Non-aqueous electrolyte secondary battery

Abstract

In a non-aqueous electrolyte secondary battery (10), a negative electrode (12) has a negative electrode core and a negative electrode mixture layer provided on the negative electrode core. The thickness of the negative electrode core is 15 μm-30 μm inclusive. The negative electrode mixture layer contains a silicon-containing material and a carbon material as negative electrode active materials, and the ratio of the total thickness of the negative electrode mixture layer to the thickness of the negative electrode core is 4-10 inclusive. The silicon-containing material includes an amorphous carbon phase and a silicon phase dispersed in the amorphous carbon phase, and the content of the silicon-containing material in relation to the negative electrode active materials is 30% by mass-60% by mass inclusive.

Description

Nonaqueous electrolyte secondary battery

[0001] The present disclosure relates to a non-aqueous electrolyte secondary battery.

[0002] In non-aqueous electrolyte secondary batteries such as lithium ion secondary batteries, the negative electrode has a significant effect on battery performance such as capacity, input / output characteristics, and durability, and therefore much research has been conducted on the negative electrode. For example, Patent Document 1 discloses a negative electrode for a non-aqueous electrolyte secondary battery that aims to achieve both high capacity and high durability of the battery, in which the ratio of the silicon-containing material to the total amount of the silicon-containing material and flake graphite, which are negative electrode active materials, is 39% by mass or more and 51% by mass or less. Patent Document 1 discloses a silicon-containing material having the general formula SiO ｘ Materials expressed as (0<x≦2) are described.

[0003] JP 2015-032419 A

[0004] In high-capacity nonaqueous electrolyte secondary batteries, the energy required for a short circuit is large, so various safety measures are taken to prevent contact between the positive and negative electrodes and the occurrence of an internal short circuit. On the other hand, as the capacity of batteries increases, the volume change of the electrodes during charging and discharging increases, which may result in the negative electrode stretching and bending of the end of the negative electrode. If the end of the negative electrode breaks and approaches the positive electrode, the risk of a short circuit increases, so preventing such bending of the negative electrode is an important issue.

[0005] A non-aqueous electrolyte secondary battery according to one aspect of the present disclosure is a non-aqueous electrolyte secondary battery including an electrode assembly including a positive electrode, a negative electrode, and a separator, a non-aqueous electrolyte, and an exterior housing that accommodates the electrode assembly and the non-aqueous electrolyte, wherein the negative electrode has a core and a mixture layer provided on both sides of the core, the core having a thickness of 15 μm or more and 30 μm or less, the mixture layer contains a silicon-containing material and a carbon material as a negative electrode active material, the ratio of the total thickness of the mixture layer to the thickness of the core being 4 or more and 10 or less, the silicon-containing material containing an amorphous carbon phase and a silicon phase dispersed in the amorphous carbon phase, and the content of the silicon-containing material relative to the negative electrode active material being 30% by mass or more and 60% by mass or less.

[0006] According to one aspect of the present disclosure, in a high-capacity nonaqueous electrolyte secondary battery, negative electrode breakage, which carries a risk of short circuit, can be suppressed.

[0007] 1 is a cross-sectional view of a nonaqueous electrolyte secondary battery according to an embodiment;

[0008] Conventionally, in order to increase the capacity of non-aqueous electrolyte secondary batteries, it has been common to thicken the negative electrode mixture layer and thin the negative electrode core. However, as a result of studies by the present inventors, it has been found that when a conventional negative electrode structure is applied to a high-capacity non-aqueous electrolyte secondary battery, the negative electrode expands due to volume changes in the negative electrode mixture layer during charge and discharge, causing the end of the negative electrode to bend. When the end of the negative electrode bends and approaches the positive electrode, the risk of a short circuit increases.

[0009] As a result of extensive research to solve the above problems, the inventors have succeeded in suppressing negative electrode breakage while increasing the capacity of the battery by using a predetermined amount of silicon-containing material containing a silicon phase dispersed in an amorphous carbon phase as the negative electrode active material, setting the thickness of the negative electrode core to 15 μm or more and 30 μm or less, and setting the ratio of the total thickness of the negative electrode mixture layer to the thickness of the negative electrode core to 4 or more and 10 or less. It is believed that by increasing the thickness of the negative electrode core and using a large amount of the silicon-containing material, it is possible to achieve both suppression of negative electrode elongation and high capacity. Suppressing negative electrode elongation can effectively suppress negative electrode breakage, which poses a risk of short circuit.

[0010] Hereinafter, an example of an embodiment of a nonaqueous electrolyte secondary battery according to the present disclosure will be described in detail with reference to the drawings. Note that configurations obtained by selectively combining the components of the multiple embodiments and variations described below are included within the scope of the present disclosure.

[0011] In the following, a cylindrical battery in which a wound-type electrode assembly 14 is housed in a cylindrical outer can 16 with a bottom is exemplified, but the outer can is not limited to a cylindrical outer can. A nonaqueous electrolyte secondary battery, which is another example of an embodiment, may include, for example, a prismatic outer can (prismatic battery), a coin-shaped outer can (coin battery), or an outer can (pouch battery) made of a laminate sheet including a metal layer and a resin layer. Furthermore, the electrode assembly is not limited to a wound type, and may be a stacked electrode assembly in which multiple positive electrodes and multiple negative electrodes are alternately stacked with separators interposed therebetween.

[0012] FIG. 1 is a cross-sectional view of a nonaqueous electrolyte secondary battery 10 according to an embodiment. As shown in FIG. 1 , the nonaqueous electrolyte secondary battery 10 includes a wound electrode assembly 14, a nonaqueous electrolyte (not shown), and an outer can 16 that accommodates the electrode assembly 14 and the nonaqueous electrolyte. The electrode assembly 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 spirally wound with the separator 13 interposed therebetween. The outer can 16 is a cylindrical metal container with a bottom that is open on one axial side, and the opening of the outer can 16 is closed by a sealing member 17. Hereinafter, for convenience of explanation, the sealing member 17 side of the battery will be referred to as the top, and the bottom side of the outer can 16 will be referred to as the bottom.

[0013] The positive electrode 11, negative electrode 12, and separator 13 that constitute the electrode assembly 14 are all long, strip-like bodies that are spirally wound and alternately stacked in the radial direction of the electrode assembly 14. The negative electrode 12 is formed to be slightly larger than the positive electrode 11 in order to prevent lithium precipitation. That is, the negative electrode 12 is formed to be longer than the positive electrode 11 in both the longitudinal direction and the width direction (short direction). The separator 13 is formed to be at least slightly larger than the positive electrode 11, and, for example, two separators 13 are arranged to sandwich the positive electrode 11. The electrode assembly 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.

[0014] Insulating plates 18 and 19 are disposed above and below the electrode body 14. In the example shown in FIG. 1 , a positive electrode lead 20 extends from positions away from the inner and outer winding ends of the electrode body 14 and passes through a through hole in the insulating plate 18 to be connected by welding or the like to the underside of the internal terminal plate 23. In this case, a cap 27, which is the top plate of the sealing body 17 electrically connected to the internal terminal plate 23, serves as the positive electrode terminal. A negative electrode lead 21 extends from the inner winding end of the electrode body 14 and passes through a through hole in the insulating plate 19 to be connected by welding or the like to the inner bottom surface of the outer can 16. Furthermore, an exposed portion is provided on the outer peripheral surface of the electrode body 14, where the surface of the negative electrode core constituting the negative electrode 12 is exposed, and this exposed portion is in contact with the inner peripheral surface of the outer can 16. In this case, the outer can 16 serves as the negative electrode terminal.

[0015] A gasket 28 is provided between the exterior can 16 and the sealing body 17 to ensure airtightness inside the battery. The exterior can 16 has a grooved portion 22 formed on its side surface that protrudes inward and supports the sealing body 17. The grooved portion 22 is preferably formed in an annular shape along the circumferential direction of the exterior can 16, and supports the sealing body 17 on its top surface. The sealing body 17 is fixed to the top of the exterior can 16 by the grooved portion 22 and the open end of the exterior can 16 that is crimped to the sealing body 17.

[0016] The sealing body 17 has a structure in which, in order from the electrode body 14 side, 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. Each member constituting the sealing body 17 has, for example, a disk or ring shape, and each member except for the insulating member 25 is electrically connected to one another. The lower valve body 24 and the upper valve body 26 are connected at their respective centers, and the insulating member 25 is interposed between their respective peripheral edges. When the internal pressure of the battery increases due to abnormal heat generation, the lower valve body 24 deforms and breaks, pushing the upper valve body 26 toward the cap 27, thereby interrupting the current path between the lower valve body 24 and the upper valve body 26. When the internal pressure further increases, the upper valve body 26 breaks, and gas is discharged from the opening of the cap 27.

[0017] The positive electrode 11, the negative electrode 12, the separator 13, and the nonaqueous electrolyte that constitute the nonaqueous electrolyte secondary battery 10 will be described in detail below, with the negative electrode 12 being particularly described below.

[0018] [Positive Electrode] The positive electrode 11 includes a positive electrode core and a positive electrode mixture layer provided on the positive electrode core. The positive electrode core can be a foil of a metal stable within the potential range of the positive electrode 11, such as aluminum, an aluminum alloy, stainless steel, or titanium, or a film with such a metal disposed on the surface. The thickness of the positive electrode core is, for example, 10 μm or more and 30 μm or less. The positive electrode mixture layer preferably contains a positive electrode active material, a conductive agent, and a binder, and is provided on both sides of the positive electrode core. The average thickness of the positive electrode mixture layer is, for example, 60 μm or more and 120 μm or less on one side of the positive electrode core. The positive electrode 11 can be produced, for example, by applying a positive electrode mixture slurry containing a positive electrode active material, a conductive agent, and a binder onto the positive electrode core, drying the coating, and then compressing it to form a positive electrode mixture layer on both sides of the positive electrode core.

[0019] The positive electrode active material is a lithium transition metal composite oxide containing transition metal elements such as Ni, Co, and Mn. Examples of metal elements contained in the composite oxide include Ni, Co, Mn, Al, Be, B, Na, Mg, Si, K, Ca, Sc, Ti, V, Cr, Fe, Cu, Zn, Ga, Sr, Y, Zr, Nb, Mo, In, Sn, Sb, Ba, Ta, W, Pb, and Bi. Among these, it is preferable to contain at least one of Ni, Co, and Mn. The lithium transition metal composite oxide has, for example, a layered rock salt structure. One type of lithium transition metal composite oxide may be used alone, or multiple types may be used in combination. The content of the positive electrode active material is, for example, 90% by mass or more and 99.8% by mass or less relative to the mass of the positive electrode mixture layer.

[0020] Examples of the conductive agent contained in the positive electrode mixture layer include carbon black such as acetylene black and ketjen black, graphite, carbon nanotubes (CNT), carbon nanofibers, graphene, metal fibers, metal powder, and conductive whiskers. One type of conductive agent may be used alone, or multiple types may be used in combination. The content of the conductive agent is not particularly limited, but is, for example, 0.1% by mass or more and 5% by mass or less relative to the mass of the positive electrode mixture layer.

[0021] Examples of binders contained in the positive electrode mixture layer include fluorine-containing resins such as polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVDF); olefin resins such as polyethylene, polypropylene, ethylene-propylene-isoprene copolymer, and ethylene-propylene-butadiene copolymer; and acrylic resins such as polyacrylonitrile (PAN), polyimide, polyamide, and ethylene-acrylic acid copolymer. These resins may also be used in combination with carboxymethyl cellulose (CMC) or a salt thereof, polyethylene oxide (PEO), or the like. One type of binder may be used alone, or multiple types may be used in combination. The content of the binder is not particularly limited, but is, for example, 0.1% by mass or more and 5% by mass or less, relative to the mass of the positive electrode mixture layer.

[0022] [Negative Electrode] The negative electrode 12 has a negative electrode core and a negative electrode mixture layer provided on the negative electrode core. The negative electrode core can be made of a foil of a metal stable within the potential range of the negative electrode 12, such as copper, a copper alloy, stainless steel, nickel, or a nickel alloy, or a film having such a metal disposed on its surface. Among these, copper foil or a copper alloy foil is preferably used. The negative electrode mixture layer contains a negative electrode active material and a binder, and is preferably provided on both sides of the negative electrode core. The negative electrode mixture layer may contain the same conductive agent as the positive electrode mixture layer. The negative electrode 12 can be produced, for example, by applying a negative electrode mixture slurry containing a negative electrode active material and a binder onto the negative electrode core, drying the coating, and then compressing it to form a negative electrode mixture layer on both sides of the negative electrode core.

[0023] The thickness of the negative electrode core is 15 μm or more and 30 μm or less, more preferably 20 μm or more and 25 μm or less, from the viewpoint of achieving both high battery capacity and suppression of negative electrode breakage. It has been common practice to reduce the thickness of the negative electrode core to 10 μm or less to achieve high capacity. However, the inventors' studies have found that if the thickness of the negative electrode core is less than 15 μm, the negative electrode stretches significantly during charge and discharge, and the end of the negative electrode is prone to bending toward the positive electrode. On the other hand, if the thickness of the negative electrode core exceeds 30 μm, it becomes difficult to achieve high battery capacity. By setting the thickness of the negative electrode core to 15 μm or more and 30 μm or less and satisfying the specific conditions described below, negative electrode breakage can be efficiently and effectively suppressed while achieving high battery capacity. The thickness of the negative electrode core is substantially constant throughout its entire length.

[0024] The elongation percentage of the negative electrode core is preferably 6% or more and 12% or less. If the elongation percentage of the negative electrode core is within this range, breakage of the negative electrode core during the manufacture of the negative electrode and during charge and discharge of the battery can be prevented, while elongation of the negative electrode due to charge and discharge can be suppressed. The negative electrode core is preferably made of copper foil, and in the case of copper foil, the elongation percentage can be adjusted by heat treatment. In general, the elongation percentage increases with increasing temperature and longer treatment time. The heat treatment temperature is, for example, 130°C or more and 220°C or less.

[0025] In this specification, the elongation percentage of a negative electrode core means the ratio (%) of the elongation to the original length of a test piece of a negative electrode core stretched until it breaks using a tensile tester. Specific measurement conditions are as follows: Tester: Tensilon RTC1210 Test piece: Width 15 mm × Length 150 mm Pull speed: 5 mm / min Chuck distance: 50 mm

[0026] From the viewpoint of achieving both high battery capacity and suppression of negative electrode breakage, the ratio of the total thickness of the negative electrode mixture layer to the thickness of the negative electrode core is 4 to 10, more preferably 6 to 8. The total thickness of the negative electrode mixture layer means the sum of the thicknesses of the negative electrode mixture layers overlapping in the thickness direction of the negative electrode, and is obtained, for example, by subtracting the thickness of the negative electrode core from the thickness of the negative electrode. In this embodiment, the negative electrode mixture layer is formed on both sides of the negative electrode core, so the total thickness of the negative electrode mixture layer is the sum of the thicknesses of the negative electrode mixture layers on both sides. The thicknesses of the negative electrode mixture layers formed on both sides of the negative electrode core are substantially the same. Furthermore, the thickness of the negative electrode mixture layer means an average value, and is substantially constant over the entire surface of the negative electrode core except for the exposed portion.

[0027] If the ratio of the total thickness of the negative electrode mixture layer to the thickness of the negative electrode core is less than 4, the strength of the negative electrode will be significantly reduced, and the negative electrode will expand significantly during charge and discharge, making it more likely to break. It will also be difficult to increase the capacity of the battery. On the other hand, if the ratio of the total thickness of the negative electrode mixture layer to the thickness of the negative electrode core exceeds 10, the volume change of the negative electrode during charge and discharge will be too large, making it more likely to expand and break. The total thickness of the negative electrode mixture layer is not particularly limited as long as the ratio satisfies the range of 4 to 10, but is preferably 60 μm to 180 μm, more preferably 75 μm to 150 μm.

[0028] The negative electrode mixture layer contains a silicon-containing material (Si-containing material) and a carbon material as negative electrode active materials. The Si-containing material contains an amorphous carbon phase and a silicon phase (Si phase) dispersed in the amorphous carbon phase, and its content relative to the negative electrode active material is 30% by mass or more and 60% by mass or less. When the content of the Si-containing material is within this range and the thicknesses of the negative electrode core and the negative electrode mixture layer satisfy the above-mentioned specific conditions, it is possible to increase the capacity of the battery while effectively suppressing negative electrode breakage, which poses a risk of short circuit.

[0029] The carbon material functioning as the negative electrode active material is, for example, at least one selected from the group consisting of natural graphite, artificial graphite, soft carbon, and hard carbon. Among them, it is preferable to use at least artificial graphite such as massive artificial graphite (MAG) and graphitized mesophase carbon microbeads (MCMB), natural graphite such as flake graphite, massive graphite, and amorphous graphite, or a mixture thereof. The content of the carbon material is, for example, 40% by mass or more and 70% by mass or less with respect to the total mass of the negative electrode active material.

[0030] The soft carbon and hard carbon are classified as amorphous carbons with an undeveloped graphite crystal structure. More specifically, they refer to carbon components having a d(002) interplanar spacing of 0.342 nm or more as determined by X-ray diffraction. Soft carbon is also called graphitizable carbon, and is carbon that is more easily graphitized by high-temperature treatment than hard carbon. Hard carbon is also called non-graphitizable carbon. It is not necessary to clearly distinguish between soft carbon and hard carbon. Graphite and at least one amorphous carbon of soft carbon and hard carbon may be used in combination as the negative electrode active material.

[0031] The volume-based median diameter (D50) of the carbon material is, for example, 1 μm or more and 30 μm or less, and preferably 5 μm or more and 25 μm or less. D50 means the particle size at which the cumulative frequency in the volume-based particle size distribution is 50% from the smallest particle size. The particle size distribution of the carbon material can be measured using a laser diffraction particle size distribution measuring device (for example, SALD-2000A manufactured by Shimadzu Corporation) using water as a dispersion medium.

[0032] As described above, the negative electrode mixture layer contains a composite material (hereinafter referred to as "SiC") containing an amorphous carbon phase and a Si phase dispersed in the amorphous carbon phase as the Si-containing material. SiC has a higher charge capacity per unit mass and a smaller irreversible capacity than other negative electrode active materials, making it suitable for increasing the capacity of the battery. In this embodiment, substantially only SiC and graphite are used as the negative electrode active material. However, the negative electrode mixture layer may contain a Si-containing material other than SiC, or a negative electrode active material other than a Si-containing material and a carbon material, as long as the object of the present disclosure is not impaired.

[0033] From the viewpoint of achieving both high battery capacity and suppression of negative electrode breakage, the content of SiC is 30% by mass or more and 60% by mass or less, more preferably 40% by mass or more and 50% by mass or less, relative to the total mass of the negative electrode active material. If the SiC content is less than 30% by mass, it is difficult to achieve high battery capacity. On the other hand, if the SiC content exceeds 60% by mass, the volume change of the negative electrode during charge and discharge becomes too large, causing the negative electrode to elongate significantly and making the negative electrode more likely to break.

[0034] SiC has a sea-island structure in which fine Si phases are dispersed in a continuous phase composed of amorphous carbon. The Si phase is dispersed in the amorphous carbon phase in the form of fine particles and repeatedly absorbs and releases Li ions as the battery is charged and discharged. The Si phase changes volume with charging and discharging, but because it is dispersed in the amorphous carbon phase, the stress caused by the volume change of the Si phase is alleviated by the amorphous carbon phase. Therefore, by using SiC, the volume change of the negative electrode during charging and discharging can be kept small even when a thick negative electrode mixture layer is formed.

[0035] The SiC is, for example, a composite particle smaller than the carbon material. The D50 of the SiC is, for example, 2 μm or more and 20 μm or less, preferably 3 μm or more and 15 μm or less. A conductive layer having higher electronic conductivity than the amorphous carbon phase may be formed on the surface of the SiC particle. The thickness of the conductive layer is, for example, 5 nm or more and 100 nm or less, which is thin enough not to affect the D50 of the SiC.

[0036] The Si content in SiC is preferably 40% by mass or more and 70% by mass or less. If the Si content is too low, the capacity of SiC decreases, so the Si content relative to the mass of SiC is more preferably 50% by mass or more. On the other hand, if the Si content is too high, for example, the volume change of the particles during charge and discharge increases, making particle cracking more likely to occur. As a result, it is thought that side reactions between the non-aqueous electrolyte and Si are more likely to occur, resulting in a decrease in the cycle characteristics of the battery.

[0037] The average particle size of the Si phase is preferably 500 nm or less, more preferably 200 nm or less, and particularly preferably 50 nm or less, at least before the first charge / discharge. By refining and dispersing the Si phase, the volume change of SiC during charge / discharge is reduced, and the stability of the particle structure is further improved. The average particle size of the Si phase is determined by observing the cross section of SiC particles using a scanning electron microscope (SEM). Specifically, 100 Si phases are arbitrarily selected from an SEM image of the cross section of SiC, and the diameters of the circumscribed circles are measured, and the average of the measured values ​​is used to determine the average particle size.

[0038] The amorphous carbon phase is an ion-conducting phase having Li ion conductivity. The amorphous carbon phase is formed, for example, from a carbon material having an average interplanar spacing of (002) planes greater than 0.34 nm as determined by XRD measurement, and is obtained by heat-treating a pitch having a softening point of 200°C or higher at a temperature of 700°C to 900°C. The amorphous carbon phase may contain a metal oxide that does not react with Li, such as zirconium oxide, aluminum oxide, titanium oxide, nickel oxide, or yttrium oxide.

[0039] The elemental content in the amorphous carbon phase can be measured by inductively coupled plasma optical emission spectroscopy (ICP). Specifically, SiC is dissolved in a heated acid solution (a mixed acid of hydrofluoric acid, nitric acid, and sulfuric acid), and the carbon remaining in the solution is removed by filtration. The filtrate obtained is then analyzed by ICP to determine the elemental content in the amorphous carbon phase. The carbon content in SiC can also be measured using a carbon / sulfur analyzer (e.g., EMIA-520, manufactured by Horiba, Ltd.).

[0040] SiC can be produced, for example, by mixing Si powder with amorphous carbon such as pitch, heat-treating the mixture, and pulverizing it. Heat-treating the mixture yields composite particles in which the Si phase is dispersed in the amorphous carbon phase. Mixing and heat-treating the raw materials are preferably performed in an inert atmosphere (e.g., an argon, nitrogen, or other atmosphere). For example, a ball mill is used to mix the raw materials.

[0041] The heat treatment is carried out in an inert atmosphere at a temperature of, for example, 450°C or higher and 1000°C or lower. If the heat treatment temperature is within this range, it becomes easy to form a sea-island structure in which minute Si phases are dispersed in an amorphous carbon phase with low crystallinity. The heat treatment temperature is preferably 600°C or higher and 950°C or lower, and more preferably 700°C or higher and 900°C or lower. The heat treatment time is, for example, 1 hour or higher and 10 hours or lower.

[0042] As in the case of the positive electrode 11, the binder contained in the negative electrode mixture layer can be a fluororesin, an olefin resin, PAN, a polyimide, a polyamide, an acrylic resin, or the like. However, polyvinyl acetate, styrene-butadiene rubber (SBR), or the like may also be used. Among these, it is preferable to use SBR. One type of binder may be used alone, or multiple types may be used in combination. In addition, the negative electrode mixture layer preferably contains CMC or a salt thereof, polyacrylic acid (PAA) or a salt thereof, polyvinyl alcohol (PVA), or the like.

[0043] [Separator] 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, a woven fabric, and a nonwoven fabric. Suitable materials for the separator 13 include polyolefins such as polyethylene and polypropylene, and cellulose. The separator 13 may have a single-layer structure or a multi-layer structure. Furthermore, a highly heat-resistant resin layer such as an aramid resin may be formed on the surface of the separator 13.

[0044] A filler layer containing an inorganic filler may be formed at the interface between the separator 13 and at least one of the positive electrode 11 and the negative electrode 12. Examples of inorganic fillers include oxides and phosphate compounds containing metal elements such as Ti, Al, Si, and Mg. The filler layer can be formed by applying a slurry containing the filler to the surface of the positive electrode 11, the negative electrode 12, or the separator 13.

[0045] [Non-aqueous electrolyte] The non-aqueous electrolyte has ion conductivity (for example, lithium ion conductivity). The non-aqueous electrolyte may be a liquid electrolyte (electrolytic solution) or a solid electrolyte.

[0046] The electrolyte solution includes, for example, a non-aqueous solvent and an electrolyte salt dissolved in the non-aqueous solvent. Examples of the non-aqueous solvent that can be used include esters, ethers, nitriles such as acetonitrile, amides such as dimethylformamide, and mixed solvents of two or more of these. The non-aqueous solvent may contain a halogen-substituted compound in which at least a portion of the hydrogen atoms in these solvents are substituted with halogen atoms such as fluorine. Examples of the halogen-substituted compound include fluorinated cyclic carbonates such as fluoroethylene carbonate (FEC), fluorinated chain carbonates, and fluorinated chain carboxylic acid esters such as methyl fluoropropionate (FMP).

[0047] Examples of the esters include cyclic carbonates such as ethylene carbonate (EC), propylene carbonate (PC), and butylene carbonate; chain carbonates such as dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), methyl propyl carbonate, ethyl propyl carbonate, and methyl isopropyl carbonate; cyclic carboxylic acid esters such as γ-butyrolactone (GBL) and γ-valerolactone (GVL); and chain carboxylic acid esters such as methyl acetate, ethyl acetate, propyl acetate, methyl propionate (MP), and ethyl propionate (EP).

[0048] Examples of the ethers include 1,3-dioxolane, 4-methyl-1,3-dioxolane, tetrahydrofuran, 2-methyltetrahydrofuran, propylene oxide, 1,2-butylene oxide, 1,3-dioxane, 1,4-dioxane, 1,3,5-trioxane, furan, 2-methylfuran, 1,8-cineole, cyclic ethers such as crown ethers, 1,2-dimethoxyethane ethyl ether, dipropyl ether, diisopropyl ether, dibutyl ether, dihexyl ether, ethyl vinyl ether, butyl vinyl ether, and methyl phenyl ether. and chain ethers such as ethyl phenyl ether, butyl phenyl ether, pentyl phenyl ether, methoxytoluene, benzyl ethyl ether, diphenyl ether, dibenzyl ether, o-dimethoxybenzene, 1,2-diethoxyethane, 1,2-dibutoxyethane, diethylene glycol dimethyl ether, diethylene glycol diethyl ether, diethylene glycol dibutyl ether, 1,1-dimethoxymethane, 1,1-diethoxyethane, triethylene glycol dimethyl ether, and tetraethylene glycol dimethyl ether.

[0049] The electrolyte salt is preferably a lithium salt. The concentration of the lithium salt is, for example, 0.5 mol or more and 3 mol or less, and preferably 0.8 mol or more and 1.5 mol or less, per 1 L of the non-aqueous solvent. One type of lithium salt may be used alone, or multiple types may be used in combination.

[0050] An example of a lithium salt is LiBF ４ , LiClO ４ , LiPF ６ , LiAsF ６ , LiSbF ６ , LiAlCl ４ , LiSCN, LiCF ３ SO ３ , LiCF ３ CO ２ , Li(P(C ２ O ４ ) F ４ ), LiPF ６－ｘ (CnF ２ｎ＋１ ) ｘ(1<x<6, n is 1 or 2), LiB １０ Cl １０ , LiCl, LiBr, LiI, lithium chloroborane, lithium lower aliphatic carboxylate, Li[B(C ２ O ４ ) ２ ], Li ２ B ４ O ７ , Li(B(C ２ O ４ ) F ２ ), lithium bisfluorosulfonylimide (LiN(FSO ２ ) ２ ), lithium bistrifluoromethanesulfonyl imide (LiN(CF ３ SO ２ ) ２ ), lithium trifluoromethanesulfonate nonafluorobutanesulfonate imide (LiN(CF ３ SO ２ ) (C ４ F ９ SO ２ )), lithium bispentafluoroethanesulfonyl imide (LiN(C ２ F ５ SO ２ ) ２ ), LiN(C １ F ２ｌ＋１ SO ２ ) (C ｍ F ２ｍ＋１ SO ２ ) {l and m are integers of 0 or more}. Among them, LiPF ６ is preferred.

[0051] As the solid electrolyte, for example, a solid or gel-like polymer electrolyte, an inorganic solid electrolyte, etc. can be used. As the inorganic solid electrolyte, a material known in all-solid-state lithium ion secondary batteries, etc. (for example, an oxide-based solid electrolyte, a sulfide-based solid electrolyte, a halogen-based solid electrolyte, etc.) can be used. The polymer electrolyte includes, for example, a lithium salt and a matrix polymer, or a non-aqueous solvent, a lithium salt, and a matrix polymer. As the matrix polymer, for example, a polymer material that absorbs the non-aqueous solvent and gels is used. As the polymer material, for example, a fluororesin, an acrylic resin, a polyether resin, etc. can be used.

[0052] The present disclosure will be further described below with reference to examples, but the present disclosure is not limited to these examples.

[0053] Example 1 [Preparation of Positive Electrode] Aluminum-containing lithium nickel cobalt oxide was used as the positive electrode active material. This positive electrode active material, acetylene black (AB), and polyvinylidene fluoride (PVDF) were mixed in a mass ratio of 100:2:1, and N-methyl-2-pyrrolidone (NMP) was used as the dispersion medium to prepare a positive electrode mixture slurry. Next, the positive electrode mixture slurry was applied to both sides of a positive electrode core made of aluminum foil with a thickness of 15 μm, and the coating was dried. The coating was compressed with a rolling roller, and the core was cut to a predetermined electrode size to obtain a positive electrode with a positive electrode mixture layer formed on both sides of the positive electrode core. An exposed portion where the surface of the positive electrode core was exposed was provided approximately in the center of the positive electrode's longitudinal direction, and an aluminum positive electrode lead was welded to this exposed portion.

[0054] [Preparation of Negative Electrode] As the negative electrode active material, a mixture of artificial graphite and SiC with a Si phase content of 40% by mass was used in a mass ratio of 70:30. This negative electrode active material, styrene butadiene rubber (SBR), and carboxymethyl cellulose sodium salt (CMC-Na) were mixed in a mass ratio of 100:1:1, and a negative electrode mixture slurry was prepared using water as a dispersion medium. Next, the negative electrode mixture slurry was applied to both sides of a negative electrode core made of copper foil with a thickness of 15 μm, and the coating was dried. The coating was compressed with a rolling roller, and the core was cut to a predetermined electrode size to prepare a negative electrode in which a negative electrode mixture layer was formed on both sides of the negative electrode core. In addition, exposed portions were provided at both longitudinal ends of the negative electrode, exposing the surface of the negative electrode core, and a nickel negative electrode lead was welded to one of the exposed portions. The thickness of the negative electrode mixture layer was substantially the same on both sides of the negative electrode core, totaling 75 μm. The ratio of the total thickness of the negative electrode mixture layers to the thickness of the negative electrode substrate was 5.0.

[0055] [Preparation of non-aqueous electrolyte] 5 parts by mass of vinylene carbonate (VC) was added to 100 parts by mass of a mixed solvent in which ethylene carbonate (EC) and dimethyl carbonate (DMC) were mixed at a volume ratio of 3:7, and lithium hexafluorophosphate (LiPF ６ A non-aqueous electrolyte was prepared by dissolving 1.3 mol / L of ammonium hydroxide in water.

[0056] [Fabrication of Non-Aqueous Electrolyte Secondary Battery] A wound electrode assembly was fabricated by spirally winding a positive electrode and a negative electrode with a 14 μm-thick polyethylene separator between them. The negative electrode was positioned so that the negative electrode lead was located on the inner side of the wound electrode assembly. Insulating plates were placed on the top and bottom of the electrode assembly, and the electrode assembly was housed in a cylindrical metal outer can with a bottom. The negative electrode lead was welded to the bottom of the outer can, and the positive electrode lead was welded to a sealing member. The exposed portion of the wound outer side of the negative electrode contacted the inner circumferential surface of the outer can, and both longitudinal ends of the negative electrode were connected to the outer can. After injecting non-aqueous electrolyte into the outer can, the opening of the outer can was sealed with a sealing member via a gasket, resulting in an 18650-type non-aqueous electrolyte secondary battery. The capacity of this battery was 3.9 Ah.

[0057] Example 2 A nonaqueous electrolyte secondary battery was fabricated in the same manner as in Example 1, except that in the fabrication of the negative electrode, SiC having an Si phase content of 70 mass% was used instead of SiC having an Si phase content of 40 mass%, the mass ratio of artificial graphite to SiC was changed to 50:50, and the thickness of the negative electrode mixture layer was changed to 150 μm. The ratio of the total thickness of the negative electrode mixture layer to the thickness of the negative electrode core was 10.0.

[0058] Example 3 A nonaqueous electrolyte secondary battery was fabricated in the same manner as in Example 2, except that a 30 μm thick copper foil was used instead of the 15 μm thick copper foil in the fabrication of the negative electrode, and the thickness of the negative electrode mixture layer was changed to 120 μm. The ratio of the total thickness of the negative electrode mixture layer to the thickness of the negative electrode core was 4.0.

[0059] Comparative Example 1 A nonaqueous electrolyte secondary battery was fabricated in the same manner as in Example 2, except that a 10 μm thick copper foil was used instead of the 15 μm thick copper foil, the mass ratio of artificial graphite to SiC was changed to 40:60, and the thickness of the negative electrode mixture layer was changed to 95 μm. The ratio of the total thickness of the negative electrode mixture layer to the thickness of the negative electrode core was 9.5.

[0060] Comparative Example 2 A nonaqueous electrolyte secondary battery was fabricated in the same manner as in Example 2, except that in the preparation of the negative electrode, the mass ratio of artificial graphite to SiC was changed to 30:70 and the thickness of the negative electrode mixture layer was changed to 95 μm. The ratio of the total thickness of the negative electrode mixture layer to the thickness of the negative electrode core was 6.3.

[0061] Comparative Example 3 A nonaqueous electrolyte secondary battery was fabricated in the same manner as in Example 2, except that a 35 μm thick copper foil was used instead of the 15 μm thick copper foil in the fabrication of the negative electrode, and the thickness of the negative electrode mixture layer was changed to 95 μm. The ratio of the total thickness of the negative electrode mixture layer to the thickness of the negative electrode core was 2.7.

[0062] Comparative Example 4 A nonaqueous electrolyte secondary battery was fabricated in the same manner as in Example 2, except that the thickness of the negative electrode mixture layer was changed to 25 μm in the fabrication of the negative electrode. The ratio of the total thickness of the negative electrode mixture layer to the thickness of the negative electrode core was 1.7.

[0063] Comparative Example 5 A nonaqueous electrolyte secondary battery was fabricated in the same manner as in Example 2, except that the thickness of the negative electrode mixture layer was changed to 170 μm in the fabrication of the negative electrode. The ratio of the total thickness of the negative electrode mixture layer to the thickness of the negative electrode core was 11.3.

[0064] Comparative Example 6 A nonaqueous electrolyte secondary battery was fabricated in the same manner as in Comparative Example 1, except that in fabricating the negative electrode, a copper foil having a thickness of 30 μm was used instead of the copper foil having a thickness of 10 μm, and the thickness of the negative electrode mixture layer was changed to 60 μm. The ratio of the total thickness of the negative electrode mixture layer to the thickness of the negative electrode core was 2.0.

[0065] The nonaqueous electrolyte secondary batteries of the Examples and Comparative Examples were subjected to the following 500-cycle charge-discharge cycle test to check for the presence or absence of negative electrode breakage. The results are shown in Table 1, along with the content of the Si-containing material as the negative electrode active material, the Si content in the Si-containing material, the thickness of the negative electrode core, the total thickness of the negative electrode mixture layer, and the ratio of the total thickness of the negative electrode mixture layer to the thickness of the negative electrode core.

[0066] [Charge-Discharge Cycle Test] Each battery of the Examples and Comparative Examples was charged at a constant current of 0.3 C until the battery voltage reached 4.2 V, and then charged at a constant voltage of 4.2 V until the current reached 0.02 C. After a 20-minute rest, the battery was discharged at a constant current of 0.5 C until the battery voltage reached 2.8 V. This charge-discharge cycle was repeated 500 times. After the cycle test, the presence or absence of breakage of the negative electrode of each battery was confirmed by X-ray computed tomography (CT).

[0067]

[0068] As shown in Table 1, in the batteries of the comparative examples, negative electrode breakage, which poses a risk of short circuiting, was observed in the charge-discharge cycle test, but no negative electrode breakage was observed in the batteries of the examples. It is believed that the expansion of the negative electrode accompanying charge-discharge was effectively suppressed in the batteries of the examples, and as a result, the occurrence of negative electrode breakage was prevented. Furthermore, it is believed that the batteries of the examples suppress the expansion of the negative electrode and make the negative electrode less likely to break.

[0069] On the other hand, in the batteries of Comparative Examples 1, 4, and 6, the strength of the negative electrode was too low, which is thought to have caused the negative electrode to expand significantly and break. In the batteries of Comparative Examples 2 and 5, the volume change of the negative electrode during charging and discharging was too large, which is thought to have caused the negative electrode to expand significantly and break. In the battery of Comparative Example 3, the force acting in the planar direction on the negative electrode (the force in the radial direction of the electrode body) was too large, which is thought to have caused the negative electrode to break.

[0070] The present disclosure is further described by the following embodiments. Configuration 1: A non-aqueous electrolyte secondary battery including an electrode assembly including a positive electrode, a negative electrode, and a separator, a non-aqueous electrolyte, and an exterior housing that accommodates the electrode assembly and the non-aqueous electrolyte, wherein the negative electrode has a core and a mixture layer provided on both sides of the core, the core having a thickness of 15 μm to 30 μm, the mixture layer containing a silicon-containing material and a carbon material as a negative electrode active material, the ratio of the total thickness of the mixture layer to the core thickness being 4 to 10, the silicon-containing material containing an amorphous carbon phase and a silicon phase dispersed in the amorphous carbon phase, and the content of the silicon-containing material relative to the negative electrode active material being 30% by mass to 60% by mass. Configuration 2: The non-aqueous electrolyte secondary battery according to Configuration 1, wherein the silicon content of the silicon-containing material is 40% by mass to 70% by mass. Aspect 3: The nonaqueous electrolyte secondary battery according to Aspect 1 or 2, wherein the core has an elongation percentage of 6% or more and 12% or less. Aspect 4: The nonaqueous electrolyte secondary battery according to any one of Aspects 1 to 3, wherein the electrode assembly has a wound structure in which the positive electrode and the negative electrode are wound with the separator interposed therebetween, and the exterior body has a cylindrical exterior can with a bottom that is open on one axial side and a sealing body that closes the opening of the exterior can.

[0071] REFERENCE SIGNS LIST 10 Non-aqueous electrolyte secondary battery, 11 Positive electrode, 12 Negative electrode, 13 Separator, 14 Electrode body, 16 Outer can, 17 Sealing body, 18, 19 Insulating plate, 20 Positive electrode lead, 21 Negative electrode lead, 22 Grooved portion, 23 Internal terminal plate, 24 Lower valve body, 25 Insulating member, 26 Upper valve body, 27 Cap, 28 Gasket

Claims

1. A non-aqueous electrolyte secondary battery comprising an electrode body including a positive electrode, a negative electrode, and a separator, a non-aqueous electrolyte, and an exterior body that houses the electrode body and the non-aqueous electrolyte, wherein the negative electrode has a core body and a mixture layer provided on the core body, the thickness of the core body is 15 μm or more and 30 μm or less, the mixture layer contains a silicon-containing material and a carbon material as negative electrode active materials, and the ratio of the total thickness of the mixture layer to the thickness of the core body is 4 or more and 10 or less, and the silicon-containing material contains an amorphous carbon phase and a silicon phase dispersed in the amorphous carbon phase, and the content thereof with respect to the negative electrode active material is 30% by mass or more and 60% by mass or less.

2. The non-aqueous electrolyte secondary battery according to claim 1, wherein the silicon content in the silicon-containing material is 40% by mass or more and 70% by mass or less.

3. The non-aqueous electrolyte secondary battery according to claim 1, wherein the elongation rate of the core body is 6% or more and 12% or less.

4. The electrode body has a wound structure in which the positive electrode and the negative electrode are wound with the separator therebetween, and the exterior body has a bottomed cylindrical exterior can with one axial side open and a sealing body that closes the opening of the exterior can. The non-aqueous electrolyte secondary battery according to any one of claims 1 to 3.

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