Negative electrode for secondary battery and secondary battery

Abstract

This negative electrode for a secondary battery comprises a negative electrode mixture layer that includes: a negative electrode active material containing graphite particles and a Si-containing material; and a binder. The Si-containing material includes a Si-containing material A that includes a carbon phase and silicon particles dispersed in the carbon phase. The graphite particles include graphite particles A with 5% or less particle internal porosity, and the content of the graphite particles A is at least 10% by mass with respect to the total mass of the negative electrode active material. The content of the Si-containing material A is at least 1% by mass with respect to the total mass of the negative electrode active material.

Description

[Technical Field]

[0001] The present disclosure relates to a negative electrode for a secondary battery and a secondary battery. [Background technology]

[0002] Non-aqueous electrolyte secondary batteries that use a carbon material as a negative electrode active material are widely used as high energy density secondary batteries.

[0003] For example, Patent Document 1 discloses a non-aqueous electrolyte secondary battery that uses densified carbon with an internal particle porosity of 5% or less as the negative electrode active material.

[0004] Furthermore, for example, Patent Document 2 discloses a non-aqueous electrolyte secondary battery using graphite and a Si-containing material having different internal particle porosities as negative electrode active materials. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Publication No. 9-320600 [Patent Document 2] International Publication No. 2019 / 239947 Summary of the Invention

[0006] Graphite with low intra-particle porosity has poor adhesion to other materials, which can reduce the peel strength between the negative electrode composite layer containing low intra-particle porosity graphite and the negative electrode current collector. In particular, when combined with a Si-containing material in which silicon particles are dispersed within the silicate phase, the peel strength between the negative electrode composite layer and the negative electrode current collector decreases significantly. This reduced peel strength can lead to poor contact between the negative electrode composite layer and the negative electrode current collector, resulting in a decrease in battery capacity.

[0007] A negative electrode for a secondary battery according to one embodiment of the present disclosure includes a negative electrode composite layer having a binder and a negative electrode active material including graphite particles and a Si-containing material, wherein the Si-containing material includes Si-containing material A having a carbon phase and silicon particles dispersed within the carbon phase, and the graphite particles include graphite particles A having an internal particle porosity of 5% or less, the content of the graphite particles A being 10% by mass or more relative to the total mass of the negative electrode active material, and the content of the Si-containing material A being 1% by mass or more relative to the total mass of the negative electrode active material.

[0008] A secondary battery according to one aspect of the present disclosure includes a positive electrode, a negative electrode, and a non-aqueous electrolyte, and the negative electrode is the above-described negative electrode for a secondary battery.

[0009] According to one aspect of the present disclosure, in a negative electrode for a secondary battery having a negative electrode active material including graphite with low intra-particle porosity and a Si-containing material, the peel strength between the negative electrode mixture layer and the negative electrode current collector can be improved. [Brief explanation of the drawings]

[0010] [Figure 1] FIG. 1 is a cross-sectional view of a secondary battery according to an embodiment. [Figure 2] FIG. 2 is an enlarged view showing a cross section of a graphite particle in a negative electrode mixture layer. DETAILED DESCRIPTION OF THE INVENTION

[0011] A negative electrode for a secondary battery according to one embodiment of the present disclosure includes a negative electrode composite layer having a negative electrode active material containing graphite particles and a Si-containing material, and a binder. The Si-containing material includes a Si-containing material A having a carbon phase and silicon particles dispersed within the carbon phase. The graphite particles include graphite particles A having an internal particle porosity of 5% or less, the content of the graphite particles A being 10% by mass or more relative to the total mass of the negative electrode active material, and the content of the Si-containing material A being 1% by mass or more relative to the total mass of the negative electrode active material. This negative electrode for a secondary battery according to one embodiment of the present disclosure can improve the peel strength between the negative electrode composite layer and the negative electrode current collector. While the mechanism behind this effect is unclear, it is believed that graphite particles A having an internal particle porosity of 5% or less have a specifically high angle of repose and coefficient of friction with Si-containing materials A having silicon particles dispersed within a carbon phase. As a result, the cohesion and contact area of ​​the graphite particles A and the Si-containing material A increases, which is thought to improve the peel strength between the negative electrode mixture layer and the negative electrode current collector.

[0012] Hereinafter, an example of an embodiment will be described in detail with reference to the drawings. Note that the nonaqueous electrolyte secondary battery of the present disclosure is not limited to the embodiment described below. Furthermore, the drawings referred to in the description of the embodiment are schematic.

[0013] FIG. 1 is a cross-sectional view of a secondary battery according to an embodiment. The secondary battery 10 shown in FIG. 1 includes a wound electrode assembly 14 formed by winding a positive electrode 11 and a negative electrode 12 with a separator 13 interposed therebetween, a nonaqueous electrolyte, insulating plates 18 and 19 disposed above and below the electrode assembly 14, respectively, and a battery case 15 for accommodating the above components. The battery case 15 is composed of a cylindrical case body 16 with a bottom and a sealing member 17 that closes the opening of the case body 16. Note that, instead of the wound electrode assembly 14, other types of electrode bodies may be used, such as a laminated electrode body formed by alternately stacking positive and negative electrodes with separators interposed therebetween. Examples of the battery case 15 include cylindrical, prismatic, coin-shaped, button-shaped, or other metal exterior cans, and pouch exteriors formed by laminating a resin sheet and a metal sheet.

[0014] Case body 16 is, for example, a cylindrical metal outer can with a bottom. A gasket 28 is provided between case body 16 and sealing body 17 to ensure airtightness inside the battery. Case body 16 has, for example, a protruding portion 22, which is a portion of the side surface that protrudes inward and supports sealing body 17. Protruding portion 22 is preferably formed in an annular shape along the circumferential direction of case body 16, and supports sealing body 17 on its upper surface.

[0015] The sealing body 17 has a structure in which, in order from the electrode body 14 side, a filter 23, a lower valve body 24, an insulating member 25, an upper valve body 26, and a cap 27 are stacked. Each member constituting the sealing body 17 has, for example, a disk or ring shape, and each member except for the insulating member 25 is electrically connected to one another. The lower valve body 24 and the upper valve body 26 are connected to one another at their respective centers, and the insulating member 25 is interposed between their respective peripheral edges. When the internal pressure of the secondary battery 10 increases due to heat generation caused by an internal short circuit or the like, for example, the lower valve body 24 deforms and ruptures, pushing the upper valve body 26 toward the cap 27, thereby interrupting the current path between the lower valve body 24 and the upper valve body 26. When the internal pressure further increases, the upper valve body 26 ruptures, and gas is discharged from the opening of the cap 27.

[0016] In the secondary battery 10 shown in Fig. 1, a positive electrode lead 20 attached to the positive electrode 11 passes through a through-hole in an insulating plate 18 and extends toward the sealing body 17, and a negative electrode lead 21 attached to the negative electrode 12 passes outside an insulating plate 19 and extends toward the bottom side of the case body 16. The positive electrode lead 20 is connected to the underside of a filter 23, which is the bottom plate of the sealing body 17, by welding or the like, and a cap 27, which is the top plate of the sealing body 17 and is electrically connected to the filter 23, serves as the positive electrode terminal. The negative electrode lead 21 is connected to the inner bottom surface of the case body 16 by welding or the like, and the case body 16 serves as the negative electrode terminal.

[0017] Each component of the secondary battery 10 will be described in detail below.

[0018] [Negative electrode] The negative electrode 12 has a negative electrode current collector made of, for example, a metal foil and a negative electrode composite layer formed on the current collector. The negative electrode current collector may be, for example, a foil of a metal such as copper that is stable within the potential range of the negative electrode, or a film with such a metal disposed on the surface layer. The negative electrode composite layer contains a negative electrode active material including graphite particles and a Si-containing material, and a binder. The negative electrode composite layer preferably contains a conductive material, etc.

[0019] The negative electrode 12 can be produced, for example, by preparing a negative electrode composite slurry containing a negative electrode active material, a binder, a conductive material, and the like, applying this negative electrode composite slurry onto a negative electrode current collector, drying it to form a negative electrode composite layer, and then performing a compression step in which the negative electrode composite layer is compressed using a rolling roller or the like.

[0020] Fig. 2 is an enlarged view showing a cross section of a graphite particle in the negative electrode mixture layer. In this embodiment, as shown in Fig. 2, graphite particle 30 included in the negative electrode mixture layer has, in a cross section of graphite particle 30, closed voids 34 that do not connect from the inside of the particle to the particle surface (hereinafter referred to as internal voids 34), and voids 36 that connect from the inside of the particle to the particle surface (hereinafter referred to as external voids 36). Here, the internal void ratio of a graphite particle is a two-dimensional value calculated from the ratio of the area of ​​the internal voids 34 of the graphite particle to the cross-sectional area of ​​the graphite particle. The internal void ratio of a graphite particle can be calculated by the following procedure.

[0021] <Method for measuring internal particle porosity> (1) Exposing a cross section of the negative electrode active material. For example, a method for exposing the cross section includes cutting out a part of the negative electrode and processing it with an ion milling device (e.g., IM4000PLUS manufactured by Hitachi High-Technologies Corporation) to expose the cross section of the negative electrode composite layer. (2) A backscattered electron image of the cross section of the exposed negative electrode composite layer is taken using a scanning electron microscope at a magnification of 3,000 to 5,000 times. (3) The cross-sectional image obtained as described above is imported into a computer and binarized using image analysis software (e.g., ImageJ, manufactured by the National Institutes of Health, USA) to obtain a binarized image in which the particle cross-sections in the cross-sectional image are colored black and the voids present in the particle cross-sections are colored white. (4) From the binarized image, graphite particles with particle sizes of 5 μm to 50 μm are selected, and the cross-sectional area of ​​the graphite particles and the area of ​​the internal voids present in the cross-sectional area of ​​the graphite particles are calculated. Here, the cross-sectional area of ​​the graphite particles refers to the area of ​​the region surrounded by the outer periphery of the graphite particles, i.e., the area of ​​the entire cross-sectional portion of the graphite particles. Furthermore, for voids present in the cross-sectional area of ​​the graphite particles with a width of 3 μm or less, it may be difficult to distinguish between internal and external voids in image analysis, so voids with a width of 3 μm or less may be considered internal voids. Then, the internal void ratio of the graphite particles (area of ​​the internal voids in the cross-sectional area of ​​the graphite particles × 100 / area of ​​the cross-sectional area of ​​the graphite particles) is calculated from the calculated cross-sectional area of ​​the graphite particles and the area of ​​the internal voids in the cross-sectional area of ​​the graphite particles. The internal void ratio of the graphite particles is taken as the average value for 10 graphite particles.

[0022] In this embodiment, the graphite particles 30 contained in the negative electrode mixture layer include graphite particles A having an internal porosity of 5% or less. The internal porosity of the graphite particles A is preferably 1% or more and 5% or less, and more preferably 3% or more and 5% or less, in order to improve battery characteristics, for example.

[0023] The content of the graphite particles A is preferably 10% by mass or more, more preferably 20% by mass or more, and more preferably 30% by mass or more, based on the total mass of the negative electrode active material in order to improve the peel strength between the negative electrode composite layer and the negative electrode current collector. However, if the content of the graphite particles A is too high, the content of the Si-containing material may decrease, resulting in a decrease in battery capacity, and therefore, for example, the content is preferably 97% by mass or less.

[0024] In this embodiment, the graphite particles 30 contained in the negative electrode composite layer preferably contain graphite particles B having an internal particle porosity of 8% or more and 20% or less, in order to further improve the peel strength between the negative electrode composite layer and the negative electrode current collector. The content of the graphite particles B is preferably 10% by mass or more and 80% by mass or less, relative to the total mass of the negative electrode active material. Graphite particles B with a large internal particle porosity are appropriately crushed during the compression step in negative electrode production, contributing to an increase in the contact area with the negative electrode current collector, which is thought to improve the peel strength between the negative electrode composite layer and the negative electrode current collector. The internal particle porosity of the graphite particles B is preferably 10% by mass or more and 18% or less, more preferably 12% by mass or more and 16% or less, in order to improve the peel strength between the negative electrode composite layer and the negative electrode current collector or to improve battery characteristics.

[0025] The graphite particles A and B are produced, for example, as follows.

[0026] <Graphite particles A with internal particle porosity of 5% or less> For example, coke (precursor), which is the main raw material, is crushed to a predetermined size, agglomerated with a binder, and then fired at a temperature of 2600°C or higher to graphitize the particles, followed by sieving to obtain graphite particles A of a desired size. The internal particle porosity can be adjusted to 5% or less by adjusting the particle size of the crushed precursor or the particle size of the agglomerated precursor. For example, the average particle size (median diameter D50) of the crushed precursor is preferably in the range of 12 μm to 20 μm. To reduce the internal particle porosity to a range of 5% or less, it is preferable to increase the particle size of the crushed precursor.

[0027] <Graphite particles B with internal porosity of 8% to 20%> For example, the coke (precursor) that is the main raw material is crushed to a predetermined size, agglomerated with a binder, and then pressed into a block shape. This block is then fired at a temperature of 2600°C or higher to graphitize it. The graphitized block is crushed and sieved to obtain graphite particles B of the desired size. The internal particle porosity can be adjusted to 8% to 20% depending on the amount of volatile component added to the block. If a portion of the binder added to the coke (precursor) volatilizes during firing, the binder can be used as the volatile component. Pitch is an example of such a binder.

[0028] The graphite particles A and B used in this embodiment may be natural graphite, artificial graphite, or the like, but are not particularly limited thereto. However, artificial graphite is preferred in terms of ease of adjusting the porosity inside the particles. The interplanar spacing (d 002 ) is, for example, preferably 0.3354 nm or more, more preferably 0.3357 nm or more, and preferably less than 0.340 nm, more preferably 0.338 nm or less. The crystallite size (Lc(002)) of the graphite particles A and B used in this embodiment, determined by X-ray diffraction, is, for example, preferably 5 nm or more, more preferably 10 nm or more, and preferably 300 nm or less, more preferably 200 nm or less. The interplanar spacing (d 002 When the surface roughness (Tc) and the crystallite size (Lc(002)) satisfy the above ranges, the battery capacity of the secondary battery tends to be larger than when the surface roughness (Tc) and the crystallite size (Lc(002)) do not satisfy the above ranges.

[0029] The waveform showing the intra-particle porosity distribution of graphite particles 30 preferably has a first peak in a range of intra-particle porosity of 5% or less and a second peak in a range of intra-particle porosity greater than 5%, in order to further improve the peel strength between the negative electrode composite layer and the negative electrode current collector, and more preferably has a first peak in a range of intra-particle porosity of 5% or less and a second peak in a range of intra-particle porosity of 8% to 20%. Here, the intra-particle porosity distribution is a distribution in which the horizontal axis represents the intra-particle porosity obtained by the above-mentioned measurement method and the vertical axis represents the abundance ratio of graphite particles relative to that intra-particle porosity. In other words, in the waveform showing the internal particle porosity distribution of graphite particles 30, having a first peak in the range of internal particle porosity of 5% or less and a second peak in the range of internal particle porosity of 8% or more and 20% or less means that there are more graphite particles with internal particle porosity of 5% or less and graphite particles with internal particle porosity of 8% or more and 20% or less than graphite particles in other internal particle porosity ranges.

[0030] In this embodiment, the Si-containing material contained in the negative electrode composite layer includes Si-containing material A having a carbon phase and silicon particles dispersed within the carbon phase. The content of Si-containing material A may be 1% by mass or more relative to the total mass of the negative electrode active material. However, from the viewpoint of further improving the peel strength between the negative electrode composite layer and the negative electrode current collector, it is preferably 5% by mass or more, and more preferably 10% by mass or more. However, if the content of Si-containing material A is too high, the charge-discharge cycle characteristics may be significantly reduced. Therefore, for example, it is preferably 30% by mass or less, and more preferably 20% by mass or less.

[0031] A suitable Si-containing material A preferably contains crystalline carbon in the carbon phase of the Si-containing material A, from the viewpoint of suppressing breakage during electrode plate production by improving the strength of the particles.

[0032] In terms of increasing capacity, the content of silicon particles in the Si-containing material A is preferably 30% by mass or more and 80% by mass or less, more preferably 35% by mass or more and 75% by mass or less, and more preferably 55% by mass or more and 70% by mass or less.

[0033] The average particle diameter of the silicon particles is generally 500 nm or less before charge and discharge, preferably 200 nm or less, and more preferably 100 nm or less. After charge and discharge, it is preferably 400 nm or less, and more preferably 100 nm or less. By miniaturizing the silicon particles, the volume change during charge and discharge becomes smaller and the cycle characteristics are improved. The average particle diameter of the silicon particles is measured by observing the particle cross-section of the Si-containing material A using a scanning electron microscope (SEM) or a transmission electron microscope (TEM), and specifically, it is obtained as the average value of the longest diameters of 100 silicon particles.

[0034] In the present embodiment, the Si-containing material contained in the negative electrode composite material layer may contain, in addition to the Si-containing material A, a Si-containing material B containing a silicate phase and silicon particles dispersed in the silicate phase.

[0035] A suitable Si-containing material B has, for example, a sea-island structure in which fine silicon particles are substantially uniformly dispersed in an amorphous silicate phase, and is represented by the general formula SiO x (0.5 ≦ x ≦ 1.6). The content of the silicon particles is preferably 35 to 75% by mass with respect to the total mass of the Si-containing material B from the viewpoint of improving the battery capacity and the like. Also, the average particle diameter of the silicon particles is generally 500 nm or less before charge and discharge, preferably 200 nm or less, and more preferably 50 nm or less.

[0036] The silicate phase of the Si-containing material B preferably contains at least one element of an alkali metal element and an alkaline earth metal element, particularly preferably contains a lithium element, in terms of improving lithium ion conductivity and the like. Also, the silicate phase of the Si-containing material B preferably contains a lithium silicate represented by the general formula Li 2z SiO (2+z) (0 < z < 2).

[0037] Furthermore, a conductive layer made of a highly conductive material may be formed on the particle surfaces of Si-containing material A and Si-containing material B. One example of a suitable conductive layer is a carbon coating made of a carbon material. The carbon coating may be made of, for example, carbon black, acetylene black, ketjen black, graphite, or a mixture of two or more of these. Examples of methods for carbon-coating the particle surfaces of Si-containing materials include a CVD method using acetylene, methane, or the like, and a method of mixing particles of Si-containing material with coal pitch, petroleum pitch, phenolic resin, or the like and then heat-treating the mixture. Alternatively, a carbon coating may be formed by adhering carbon powder such as carbon black to the particle surfaces using a binder.

[0038] The mass ratio (A / B) of Si-containing material A to Si-containing material B is preferably 0.2 or more and 20 or less, and more preferably 2 or more and 10 or less, from the viewpoint of improving the peel strength between the negative electrode composite layer and the negative electrode current collector.

[0039] The total content of the Si-containing material is preferably 5% by mass or more and 20% by mass or less, and more preferably 10% by mass or more and 15% by mass or less, relative to the total mass of the negative electrode active material, in order to improve the peel strength between the negative electrode composite layer and the negative electrode current collector and to improve the battery capacity.

[0040] Examples of binders include fluorine-based resins, PAN, polyimide-based resins, acrylic-based resins, polyolefin-based resins, styrene-butadiene rubber (SBR), carboxymethyl cellulose (CMC) or salts thereof, polyacrylic acid (PAA) or salts thereof (PAA-Na, PAA-K, etc., or partially neutralized salts), polyvinyl alcohol (PVA), etc. These may be used alone or in combination of two or more.

[0041] Examples of conductive materials include carbon particles such as carbon black (CB), acetylene black (AB), ketjen black, carbon nanotubes (CNT), graphite, etc. These may be used alone or in combination of two or more.

[0042] [Positive electrode] The positive electrode 11 is composed of, for example, a positive electrode current collector such as a metal foil, and a positive electrode mixture layer formed on the positive electrode current collector. As the positive electrode current collector, a foil of a metal stable within the potential range of the positive electrode such as aluminum, a film having the metal disposed on the surface layer, or the like can be used. The positive electrode mixture layer contains, for example, a positive electrode active material, a binder, a conductive material, and the like.

[0043] The positive electrode 11 can be produced, for example, by applying a positive electrode mixture slurry containing a positive electrode active material, a binder, a conductive material, and the like onto the positive electrode current collector, drying to form a positive electrode mixture layer, and then performing a compression step of compressing this positive electrode mixture layer with a rolling roller or the like.

[0044] Examples of the positive electrode active material include lithium transition metal oxides containing transition metal elements such as Co, Mn, and Ni. The lithium transition metal oxide is, 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; at least one of Na, Mg, Sc, Y, Mn, Fe, Co, Ni, Cu, Zn, Al, Cr, Pb, Sb, B, 0 < x ≦ 1.2, 0 < y ≦ 0.9, 2.0 ≦ z ≦ 2.3). These may be used alone or in combination of multiple types. In terms of achieving a higher capacity of the secondary battery, the positive electrode active material is Li x NiO2, Li x Co y Ni 1-y , O2, Li x Ni 1-y M y Oz It is preferable to contain a lithium nickel composite oxide such as (M; at least one of Na, Mg, Sc, Y, Mn, Fe, Co, Ni, Cu, Zn, Al, Cr, Pb, Sb, B, 0 < x ≤ 1.2, 0 < y ≤ 0.9, 2.0 ≤ z ≤ 2.3).

[0045] Examples of the conductive material include carbon-based particles such as carbon black (CB), acetylene black (AB), ketjen black, carbon nanotube (CNT), and graphite. These may be used alone or in combination of two or more kinds.

[0046] Examples of the binder include fluorine-based resins such as polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVdF), polyacrylonitrile (PAN), polyimide-based resins, acrylic resins, and polyolefin-based resins. These may be used alone or in combination of two or more kinds.

[0047] [Separator] For the separator 13, for example, a porous sheet having ion permeability and insulation is used. Specific examples of the porous sheet include microporous thin films, woven fabrics, non-woven fabrics, etc. As the material of the separator, olefin-based resins such as polyethylene and polypropylene, and cellulose are suitable. The separator 13 may be a laminate having a cellulose fiber layer and a thermoplastic resin fiber layer such as an olefin-based resin. Also, a multilayer separator including a polyethylene layer and a polypropylene layer may be used, or a separator with a material such as an aramid-based resin or ceramic coated on the surface of the separator may be used.

[0048] [Non-aqueous electrolyte] The non-aqueous electrolyte includes a non-aqueous solvent and an electrolyte salt dissolved in the non-aqueous solvent. The non-aqueous electrolyte is not limited to a liquid electrolyte (electrolytic solution) and may be a solid electrolyte using a gel polymer or the like. Examples of the non-aqueous solvent that can be used include esters, ethers, nitriles such as acetonitrile, amides such as dimethylformamide, and mixed solvents of two or more of these. The non-aqueous solvent may contain a halogen-substituted product in which at least a portion of the hydrogen atoms of these solvents are substituted with halogen atoms such as fluorine.

[0049] Examples of the esters include cyclic carbonates such as ethylene carbonate (EC), propylene carbonate (PC), and butylene carbonate; chain carbonates such as dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), methyl propyl carbonate, ethyl propyl carbonate, and methyl isopropyl carbonate; cyclic carboxylic acid esters such as γ-butyrolactone and γ-valerolactone; and chain carboxylic acid esters such as methyl acetate, ethyl acetate, propyl acetate, methyl propionate (MP), ethyl propionate, and γ-butyrolactone.

[0050] Examples of the above ethers include cyclic ethers such as 1,3-dioxolane, 4-methyl-1,3-dioxolane, tetrahydrofuran, 2-methyltetrahydrofuran, propylene oxide, 1,2-butylene oxide, 1,3-dioxane, 1,4-dioxane, 1,3,5-trioxane, furan, 2-methylfuran, 1,8-cineole, crown ether, 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, etc.

[0051] As the above halogenated derivatives, it is preferable to use fluorinated cyclic carbonates such as fluoroethylene carbonate (FEC), fluorinated chain carbonates, fluorinated chain carboxylic acid esters such as methyl fluoropropionate (FMP), etc.

[0052] 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 chloroborane, lower aliphatic lithium carboxylates, borates such as Li2B4O7, Li(B(C2O4)F2), LiN(SO2CF3)2, LiN(C1F 2l+1 SO2)(C m F 2m+1 Examples of the lithium salt include imide salts such as SiO2) (where l and m are integers of 1 or more). The lithium salt may be used alone or in combination. Among these, LiPF6 is preferred from the viewpoints of ionic conductivity, electrochemical stability, etc. The concentration of the lithium salt is preferably 0.8 to 1.8 mol per 1 L of solvent.

[0053] <Example> The present disclosure will be further described below with reference to examples, but the present disclosure is not limited to these examples.

[0054] Example 1 [Preparation of negative electrode] Graphite particles A with an internal particle porosity of 5% or less and Si-containing material A with silicon particles dispersed within the carbon phase were mixed at a mass ratio of 94:6. This mixture was used as the negative electrode active material. These materials were then mixed at a mass ratio of 100:1:1 (negative electrode active material: carboxymethyl cellulose-sodium (CMC-Na): styrene-butadiene copolymer rubber (SBR)). A negative electrode composite slurry was prepared by applying this slurry to both sides of a copper foil current collector using a doctor blade method. After drying, the coating was compressed with a rolling roller to produce a negative electrode with a negative electrode composite layer formed on both sides of the negative electrode current collector. The internal particle porosity of graphite particles A in the produced negative electrode was measured and found to be 2.4%.

[0055] <Example 2> In the preparation of the negative electrode, graphite particles A having an internal particle porosity of 5% or less, Si-containing material B having silicon particles dispersed in a silicate phase, and Si-containing material A having silicon particles dispersed in a carbon phase were mixed in a mass ratio of 94:4.4:1.6. An anode was prepared in the same manner as in Example 1, except that this mixture was used as the negative electrode active material.

[0056] Example 3 In preparing the negative electrode, graphite particles A having an internal particle porosity of 5% or less, graphite particles B having an internal particle porosity of 8% or more and 20% or less, Si-containing material B having silicon particles dispersed in a silicate phase, and Si-containing material A having silicon particles dispersed in a carbon phase were mixed in a mass ratio of 18.8:75.2:4.4:1.6. A negative electrode was prepared in the same manner as in Example 1, except that this mixture was used as the negative electrode active material. The internal particle porosity of graphite particles B in the prepared negative electrode was measured and found to be 12.8%.

[0057] Example 4 In preparing the negative electrode, graphite particles A with an internal particle porosity of 5% or less, graphite particles B with an internal particle porosity of 8% or more and 20% or less, Si-containing material B with silicon particles dispersed in a silicate phase, and Si-containing material A with silicon particles dispersed in a carbon phase were mixed in a mass ratio of 37.6:56.4:4.4:1.6. An anode was prepared in the same manner as in Example 1, except that this mixture was used as the negative electrode active material.

[0058] <Example 5> In preparing the negative electrode, graphite particles A with an internal particle porosity of 5% or less, graphite particles B with an internal particle porosity of 8% to 20%, Si-containing material B with silicon particles dispersed in a silicate phase, and Si-containing material A with silicon particles dispersed in a carbon phase were mixed in a mass ratio of 37.6:56.4:5:1. An anode was prepared in the same manner as in Example 1, except that this mixture was used as the negative electrode active material.

[0059] Example 6 In preparing the negative electrode, graphite particles A with an internal particle porosity of 5% or less, graphite particles B with an internal particle porosity of 8% or more and 20% or less, Si-containing material B with silicon particles dispersed in a silicate phase, and Si-containing material A with silicon particles dispersed in a carbon phase were mixed in a mass ratio of 32.02:48.03:0.95:19. An anode was prepared in the same manner as in Example 1, except that this mixture was used as the negative electrode active material.

[0060] <Comparative Example 1> In the preparation of the negative electrode, graphite particles A having an internal particle porosity of 5% or less and Si-containing material B having silicon particles dispersed in a silicate phase were mixed in a mass ratio of 94:6. Then, a negative electrode was prepared in the same manner as in Example 1, except that this mixture was used as the negative electrode active material.

[0061] <Comparative Example 2> In preparing the negative electrode, graphite particles A having an internal particle porosity of 5% or less, graphite particles B having an internal particle porosity of 8% or more and 20% or less, and Si-containing material B having silicon particles dispersed in a silicate phase were mixed in a mass ratio of 37.6:56.4:6. A negative electrode was prepared in the same manner as in Example 1, except that this mixture was used as the negative electrode active material.

[0062] <Comparative Example 3> In the preparation of the negative electrode, graphite particles B having an internal particle porosity of 8% or more and 20% or less and Si-containing material A having silicon particles dispersed in the carbon phase were mixed in a mass ratio of 94:6. Then, a negative electrode was prepared in the same manner as in Example 1, except that this mixture was used as the negative electrode active material.

[0063] <Comparative Example 4> In the preparation of the negative electrode, graphite particles B having an internal particle porosity of 8% or more and 20% or less, Si-containing material B having silicon particles dispersed in a silicate phase, and Si-containing material A having silicon particles dispersed in a carbon phase were mixed in a mass ratio of 94:4.3:1.6. An anode was prepared in the same manner as in Example 1, except that this mixture was used as the negative electrode active material.

[0064] <Comparative Example 5> In the preparation of the negative electrode, graphite particles B having an internal particle porosity of 8% or more and 20% or less and Si-containing material B having silicon particles dispersed in a silicate phase were mixed in a mass ratio of 94:6. Then, a negative electrode was prepared in the same manner as in Example 1, except that this mixture was used as the negative electrode active material.

[0065] [Peel strength between negative electrode current collector and negative electrode composite layer] The negative electrodes of each example and comparative example were cut to prepare test specimens measuring 20 mm in width and 80 mm in length. Double-sided tape (NW-20, manufactured by Nichiban Co., Ltd.) was attached to the negative electrode composite layer on one side of the test specimen and fixed to a smooth plastic substrate. Next, one end of the negative electrode current collector in the longitudinal direction of the test specimen was fixed to a movable jig of a tensile tester (FGP-5, manufactured by Nidec-Shimpo Corporation) so that the negative electrode current collector could be pulled in a direction 90° relative to the substrate surface. The movable jig was moved so that the negative electrode composite layer of the test specimen and the negative electrode current collector were peeled off at a speed of 50 mm / min. During this process, the pulling direction was always maintained at 90° relative to the plastic substrate on which the test specimen was fixed. The stable tensile strength value was read when at least 30 mm of the test specimen had peeled off, and this was defined as the peel strength (N / m) between the negative electrode composite layer and the negative electrode current collector. The results are summarized in Table 1.

[0066] [Table 1]

[0067] As can be seen from Table 1, in all of Examples 1 to 6, the peel strength between the negative electrode current collector and the negative electrode mixture layer was improved compared to Comparative Examples 1 to 5. Therefore, by using a negative electrode active material containing graphite particles A with an internal particle porosity of 5% or less and Si-containing material A in which silicon particles are dispersed within the carbon phase, and by setting the content of graphite particles A to 10% by mass or more and the content of Si-containing material A to 1% by mass or more, relative to the total mass of the negative electrode active material, the peel strength between the negative electrode current collector and the negative electrode mixture layer can be improved. [Explanation of symbols]

[0068] 10 Secondary battery 11 Positive electrode 12 Negative electrode 13 Separator 14 Electrode body 15 Battery case 16 Case body 17 Sealing body 18,19 Insulating plate 20 Positive lead 21 Negative lead 22 Overhang 23 Filters 24 Lower valve body 25 Insulating material 26 Superior valve 27 Cap 28 Gasket 30 graphite particles 34 Internal void 36 External void

Claims

1. a negative electrode mixture layer including a negative electrode active material containing graphite particles and a Si-containing material, and a binder; The Si-containing material includes a Si-containing material A having a carbon phase and silicon particles dispersed within the carbon phase; The graphite particles include graphite particles A having an internal particle porosity of 5% or less, a content of the graphite particles A is 10% by mass or more, relative to the total mass of the negative electrode active material, and a content of the Si-containing material A is 1% by mass or more, relative to the total mass of the negative electrode active material.

2. 2. The negative electrode for a secondary battery according to claim 1, wherein a content of the graphite particles A is 97 mass% or less, based on a total mass of the negative electrode active material, and a content of the Si-containing material A is 20 mass% or less, based on a total mass of the negative electrode active material.

3. 3. The negative electrode for a secondary battery according to claim 1, wherein a waveform showing the distribution of intra-particle porosity of the graphite particles has a first peak in a range of intra-particle porosity of 5% or less and a second peak in a range of intra-particle porosity of more than 5%.

4. 4. The negative electrode for a secondary battery according to claim 3, wherein a waveform showing the distribution of intra-particle porosity of the graphite particles has a first peak in a range of intra-particle porosity of 5% or less and a second peak in a range of intra-particle porosity of 8% or more and 20% or less.

5. The graphite particles include graphite particles B having an internal particle porosity of 8% or more and 20% or less, 5. The negative electrode for secondary batteries according to claim 1, wherein the content of the graphite particles B is 10% by mass or more and 80% by mass or less with respect to the total mass of the negative electrode active material.

6. 6. The negative electrode for a secondary battery according to claim 1, wherein the Si-containing material includes a Si-containing material B having a silicate phase and silicon particles dispersed in the silicate phase.

7. 7. The negative electrode for a secondary battery according to claim 6, wherein a mass ratio (A / B) of the Si-containing material A to the Si-containing material B is 0.2 or more and 20 or less.

8. 8. The negative electrode for secondary batteries according to claim 1, wherein a total content of the Si-containing material is 5 mass % or more and 20 mass % or less with respect to a total mass of the negative electrode active material.

9. 9. The negative electrode for a secondary battery according to claim 1, wherein the carbon phase of the Si-containing material A contains crystalline carbon.

10. 8. The negative electrode for a secondary battery according to claim 6, wherein the silicate phase of the Si-containing material B contains at least one element selected from the group consisting of an alkali metal element and an alkaline earth metal element.

11. The silicate phase of the Si-containing material B has the general formula Li ２ｚ SiO （２＋ｚ） 11. The negative electrode for a secondary battery according to claim 6, 7 or 10, comprising a lithium silicate represented by (0<z<2).

12. A positive electrode, a negative electrode, and a non-aqueous electrolyte are provided. A secondary battery, wherein the negative electrode is the negative electrode for a secondary battery according to any one of claims 1 to 11.

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