The crystal grain size can be measured using SEM-EBSD (scanning electron beam backscatter diffraction) or the like.

[0024] The 1% yield strength of the negative electrode current collector is 300 MPa to 700 MPa. The 1% yield strength is the yield strength (1%) measured by the tensile test method for metallic materials according to JIS Z 2241. Figure 2 is a schematic diagram showing the relationship between strain and stress when a tensile test is performed on the negative electrode current collector. The 1% yield strength is the stress value when the strain is 1%. The 1% yield strength of the negative electrode current collector can be adjusted, for example, by the thickness of the negative electrode current collector or the grain size of the material of the negative electrode current collector. By using a negative electrode current collector with a thickness of 4 μm to 12 μm and a 1% yield strength of 300 MPa to 700 MPa together with a predetermined separator described later, it is possible to suppress fracture and elongation of the negative electrode due to repeated charging and discharging.

[0025] The fracture elongation of the negative electrode current collector is, for example, 2% to 9%. Here, fracture elongation is the value obtained by dividing the amount of elongation of the test specimen that fractured in the tensile test by the length of the test specimen before the tensile test and expressing it as a percentage, and in Figure 2, it is the value of strain when the negative electrode current collector fractured.

[0026] The negative electrode mixture layer contains a carbon-based material and a silicon-based material as negative electrode active materials. The discharge capacity of the negative electrode active material is 400 mAh / g to 750 mAh / g. The negative electrode active material may also include materials other than carbon-based and silicon-based materials that can absorb and release Li.

[0027] The proportion of the silicon-based material in the negative electrode active material may be 5% by mass to 30% by mass relative to the total of the carbon-based material and the silicon-based material. The negative electrode active material mixed at such a proportion has the above-mentioned discharge capacity.

[0028] The discharge capacity CA (mAh / g) of the negative electrode active material, the thickness CT (μm) of the negative electrode current collector, and the 1% proof stress CM (N / mm of the negative electrode current collector 2 ) preferably satisfy the relationship of CA / (CM×CT)<0.3.

[0029] The carbon-based material may be at least one selected from the group consisting of natural graphite, artificial graphite, soft carbon, and hard carbon. In particular, natural graphite or artificial graphite, which has excellent charge-discharge stability and low irreversible capacity, is preferred. Examples of natural graphite include flake graphite, massive graphite, amorphous graphite, and the like. Examples of artificial graphite include massive artificial graphite, graphitized mesophase carbon microbeads, and the like.

[0030] Examples of the silicon-based material include Si metal that alloys with Li, metal compounds containing Si, composite oxides containing Si, and the like. The silicon-based material includes, for example, an ion conductive phase, silicon particles dispersed in the ion conductive phase, and a coating layer covering the surface of the ion conductive phase.

[0031] The ion conductive phase is, for example, at least one selected from the group consisting of a silicate phase, an amorphous carbon phase, and a silicide phase.

[0032] SiO, which is an example of the silicon-based material, has a particle structure in which fine silicon particles are dispersed in a silicate phase. Suitable SiO has a sea-island structure in which fine silicon particles are substantially uniformly dispersed in an amorphous silicon oxide matrix, and has the general formula SiO x represented by (0<x≦2). The silicate phase is constituted by an aggregate of particles finer than the silicon particles. From the viewpoint of achieving both battery capacity and cycle characteristics, the content of silicon particles is preferably 35% by mass to 75% by mass relative to the total mass of SiO.

[0033] The ion conductive phase may contain at least one element selected from the group consisting of alkali metal elements and Group 2 elements. For example, the general formula Li 2z SiO (2+z) (0<z<2) is a silicon-based material having a sea-island structure in which fine Si particles are substantially uniformly dispersed in a lithium silicate matrix. The lithium silicate phase is constituted by an assembly of particles finer than the silicon particles. As in the case of SiO, the content of silicon particles in Li 2z SiO (2+z) is preferably 35% by mass to 75% by mass relative to the total mass of the compound.

[0034] Si-C, which is another example of a silicon-based material, has a particle structure in which fine silicon particles are dispersed in an amorphous carbon phase. Preferred Si-C has a sea-island structure in which fine silicon particles are substantially uniformly dispersed in an amorphous carbon phase matrix. From the viewpoint of increasing capacity, the content of Si particles is preferably 35% by mass to 75% by mass relative to the total mass of Si-C.

[0035] The ion conductive phase contains an element M, and the element M may be at least one selected from the group consisting of B, Al, Zr, Nb, Ta, V, La, Y, Ti, P, Bi, Zn, Sn, Pb, Sb, Co, Er, F, and W.

[0036] Examples of the constituent material of the coating layer include carbon materials, metals, and metal compounds, and carbon materials such as amorphous carbon are particularly preferred. The coating layer can be formed, for example, by a CVD method using acetylene, methane, or the like, or by a method of mixing coal pitch, petroleum pitch, phenolic resin, or the like with silicon compound particles and performing heat treatment. Alternatively, the coating layer may be formed by fixing a conductive filler such as carbon black to the surface of the ion conductive phase using a binder.

[0037] The negative electrode mixture layer may contain a conductive agent, a binder, and the like in addition to the negative electrode active material.

[0038] Examples of conductive agents included in the negative electrode mixture layer include particulate conductive agents such as carbon black, acetylene black, Ketjenblack, and graphite, as well as fibrous conductive agents such as vapor-grown carbon fibers (VGCF), electrospun carbon fibers, polyacrylonitrile (PAN) carbon fibers, pitch carbon fibers, carbon nanotubes, and graphene.

[0039] The binder included in the negative electrode mixture layer may be fluororesin, PAN, polyimide, acrylic resin, polyolefin, etc., as in the case of the positive electrode 11, but styrene-butadiene rubber (SBR) is preferred. The negative electrode mixture layer may further contain CMC or its salt, polyacrylic acid (PAA) or its salt, polyvinyl alcohol (PVA), etc. As the binder included in the negative electrode mixture layer, it is preferable to use SBR in combination with CMC or its salt, or PAA or its salt.

[0040] [Separator] The separator 13 separates the positive electrode 11 and the negative electrode 12 from each other. The separator 13 has a base layer, a filler layer formed on the surface of the base layer, and a resin layer formed on the surface of the base layer or the filler layer. Figure 3 shows an example of a cross-section of the separator 13. In Figure 3, the filler layer 32 is formed over the entire surface of one side of the base layer 30, and the resin layer 34 is formed on a part of the surface of the filler layer 32. That is, the base layer 30 or filler layer 32 adjacent to the resin layer 34 may have a portion covered by the resin layer 34 and a portion that is not covered. The upper surface in Figure 3 on which the filler layer 32 and the resin layer 34 are formed may face the positive electrode 11. Note that the configuration of the separator 13 is not limited to the example shown in Figure 3. In Figure 3, the resin layer 34 may be formed over the entire surface of the filler layer 32, or it may be formed on the surface of the base layer 30 opposite to the surface on which the filler layer 32 is formed. Furthermore, both the filler layer 32 and the resin layer 34 may be formed on both sides of the base layer 30.

[0041] As the base layer 30, for example, a porous base material having ion permeability and insulating properties can be used. Specific examples of porous base materials include microporous thin films, woven fabrics, and nonwoven fabrics. Examples of materials for the base layer 30 include polyethylene, polypropylene, polyolefins such as copolymers of polyethylene and α-olefin, acrylic resins, polystyrene, polyester, and cellulose. The thickness of the base layer 30 is, for example, 5 μm to 20 μm.

[0042] The filler layer 32 contains a filler and a binder. The thickness of the filler layer 32 is, for example, 1 μm to 20 μm. The filler layer 32 can be formed on the surface of the substrate layer 30, for example, using a doctor blade method, gravure coating method, transfer method, or die coating method.

[0043] The filler content in the filler layer 32 is, for example, 50% to 90% by mass. Here, the filler content in the filler layer 32 is the ratio of the filler content in the filler layer 32 to the total mass of the filler layer 32.

[0044] Examples of fillers include metal oxide particles, metal nitride particles, metal fluoride particles, metal carbide particles, and sulfide particles. Examples of metal oxide particles include aluminum oxide (e.g., α-Al2O3), titanium oxide, magnesium oxide, zirconium oxide, nickel oxide, silicon oxide, and manganese oxide. Examples of metal nitride particles include titanium nitride, boron nitride, aluminum nitride, magnesium nitride, and silicon nitride. Examples of metal fluoride particles include aluminum fluoride, lithium fluoride, sodium fluoride, magnesium fluoride, calcium fluoride, and barium fluoride. Examples of metal carbide particles include silicon carbide, boron carbide, titanium carbide, and tungsten carbide. Examples of sulfide particles include barium sulfate. These may be used individually or in combination of two or more types.

[0045] The binder contained in the filler layer 32 has the function of bonding individual fillers to each other and to the substrate layer 30. Examples of binders include fluororesins such as polyvinylidene fluoride (PVDF) and polytetrafluoroethylene (PTFE), acrylic resins such as polyacrylonitrile and polyacrylic acid, polyimide resins, polyamide resins, polyolefin resins, styrene-butadiene rubber (SBR), nitrile-butadiene rubber (NBR), carboxymethylcellulose (CMC) or its salts, polyacrylic acid (PAA) or its salts, and polyvinyl alcohol (PVA). These may be used individually or in combination of two or more types.

[0046] The resin layer 34 is preferably a fluororesin or an acrylic resin. Examples of fluororesins include PVDF and PTFE. Examples of acrylic resins include polyacrylonitrile and polyacrylic acid. The resin forming the resin layer and the binder contained in the filler layer may be different, but if they are the same, the adhesion between the two can be improved.

[0047] The porosity of the resin layer 34 is 30% to 80%. Having a porosity within this range reduces the impact of volume changes during charging and discharging of the negative electrode active material.

[0048] The separator 13 can be manufactured, for example, as follows. (1) Prepare a dispersion by mixing the filler and binder and dispersing them in a solvent. (2) The dispersion is applied to the entire surface of one side of the substrate layer 30, and the coating film is dried to form the filler layer 32. (3) The resin and plasticizer are mixed together to prepare the resin composition. (4) The resin composition is applied to a portion of the surface of the filler layer 32, and the plasticizer is extracted and washed to form a resin layer 34 having voids. The void ratio of the resin layer 34 can be changed by adjusting the ratio of plasticizer, particle shape, particle size distribution, etc.

[0049] The thickness Tr of the resin layer, the thickness Tb of the base material layer, and the thickness Tf of the filler layer may satisfy the relationship of Tr≧(Tb+Tf)×0.3. The thickness of the base material layer 30 is, for example, 2 μm to 30 μm.

Examples

[0050] Hereinafter, the present disclosure will be further described with reference to Examples, but the present disclosure is not limited to these Examples.

[0051] <Example 1> [Preparation of Positive Electrode] As the positive electrode active material, LiCo 1 / 3 Mn 1 / 3 Ni 1 / 3 A lithium transition metal composite oxide represented by O2 was used. 98 parts by mass of the positive electrode active material, 1 part by mass of acetylene black, and 1 part by mass of polyvinylidene fluoride were mixed, and N-methyl-2-pyrrolidone (NMP) was used as a dispersion medium to prepare a positive electrode mixture slurry. Next, the positive electrode mixture slurry was applied to both surfaces of a positive electrode current collector made of aluminum foil, the coating film was dried and compressed, and then cut into a predetermined electrode size, thereby producing a positive electrode in which positive electrode mixture layers were formed on both surfaces of the positive electrode current collector. An exposed portion where the surface of the current collector was exposed was provided in the central portion in the longitudinal direction of the positive electrode, and a positive electrode lead was welded to the exposed portion.

[0052] [Preparation of Negative Electrode] Artificial graphite and SiO x (0<x≦2) were mixed at a mass ratio of 92:8, and the mixture was used as a negative electrode active material. The discharge capacity of this negative electrode active material was 430 mAh / g. The negative electrode active material, carboxymethyl cellulose (CMC), and a dispersion of styrene butadiene rubber (SBR) were mixed at a solid content mass ratio of 98:1:1, and water was used as a dispersion medium to prepare a negative electrode mixture slurry. The negative electrode mixture slurry was applied to both surfaces of a copper foil having a thickness of 6 μm serving as a negative electrode current collector, and the coating film was dried and compressed. Thereafter, the resultant was cut into a predetermined electrode size, thereby producing a negative electrode in which negative electrode mixture layers were formed on both surfaces of the negative electrode current collector. An exposed portion where the surface of the current collector was exposed was provided at an end portion in the longitudinal direction of the negative electrode, and a negative electrode lead was welded to the exposed portion.

[0053] [Preparation of electrolytes] Ethylene carbonate (EC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC) were mixed in a volume ratio of 3:3:4. Lithium hexafluoride phosphate (LiPF6) was dissolved in this mixed solvent at a concentration of 1 mole / liter to prepare an electrolyte.

[0054] [Separator fabrication] A porous polyethylene substrate with a thickness of 12 μm was prepared. Alumina powder and polyvinylidene fluoride (PVDF) were mixed in a solid content mass ratio of 97:3, and then an appropriate amount of NMP was added to prepare a dispersion. The dispersion was applied to the entire surface of one side of the substrate using a microgravure coater, and the coating was heated and dried in an oven to form a 4 μm thick filler layer on the surface of the substrate. Furthermore, an appropriate amount of NMP was added to the PVDF and kneaded to prepare a resin composition. The resin composition was applied to the entire surface of the filler layer using a microgravure coater, and a 10 μm thick resin layer was formed by extracting and washing away the NMP.

[0055] [Preparation of test cells] An aluminum lead was attached to the exposed portion of the positive electrode, and a nickel lead was attached to a predetermined position on the negative electrode. The positive and negative electrodes were then wound in a spiral shape via a separator to create a wound electrode body. This electrode body was placed inside a bottomed cylindrical outer container, the electrolyte was injected, and a sealing body was attached to the opening of the outer container via a gasket to create a cylindrical test cell.

[0056] [Cycle Test] The test cell was charged at a constant current of 0.2It at a temperature of 25°C until the battery voltage reached 4.2V. Then, it was charged at a constant voltage until the current value was 0.02It at 4.2V. After that, it was discharged at a constant current of 0.2It until the battery voltage reached 3.0V. This charge-discharge cycle was considered one cycle, and 200 cycles were performed.

[0057] [Evaluation of foil cutting and stretching properties] The test cell was disassembled after the cycle test, and the negative electrode was removed. The negative electrode foil was visually inspected for breakage. If there was no breakage, it was considered OK; if there was breakage, it was considered NG. For elongation, the length of the side in the width direction of the negative electrode (the side parallel to the axial direction of the electrode body) was measured, and the amount of elongation was calculated from the length of the side in the width direction of the negative electrode before it was assembled into the test cell.

[0058] <Examples 2-8, Comparative Examples 1-6> In the preparation of the negative electrode, as shown in Table 1, the ratio of SiO mass to the total mass of artificial graphite and SiO was varied, and copper foils with different properties (thickness, 1% yield strength, elongation at break) were used. In the preparation of the separator, as shown in Table 1, the thickness and porosity of the resin layer were varied, but otherwise, a test cell was prepared and evaluated in the same manner as in Example 1. Table 1 also shows the discharge capacity of the negative electrode active material in the example and comparative example.

[0059] Table 1 shows the evaluation results of the test cells for the examples and comparative examples. The elongation of the foil in the test cells is a relative value with the elongation of Comparative Example 2 as the baseline (100). A relative value of 90 or less was considered OK, and a relative value greater than 90 was considered NG.

[0060] [Table 1]

[0061] As shown in Table 1, the negative electrode of the example showed no foil breakage and minimal elongation even after charge-discharge cycles. On the other hand, the negative electrode of the comparative example failed in either foil breakage or elongation. [Explanation of Symbols]

[0062] 10 Lithium-ion secondary battery, 11 Positive electrode, 12 Negative electrode, 13 Separator, 14 Electrode body, 16 Outer casing, 17 Sealing body, 18,19 Insulating plate, 20 Positive electrode lead, 21 Negative electrode lead, 22 Grooved section, 23 Internal terminal plate, 24 Lower valve body, 25 Insulating material, 26 Upper valve body, 27 Cap, 28 Gasket, 30 Base layer, 32 Filler layer, 34 Resin layer

Claims

1. It comprises a positive electrode, a negative electrode, a separator that isolates the positive electrode and the negative electrode from each other, and an electrolyte. The negative electrode comprises a negative electrode current collector and a negative electrode mixture layer formed on the surface of the negative electrode current collector. The anode mixture layer comprises a carbon-based material and a silicon-based material as a negative electrode active material, and the discharge capacity of the negative electrode active material is 400 mAh / g to 750 mAh / g. The thickness of the negative electrode current collector is 4 μm to 12 μm, and the 1% proof stress of the negative electrode current collector is 300 MPa to 700 MPa. The discharge capacity CA (mAh / g) of the negative electrode active material, the thickness CT (μm) of the negative electrode current collector, and the 1% proof stress CM (N / mm²) of the negative electrode current collector satisfy the relationship CA / (CM × CT) < 0.

3. The separator comprises a base layer, a filler layer formed on the surface of the base layer, and a resin layer formed on the surface of the base layer or the filler layer. A lithium-ion secondary battery in which the porosity of the resin layer is 30% to 80%.

2. The lithium-ion secondary battery according to claim 1, wherein the carbon-based material is at least one selected from the group consisting of natural graphite, artificial graphite, soft carbon, and hard carbon.

3. The lithium-ion secondary battery according to claim 1 or 2, wherein the silicon-based material comprises an ion-conducting phase, silicon particles dispersed within the ion-conducting phase, and a coating layer covering the surface of the ion-conducting phase.

4. The lithium-ion secondary battery according to claim 3, wherein the ion conducting phase is at least one selected from the group consisting of silicate phase, amorphous carbon phase, and silicide phase.

5. The lithium-ion secondary battery according to claim 3, wherein the ion conducting phase includes at least one element selected from the group consisting of alkali metal elements and group 2 elements.

6. The lithium-ion secondary battery according to claim 3, wherein the ion conducting phase contains element M, and element M is at least one selected from the group consisting of B, Al, Zr, Nb, Ta, V, La, Y, Ti, P, Bi, Zn, Sn, Pb, Sb, Co, Er, F, and W.

7. The lithium-ion secondary battery according to claim 1 or 2, wherein the proportion of the silicon-based material in the negative electrode active material is 5% by mass to 30% by mass relative to the total of the carbon-based material and the silicon-based material.

8. The lithium-ion secondary battery according to claim 1 or 2, wherein the negative electrode current collector is copper with a crystal grain size of 0.2 μm to 2 μm.

9. The lithium-ion secondary battery according to claim 1 or 2, wherein the break elongation of the negative electrode current collector is 2% to 9%.

10. The lithium-ion secondary battery according to claim 1 or 2, wherein the resin layer is a fluororesin or an acrylic resin.

11. The lithium-ion secondary battery according to claim 1 or 2, wherein the thickness Tr of the resin layer, the thickness Tb of the substrate layer, and the thickness Tf of the filler layer satisfy the relationship Tr ≥ (Tb + Tf) × 0.

3.

12. The lithium-ion secondary battery according to claim 1 or 2, wherein the substrate layer or filler layer adjacent to the resin layer has a portion covered by the resin layer and a portion that is not covered.

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