Non-aqueous electrolyte secondary battery

Abstract

A nonaqueous electrolyte secondary battery has a positive electrode, a negative electrode, and a nonaqueous electrolyte. The negative electrode has a negative electrode core and a negative electrode mixture layer formed on at least one face of the negative electrode core, a ratio (E1 / E2) of a charge expansion ratio (E1) to a discharge expansion ratio (E2) of the negative electrode is 1.05 or more and less than 1.15, and a content of a silicon material is 3 to 12 mass% with respect to a mass of a negative electrode active material in the negative electrode mixture layer. In the negative electrode mixture layer, it is preferable that graphite, a first silicon material (SiO), and a second silicon material (LSX) be included as the negative electrode active material.

Description

TECHNICAL FIELD

[0001] The present application relates to a nonaqueous electrolyte secondary battery, and more particularly to a high-capacity nonaqueous electrolyte secondary battery provided with a Si-containing negative electrode. BACKGROUND

[0002] It is known that Si or a compound containing Si can occlude more lithium ions per unit volume than carbon-based active materials such as graphite. For example, Patent Literature 1 discloses a nonaqueous electrolyte secondary battery in which a compound containing Si is used as a negative electrode active material. In addition, Patent Literature 1 describes that by adjusting the equivalent ratio of Li to Si of the negative electrode material at the time of charging the battery to the charge termination voltage to a specific range by making the expansion rate accompanying lithium insertion into the negative electrode 1.05 or more and 3.0 or less, the cycle characteristics of the battery are improved.

[0003] PRIOR ART DOCUMENTS

[0004] PATENT LITERATURE

[0005] Patent Literature 1: Japanese Patent No. 5229239 SUMMARY

[0006] The Si-containing negative electrode has a problem in that the volume changes greatly accompanying charge and discharge, and thus the charge and discharge cycle characteristics of the battery deteriorate. The great volume change of the negative electrode accompanying charge and discharge promotes decomposition of the electrolyte, which is one cause of the cycle characteristics deterioration. Note that the nonaqueous electrolyte secondary battery of Patent Literature 1 has room for improvement in terms of cycle characteristics.

[0007] The nonaqueous electrolyte secondary battery of the present application is a nonaqueous electrolyte secondary battery provided with a positive electrode, a negative electrode, and a nonaqueous electrolyte, the negative electrode having a negative electrode core and a negative electrode mixture layer formed on at least one face of the negative electrode core, the ratio (E1 / E2) of the charge expansion rate (E1) to the discharge expansion rate (E2) of the negative electrode being 1.05 or more and less than 1.15, and the content of a silicon material with respect to the mass of the negative electrode active material in the negative electrode mixture layer being 3 to 12 mass%.

[0008] EFFECT OF THE INVENTION

[0009] The nonaqueous electrolyte secondary battery of the present application is a high-capacity battery provided with a Si-containing negative electrode, and has excellent cycle characteristics. BRIEF DESCRIPTION OF DRAWINGS

[0010] Figure 1 Cross-sectional view of a nonaqueous electrolyte secondary battery as one example of an embodiment. DETAILED DESCRIPTION

[0011] As described above, non-aqueous electrolyte secondary batteries with Si-containing negative electrodes have high capacity, but they suffer from the technical problem of large volume changes in the negative electrode during charging and discharging, leading to a decline in cycle performance. To improve the cycle performance of this battery, the inventors conducted in-depth research on the ratio (E1 / E2) of the negative electrode's charge expansion rate (E1) to its discharge expansion rate (E2). The results showed that when the ratio (E1 / E2) is 1.05 or higher and lower than 1.15, the cycle performance is specifically improved. Non-aqueous electrolyte secondary batteries that meet the condition 1.05 ≤ (E1 / E2) < 1.15 exhibit superior cycle performance compared to batteries that do not meet this condition.

[0012] An example embodiment of the non-aqueous electrolyte secondary battery of the present invention will now be described in detail with reference to the accompanying drawings. It should be noted that, from the outset, it was envisioned that the various embodiments and modifications described below be selectively combined.

[0013] The following describes a cylindrical battery in which the wound electrode body 14 is housed in a bottomed cylindrical outer casing 16. However, the outer casing of the battery is not limited to a cylindrical outer casing. For example, it can be a square outer casing (square battery), a coin-shaped outer casing (coin-shaped battery), or an outer casing made of a laminate containing a metal layer and a resin layer (laminated battery). In addition, the electrode body can also be a stacked electrode body in which multiple positive electrodes and multiple negative electrodes are alternately stacked with separators.

[0014] Figure 1 This is a cross-sectional view of a non-aqueous electrolyte secondary battery 10, as an example of an embodiment. (See attached image.) Figure 1 As shown, the non-aqueous electrolyte secondary battery 10 includes a wound electrode body 14, a non-aqueous electrolyte, and an outer casing 16 for housing the electrode body 14 and the non-aqueous 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 into a spiral shape with the separator 13 in between. The outer casing 16 is a bottomed cylindrical metal container with an opening at one end along the axial direction, and the opening of the outer casing 16 is closed by a sealing body 17. Hereinafter, for ease of explanation, the sealing body 17 side of the battery is referred to as the upper side, and the bottom side of the outer casing 16 is referred to as the lower side.

[0015] The positive electrode 11, the negative electrode 12, and the separator 13 that constitute the electrode body 14 are each a long strip body in a band shape, and 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 one turn larger than the positive electrode 11 in order to prevent precipitation of lithium. That is, the negative electrode 12 is formed to be longer than the positive electrode 11 in the length direction and the width direction (short side direction). The separator 13 is formed to be at least one turn larger than the positive electrode 11, and is disposed in two pieces in a manner of sandwiching 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.

[0016] The insulating plates 18 and 19 are disposed on the upper and lower sides of the electrode body 14, respectively. Figure 1 In the illustrated example, the positive electrode lead 20 extends to the side of the sealing body 17 through a through-hole of the insulating plate 18, and the negative electrode lead 21 extends to the side of the bottom of the outer can 16 through the outside of the insulating plate 19. The positive electrode lead 20 is connected to the lower surface of the inner terminal plate 23 of the sealing body 17 by welding or the like, and the top plate of the sealing body 17, that is, the lid 27, which is electrically connected to the inner terminal plate 23, is 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 is a negative electrode terminal.

[0017] As described above, the outer can 16 is a metal container of a bottomed cylindrical shape that is open on one end side in the axial direction. A gasket 28 is provided between the outer can 16 and the sealing body 17 to ensure the airtightness of the inside of the battery and the insulation of the outer can 16 and the sealing body 17. A groove portion 22, which is bulged inward by a portion of the side surface portion of the outer can 16, is formed in the outer can 16 to support the sealing body 17. The groove portion 22 is preferably formed in a ring 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 groove portion 22 and the open end portion of the outer can 16, which is caulked to the sealing body 17.

[0018] The sealing body 17 has a structure in which an inner terminal plate 23, a lower valve body 24, an insulating member 25, an upper valve body 26, and a lid 27 are sequentially stacked from the side of the electrode body 14. Each of the members that constitute the sealing body 17 has, for example, a circular plate shape or a ring shape, and each of the members except for the insulating member 25 is electrically connected to each other. The lower valve body 24 and the upper valve body 26 are connected at the respective central portions thereof, and the insulating member 25 is interposed between the respective peripheral edge portions thereof. When the internal pressure of the battery increases due to an abnormality, the lower valve body 24 is deformed to be broken in a manner of pushing the upper valve body 26 upward toward the lid 27, and thus the current path between the lower valve body 24 and the upper valve body 26 is cut off. If the internal pressure further increases, the upper valve body 26 is broken, and gas is discharged from the opening portion of the lid 27.

[0019] 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, particularly the negative electrode 12.

[0020] [positive electrode]

[0021] The positive electrode 11 has a positive electrode core 30 and a positive electrode mixture layer 31 formed on at least one face of the positive electrode core 30. The positive electrode core 30 can use a foil of aluminum, an aluminum alloy, or the like, a film in which such a metal is disposed on the surface layer, or the like, which is stable in the potential range of the positive electrode 11. The positive electrode mixture layer 31 preferably contains a positive electrode active material, a conductive agent, and a binder, and is formed on both faces of the positive electrode core 30. The positive electrode 11 can be manufactured by applying a positive electrode mixture slurry containing a positive electrode active material, a conductive agent, a binder, and the like on the positive electrode core 30, drying the coating film, and compressing the same to form the positive electrode mixture layer 31 on both faces of the positive electrode core 30.

[0022] The positive electrode active material can use a lithium transition metal composite oxide. As elements contained in the lithium transition metal composite oxide, Ni, Co, Mn, Al, B, Mg, Ti, V, Cr, Fe, Cu, Zn, Ga, Sr, Zr, Nb, In, Sn, Ta, W, and the like can be listed. One example of the preferable lithium transition metal composite oxide is a composite oxide containing at least one selected from Ni, Co, Mn, and Al. As specific examples, an NCM-based composite oxide containing Ni, Co, and Mn, an NCA-based composite oxide containing Ni, Co, and Al, and the like can be listed. Note that the surface of the lithium transition metal composite oxide can also have inorganic compound particles such as alumina, lanthanide-containing compounds, or the like fixed thereto.

[0023] As the conductive agent contained in the positive electrode mixture layer 31, carbon black, acetylene black, Ketjen black, graphite, and the like can be listed. As the binder contained in the positive electrode mixture layer 31, fluororesins such as polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVdF), polyacrylonitrile (PAN), polyimide, acrylic resins, polyolefins, and the like can be listed. These resins can also be used in combination with carboxymethylcellulose (CMC), a salt of CMC, polyethylene oxide (PEO), and the like.

[0024] [negative electrode]

[0025] The negative electrode 12 has a negative electrode core 40 and a negative electrode mixture layer 41 formed on at least one face of the negative electrode core 40. The negative electrode core 40 can use a foil of copper, a copper alloy, or the like, a film in which such a metal is disposed on the surface layer, or the like, which is stable in the potential range of the negative electrode. The negative electrode mixture layer 41 preferably contains a negative electrode active material, a conductive agent, and a binder, and is formed on both faces of the negative electrode core 40. The negative electrode 12 can be manufactured by applying a negative electrode mixture slurry containing a negative electrode active material, a conductive agent, a binder, and the like on the negative electrode core 40, drying the coating film, and compressing the same to form the negative electrode mixture layer 41 on both faces of the negative electrode core 40.

[0026] As will be described later in detail, the ratio (E1 / E2) of the charging expansion rate (E1) to the discharging expansion rate (E2) of the negative electrode 12 is 1.05 or greater and less than 1.15, and the content of the silicon material is 3 to 12% by mass relative to the mass of the negative electrode active material in the negative electrode mixture layer 41. According to the negative electrode 12 satisfying this condition, the cycle characteristics of the nonaqueous electrolyte secondary battery 10 can be specifically improved. The negative electrode mixture layer 41 can include Si single substance, but it is preferable to exist in the form of a silicon material that is a Si-containing compound having a smaller volume change ratio than Si at the time of charging and discharging.

[0027] The thickness of the negative electrode mixture layer 41 is, for example, 30 μm to 120 μm or 50 μm to 80 μm on one side of the negative electrode core 40. From the viewpoint of high capacity and the like, the packing density of the negative electrode mixture layer 41 is preferably 1.5 g / cm 3 or greater and 2.0 g / cm 3 or less. One example of the preferable packing density of the negative electrode mixture layer 41 is 1.5 to 1.9 g / cm 3 or 1.6 to 1.8 g / cm 2 . Note that the negative electrode mixture layer 41 is not limited to a single layer structure, and can be composed of a plurality of layers.

[0028] In the negative electrode mixture layer 41, it is preferable to include graphite, a first silicon material (SiO) containing a silicon oxide phase and Si dispersed in the silicon oxide phase, and a second silicon material (LSX) containing a lithium silicate phase and Si dispersed in the lithium silicate phase, as the negative electrode active material. By using graphite together with the silicon material, it is easy to achieve both high capacity and good cycle characteristics. Note that, within a range not impairing the object of the present application, the negative electrode mixture layer 41 can include a negative electrode active material other than graphite and the two kinds of silicon materials.

[0029] One example of the preferable content of graphite is 70 to 97% by mass or 80 to 96% by mass or 85 to 95% by mass relative to the mass of the negative electrode active material. That is, one example of the preferable content of the silicon material is 3 to 30% by mass or 4 to 20% by mass or 5 to 15% by mass relative to the mass of the negative electrode active material. The content of the negative electrode active material is, for example, 90 to 99% by mass or 93 to 98% by mass relative to the total mass of the negative electrode mixture layer 41. As described above, in the negative electrode mixture layer 41, it is preferable to include a conductive agent and a binder as components other than the negative electrode active material.

[0030] Graphite can use natural graphite such as flaky graphite, blocky artificial graphite, artificial graphite such as graphitized mesocarbon microbeads, and the like. The BET specific surface area of graphite is, for example, 0.5 to 7.5 m 2 / g or 1 to 5 m 2The BET specific surface area is measured according to the BET method (nitrogen adsorption method) described in JIS R 1626. One example of the median particle size (D50) of the graphite on a volume basis is 1 to 20 μm or 2 to 15 μm. The D50 is the particle size at which the cumulative value of the particle size distribution on a volume basis is 50% as measured by the laser diffraction scattering method. The graphite can be composed of one primary particle or a secondary particle in which a plurality of primary particles are aggregated.

[0031] The first silicon material (SiO) and the second silicon material (LSX) are, for example, particles having a D50 smaller than the D50 of the graphite. One example of the D50 of the SiO and the LSX is 1 μm to 15 μm or 3 μm to 10 μm. An electrically conductive layer composed of a material having high electrical conductivity can be formed on the surface of the particles of the SiO and the LSX. One example of the preferred electrically conductive layer is a carbon coating film composed of a carbon material. The thickness of the electrically conductive layer is preferably 1 to 200 nm or 5 to 100 nm in view of the securing of electrical conductivity and the diffusivity of lithium ions into the interior of the particles.

[0032] The carbon coating film is composed of, for example, carbon black, acetylene black, ketjen black, graphite, and a mixture of two or more of these. As a method of carbon-coating the surface of the particles of the SiO and the LSX, there are, for example, a CVD method using acetylene, methane, or the like; a method in which coal tar pitch, petroleum pitch, phenol resin, or the like is mixed with the particles of the SiO and the LSX and heat-treated; and the like. Alternatively, the carbon coating film can be formed by fixing carbon powder such as carbon black on the surface of the particles with a binder. The carbon coating film is formed, for example, at a mass of 0.5 to 10 mass% with respect to the mass of the particles of the SiO and the LSX.

[0033] The SiO has a particle structure in which fine Si particles are dispersed in a silicon oxide phase. The preferred SiO has an island-in-sea structure in which fine Si particles are substantially uniformly dispersed in a matrix of amorphous silicon oxide, represented by the general formula SiO x (0.5 < x < 1.6). From the viewpoint of balancing the battery capacity and the cycle characteristics, the content ratio of the Si particles is preferably 35 to 75 mass% with respect to the total mass of the SiO. For example, if the content ratio of the Si particles is too low, the charge and discharge capacity decreases, and if the content ratio of the Si particles is too high, a part of the Si particles exposed without being covered with silicon oxide come into contact with the electrolyte, and the cycle characteristics decrease.

[0034] The average particle diameter of the Si particles dispersed in the silicon oxide phase is, for example, 500 nm or less, preferably 200 nm or less or 50 nm or less, before charge and discharge. It is, for example, 400 nm or less or 100 nm or less, after charge and discharge. By making the Si particles fine, the volume change at the time of charge and discharge becomes small, and the cycle characteristics are improved. The average particle diameter of the Si particles is obtained by observing the cross section of the SiO with a scanning electron microscope (SEM) or a transmission electron microscope (TEM), and taking the average of the longest diameters of 100 Si particles. The silicon oxide phase is composed of a collection of particles finer than the Si particles.

[0035] The LSX has a particle structure in which fine Si particles are dispersed in a lithium silicate phase. The preferred LSX has fine Si particles dispersed substantially uniformly in a matrix of lithium silicate represented by the general formula Li 2z SiO (2+z) (0 < z < 2). As in the case of SiO, the content of the Si particles is preferably 35 to 75 mass% relative to the total mass of the LSX. In addition, the average particle diameter of the Si particles is, for example, 500 nm or less, preferably 200 nm or less or 50 nm or less, before charge and discharge. The lithium silicate phase is composed of a collection of particles finer than the Si particles.

[0036] The lithium silicate phase is preferably composed of a compound represented by Li 2z SiO (2+z) (0 < z < 2). That is, the lithium silicate phase does not contain Li4SiO4 (Z = 2). Li4SiO4 is an unstable compound, and reacts with water to become alkaline, and thus sometimes deteriorates Si to cause a decrease in charge and discharge capacity. From the viewpoints of stability, ease of production, lithium ion conductivity, and the like, the lithium silicate phase is preferably composed mainly of Li2SiO3 (Z = 1) or Li2Si2O5 (Z = 1 / 2). When the lithium silicate phase is composed mainly of Li2SiO3 or Li2Si2O5, the content of the main component is preferably more than 50 mass%, more preferably 80 mass% or more, relative to the total mass of the lithium silicate phase.

[0037] The SiO can be produced, for example, by the following processes 1 to 3.

[0038] (1) Si and silicon oxide are mixed at a weight ratio of 20:80 to 95:5 to produce a mixture.

[0039] (2) At least before or after the production of the mixture, the Si and silicon oxide are pulverized to make them fine particles by using a ball mill.

[0040] (3) The pulverized mixture is heat-treated at 600 to 1000°C in a non-active atmosphere.

[0041] Note that, in the above process, by using lithium silicate instead of silicon oxide, LSX can be produced.

[0042] In the above heat treatment, pressure can be applied as in hot pressing to produce a sintered body of the above mixture. At this time, the sintered body is pulverized to a predetermined particle size. Note that, Li 2z SiO (2+z) Lithium silicate represented by SiOz (0 < z < 2) is stable in the temperature range of 600 to 1000°C and does not react with Si, so the capacity does not decrease. LSX can be produced by synthesizing nanoparticles of Si and nanoparticles of lithium silicate, mixing them, and performing heat treatment, without using a ball mill.

[0043] As described above, the ratio (E1 / E2) of the charge expansion ratio (E1) of the negative electrode 12 to the discharge expansion ratio (E2) is 1.05 or more and less than 1.15. When the ratio (E1 / E2) is less than 1.05 or 1.15 or more, the cycle characteristics greatly decrease, so it is necessary to produce the negative electrode 12 in a manner that satisfies the condition of 1.05 ≤ (E1 / E2) < 1.15. In this specification, the charge expansion ratio (E1) and the discharge expansion ratio (E2) are measured after the nonaqueous electrolyte secondary battery 10 is produced, an aging process is performed, and the battery is disassembled after the second charge and discharge, respectively.

[0044] The charge and discharge conditions of the nonaqueous electrolyte secondary battery 10 in the measurement of the charge expansion ratio (E1) and the discharge expansion ratio (E2) are as follows. The negative electrode is taken out from the disassembled battery, the thickness of the negative electrode mixture layer is measured, and the thickness of the negative electrode mixture layer immediately after the production of the negative electrode is compared to find the expansion ratio.

[0045] [Charge and discharge conditions]

[0046] The nonaqueous electrolyte secondary battery 10 is subjected to constant current charging at a constant current of 0.2 It until the battery voltage becomes 4.2 V at a temperature environment of 25°C, and then subjected to constant voltage charging at 4.2 V until the current value becomes 0.02 It. Thereafter, the battery is subjected to constant current discharging at a constant current of 0.2 It until the battery voltage becomes 3.0 V. The charging is performed again under the same conditions, the charged battery is disassembled, and the charge expansion ratio (E1) is measured. In addition, the charge and discharge are performed again under the same conditions, the discharged battery is disassembled, and the discharge expansion ratio (E2) is measured.

[0047] A more preferable range of the ratio (E1 / E2) is, for example, 1.06 to 1.13 (1.06 or more and 1.13 or less) or 1.07 to 1.10. Although the presence or absence and the content of SiO and LSX in the negative electrode mixture layer 41 greatly affect the ratio (E1 / E2), the ratio (E1 / E2) also varies depending on the kind of constituent components other than the silicon material, such as the kind of the conductive agent and the binder, the compounding ratio, and the like, and there are a plurality of control factors. Therefore, it is necessary to manufacture the negative electrode 12 in such a manner that the ratio (E1 / E2) is within the above range by adjusting these plurality of factors.

[0048] The content of the silicon material (SiO, LSX) in the negative electrode mixture layer 41 is 3 to 12% by mass with respect to the mass of the negative electrode active material, and is preferably 3 to 9% by mass. SiO and LSX are added to the negative electrode mixture slurry used for forming the negative electrode mixture layer 41 in such a manner that the content with respect to the mass of the negative electrode active material is 3 to 12% by mass. When the content of the silicon material deviates from this range, it is difficult to satisfy the condition of the ratio (E1 / E2), and it is not possible to achieve both high capacity and good cycle characteristics.

[0049] It is preferable that the negative electrode mixture layer 41 contain LSX in an amount of 1% by mass or more with respect to the mass of the negative electrode active material. LSX has a higher charge-discharge efficiency than SiO, and in addition, the volume change accompanying charge and discharge is also slow, and by adding 1% by mass or more of LSX, it is easy to adjust the ratio (E1 / E2). On the other hand, it is necessary to add LSX in such a manner that the content of the silicon material is 12% by mass or less with respect to the negative electrode active material. The upper limit of the content of LSX is, for example, 10% by mass with respect to the mass of the negative electrode active material. One example of a preferable content of LXS is 1 to 10% by mass or 2 to 8% by mass.

[0050] The mass ratio of LSX with respect to the mass of SiO is, for example, 0.2 to 6. By controlling the mass ratio of SiO to LSX to be 0.2 to 6, it is easy to adjust the ratio (E1 / E2), and it is possible to achieve both high capacity and good cycle characteristics. Also, in order to improve the cycle characteristics of the battery, it is also possible to make the charge-discharge efficiency of the positive electrode 11 and the negative electrode 12 the same degree. The use of SiO and LSX makes it easy to adjust this charge-discharge efficiency. A more preferable range of the mass ratio of SiO to LSX is 0.5 to 5 or 0.6 to 3 or 0.8 to 2 or 1 to 1.5.

[0051] The negative electrode mixture layer 41 preferably contains carbon nanotubes (CNT) at 0.01 to 0.1% by mass or 0.05 to 0.1% by mass relative to the mass of the negative electrode active material. The CNT functions as a conductive material and forms a good conductive path in the negative electrode mixture layer 41. The CNT can be a single-layer CNT, a 2-layer CNT, a multi-layer CNT, or a mixture thereof. In addition, the CNT can be a vapor-grown carbon fiber called VGCF (registered trademark). The CNT has a diameter of 2 nm to 20 μm and a total length of 0.03 μm to 500 μm, for example.

[0052] The negative electrode mixture layer 41 preferably contains carboxymethyl cellulose (CMC) or a salt thereof, styrene-butadiene copolymer (SBR), and polyacrylic acid (PAA) or a salt thereof at 0.1 to 3% by mass relative to the mass of the negative electrode active material. The use of CNT as a conductive agent and the use of these three compounds as binders make it possible to easily achieve both high capacity and good cycle characteristics. The CNT and these binders are control factors for the ratio (E1 / E2). Note that other conductive agents, binders, and the like can also be contained in the negative electrode mixture layer 41.

[0053] [Separator]

[0054] The separator 13 can use a porous sheet having ion permeability and insulating properties. As specific examples of the porous sheet, a microporous film, a woven fabric, a nonwoven fabric, and the like can be given. As the material of the separator 13, polyethylene, polypropylene, a copolymer of ethylene and an α-olefin, and the like polyolefin, cellulose, and the like are preferable. The separator 13 can be either of a single-layer structure and a laminated structure. A heat-resistant layer containing inorganic particles, a heat-resistant layer composed of a resin having high heat resistance such as an aromatic polyamide resin, a polyimide, a polyamide-imide, and the like, and the like can be formed on the surface of the separator 13.

[0055] [Non-aqueous electrolyte]

[0056] The non-aqueous electrolyte contains, for example, a non-aqueous solvent and an electrolyte salt dissolved in the non-aqueous solvent. The non-aqueous solvent can use, for example, an ester, an ether, acetonitrile, an amide such as dimethylformamide, and a mixed solvent of two or more of these solvents. The non-aqueous solvent can also contain a halogen-substituted product in which at least a part of the hydrogen of these solvents is replaced with a halogen atom such as fluorine. As the halogen-substituted product, a fluorinated cyclic carbonate such as fluoroethylene carbonate (FEC), a fluorinated chain carbonate, a fluorinated chain carboxylic acid ester such as fluoropropionates methyl (FMP), and the like can be given.

[0057] As examples of the above-mentioned esters, there can be mentioned cyclic carbonates such as ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate and the like; chain carbonates such as dimethyl carbonate (DMC), ethylmethyl carbonate (EMC), diethyl carbonate (DEC), methylpropyl carbonate, ethylpropyl carbonate, methylisopropyl carbonate and the like; cyclic carboxylic acid esters such as γ-butyrolactone (GBL), γ-valerolactone (GVL) and the like; chain carboxylic acid esters such as methyl acetate, ethyl acetate, propyl acetate, methyl propionate (MP), ethyl propionate and the like.

[0058] As examples of the above-mentioned ethers, there can be mentioned cyclic ethers such as 1,3-dioxolane, 4-methyl-1,3-dioxolane, tetrahydrofuran, 2-methyltetrahydrofuran, propylene oxide, 1,2-epoxybutane, 1,3-dioxane, 1,4-dioxane, 1,3,5-trioxane, furan, 2-methylfuran, 1,8-cineole, crown ether and the like; chain ethers such as 1,2-dimethoxyethane, diethyl ether, dipropyl ether, diisopropyl ether, dibutyl ether, dihexyl ether, ethylvinyl ether, butylvinyl ether, methylphenyl ether, ethylphenyl ether, butylphenyl ether, amylphenyl 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-dimethoxy methane, 1,1-diethoxy ethane, triethylene glycol dimethyl ether, tetraethylene glycol dimethyl ether and the like.

[0059] The electrolyte salt is preferably a lithium salt. As examples of the lithium salt, there can be mentioned LiBF4, LiClO4, LiPF6, LiAsF6, LiSbF6, LiAlCl4, LiSCN, LiCF3SO3, LiCF3CO2, Li(P(C2O4)F4), LiPF 6-x (C n F 2n+1 )x(1 10 Cl 10 , LiCl, LiBr, LiI, chloroborane lithium, lithium lower aliphatic carboxylate, Li2B4O7, borate such as Li(B(C2O4)F2), LiN(SO2CF3)2, LiN(C1F 2l+1 SO2)(C m F 2m+1 SO2){l, m are integers of 0 or more} and the like. As to the lithium salt, one kind thereof can be used alone, or a plurality of kinds thereof can be used in mixture. Of these, from the viewpoints of ionic conductivity, electrochemical stability and the like, LiPF6is preferably used. As to the concentration of the lithium salt, for example, it is 0.8 mol or more and 1.8 mol or less per 1 L of the nonaqueous solvent. Furthermore, vinylene carbonate, a propanesulfonic acid lactone additive and the like can be added.

[0060] <Example>

[0061] The present invention will be further illustrated below by way of examples, but the present invention is not limited to these examples.

[0062] <Example 1>

[0063] [positive electrode]

[0064] Lithium nickel cobalt aluminum oxide was used as the positive electrode active material. 98.0 parts by weight of the positive electrode active material, 1.0 part by weight of acetylene black, and 1.0 part by weight of polyvinylidene fluoride (PVDF) were mixed, and an appropriate amount of N-methyl-2-pyrrolidone (NMP) was further added to prepare a positive electrode slurry. Next, the portion of the positive electrode core formed from aluminum foil for connecting the leads was coated with the positive electrode slurry and dried. After rolling the coating, it was cut to the specified electrode size to fabricate a positive electrode with positive electrode slurry layers formed on both sides of the positive electrode current collector.

[0065] [negative electrode]

[0066] As the negative electrode active material, SiO is used, in which graphite and Si particles are dispersed in a silicon oxide phase. x (x=1) represents a substance obtained by mixing a first silicon material (SiO) and a second silicon material (LSX) with Si particles dispersed in a lithium silicate phase mainly composed of Li2Si2O5 at a mass ratio of 94:3:3. It should be noted that a carbon coating is formed on the surface of both SiO and LSX particles. The mass ratio of LSX / SiO is 1.

[0067] The above-mentioned negative electrode active material, sodium salt of carboxymethyl cellulose (CMC-Na), dispersion of styrene-butadiene copolymer (SBR), lithium salt of polyacrylic acid (PAA) (PAA-Na), and carbon nanotubes (CNTs) were mixed in a solid component mass ratio of 100:0.9:1.2:0.7:0.05, and an appropriate amount of water was added to prepare a negative electrode composite slurry. Next, the portion of the negative electrode core formed by copper foil for connecting the leads was coated with the negative electrode slurry and dried. After rolling the coating, it was cut to the specified electrode size to fabricate a negative electrode with an electrode slurry layer formed on both sides of the negative electrode core.

[0068] [Preparation of non-aqueous electrolytes]

[0069] A non-aqueous electrolyte was prepared by adding LiPF6 at a concentration of 1 mol / L to a mixed solvent obtained by mixing ethylene carbonate (EC) and methyl ethyl carbonate (MEC) at a volume ratio of 3:7 (25°C, 1 atm), and further adding ethylene carbonate at a concentration of 2% by mass.

[0070] [Test Battery]

[0071] A lead wire was attached to each of the above positive electrode and the above negative electrode, and the positive electrode and the negative electrode were wound into a spiral shape with a separator interposed therebetween. After a tape was attached to the outermost circumferential surface of the wound body, the wound body was pressed in the radial direction to produce a flat-shaped wound electrode body. The separator was a single-layer polypropylene separator. After the produced electrode body was inserted into an exterior body composed of an aluminum laminate sheet, the above non-aqueous electrolyte was injected, the opening of the exterior body was sealed, and a test cell (laminate cell) was produced.

[0072] For the above test cell, the charge expansion ratio (El) and the discharge expansion ratio (E2) of the negative electrode were measured by the above method, and the ratio (El / E2) of the charge expansion ratio (El) to the discharge expansion ratio (E2) was calculated. As a result, the ratio (El / E2) was 1.08 (the same calculation was performed for other examples and comparative examples).

[0073] <Example 2>

[0074] As the negative electrode active material, a material obtained by mixing graphite, SiO, and LSX at a mass ratio of 92.5:4.5:3.0 was used, and a negative electrode and a test cell were produced in the same manner as in Example 1. Note that in the negative electrode of Example 2, the mass ratio of LSX / SiO was 1.5.

[0075] <Example 3>

[0076] As the negative electrode active material, a material obtained by mixing graphite, SiO, and LSX at a mass ratio of 92.0:5.0:3.0 was used, and a negative electrode and a test cell were produced in the same manner as in Example 1. Note that in the negative electrode of Example 3, the mass ratio of LSX / SiO was 1.7, and the Si content in the negative electrode mixture layer was 8.0 mass%.

[0077] <Example 4>

[0078] As the negative electrode active material, a material obtained by mixing graphite and LSX at a mass ratio of 94.0:6.0 was used, and a negative electrode and a test cell were produced in the same manner as in Example 1.

[0079] <Comparative Example 1>

[0080] As the negative electrode active material, a material obtained by mixing graphite and SiO at a mass ratio of 92.5:7.5 was used, and a negative electrode and a test cell were produced in the same manner as in Example 1.

[0081] <Comparative Example 2>

[0082] As the negative active material, a material obtained by mixing graphite and SiO in a mass ratio of 90:10 was used, and a negative electrode and a test cell were produced in the same manner as in Example 1 except for this.

[0083] <Comparative Example 3>

[0084] As the negative active material, a material obtained by mixing graphite and SiO in a mass ratio of 80:20 was used, and a negative electrode and a test cell were produced in the same manner as in Example 1 except for this.

[0085] <Comparative Example 4>

[0086] As the negative active material, a material obtained by mixing graphite and LSX in a mass ratio of 90:10 was used, and a negative electrode and a test cell were produced in the same manner as in Example 1 except for this.

[0087] For each of the above test cells, evaluation of the cycle characteristics was performed by the following method. The evaluation results are shown in Table 1 together with the constitution of the negative electrode.

[0088] [Evaluation of cycle characteristics (capacity retention rate)]

[0089] In a temperature environment of 25°C, the test cell was subjected to constant current charging at a constant current of 0.2 It until the cell voltage became 4.2 V, and then to constant voltage charging at 4.2 V until the current value became 0.02 It. Thereafter, constant current discharging was performed at a constant current of 0.2 It until the cell voltage became 3.0 V. This charging and discharging was performed for 100 cycles, and the capacity retention rate in the charging and discharging cycles was calculated based on the following equation. The capacity retention rates shown in Table 1 are relative values when the capacity retention rate of the test cell of Example 1 is taken as a reference (100).

[0090] Capacity retention rate = (discharge capacity at the 100th cycle / discharge capacity at the 1st cycle) x 100

[0091] [Table 1]

[0092] E1 / E2 LSX SiO LSX / SiO Capacity retention rate Example 1 1.08 2.0 mass% 2.0 mass% 1.0 100 Example 2 1.11 4.5 mass% 3.0 mass% 1.5 99 Example 3 1.12 5.0 mass% 3.0 mass% 1.9 98 Example 4 1.06 6.0 mass% 0 mass% 0 97 Comparative Example 1 1.15 0 mass% 7.5 mass% 0 93 Comparative Example 2 1.17 0 mass% 10.0 mass% 0 88 Comparative Example 3 1.19 0 mass% 20.0 mass% 0 85 Comparative Example 4 1.03 10.0 mass% 0 mass% 0 83

[0093] As is clear from the results shown in Table 1, the capacity retention rates of the test cells of the Examples were all high, and the cycle characteristics were all excellent, as compared with the test cells of the Comparative Examples. When the ratio (El / E2) of the charging expansion rate (El) to the discharging expansion rate (E2) of the negative electrode was 1.15 or more (Comparative Examples 1 to 3), a sharp decrease in the capacity retention rate was confirmed. More interestingly, when the ratio (El / E2) was less than 1.05 (Comparative Example 4), a further large decrease in the capacity retention rate was seen. That is, when the ratio (El / E2) satisfies the condition of being 1.05 or more and less than 1.15, the cycle characteristics are specifically improved.

[0094] Reference Signs List

[0095] 10 nonaqueous electrolyte secondary battery

[0096] 11 positive electrode

[0097] 12 negative electrode

[0098] 13 separator

[0099] 14 electrode body

[0100] 16 outer can

[0101] 17 sealing body

[0102] 18, 19 insulating plate

[0103] 20 positive electrode lead

[0104] 21 negative electrode lead

[0105] 22 groove portion

[0106] 23 inner terminal plate

[0107] 24 lower valve body

[0108] 25 insulating member

[0109] 26 upper valve body

[0110] 27 lid

[0111] 28 gasket

[0112] 30 positive electrode core

[0113] 31 positive electrode mixture layer

[0114] 40 negative electrode core

[0115] 41 negative electrode mixture layer

Claims

1. A nonaqueous electrolyte secondary battery, wherein The nonaqueous electrolyte secondary battery includes a positive electrode, a negative electrode, and a nonaqueous electrolyte, The negative electrode has a negative electrode core and a negative electrode mixture layer formed on at least one surface of the negative electrode core, A ratio (E1 / E2) of a charge expansion rate (E1) of the negative electrode to a discharge expansion rate (E2) is 1.05 or greater and lower than 1.15, and a content of a silicon material is 3 to 12 mass% relative to a mass of a negative electrode active material in the negative electrode mixture layer; The negative electrode mixture layer contains, as the negative electrode active material, graphite, a first silicon material SiO containing a silicon oxide phase and Si dispersed in the silicon oxide phase, and a second silicon material LSX containing a lithium silicate phase and Si dispersed in the lithium silicate phase, A ratio of a mass of the LSX to a mass of the SiO is 0.8 to 2.

2. The nonaqueous electrolyte secondary battery according to claim 1, wherein The negative electrode mixture layer contains the LSX at 1 mass% or greater relative to a mass of the negative electrode active material.

3. The nonaqueous electrolyte secondary battery according to claim 1 or 2, wherein The negative electrode mixture layer contains carbon nanotubes at 0.01 to 0.1 mass% relative to a mass of the negative electrode active material.

4. The nonaqueous electrolyte secondary battery according to any one of claims 1 to 3, wherein The negative electrode mixture layer contains carboxymethyl cellulose or a salt thereof, a styrene-butadiene copolymer, and polyacrylic acid or a salt thereof in an amount of 0.1 to 3 mass% relative to a mass of the negative electrode active material.

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