Lithium-ion rechargeable battery

Abstract

Provided is a high-capacity, long-life lithium-ion secondary battery. A lithium ion secondary battery that is an example of an embodiment comprises a positive electrode, a negative electrode, a separator that separates the positive electrode and the negative electrode from each other, and an electrolyte, wherein the negative electrode has a negative electrode collector and a negative electrode mixture layer formed on the surface of the negative electrode collector, the negative electrode mixture layer includes a negative electrode active material having a discharge capacity of 400-750 mAh / g and a conductive agent, the negative electrode active material includes a carbon-based material and a silicon-based material, the negative electrode current collector has a thickness of 4-12 μm, the negative electrode current collector has a 1% yield strength of 300-700MPa, and the conductive agent includes carbon nanotubes.

Description

[Technical Field]

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

[0002] The electrodes of a lithium-ion secondary battery generally consist of a metal current collector and a composite layer formed on the surface of the current collector. In addition to the active material, which is the main component, the composite layer contains a conductive agent that interposes between the active materials to form conductive paths. Patent Document 1 discloses a technique for incorporating carbon nanotubes as a conductive agent into the negative electrode composite layer in order to maintain conductive paths between the negative electrode active materials even after repeated charging and discharging. [Prior art documents] [Patent Documents]

[0003] [Patent Document 1] Japanese Patent Publication No. 2007-35472 [Overview of the project] [Problems that the invention aims to solve]

[0004] In recent years, in order to increase the capacity of negative electrodes, silicon-based materials, which have a larger capacity per unit volume than carbon-based materials such as graphite, have been considered as negative electrode active materials. As a result of diligent research by the present inventors, it has been found that because silicon-based materials undergo large volume changes during charging and discharging, the effect of carbon nanotubes becomes less pronounced when the proportion of silicon-based materials in the negative electrode active material increases. The technology described in Patent Document 1 has not been examined in the case of negative electrode active materials that undergo large volume changes during charging and discharging, and there is still room for improvement.

[0005] The purpose of this disclosure is to provide high-capacity, long-life lithium-ion secondary batteries. pond The objective is to provide. [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 negative electrode active material having a discharge capacity of 400 mAh / g to 750 mAh / g and a conductive agent. The negative electrode active material includes a carbon-based material and a silicon-based material. 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 conductive agent contains carbon nanotubes. [Effects of the Invention]

[0007] A lithium-ion secondary battery, according to one aspect of this disclosure, has high capacity and can suppress the decrease in battery capacity 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. [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 battery capacity decreases significantly with repeated charging and discharging, even if carbon nanotubes are included. 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 fracture and elongation of the negative electrode due to repeated charging and discharging can be suppressed by using carbon nanotubes and a predetermined negative electrode current collector. More specifically, 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, the expansion of the negative electrode can be suppressed, while the conductive path is maintained with carbon nanotubes, thereby suppressing the deterioration of cycle characteristics. Here, cycle characteristics refer to the change in battery capacity due to repeated charging and discharging.

[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 disposed 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 a through hole of the insulating plate 18 toward the sealing body 17 side, and a negative electrode lead 21 extends through the outer side of the insulating plate 19 toward the bottom side of the outer can 16. The positive electrode lead 20 is connected to the lower surface of an 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 electrically connected to the internal terminal plate 23, serves as a 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 a negative electrode terminal.

[0016] As described above, the outer can 16 is a bottomed cylindrical metal container having an opening on one axial side. A gasket 28 is provided between the outer can 16 and the sealing body 17, so that the hermeticity of the inside of the battery and the insulation between the outer can 16 and the sealing body 17 are ensured. The outer can 16 is formed with a grooved portion 22 that protrudes inward at a part of a side surface portion thereof and supports the sealing body 17. The grooved portion 22 is preferably formed annularly 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 part of the outer can 16 by the grooved portion 22 and an opening end portion of the outer can 16 crimped to 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 stacked 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 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, particularly the negative electrode 12, will be described in detail.

[0019] [Positive Electrode] The positive electrode 11 includes 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 that is stable in 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 coating to form the positive electrode mixture layer 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. An 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 fluororesins such as polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVDF), polyacrylonitrile (PAN), polyimide resins, acrylic resins, and polyolefin resins. 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. 、S Measurements can be taken using methods such as EM-EBSD (electron beam backscatter diffraction).

[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 negative electrode active material with a discharge capacity of 400 mAh / g to 750 mAh / g and a conductive agent. The negative electrode active material includes carbon-based materials and silicon-based materials. 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 in 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 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, and amorphous graphite, and examples of artificial graphite include massive artificial graphite and graphitized mesophase carbon microbeads.

[0030] Examples of the silicon-based material include Si metal that alloys with Li, metal compounds containing Si, and composite oxides containing Si. The silicon-based material includes, for example, an ion conductive phase, silicon particles dispersed in the ion conductive phase, and a coating layer covering a 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 the 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, and has 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 aggregate of particles finer than the silicon particles. The content of silicon particles is the same as in the case of SiO, and the 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. Suitable Si-C has a sea-island structure in which fine silicon particles are substantially uniformly dispersed in a matrix of an amorphous carbon phase. From the viewpoint of increasing the 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 constituent materials for 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 in which coal pitch, petroleum pitch, phenol resin, or the like is mixed with silicon compound particles and then heat-treated. 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 conductive agent contained in the negative electrode mixture layer includes carbon nanotubes (CNTs). In addition to CNTs, the conductive agent may also contain carbon black, acetylene black, Ketjen black, graphite, etc., but preferably contains only CNTs. CNTs contribute to reducing the resistance of the negative electrode mixture layer by forming good conductive paths within it.

[0038] The carbon nanotubes (CNTs) may be either multilayer carbon nanotubes (MWCNTs) or single-walled carbon nanotubes (SWCNTs), with SWCNTs being preferred. Both SWCNTs and MWCNTs may be included in the negative electrode mixture layer.

[0039] The average fiber diameter of CNTs is, for example, 0.01 nm to 5 nm. If the average fiber diameter of the CNTs is within this range, the improvement in cycle characteristics is enhanced compared to using CNTs with an average fiber diameter outside this range. The average fiber diameter of the CNTs is determined by selecting 100 CNTs from a cross-sectional image of the negative electrode mixture layer, measuring their diameters, and averaging these measurements.

[0040] The CNT content in the negative electrode mixture layer is, for example, 0.005% to 0.6% by mass relative to the total amount of negative electrode active material. If the CNT content is within this range, the content of the negative electrode active material can be increased in the negative electrode mixture layer while ensuring conductive paths, thereby improving cycle characteristics and increasing battery capacity.

[0041] The negative electrode mixture layer may further contain a binder. 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.

[0042] [Separator] The separator 13 isolates the positive electrode 11 and the negative electrode 12 from each other. A porous sheet having ion permeability and insulating properties is used for the separator 13. Specific examples of the porous sheet include a microporous thin film, woven fabric, nonwoven fabric, and the like. Preferable materials for the separator 13 include polyolefins such as polyethylene, polypropylene, and copolymers of ethylene and α-olefin, and cellulose. The separator 13 may have either a single-layer structure or a laminated structure. A heat-resistant layer containing inorganic particles or a heat-resistant layer formed of a resin having high heat resistance such as aramid resin, polyimide, or polyamide-imide may be formed on the surface of the separator 13.

Examples

[0043] Hereinafter, the present disclosure will be further described with reference to examples, but the present disclosure is not limited to these examples.

[0044] <Example 1> [Production of Positive Electrode] As the positive electrode active material, LiCo 1 / 3 Mn 1 / 3 Ni 1 / 3 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 sides of a positive electrode current collector made of aluminum foil, the coating film was dried and compressed, then cut into a predetermined electrode size, to produce a positive electrode in which positive electrode mixture layers were formed on both sides of the positive electrode current collector. An exposed portion where the surface of the current collector was exposed was provided at the center in the longitudinal direction of the positive electrode, and a positive electrode lead was welded to the exposed portion.

[0045] [Production of Negative Electrode] Artificial graphite and SiO x (0 < x ≤ 2) were mixed at a mass ratio of 92:8, and this mixture was used as a negative electrode active material. The discharge capacity of this negative electrode active material was 430 mAh / g. Further, carbon nanotubes (CNT) having an average fiber diameter of 0.1 nm were used as the conductive agent. The negative electrode active material, CNT, and ,mosquitoA negative electrode slurry was prepared by mixing a dispersion of ruboxymethylcellulose (CMC) and styrene-butadiene rubber (SBR) in a solid content mass ratio of 100:0.05:1:1, and using water as the dispersion medium. This negative electrode slurry was applied to both sides of a 6 μm thick copper foil to be used as the negative electrode current collector, and the coating was dried and compressed. Subsequently, it was cut to a predetermined electrode size, and a negative electrode was fabricated in which the negative electrode slurry layer was formed on both sides of the negative electrode current collector. An exposed portion was provided at the longitudinal end of the negative electrode, where the surface of the current collector was exposed, and a negative electrode lead was welded to this exposed portion.

[0046] [Electrolyte preparation] 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.

[0047] [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 polyethylene 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.

[0048] [Evaluation of cycle characteristics] Under an ambient temperature of 25°C, the above test cell was charged with a constant current of 0.2It to 4.2V, and then charged with a constant voltage of 4.2V to 0.02It. Afterward, it was discharged with a constant current of 0.2It to 2.5V. This charge-discharge cycle was performed 200 times, and the capacity degradation rate was calculated using the following formula. Capacity degradation rate = (Discharge capacity in the first cycle - Discharge capacity in the 200th cycle) / Discharge capacity in the first cycle × 100

[0049] <Examples 2-5, Comparative Examples 1-4> In the preparation of the negative electrode, as shown in Tables 1-3, the ratio of SiO mass to the total mass of artificial graphite and SiO was varied, copper foils with different properties (thickness, 1% yield strength, elongation at break) were used, and the CNT content was varied. Except for these variations, test cells were prepared and evaluated in the same manner as in Example 1. Table 1 also shows the discharge capacity of the negative electrode active materials in the examples and comparative examples.

[0050] Tables 1-3 show the evaluation results of the test cells for the examples and comparative examples. Table 1 shows the results when the proportion of silicon-based material is 8% by mass, and the capacity degradation rate for Example 1 is a relative value with the capacity degradation rate of Comparative Example 1 as the base (100). Table 2 shows the results when the proportion of silicon-based material is 15% by mass, and for Example 2- 7 and comparative examples 3 The capacity degradation rate is, in comparison to 2 This is a relative value with the capacity degradation rate set as the baseline (100). Table 3 shows the results when the proportion of silicon-based material is 20% by mass, and is an example. 8 The capacity degradation rate is a relative value with the capacity degradation rate of Comparative Example 4 as the baseline (100).

[0051] [Table 1]

[0052] [Table 2]

[0053] [Table 3]

[0054] As shown in Tables 1-3, the test cells in the examples showed a lower capacity degradation rate compared to the test cells in the comparative examples. [Explanation of Symbols]

[0055] 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

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 aforementioned negative electrode mixture layer comprises a negative electrode active material having a discharge capacity of 400 mAh / g to 750 mAh / g and a conductive agent. The negative electrode active material includes a carbon-based material and a silicon-based material. 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. A lithium-ion secondary battery comprising carbon nanotubes as the conductive agent.

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, 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, 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, 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, wherein the break elongation of the negative electrode current collector is 2% to 9%.

10. The lithium-ion secondary battery according to claim 1, wherein the average fiber diameter of the carbon nanotubes is 0.01 nm to 5 nm.

11. The lithium-ion secondary battery according to claim 1, wherein the carbon nanotube content in the negative electrode mixture layer is 0.005% by mass to 0.6% by mass relative to the total amount of the negative electrode active material.

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