Negative electrode for non-aqueous electrolyte secondary battery and non-aqueous electrolyte secondary battery

Abstract

The object of the present application is to provide a negative electrode that takes into account both the battery capacity and the charge-discharge cycle characteristics. The negative electrode for a non-aqueous electrolyte secondary battery as an example of an embodiment has: a strip-shaped negative electrode current collector, and a negative electrode mixture layer formed on the surface of the negative electrode current collector. The negative electrode mixture layer contains a first Si-based active material and a second Si-based active material. Both the first Si-based active material and the second Si-based active material have a structure in which Si particles are dispersed in an oxide phase, and the content rate of Si particles in the first Si-based active material is higher than the content rate of Si particles in the second Si-based active material. In the negative electrode mixture layer, the ratio of the mass of the first Si-based active material to the total mass of the first Si-based active material and the second Si-based active material is larger at the central portion than at the end portions in the width direction of the negative electrode current collector.

Description

Technical Field

[0001] The present application relates to a negative electrode for a non-aqueous electrolyte secondary battery and a non-aqueous electrolyte secondary battery, and particularly to a negative electrode for a non-aqueous electrolyte secondary battery containing a Si-based active material and a non-aqueous electrolyte secondary battery. Background Art

[0002] As a negative electrode active material for a non-aqueous electrolyte secondary battery, a Si-based active material containing silicon (Si), a carbon-based active material such as graphite, etc. are used. It is known that a Si-based active material can occlude more lithium ions per unit mass than a carbon-based active material such as graphite. In particular, a Si-based active material in which Si particles are dispersed in an oxide phase containing silicon oxide and lithium silicate has a smaller volume change caused by the occlusion of lithium ions than when Si is used in its elemental form. Therefore, it is suitable as a negative electrode active material for a non-aqueous electrolyte secondary battery. For example, Patent Document 1 discloses a negative electrode active material composed of silicon oxide represented by SiO x (0.5 ≤ x < 1.6), and a silicon-lithium silicate composite in which a silicon phase is dispersed in a lithium silicate phase represented by Li 2z SiO (2+z) (0 < z < 2).

[0003] Prior Art Documents

[0004] Patent Documents

[0005] Patent Document 1: WO 2019 / 142744 Summary of the Invention

[0006] Problems to be Solved by the Invention

[0007] However, for non-aqueous electrolyte secondary batteries such as lithium ion batteries, it is required to maintain the ratio of the discharge capacity to the charge capacity, that is, the charge-discharge efficiency, at a high level even after repeated charge and discharge. Improving such charge-discharge cycle characteristics is an important issue. In particular, it is required to improve the charge-discharge cycle characteristics while achieving a high capacity. The technology disclosed in Patent Document 1 has not studied the balance between battery capacity and charge-discharge cycle characteristics, and there is still room for improvement.

[0008] Therefore, an object of the present application is to provide a negative electrode that balances battery capacity and charge-discharge cycle characteristics.

[0009] Means for Solving the Problems

[0010] The negative electrode for a non-aqueous electrolyte secondary battery according to an aspect of the present application is characterized by having: a strip-shaped negative electrode current collector, and a negative electrode mixture layer formed on the surface of the negative electrode current collector. The negative electrode mixture layer contains a first Si-based active material and a second Si-based active material. Both the first Si-based active material and the second Si-based active material have a structure in which Si particles are dispersed in an oxide phase, and the content rate of Si particles in the first Si-based active material is higher than the content rate of Si particles in the second Si-based active material. In the negative electrode mixture layer, the ratio of the mass of the first Si-based active material to the total mass of the first Si-based active material and the second Si-based active material is larger at the central portion than at the end portions in the width direction of the negative electrode current collector.

[0011] The non-aqueous electrolyte secondary battery according to an aspect of the present application is characterized by including: the negative electrode for a non-aqueous electrolyte secondary battery described above, a positive electrode, and a non-aqueous electrolyte.

[0012] Advantageous Effects of the Invention

[0013] According to the negative electrode according to an aspect of the present application, a non-aqueous electrolyte secondary battery that takes both battery capacity and charge-discharge cycle characteristics into account can be provided. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Figure 1 is an axial sectional view of a cylindrical secondary battery as an example of an embodiment.

[0015] Figure 2 is a sectional view schematically showing the first Si-based active material and the second Si-based active material in an example of the embodiment.

[0016] Figure 3 is a front view of the negative electrode as an example of the embodiment. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0017] Hereinafter, an example of an embodiment of the non-aqueous electrolyte secondary battery of the present application will be described in detail. Hereinafter, a cylindrical battery in which a wound electrode body is housed in a cylindrical battery case is exemplified, but the electrode body is not limited to the wound type, and may be a laminated type in which a plurality of positive electrodes and a plurality of negative electrodes are alternately laminated with a spacer therebetween. In addition, the battery case is not limited to the cylindrical shape, and may be, for example, a square shape, a coin shape, etc., or may be a bag shape formed of a laminated sheet including a metal layer and a resin layer.

[0018] Figure 1 is an axial sectional view of a cylindrical secondary battery 10 as an example of the embodiment. In Figure 1In the secondary battery 10 shown, an electrode body 14 and a non-aqueous electrolyte (not shown) are accommodated in an outer packaging body 15. The electrode body 14 has a wound structure in which a positive electrode 11 and a negative electrode 12 are wound with a separator 13 therebetween. Hereinafter, for convenience of explanation, the side of the sealing body 16 is referred to as "upper", and the bottom side of the outer packaging body 15 is referred to as "lower" for explanation.

[0019] By blocking the opening end portion at the upper part of the outer body 15 with the sealing body 16, the inside of the secondary battery 10 is sealed. Insulating plates 17 and 18 are provided above and below the electrode body 14, respectively. The positive electrode lead 19 passes through the through hole of the insulating plate 17 and extends upward, and is welded to the lower surface of the bottom plate of the sealing body 16, that is, the filter member 22. In the secondary battery 10, the top plate of the sealing body 16, that is, the lid 26, which is electrically connected to the filter member 22, becomes the positive electrode terminal. On the other hand, the negative electrode lead 20 passes outside the insulating plate 18 and extends to the bottom side of the outer body 15, and is welded to the inner surface of the bottom of the outer body 15. In the secondary battery 10, the outer body 15 becomes the negative electrode terminal.

[0020] The outer body 15 is, for example, a bottomed cylindrical metal outer can. A gasket 27 is provided between the outer body 15 and the sealing body 16 to ensure the airtightness of the inside of the secondary battery 10. The outer body 15 has, for example, a groove portion 21 formed by pressing the side surface portion from the outside to support the sealing body 16. The groove portion 21 is preferably formed in a ring shape along the circumferential direction of the outer body 15, and supports the sealing body 16 with the gasket 27 interposed therebetween on its upper surface.

[0021] The sealing body 16 has a filter member 22, a lower valve body 23, an insulating member 24, an upper valve body 25, and a lid 26 laminated in this order from the side of the electrode body 14. Each member constituting the sealing body 16 has, for example, a disc shape or a ring shape, and the members other than the insulating member 24 are electrically connected to each other. The lower valve body 23 and the upper valve body 25 are connected to each other at their central portions, and the insulating member 24 is interposed between their peripheral portions. If the internal pressure of the battery rises due to abnormal heat generation, for example, the lower valve body 23 breaks, and thus, the upper valve body 25 bulges toward the lid 26 side and separates from the lower valve body 23, thereby blocking the electrical connection between the two. If the internal pressure further rises, the upper valve body 25 breaks, and gas is discharged from the opening portion 26a of the lid 26.

[0022] Hereinafter, the positive electrode 11, the negative electrode 12, the separator 13, and the non-aqueous electrolyte constituting the secondary battery 10, particularly the negative electrode active material contained in the negative electrode mixture layer constituting the negative electrode 12, will be described in detail.

[0023] [Positive Electrode]

[0024] The positive electrode 11 has a strip-shaped positive electrode current collector and a positive electrode mixture layer formed on the surface of the positive electrode current collector. As Figure 1As shown, the positive electrode 11 may have a positive electrode mixture layer 32 on both sides of the positive electrode current collector 30. As the positive electrode current collector 30, a foil of a metal that is stable within the potential range of the positive electrode 11 such as aluminum, a film having the metal disposed on the surface layer, or the like can be used. The positive electrode mixture layer 32 may contain, for example, a positive electrode active material, a binder, a conductive agent, and the like. The positive electrode 11 can be manufactured, for example, by coating a positive electrode mixture slurry containing a positive electrode active material, a conductive agent, a binder, and the like on the positive electrode current collector 30, drying the coated slurry, and then compressing it to form the positive electrode mixture layer 32.

[0025] Examples of the positive electrode active material include lithium transition metal composite oxides containing transition metal elements such as Co, Mn, and Ni. Examples of the lithium transition metal composite oxide include, for example: Li x CoO2, Li x NiO2, Li x MnO2, Li x Co y Ni 1-y O2, Li x Co y M 1-y O z 、Li x Ni 1-y M y O z 、Li x Mn2O4, Li x Mn 2-y M y O4, LiMPO4, Li2MPO4F (M is at least one of Na, Mg, Sc, Y, Mn, Fe, Co, Ni, Cu, Zn, Al, Cr, Pb, Sb, and B, 0 < x ≤ 1.2, 0 < y ≤ 0.9, 2.0 ≤ z ≤ 2.3). They can be used alone or in combination of multiple kinds. From the viewpoint of enabling high capacity of the non-aqueous electrolyte secondary battery, the positive electrode active material preferably contains Li x NiO2, Li x Co y Ni 1-y O2, Li x Ni 1-y M y O z (M is at least one of Na, Mg, Sc, Y, Mn, Fe, Co, Ni, Cu, Zn, Al, Cr, Pb, Sb, and B, 0 < x ≤ 1.2, 0 < y ≤ 0.9, 2.0 ≤ z ≤ 2.3) and other lithium nickel composite oxides. It should be noted that inorganic compound particles such as alumina and compounds containing lanthanide elements can be adhered to the surface of the particles of the lithium transition metal composite oxide.

[0026] Examples of the conductive agent contained in the positive electrode mixture layer 32 include carbon materials such as carbon black (CB), acetylene black (AB), Ketjen black, and graphite. They can be used alone or in combination of two or more.

[0027] Examples of the binder contained in the positive electrode mixture layer 32 include fluorine-based resins such as polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVdF), polyacrylonitrile (PAN), polyimide (PI), acrylic resins, and polyolefin resins. They can be used alone or in combination of two or more. In addition, these resins can be used in combination with hydroxymethyl cellulose (CMC) or its salts, polyethylene oxide (PEO), etc.

[0028] [Negative electrode]

[0029] The negative electrode 12 has: a strip-shaped negative electrode current collector, and a negative electrode mixture layer formed on the surface of the negative electrode current collector. As Figure 1 shown, the negative electrode 12 can have negative electrode mixture layers 36 on both surfaces of the negative electrode current collector 34. As the negative electrode current collector 34, a foil of a metal stable within the potential range of the negative electrode 12 such as copper, a film having the metal disposed on the surface layer, etc. can be used. The negative electrode mixture layer 36 can contain, for example, a negative electrode active material, a binder, etc.

[0030] As the binder contained in the negative electrode mixture layer 36, similar to the case of the positive electrode 11, fluorine-based resins such as PTFE and PVdF, PAN, PI, acrylic resins, polyolefin resins, etc. can be used, and styrene-butadiene rubber (SBR) is preferably used. In addition, CMC or its salts, polyacrylic acid (PAA) or its salts, polyvinyl alcohol (PVA), etc. can be contained in the negative electrode mixture layer.

[0031] The negative electrode mixture layer 36 contains a first Si-based active material 40 and a second Si-based active material 50 exemplified as Figure 2 the negative electrode active material (hereinafter, the first Si-based active material and the second Si-based active material may be collectively referred to as Si-based active materials). The first Si-based active material 40 contains at least an oxide phase 42 and Si particles 44, and has a structure in which the Si particles 44 are dispersed in the oxide phase 42. The first Si-based active material 40 can further contain a conductive coating film 48 that covers the surface of the mother particle 46 composed of the oxide phase 42 and the Si particles 44. Similarly, the second Si-based active material 50 contains at least an oxide phase 52 and Si particles 54, and has a structure in which the Si particles 54 are dispersed in the oxide phase 52. The second Si-based active material 50 can further contain a conductive coating film 58 that covers the surface of the mother particle 56 composed of the oxide phase 52 and the Si particles 54.

[0032] Si particles 44 and 54 can be dispersed in oxide phases 42 and 52, respectively, in a substantially uniform manner. That is, the mother particles 46 and 56 have a sea-island structure in which fine Si particles 44 and 54 are dispersed in oxide phases 42 and 52, respectively. In any cross-section, Si particles 44 and 54 can be substantially uniformly distributed without unevenly existing in a part of the region. The average particle diameter of Si particles 44 and 54 is preferably 200 nm or less, more preferably 100 nm or less. The average particle diameter of Si particles 44 and 54 is measured by observing the cross-section of the negative electrode mixture layer using a scanning electron microscope (SEM) or a transmission electron microscope (TEM). Specifically, the diameter of the circumscribed circle of any 100 particles selected from the SEM or TEM image is measured, and the measured values are averaged to obtain the average particle diameter.

[0033] Oxide phases 42 and 52 may have a metal oxide containing at least Si as a main component (the component with the largest mass). In addition, oxide phases 42 and 52 may be composed of a collection of particles finer than Si particles 44 and 54. Oxide phases 42 and 52 may contain, for example, at least one of Li, Si, Al, and B.

[0034] Before the first charge, oxide phases 42 and 52 may have at least one of lithium silicate and silicon oxide as a main component. The above lithium silicate is preferably represented by, for example, Li 2z SiO (2+z) (0 < z < 2). From the viewpoints of stability, ease of production, lithium ion conductivity, etc., lithium silicate preferably has Li2SiO3 (z = 1) or Li2Si2O5 (z = 1 / 2) as a main component. When Li2SiO3 or Li2Si2O5 is used as a main component, the content of the main component is preferably more than 50% by mass, more preferably 80% by mass or more, and may substantially be 100% by mass with respect to the total mass of oxide phases 42 and 52. In addition, the above silicon oxide may be, for example, silicon dioxide (SiO2). When oxide phases 42 and 52 have silicon dioxide as a main component, Si-based active materials 40 and 50 have, for example, a structure in which Si particles 44 and 54 are dispersed in an amorphous SiO2 matrix, and are represented by SiO x (0.5 ≤ x ≤ 1.5). It should be noted that SiO2 is mainly changed to Li4SiO4 by the first charge.

[0035] Both of the oxide phases 42 and 52 before the first charge can be lithium silicate phases, or both can be silicon oxide phases. Preferably, the oxide phase 42 has lithium silicate as the main component, and the oxide phase 52 has silicon oxide as the main component. That is, preferably, the first Si-based active material 40 is particles in which Si particles 44 are dispersed in a lithium silicate phase, and the second Si-based active material 50 is particles in which Si particles 54 are dispersed in a silicon oxide phase. In this case, the effect of improving the charge-discharge cycle characteristics of the battery is more significant.

[0036] The content rate of the Si particles 44 in the first Si-based active material 40 is higher than the content rate of the Si particles 54 in the second Si-based active material 50. Here, the content rate of the Si particles 44 in the first Si-based active material 40 refers to the ratio of the mass of the Si particles 44 to the mass of the mother particles 46, and is preferably 40% by mass to 70% by mass, more preferably 40% by mass to 60% by mass. In addition, the content rate of the Si particles 54 in the second Si-based active material 50 refers to the ratio of the mass of the Si particles 54 to the mass of the mother particles 56, and is preferably 20% by mass to 40% by mass, more preferably 25% by mass to 35% by mass. For example, the content rate of the Si particles 44 in the first Si-based active material 40 can be set to 40% by mass or more, and the content rate of the Si particles 54 in the second Si-based active material 50 can be set to less than 40% by mass.

[0037] Each of the Si-based active materials 40 and 50 can be composed only of the mother particles 46 and 56, or can have conductive coatings 48 and 58 made of a material having a higher conductivity than the oxide phases 42 and 52 on the surfaces of the mother particles 46 and 56. The conductive material constituting the conductive coatings 48 and 58 can be, for example, at least one selected from carbon materials, metals, and metal compounds, and a carbon material is preferably used. As a method for carbon coating the surfaces of the mother particles 46 and 56, a CVD method using acetylene, methane, etc. can be exemplified; a method of mixing coal tar pitch, petroleum pitch, phenolic resin, etc. with the mother particles 46 and 56 and performing heat treatment, etc. In addition, a carbon coating layer can also be formed by adhering a conductive agent such as carbon black or Ketjen black to the surfaces of the mother particles 46 and 56 using a binder.

[0038] The conductive films 48 and 58 can be formed to cover substantially the entire surface of the mother particles 46 and 56. Considering the ensuring of conductivity and the diffusibility of lithium ions to the mother particles 46 and 56, the thickness of the conductive films 48 and 58 is preferably 1 nm to 200 nm, more preferably 5 nm to 100 nm. When the thickness of the conductive films 48 and 58 is too thin, the conductivity decreases, and it is difficult to uniformly coat the mother particles 46 and 56. On the other hand, when the thickness of the conductive films 48 and 58 is too thick, there is a tendency that the diffusion of lithium ions to the mother particles 46 and 56 is hindered and the capacity decreases. The thickness of the conductive films 48 and 58 can be measured by observing the cross-section of the particles using SEM or TEM, etc.

[0039] The volume-based median diameter (D50) of the first Si-based active material 40 and the D50 of the second Si-based active material 50 can be, for example, 2 μm to 20 μm. Here, the volume-based median diameter (D50) is the diameter at which the volume accumulation value becomes 50% in the particle size distribution measured by the laser diffraction scattering method, and is also called the 50% diameter or the median diameter. In addition, the D50 of the first Si-based active material 40 can be larger than the D50 of the second Si-based active material 50. In this case, the D50 of the first Si-based active material 40 is preferably 7 μm to 20 μm, more preferably 8 μm to 15 μm. In addition, the D50 of the second Si-based active material 50 is preferably 2 μm to 7 μm, more preferably 3 μm to 6 μm. It should be noted that the D50 of the Si-based active materials 40 and 50 can be smaller than the D50 of the carbon-based active material described later.

[0040] The Si-based active materials 40 and 50 can be manufactured, for example, through the following steps 1 to 3. (1) Mix Si particles with Si-containing inorganic compounds such as lithium silicate and silicon oxide in a specified mass ratio. It should be noted that this inorganic compound becomes the oxide phases 42 and 52. When manufacturing the first Si-based active material 40, the mixing ratio of Si particles is increased compared to manufacturing the second Si-based active material 50.

[0041] (2) After pulverizing and mixing the above raw material powders in an inert atmosphere using a ball mill or the like, heat treatment (sintering) is performed, for example, at 500 °C to 700 °C. The sintered body is pulverized and classified so that the D50 falls within a specified range, thereby obtaining mother particles 46 and 56 in which Si particles 44 and 54 are dispersed in the oxide phases 42 and 52.

[0042] (3) Next, the mother particles 46 and 56 are mixed with a carbon material such as coal tar pitch, and heat treatment is performed in an inert atmosphere. In this way, Si-based active materials 40 and 50 having a conductive film such as a carbon film formed on the surface of the mother particles 46 and 56 are obtained.

[0043] The Si-based active materials 40 and 50 can occlude more lithium ions compared to the carbon-based active materials. Therefore, by using the Si-based active materials 40 and 50 in the negative electrode active material, high capacity of the battery can be achieved. The Si-based active materials 40 and 50 have a larger volume change caused by charge and discharge compared to the carbon-based active materials. Therefore, it is preferable to use the carbon-based active material in combination with the Si-based active materials 40 and 50.

[0044] As the carbon-based active material, graphite that has been used as a negative electrode active material in the past, such as natural graphite like flake graphite, massive graphite, amorphous graphite, and artificial graphite such as massive artificial graphite (MAG) and graphitized mesocarbon microbeads (MCMB) can be used. The D50 of the graphite can be, for example, 18 μm to 24 μm.

[0045] The content of the Si-based active materials 40 and 50 is preferably 2% by mass to 20% by mass, more preferably 3% by mass to 15% by mass, and particularly preferably 4% by mass to 10% by mass with respect to the total mass of the negative electrode active material. That is, the mixing ratio of the carbon-based active material and the Si-based active materials 40 and 50 is preferably 98:2 to 80:20 by mass ratio, more preferably 97:3 to 85:15, and particularly preferably 96:4 to 90:10. When the mass ratio of the carbon-based active material and the Si-based active materials 40 and 50 is within this range, it is easy to achieve high capacity and significantly improve the charge and discharge cycle characteristics at the same time.

[0046] Next, refer to Figure 3 The distribution of the negative electrode active material in the negative electrode mixture layer 36 formed on the surface of the negative electrode current collector 34 will be described. Figure 3 is a front view of the negative electrode 12 as an example of an embodiment. The end portions 34a are regions at both ends in the width direction of the strip-shaped negative electrode current collector 34, and the central portion 34b is the region at the center in the width direction of the negative electrode current collector 34. The ratio of the lengths in the width direction (hereinafter referred to as the width) of one end portion 34a, the central portion 34b, and the other end portion 34a can be set to, for example, 1:8:1 to 3:4:3.

[0047] In the above-mentioned negative electrode mixture layer 36, the ratio of the mass of the first Si-based active material 40 to the total mass of the first Si-based active material 40 and the second Si-based active material 50 is larger in the central portion 34b than in the end portion 34a. Thus, by including a large amount of the first Si-based active material 40 in the central portion 34b where the negative electrode potential is lower than that in the end portion 34a due to repeated charge and discharge, the charge and discharge depth of the first Si-based active material 40 can be reduced, side reactions can be suppressed, and the charge and discharge cycle characteristics can be improved. The first Si-based active material 40 with a high content of Si particles 44 is beneficial for achieving a high capacity, but has a large expansion and contraction during charge and discharge and is prone to side reactions on the surface. During charge and discharge, the negative electrode 12 expands and contracts, and the intrusion and discharge of the electrolyte are repeated, generating a concentration gradient of lithium ions in the electrolyte along the width direction of the negative electrode current collector 34 inside the electrode body 14. In the central portion 34b, the concentration of lithium ions in the electrolyte is higher than that in the end portion 34a. Due to this concentration gradient of lithium ions, the negative electrode potential during charge and discharge is higher in the end portion 34a and lower in the central portion 34b. Therefore, by arranging a large amount of the first Si-based active material 40 in the central portion 34b where side reactions are less likely to occur, it is possible to achieve both high capacity and good charge and discharge cycle characteristics.

[0048] The above-mentioned negative electrode 12 can be manufactured, for example, through the following steps 1 to 3.

[0049] (1) By mixing the first Si-based active material 40 and the second Si-based active material 50, Si-based active material A with a high ratio of the mass of the first Si-based active material 40 to the total mass of the first Si-based active material 40 and the second Si-based active material 50, and Si-based active material B with a low ratio of the mass of the first Si-based active material 40 to the total mass of the first Si-based active material 40 and the second Si-based active material 50 are respectively prepared. It should be noted that Si-based active material A can substantially contain only the first Si-based active material, and Si-based active material B can substantially contain only the second Si-based active material.

[0050] (2) Si-based active material A is mixed with a carbon-based active material in an appropriate ratio to prepare negative electrode active material A, which is then mixed with a binder, a thickener, etc. in an appropriate ratio to prepare a negative electrode mixture slurry A using water as a dispersion medium. Similarly, Si-based active material B is mixed with a carbon-based active material in an appropriate ratio to prepare negative electrode active material B, which is then mixed with a binder, a thickener, etc. in an appropriate ratio to prepare a negative electrode mixture slurry B using water as a dispersion medium.

[0051] (3) The negative electrode 12 can be manufactured by applying the negative electrode mixture paste A to the surface of the negative electrode current collector 34 at the central portion 34b, drying it, then applying the negative electrode mixture paste B to the end portion 34a and drying it, and then performing compression to form the negative electrode mixture layer 36. It should be noted that it is also possible not to perform drying after applying the negative electrode mixture paste A, but to perform drying and compression after applying the negative electrode mixture paste B to form the negative electrode mixture layer 36. Additionally, it is also possible to apply the negative electrode mixture paste A to the central portion 34b after applying the negative electrode mixture paste B to the end portion 34a.

[0052] [Spacer]

[0053] As the spacer 13, a porous sheet having ion permeability and insulation can be used. Specific examples of the porous sheet include microporous films, woven fabrics, non-woven fabrics, etc. As the material of the spacer 13, polyolefins such as polyethylene and polypropylene, cellulose, etc. are preferred. The spacer 13 can be any of a single-layer structure and a laminated structure. A heat-resistant layer or the like can be formed on the surface of the spacer 13.

[0054] [Non-aqueous electrolyte]

[0055] The non-aqueous electrolyte contains a non-aqueous solvent and an electrolyte salt dissolved in the non-aqueous solvent. As the non-aqueous solvent, for example, esters, ethers, nitriles such as acetonitrile, amides such as dimethylformamide, and a mixed solvent of two or more of them can be used. The non-aqueous solvent can contain halogenated substituents in which at least a part of the hydrogen of these solvents is substituted by halogen atoms such as fluorine. Examples of the halogenated substituents include fluorinated cyclic carbonates such as fluoroethylene carbonate (FEC), fluorinated linear carbonates, and fluorinated linear carboxylates such as methyl fluoropropionate (FMP).

[0056] Examples of the above esters include cyclic carbonates such as ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate, linear carbonates such as dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), methyl propyl carbonate, ethyl propyl carbonate, methyl isopropyl carbonate, cyclic carboxylates such as γ-butyrolactone (GBL), γ-valerolactone (GVL), and linear carboxylates such as methyl acetate, ethyl acetate, propyl acetate, methyl propionate (MP), ethyl propionate.

[0057] Examples of the above-mentioned ethers include 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 ethers, etc.; and chain ethers such as 1,2-dimethoxyethane, diethyl ether, dipropyl ether, diisopropyl ether, dibutyl ether, dihexyl ether, ethyl vinyl ether, butyl vinyl ether, methyl phenyl ether, 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, tetraethylene glycol dimethyl ether, etc.

[0058] The electrolyte salt is preferably a lithium salt. Examples of the lithium salt include: LiBF4, LiClO4, LiPF6, LiAsF6, LiSbF6, LiAlCl4, LiSCN, LiCF3SO3, LiCF3CO2, Li(P(C2O4)F4), LiPF 6-x (C n F 2n+1 ) x (1 < x < 6, n is 1 or 2), LiB 10 Cl 10 、LiCl、LiBr、LiI、lithium chloroborate, lithium lower aliphatic carboxylate, borate salts such as Li2B4O7, Li(B(C2O4)F2), etc.; imide salts such as LiN(SO2CF3)2, LiN(C1F 2l+1 SO2)(C m F 2m+1 SO2){l, m are integers of 0 or more}, etc. The lithium salts can be used alone or in combination of multiple kinds. Among these, from the viewpoints of ionic conductivity, electrochemical stability, etc., LiPF6 is preferably used. The concentration of the lithium salt can be set, for example, to be 0.8 mol to 1.8 mol per 1 L of non-aqueous solvent.

[0059] Examples

[0060] Hereinafter, the present invention will be further described by way of examples, but the present invention is not limited to these examples.

[0061] <Example>

[0062] [Fabrication of positive electrode]

[0063] As the positive electrode active material, LiNi 0.91 Co 0.045 Al0.045 Lithium nickelate containing cobalt and aluminum represented by O2. The positive electrode active material, acetylene black (AB), and polyvinylidene fluoride (PVdF) are mixed at a mass ratio of 100:0.75:0.6 to prepare a positive electrode binder slurry using N-methyl-2-pyrrolidone (NMP) as a dispersion medium. The positive electrode binder slurry is coated on both sides of a strip-shaped positive electrode current collector formed of aluminum foil by a doctor blade method. After drying the coating film, the coating film is compressed with a roller to form a positive electrode binder layer on both sides of the positive electrode current collector. The positive electrode current collector having the positive electrode binder layer formed thereon is cut into a predetermined electrode size to fabricate a positive electrode.

[0064] [Fabrication of the First Si-based Active Material]

[0065] In an inert atmosphere, Si particles (3N, 10 μm pulverized product) and lithium silicate particles (10 μm pulverized product) represented by Li 2z SiO (2+z) (0 < z < 2) are mixed such that the content rate of Si particles becomes 52 mass%. The mixture is pulverized by a ball mill. Then, the mixed powder is taken out in an inert atmosphere and heat-treated in an inert atmosphere at 600 °C for 4 hours. After pulverizing the heat-treated powder (hereinafter referred to as mother particles) with a jet mill, it is mixed with coal tar pitch and heat-treated in an inert atmosphere at 800 °C for 5 hours to form a carbon conductive coating film on the surface of the mother particles. The coating amount of carbon is 2 mass% with respect to the total mass of the particles including the mother particles and the conductive coating film. The particles having the conductive coating film formed thereon are crushed and classified with a sieve to obtain a first Si-based active material having a D50 of 11 μm in which Si particles are dispersed at a content rate of 52 mass% in the lithium silicate phase.

[0066] [Fabrication of the Second Si-based Active Material]

[0067] In an inert atmosphere, Si particles (3N, 10 μm pulverized product) and silica particles (10 μm pulverized product) are mixed such that the content rate of Si particles becomes 30 mass%. The mixture is pulverized by a ball mill. Then, the mixed powder is taken out in an inert atmosphere and heat-treated in an inert atmosphere at 600 °C for 4 hours. After pulverizing the heat-treated powder (hereinafter referred to as mother particles) with a jet mill, a carbon conductive coating film is formed on the surface of the mother particles by CVD method (1000 °C). The coating amount of carbon is 5 mass% with respect to the total mass of the particles including the mother particles and the conductive coating film. The particles having the conductive coating film formed thereon are crushed and classified using a sieve to obtain a second Si-based active material having a D50 of 5 μm in which Si particles are dispersed at a content rate of 30 mass% in the silica phase.

[0068] [Analysis of Si-based Active Material]

[0069] The cross-sections of the Si-based active material particles were observed by SEM, and as a result, it was confirmed that Si particles were roughly uniformly dispersed in the oxide phase. In addition, the average particle diameter of the Si particles was less than 50 nm. The carbon coating amount was analyzed by a CS analyzer. The D50 of the Si-based active material was measured using a laser diffraction particle size distribution measuring device (manufactured by Shimadzu Corporation, SALD-2000A). Water was used as the dispersion medium, and the refractive index of the particles was set to 1.70 - 0.01i for measurement.

[0070] [Fabrication of the negative electrode]

[0071] Graphite with a D50 of 22 μm was mixed with the above-mentioned first Si-based active material at a mass ratio of 91.7:8.3 and used as the negative electrode active material A. The negative electrode active material A, carboxymethyl cellulose (CMC), polyacrylic acid (PAA), and styrene-butadiene rubber (SBR) were mixed at a mass ratio of 100:1:1:1 to prepare a negative electrode binder paste A using water as the dispersion medium.

[0072] Graphite with a D50 of 22 μm was mixed with the above-mentioned second Si-based active material at a mass ratio of 96.2:3.8 and used as the negative electrode active material B. The negative electrode binder paste B was prepared in the same manner as the above-mentioned negative electrode binder paste A, except that the negative electrode active material B was used instead of the negative electrode active material A.

[0073] A negative electrode current collector made of copper foil was prepared, and in the width direction, it was divided into an end part, a central part, and an end part at a length ratio of 2:6:2. The negative electrode binder paste A was coated on the central parts of both sides of the negative electrode current collector by the doctor blade method. After the coating film was dried, the negative electrode binder paste B was coated on both end parts by the doctor blade method, and the coating film was dried and compressed with a roller to form a negative electrode binder layer on both sides of the negative electrode current collector. The negative electrode current collector with the negative electrode binder layer formed thereon was cut into a specified electrode size to fabricate the negative electrode.

[0074] [Preparation of the non-aqueous electrolyte]

[0075] LiPF6 was dissolved in a mixed solvent composed of ethylene carbonate (EC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC) at a volume ratio of 20:5:75 (25 °C, 1 atm) at a concentration of 1.3 mol / L. Further, vinylene carbonate (VC) was dissolved in the above-mentioned mixed solvent at a concentration of 4% by mass to prepare a non-aqueous electrolyte (electrolyte solution).

[0076] [Fabrication of the secondary battery]

[0077] An aluminum lead is installed at the positive electrode and a nickel lead is installed at the negative electrode. The positive electrode and the negative electrode are wound in a spiral shape with a spacer formed of a microporous film made of polyethylene therebetween to produce a wound electrode body. The electrode body is housed in an outer package, and the nickel lead is welded to the bottom of the outer package. Next, the aluminum lead is welded to a sealing body, the above-mentioned electrolyte is injected, and then the opening of the outer package is sealed with the sealing body to obtain a non-aqueous electrolyte secondary battery.

[0078] <Comparative Example 1>

[0079] In the production of the negative electrode, graphite, a first Si-based active material, and a second Si-based active material are mixed at a mass ratio of 93.5:5:1.5 and used as the negative electrode active material C. The negative electrode active material C is used to replace the negative electrode active material A to prepare a negative electrode mixture paste C. The negative electrode mixture paste C is coated on the entire surface of the central portion and both end portions of the negative electrode current collector to form a negative electrode mixture layer. Except for this, the negative electrode is produced in the same manner as in the example, and a non-aqueous electrolyte secondary battery is produced.

[0080] <Comparative Example 2>

[0081] In the production of the negative electrode, graphite and a second Si-based active material are mixed at a mass ratio of 97.5:2.5 and used as the negative electrode active material D. Graphite and a first Si-based active material are mixed at a mass ratio of 87.5:12.5 and used as the negative electrode active material E. The negative electrode active material D is used to replace the negative electrode active material A to prepare a negative electrode mixture paste D. The negative electrode active material E is used to replace the negative electrode active material B to prepare a negative electrode mixture paste E. Except for this, the negative electrode is produced in the same manner as in the example, and a non-aqueous electrolyte secondary battery is produced.

[0082] <Comparative Example 3>

[0083] In the production of the negative electrode, graphite and a second Si-based active material are mixed at a mass ratio of 94:6 and used as the negative electrode active material F. The negative electrode active material F is used to replace the negative electrode active material A to prepare a negative electrode mixture paste F. The negative electrode mixture paste F is coated on the entire surface of the central portion and both end portions of the negative electrode current collector to form a negative electrode mixture layer. Except for this, the negative electrode is produced in the same manner as in the example, and a non-aqueous electrolyte secondary battery is produced.

[0084] The initial discharge capacity and charge-discharge cycle characteristics of the above-mentioned respective batteries are evaluated by the following method. For the charge-discharge cycle characteristics, the number of cycles at which the discharge capacity sharply decreases is used as an index, and the relative value of each battery when the number of cycles at which the discharge capacity of the example sharply decreases is set to 100 is shown. While showing the evaluation results in Table 1, the composition (mass ratio) of the negative electrode active material at the end portion and the central portion of the negative electrode and the average composition of the entire negative electrode mixture layer are also shown.

[0085] [Evaluation of Initial Discharge Capacity]

[0086] After charging each battery of the examples and comparative examples at a constant current of 0.3C in a temperature environment of 25°C until the battery voltage reached 4.2V, charging at a constant voltage of 4.2V until the current value became 1 / 50C, and then discharging at a constant current of 0.5C until the battery voltage reached 2.85V. The discharge capacity at this time was taken as the initial discharge capacity.

[0087] [Evaluation of charge-discharge cycle characteristics]

[0088] After charging each battery of the examples and comparative examples at a constant current of 0.3C in a temperature environment of 25°C until the battery voltage reached 4.2V, charging at a constant voltage of 4.2V until the current value became 1 / 50C, and then discharging at a constant current of 0.5C until the battery voltage reached 2.85V. This was taken as the charge-discharge of one cycle, and the ratio of the discharge capacity to the charge capacity in each cycle, that is, the charge-discharge efficiency, was calculated. The number of cycles when the charge-discharge efficiency became less than 99.8% after 5 consecutive cycles was taken as the number of cycles with a sharp drop in discharge capacity. The charge-discharge efficiency of the battery with a large number of cycles with a sharp drop in discharge capacity was maintained at a relatively high level, and the charge-discharge cycle characteristics were good.

[0089] [Table 1]

[0090]

[0091] As shown in Table 1, compared with the batteries of Comparative Examples 1 and 2, the batteries of the examples had excellent charge-discharge cycle characteristics, and compared with Comparative Example 3, they showed high capacity. In this way, one of the battery capacity and charge-discharge cycle characteristics of the batteries of Comparative Examples 1 to 3 was insufficient. In contrast, only the batteries of the examples achieved both battery capacity and charge-discharge cycle characteristics. That is to say, in order to balance battery capacity and charge-discharge cycle characteristics, it is not only necessary to simply mix two Si-based active materials (the first Si-based active material and the second Si-based active material) with different Si particle contents, but also to appropriately arrange them in the negative electrode mixture layer.

[0092] Explanation of reference numerals

[0093] 10 Secondary battery, 11 Positive electrode, 12 Negative electrode, 13 Spacer, 14 Electrode body, 15 Outer package, 16 Sealing body, 17, 18 Insulating plate, 19 Positive electrode lead, 20 Negative electrode lead, 21 Groove portion, 22 Filter, 23 Lower valve body, 24 Insulating member, 25 Upper valve body, 26 Cover, 26a Opening portion, 27 Gasket, 30 Positive electrode current collector, 32 Positive electrode mixture layer, 34 Negative electrode current collector, 34a End portion, 34b Central portion, 36 Negative electrode mixture layer, 40 First Si-based active material, 50 Second Si-based active material, 42, 52 Oxide phase, 44, 54 Si particles, 46, 56 Mother particles, 48, 58 Conductive coating film.

Claims

1. A negative electrode for a non-aqueous electrolyte secondary battery, comprising: a strip-shaped negative electrode current collector, and a negative electrode mixture layer formed on the surface of the negative electrode current collector, wherein, the negative electrode mixture layer contains a first Si-based active material and a second Si-based active material, both the first Si-based active material and the second Si-based active material have a structure in which Si particles are dispersed in an oxide phase, and the content ratio of the Si particles in the first Si-based active material is higher than the content ratio of the Si particles in the second Si-based active material, in the negative electrode mixture layer, the ratio of the mass of the first Si-based active material to the total mass of the first Si-based active material and the second Si-based active material is larger at the central portion than at the end portion in the width direction of the negative electrode current collector.

2. The negative electrode for a non-aqueous electrolyte secondary battery according to claim 1, wherein, before the first charging, the oxide phase contains at least one of lithium silicate and silicon oxide as a main component.

3. The negative electrode for a non-aqueous electrolyte secondary battery according to claim 2, wherein, before the first charging, the oxide phase of the first Si-based active material contains the lithium silicate as a main component, the oxide phase of the second Si-based active material contains the silicon oxide as a main component.

4. A non-aqueous electrolyte secondary battery, comprising: the negative electrode according to any one of claims 1 to 3, a positive electrode, and a non-aqueous electrolyte.

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