Negative electrode for secondary battery and secondary battery

Abstract

This negative electrode for secondary batteries is provided with a negative electrode mixture layer which comprises a binder material and a negative electrode active material that contains graphite particles and an Si-containing material; the Si-containing material contains an Si-containing material A which comprises a carbon phase and silicon particles that are dispersed in the carbon phase; in cases where the negative electrode mixture layer is divided into equal halves in the thickness direction, the average intraparticle void fraction of the graphite particles contained in the upper half region is 13% or less, and is smaller than the average intraparticle void fraction of the graphite particles contained in the lower half region.

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] When a Si-containing material is used as a negative electrode active material, the capacity of the secondary battery can be increased, but the charge-discharge cycle characteristics of the secondary battery tend to be reduced.

[0007] A secondary battery negative electrode according to one embodiment of the present disclosure includes a negative electrode composite layer having a negative electrode active material including graphite particles and a Si-containing material, the Si-containing material including Si-containing material A having a carbon phase and silicon particles dispersed within the carbon phase, and when the negative electrode composite layer is divided into two equal parts in the thickness direction, the graphite particles contained in the upper half region have an average intra-particle porosity of 13% or less, which is smaller than the average intra-particle porosity of the graphite particles contained in the lower half region.

[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 embodiment of the present disclosure, the charge-discharge cycle characteristics of a secondary battery can be improved by using a negative electrode active material including a Si-containing material. [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 a cross-sectional view of a negative electrode according to an embodiment of the present invention. [Figure 3] FIG. 3 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. The Si-containing material includes a Si-containing material A having a carbon phase and silicon particles dispersed within the carbon phase. When the negative electrode composite layer is divided into two equal parts in the thickness direction, the graphite particles contained in the upper half have an average intra-particle porosity of 13% or less, which is smaller than the average intra-particle porosity of the graphite particles contained in the lower half. The negative electrode for a secondary battery according to one embodiment of the present disclosure can improve the charge-discharge cycle characteristics of a secondary battery. The mechanism by which this effect is achieved is unclear, but the following is thought to be the case. It is believed that mixing graphite particles having an intra-particle porosity of 13% or less with a Si-containing material A having silicon particles dispersed within the carbon phase specifically increases inter-particle friction, resulting in a layer that is less likely to be crushed. By disposing such a crush-resistant layer in the upper half of the negative electrode composite layer when the layer is divided into two equal parts in the thickness direction, the absorption of the electrolyte used in the secondary battery is increased, which is thought to improve the charge-discharge cycle characteristics of the secondary battery. Furthermore, when the negative electrode composite layer is divided into two equal parts in the thickness direction, the lower half has a higher internal porosity of the graphite particles than the upper half, making it a layer that is relatively easy to crush. As a result, the contact area and adhesion with the negative electrode current collector are increased in the lower half, which is thought to also improve the charge-discharge cycle characteristics of the secondary battery.

[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] 2 is a cross-sectional view of a negative electrode according to an embodiment of the present invention. Negative electrode 12 includes a negative electrode current collector 40 and a negative electrode mixture layer 42 provided on negative electrode current collector 40.

[0019] The negative electrode current collector 40 may be, for example, a foil of a metal such as copper that is stable in the potential range of the negative electrode, or a film having such a metal disposed on the surface thereof.

[0020] The negative electrode mixture layer 42 includes a negative electrode active material that includes graphite particles and a Si-containing material. The negative electrode mixture layer preferably includes a binder, a conductive material, and the like. The negative electrode 12 can be produced, for example, by preparing a negative electrode mixture slurry that includes the negative electrode active material, the binder, and the like, applying this negative electrode mixture slurry onto the negative electrode current collector 40, drying it to form the negative electrode mixture layer 42, and rolling this negative electrode mixture layer 42. The method for producing the negative electrode mixture layer 42 will be described in detail below.

[0021] In this embodiment, when the negative electrode mixture layer 42 shown in Fig. 2 is divided into two equal parts in the thickness direction, the average intra-particle porosity of the graphite particles contained in the upper half region 42a is 13% or less and is smaller than the average intra-particle porosity of the graphite particles contained in the lower half region. Note that dividing the negative electrode mixture layer 42 into two equal parts in the thickness direction means dividing the negative electrode mixture layer 42 in half at the midpoint Z of the thickness of the negative electrode mixture layer 42 when the stacking direction of the negative electrode current collector 40 and the negative electrode mixture layer 42 is the thickness direction of the negative electrode mixture layer 42. Of the two equal parts in the thickness direction, the negative electrode mixture layer 42 located closer to the negative electrode current collector 40 is the lower half region 42b, and the negative electrode mixture layer 42 located farther from the negative electrode current collector 40 is the upper half region 42a.

[0022] Fig. 3 is an enlarged view showing a cross section of a graphite particle in the negative electrode composite layer. In this embodiment, as shown in Fig. 3, graphite particle 30 included in the negative electrode composite layer has, in a cross section of graphite particle 30, closed voids 34 that do not connect the interior of the particle to the particle surface (hereinafter referred to as internal voids 34), and voids 36 that connect the interior 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 average value of the internal void ratio of graphite particles included in upper half region 42a and lower half region 42b is calculated by the following procedure.

[0023] <Method for measuring internal particle porosity> (1) A cross section of the negative electrode composite layer (upper half region 42a or lower half region 42b) is exposed. 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) Using a scanning electron microscope, a backscattered electron image of the cross section of the exposed negative electrode composite layer (upper half region 42a and lower half region 42b) is taken. The magnification for taking the backscattered electron image is 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 porosity of the graphite particles (area of ​​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. (5) For all graphite particles with particle sizes of 5 μm to 50 μm present in the cross-sectional image of the negative electrode composite layer (upper half region 42a and lower half region 42b), the intra-particle porosity is calculated, and the average value is obtained from the calculated intra-particle porosities.

[0024] In this embodiment, the average intra-particle porosity of the graphite particles contained in the upper half region 42a of the negative electrode mixture layer 42 may be 13% or less, and is preferably 2.5% to 13%, and more preferably 2.5% to 5% in order to improve the charge-discharge cycle characteristics of the secondary battery. The average intra-particle porosity of the graphite contained in the lower half region 42b of the negative electrode mixture layer 42 may be greater than the average intra-particle porosity of the graphite particles contained in the upper half region 42a, and is preferably 8% to 20%, and more preferably 10% to 18%.

[0025] In this embodiment, the upper half region 42a of the negative electrode mixture layer 42 may contain graphite particles with any internal porosity as long as the average internal porosity of the graphite particles is 13% or less, but it is preferable to include graphite particles A with an internal porosity of 5% or less, for example, in order to lower the average internal porosity of the graphite particles contained in the upper half region 42a. Furthermore, the lower half region 42b of the negative electrode mixture layer 42 may contain graphite particles with any internal porosity as long as the internal porosity is higher than the average internal porosity of the graphite particles contained in the upper half region 42a, but it is preferable to include graphite particles B with an internal porosity of 8% or more and 20% or less, for example, in order to increase the average internal porosity of the graphite particles contained in the lower half region 42b.

[0026] In this embodiment, the upper half region 42a of the negative electrode mixture layer 42 may contain, for example, graphite particles B having an internal porosity of 8% or more and 20% or less, as long as the average internal porosity of the graphite particles is 13% or less. The lower half region 42b of the negative electrode mixture layer 42 may contain, for example, graphite particles A having an internal porosity of 5% or less, as long as the internal porosity is higher than the average internal porosity of the graphite particles contained in the upper half region 42a.

[0027] The internal porosity of the graphite particles A is preferably 1% to 5%, more preferably 3% to 5%. The internal porosity of the graphite particles B is preferably 10% or more and 18% or less, more preferably 12% or more and 16% or less.

[0028] The graphite particles A and B are produced, for example, as follows.

[0029] <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.

[0030] <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.

[0031] 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 002When 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.

[0032] In this embodiment, the Si-containing material contained in negative electrode composite layer 42 includes Si-containing material A having a carbon phase and silicon particles dispersed in the carbon phase. In order to improve the charge / discharge cycle characteristics of the secondary battery, the content of Si-containing material A is preferably 1% by mass or more and 30% by mass or less, more preferably 5% by mass or more and 20% by mass or less, and even more preferably 10% by mass or more and 20% by mass or less, relative to the total mass of the negative electrode active material.

[0033] 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, from the viewpoint of increasing capacity, etc. The carbon phase of the Si-containing material A preferably contains crystalline carbon from the viewpoint of suppressing breakage of electrode plates during electrode plate fabrication by improving strength, etc.

[0034] The average particle size of silicon particles is generally 500 nm or less, preferably 200 nm or less, and more preferably 100 nm or less before charge and discharge. 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 is reduced, improving cycle characteristics. The average particle size of silicon particles is measured by observing the particle cross-section of Si-containing material A using a scanning electron microscope (SEM) or a transmission electron microscope (TEM), and is specifically determined as the average of the longest diameters of 100 silicon particles.

[0035] In this embodiment, the Si-containing material contained in negative electrode composite material layer 42 may include, in addition to Si-containing material A, Si-containing material B which includes a silicate phase and silicon particles dispersed in the silicate phase.

[0036] 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). From the viewpoint of improving the battery capacity and the like, the content of the silicon particles is preferably 35 to 75% by mass based on the total mass of the Si-containing material B. Further, 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.

[0037] The silicate phase of the Si-containing material B preferably contains at least one of an alkali metal element and an alkaline earth metal element in terms of improving lithium ion conductivity, and particularly preferably contains a lithium element. Further, 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).

[0038] In addition, a conductive layer made of a material with high conductivity may be formed on the particle surface of the Si-containing material A or the Si-containing material B. An example of a suitable conductive layer is a carbon coating made of a carbon material. The carbon coating is composed of, for example, carbon black, acetylene black, ketjen black, graphite, and a mixture of two or more of these. As a method for carbon coating the particle surface of the Si-containing material, examples include a CVD method using acetylene, methane, etc., a method of mixing coal pitch, petroleum pitch, phenol resin, etc. with the particles of the Si-containing material and performing heat treatment. Also, a carbon coating may be formed by fixing carbon powder such as carbon black to the particle surface using a binder.

[0039] The mass ratio (A / B) of the Si-containing material A to the Si-containing material B is preferably 0.2 or more and 20 or less, and more preferably 2 or more and 10 or less, in terms of improving the charge and discharge cycle characteristics of the secondary battery and the like.

[0040] 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 charge / discharge cycle characteristics of the secondary battery and to increase the battery capacity.

[0041] 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.

[0042] 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.

[0043] An example of a method for producing the negative electrode composite layer 42 will be described. For example, a negative electrode active material containing graphite particles B and Si-containing material A (and optionally Si-containing material B), a binder, and the like are mixed together with a solvent such as water to prepare a negative electrode composite slurry for the lower half region 42b. Note that graphite particles A may be added to the slurry, for example, to adjust the average value of the internal porosity of the graphite particles. Separately from the slurry, a negative electrode active material containing graphite particles A and Si-containing material A (and optionally Si-containing material B), a binder, and the like are mixed together with a solvent such as water to prepare a negative electrode composite slurry for the upper half region 42a. Note that graphite particles B may be added to the slurry, for example, to adjust the average value of the internal porosity of the graphite particles. Then, the negative electrode composite slurry for the lower half region 42b is applied to both surfaces of the negative electrode current collector 40 and dried, and then the negative electrode composite slurry for the upper half region 42a is applied to both surfaces of the coating film of the negative electrode composite slurry for the lower half region 42b and dried, thereby forming the negative electrode composite layer 42. In the above method, the negative electrode composite slurry for the lower half region 42b is applied and dried, and then the negative electrode composite slurry for the upper half region 42a is applied, but a method in which the negative electrode composite slurry for the upper half region 42a is applied after the negative electrode composite slurry for the lower half region 42b is applied and before drying may also be used, or the negative electrode composite slurry for the lower half region 42b and the negative electrode composite slurry for the upper half region 42a may be applied simultaneously.

[0044] [Positive electrode] The positive electrode 11 is composed of a positive electrode current collector, such as a metal foil, and a positive electrode composite layer formed on the positive electrode current collector. The positive electrode current collector can be a foil of a metal, such as aluminum, that is stable within the potential range of the positive electrode, or a film with such a metal disposed on its surface. The positive electrode composite layer contains, for example, a positive electrode active material, a binder, a conductive material, etc.

[0045] The positive electrode 11 can be produced, for example, by applying a positive electrode composite slurry containing a positive electrode active material, a binder, a conductive material, etc. onto a positive electrode current collector, drying the slurry to form a positive electrode composite layer, and then performing a compression step in which the positive electrode composite layer is compressed using a rolling roller or the like.

[0046] 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 high capacity of the secondary battery, the positive electrode active material preferably contains lithium nickel composite oxides such as Li x NiO2, Li x Co y Ni 1-y O2, Li x Ni 1-y M y O z (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), etc.

[0047] 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 types.

[0048] Examples of binders include fluorine-based resins such as polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVdF), polyacrylonitrile (PAN), polyimide-based resins, acrylic-based resins, polyolefin-based resins, etc. These may be used alone or in combination of two or more.

[0049] [Separator] The separator 13 may be, for example, a porous sheet having ion permeability and insulating properties. Specific examples of porous sheets include a microporous thin film, a woven fabric, and a nonwoven fabric. Suitable materials for the separator include olefin-based resins such as polyethylene and polypropylene, and cellulose. The separator 13 may be a laminate having a cellulose fiber layer and a thermoplastic resin fiber layer such as an olefin-based resin. Alternatively, the separator 13 may be a multilayer separator including a polyethylene layer and a polypropylene layer, and a separator whose surface is coated with a material such as an aramid-based resin or ceramic may be used.

[0050] [Non-aqueous electrolyte] The non-aqueous electrolyte includes a non-aqueous solvent and an electrolyte salt dissolved in the non-aqueous solvent. Examples of the non-aqueous solvent that can be used include esters, ethers, nitriles such as acetonitrile, amides such as dimethylformamide, and mixed solvents of two or more of these. The non-aqueous solvent may contain a halogen-substituted product in which at least a portion of the hydrogen atoms of these solvents are substituted with halogen atoms such as fluorine.

[0051] 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.

[0052] Examples of the ethers include 1,3-dioxolane, 4-methyl-1,3-dioxolane, tetrahydrofuran, 2-methyltetrahydrofuran, propylene oxide, 1,2-butylene oxide, 1,3-dioxane, 1,4-dioxane, 1,3,5-trioxane, furan, 2-methylfuran, 1,8-cineole, cyclic ethers such as crown ethers, 1,2-dimethoxyethane, diethyl ether, dipropyl ether, diisopropyl ether, dibutyl ether, dihexyl ether, ethyl vinyl ether, butyl vinyl ether, methylphenyl ether, and the like. and chain ethers such as ethyl 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, and tetraethylene glycol dimethyl.

[0053] As the halogen-substituted compound, it is preferable to use fluorinated cyclic carbonates such as fluoroethylene carbonate (FEC), fluorinated chain carbonates, fluorinated chain carboxylates such as methyl fluoropropionate (FMP), and the like.

[0054] The electrolyte salt is preferably a lithium salt. Examples of lithium salts 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、chloroborane lithium, lithium lower aliphatic carboxylic acid, borate salts such as Li2B4O7, Li(B(C2O4)F2), imide salts such as LiN(SO2CF3)2, LiN(C1F 2l+1 SO2)(C m F 2m+1 SO2){l, m are integers of 1 or more}, etc. Among these, from the viewpoints of ionic conductivity, electrochemical stability, etc., it is preferable to use LiPF6. The concentration of the lithium salt is preferably 0.8 to 1.8 mol per 1 L of the solvent.

[0055] <Example> Hereinafter, the present disclosure will be further described by examples, but the present disclosure is not limited to these examples.

[0056] <Example 1> [Preparation of negative electrode] A negative electrode active material A was prepared by mixing 94 parts by mass of graphite particles B having an internal particle porosity of 8% or more and 20% or less and 6 parts by mass of Si-containing material A having silicon particles dispersed within the carbon phase. A slurry was prepared by mixing these materials so that the mass ratio of negative electrode active material A, carboxymethyl cellulose-sodium (CMC-Na), and styrene-butadiene copolymer rubber (SBR) was 100:1:1. This slurry was used as the negative electrode composite slurry for the lower half region. A negative electrode active material B was prepared by mixing 38 parts by mass of graphite particles A having an internal particle porosity of 5% or less, 56 parts by mass of graphite particles B having an internal particle porosity of 8% or more and 20% or less, and 6 parts by mass of Si-containing material A having silicon particles dispersed within the carbon phase. A negative electrode active material B, carboxymethyl cellulose-sodium (CMC-Na), and styrene-butadiene copolymer rubber (SBR) were mixed at a mass ratio of 100:1:1 to prepare a slurry, which was used as the negative electrode composite slurry for the upper half region.

[0057] The negative electrode composite slurry for the lower half region was applied to both sides of an 8 μm-thick copper foil, the coating was dried, and then the negative electrode composite slurry for the upper half region was applied to the coating and dried. The coating was then rolled using 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 average internal porosity of the graphite particles in the produced negative electrode was also measured. As a result, when the negative electrode composite layer was divided into two equal parts in the thickness direction, the average internal porosity of the graphite particles in the upper half region was 13%, and the average internal porosity of the graphite particles in the lower half region was 14%.

[0058] [Preparation of positive electrode] The positive electrode active material is aluminum-containing lithium nickel cobalt oxide (LiNi 0.88 Co 0.09 Al 0.03O2) was used. 100 parts by mass of the above positive electrode active material, 1 part by mass of acetylene black, and 0.9 parts by mass of polyvinylidene fluoride were mixed in a solvent of N-methyl-2-pyrrolidone (NMP) to prepare a positive electrode composite slurry. This slurry was applied to both sides of a 15 μm-thick aluminum foil, the coating was dried, and then the coating was rolled using a rolling roller to produce a positive electrode in which a positive electrode composite layer was formed on both sides of the positive electrode current collector.

[0059] [Preparation of non-aqueous electrolyte] LiPF6 was dissolved at a concentration of 1.4 mol / L in a non-aqueous solvent made by mixing ethylene carbonate (EC), methyl ethyl carbonate (MEC), and dimethyl carbonate (DMC) in a volume ratio of 20:5:75, and then 3 mass% of vinylene carbonate and 0.5 mass% of 1,6-diisocyanate hexane were added to prepare a non-aqueous electrolyte.

[0060] [Secondary battery production] (1) An aluminum positive electrode lead was attached to the positive electrode current collector, and a nickel-copper-nickel negative electrode lead was attached to the negative electrode current collector. After that, a polyethylene separator was placed between the positive and negative electrodes and wound to prepare a wound electrode body. (2) Insulating plates were placed above and below the electrode body, the negative electrode lead was welded to the case body, and the positive electrode lead was welded to the sealing member, and the electrode body was housed within the case body. (3) After the non-aqueous electrolyte was injected into the case body under reduced pressure, the open end of the case body was crimped to a sealing member via a gasket, completing the secondary battery.

[0061] <Example 2> A secondary battery was fabricated in the same manner as in Example 1, except that the negative electrode active material A used in the negative electrode composite slurry for the lower half region was a mixture of 93.6 parts by mass of graphite particles B having an internal particle porosity of 8% or more and 20% or less, 2.3 parts by mass of Si-containing material A in which silicon particles are dispersed in a carbon phase, and 4.1 parts by mass of Si-containing material B in which silicon particles are dispersed in a silicate phase, and the negative electrode active material B used in the negative electrode composite slurry for the upper half region was a mixture of 37.4 parts by mass of graphite particles A having an internal particle porosity of 5% or less, 56.2 parts by mass of graphite particles B having an internal particle porosity of 8% or more and 20% or less, 2.3 parts by mass of Si-containing material A in which silicon particles are dispersed in a carbon phase, and 4.1 parts by mass of Si-containing material B in which silicon particles are dispersed in a silicate phase.

[0062] In the negative electrode produced in Example 2, when the negative electrode composite layer was divided into two equal parts in the thickness direction, the average internal porosity of the graphite particles contained in the upper half region was 9%, and the average internal porosity of the graphite particles contained in the lower half region was 14%.

[0063] Example 3 A secondary battery was fabricated in the same manner as in Example 1, except that the negative electrode active material A used in the negative electrode composite slurry for the lower half region was a mixture of 93.6 parts by mass of graphite particles B having an internal particle porosity of 8% or more and 20% or less, 2.3 parts by mass of Si-containing material A in which silicon particles are dispersed in a carbon phase, and 4.1 parts by mass of Si-containing material B in which silicon particles are dispersed in a silicate phase, and the negative electrode active material B used in the negative electrode composite slurry for the upper half region was a mixture of 37.4 parts by mass of graphite particles A having an internal particle porosity of 5% or less, 56.2 parts by mass of graphite particles B having an internal particle porosity of 8% or more and 20% or less, 2.3 parts by mass of Si-containing material A in which silicon particles are dispersed in a carbon phase, and 4.1 parts by mass of Si-containing material B in which silicon particles are dispersed in a silicate phase.

[0064] In the negative electrode produced in Example 3, when the negative electrode composite layer was divided into two equal parts in the thickness direction, the average internal porosity of the graphite particles contained in the upper half region was 13%, and the average internal porosity of the graphite particles contained in the lower half region was 14%.

[0065] Example 4 A secondary battery was fabricated in the same manner as in Example 1, except that the negative electrode active material A used in the negative electrode composite slurry for the lower half region was a mixture of 95.0 parts by mass of graphite particles B having an internal particle porosity of 8% or more and 20% or less, 1.0 part by mass of Si-containing material A in which silicon particles are dispersed in a carbon phase, and 4.0 parts by mass of Si-containing material B in which silicon particles are dispersed in a silicate phase, and the negative electrode active material B used in the negative electrode composite slurry for the upper half region was a mixture of 38 parts by mass of graphite particles A having an internal particle porosity of 5% or less, 57 parts by mass of graphite particles B having an internal particle porosity of 8% or more and 20% or less, 1.0 part by mass of Si-containing material A in which silicon particles are dispersed in a carbon phase, and 4.0 parts by mass of Si-containing material B in which silicon particles are dispersed in a silicate phase.

[0066] In the negative electrode produced in Example 4, when the negative electrode composite layer was divided into two equal parts in the thickness direction, the average internal porosity of the graphite particles contained in the upper half region was 9%, and the average internal porosity of the graphite particles contained in the lower half region was 14%.

[0067] <Example 5> A secondary battery was fabricated in the same manner as in Example 1, except that the negative electrode active material A used in the negative electrode composite slurry for the lower half region was a mixture of 80 parts by mass of graphite particles B having an internal particle porosity of 8% or more and 20% or less, 19 parts by mass of Si-containing material A in which silicon particles are dispersed in a carbon phase, and 1 part by mass of Si-containing material B in which silicon particles are dispersed in a silicate phase, and the negative electrode active material B used in the negative electrode composite slurry for the upper half region was a mixture of 32 parts by mass of graphite particles A having an internal particle porosity of 5% or less, 48 ​​parts by mass of graphite particles B having an internal particle porosity of 8% or more and 20% or less, 19 parts by mass of Si-containing material A in which silicon particles are dispersed in a carbon phase, and 1 part by mass of Si-containing material B in which silicon particles are dispersed in a silicate phase.

[0068] In the negative electrode produced in Example 5, when the negative electrode composite layer was divided into two equal parts in the thickness direction, the average internal porosity of the graphite particles contained in the upper half region was 9%, and the average internal porosity of the graphite particles contained in the lower half region was 14%.

[0069] <Comparative Example 1> A secondary battery was produced in the same manner as in Example 1, except that the negative electrode composite slurry for the upper half region used in Example 1 was applied to both sides of a copper foil with a thickness of 8 μm, the coating film was dried, and then the coating film was rolled with a rolling roller to produce a negative electrode in which a negative electrode composite layer was formed on both sides of the negative electrode current collector.

[0070] In the negative electrode produced in Comparative Example 1, when the negative electrode composite layer was divided into two equal parts in the thickness direction, the average intra-particle porosity of the graphite particles contained in the upper half region was 13%, and the average intra-particle porosity of the graphite particles contained in the lower half region was 13%.

[0071] <Comparative Example 2> A secondary battery was produced in the same manner as in Example 1, except that the negative electrode composite slurry for the lower half region used in Example 1 was applied to both sides of a copper foil with a thickness of 8 μm, the coating film was dried, and then the coating film was rolled with a rolling roller to produce a negative electrode in which a negative electrode composite layer was formed on both sides of the negative electrode current collector.

[0072] In the negative electrode produced in Comparative Example 2, when the negative electrode composite layer was divided into two equal parts in the thickness direction, the average intra-particle porosity of the graphite particles contained in the upper half region was 14%, and the average intra-particle porosity of the graphite particles contained in the lower half region was 14%.

[0073] <Comparative Example 3> A secondary battery was fabricated in the same manner as in Example 1, except that the negative electrode active material A used in the negative electrode composite slurry for the lower half region was a mixture of 94 parts by mass of graphite particles B having an internal particle porosity of 8% or more and 20% or less, and 6 parts by mass of Si-containing material A having silicon particles dispersed in the carbon phase, and the negative electrode active material B used in the negative electrode composite slurry for the upper half region was a mixture of 38 parts by mass of graphite particles A having an internal particle porosity of 5% or less, 56 parts by mass of graphite particles B having an internal particle porosity of 8% or more and 20% or less, and 6 parts by mass of Si-containing material A having silicon particles dispersed in the carbon phase.

[0074] In the negative electrode produced in Comparative Example 3, when the negative electrode composite layer was divided into two equal parts in the thickness direction, the average intra-particle porosity of the graphite particles contained in the upper half region was 14%, and the average intra-particle porosity of the graphite particles contained in the lower half region was 16%.

[0075] <Comparative Example 4> A secondary battery was fabricated in the same manner as in Example 1, except that the negative electrode active material A used in the negative electrode composite slurry for the lower half region was a mixture of 94 parts by mass of graphite particles B having an internal particle porosity of 8% or more and 20% or less, and 6 parts by mass of Si-containing material B having silicon particles dispersed in a silicate phase, and the negative electrode active material B used in the negative electrode composite slurry for the upper half region was a mixture of 38 parts by mass of graphite particles A having an internal particle porosity of 5% or less, 56 parts by mass of graphite particles B having an internal particle porosity of 8% or more and 20% or less, and 6 parts by mass of Si-containing material B having silicon particles dispersed in a silicate phase.

[0076] In the negative electrode produced in Comparative Example 4, when the negative electrode composite layer was divided into two equal parts in the thickness direction, the average intra-particle porosity of the graphite particles contained in the upper half region was 13%, and the average intra-particle porosity of the graphite particles contained in the lower half region was 14%.

[0077] <Comparative Example 5> A secondary battery was fabricated in the same manner as in Example 1, except that the negative electrode active material A used in the negative electrode composite slurry for the lower half region was a mixture of 80 parts by mass of graphite particles B having an internal particle porosity of 8% or more and 20% or less, and 20 parts by mass of Si-containing material B having silicon particles dispersed in a silicate phase, and the negative electrode active material B used in the negative electrode composite slurry for the upper half region was a mixture of 32 parts by mass of graphite particles A having an internal particle porosity of 5% or less, 48 ​​parts by mass of graphite particles B having an internal particle porosity of 8% or more and 20% or less, and 20 parts by mass of Si-containing material B having silicon particles dispersed in a silicate phase.

[0078] In the negative electrode produced in Comparative Example 5, when the negative electrode composite layer was divided into two equal parts in the thickness direction, the average intra-particle porosity of the graphite particles contained in the upper half region was 9%, and the average intra-particle porosity of the graphite particles contained in the lower half region was 14%.

[0079] [Charge / discharge cycle test] Each secondary battery of the Examples and Comparative Examples was subjected to constant current charging at a constant current of 0.5 It in a temperature environment of 25°C until the battery voltage reached 4.2 V, and then constant voltage charging at 4.2 V until the current value reached 1 / 100 It. Thereafter, constant current discharging was performed at a constant current of 0.5 It until the battery voltage reached 2.5 V. This charge / discharge cycle was repeated 450 times, and the capacity retention rate was calculated using the following formula.

[0080] Capacity retention rate (%) = (450th cycle discharge capacity ÷ 1st cycle discharge capacity) × 100 The results of the capacity retention rate of each secondary battery of the Examples and Comparative Examples are summarized in Table 1. Note that a higher value of the capacity retention rate indicates that the charge / discharge cycle characteristics of the secondary battery are more improved.

[0081] [Table 1]

[0082] As can be seen from Table 1, Examples 1 to 4 all exhibited higher capacity retention values ​​than Comparative Examples 1 to 4, which contained the same amount of Si-containing material. Furthermore, Example 5 also exhibited a higher capacity retention value than Comparative Example 5, which contained the same amount of Si-containing material. Therefore, it can be said that the charge-discharge cycle characteristics of a secondary battery can be improved by using a negative electrode active material containing graphite particles and Si-containing material A in which silicon particles are dispersed within a carbon phase, and by dividing the negative electrode composite layer into two equal parts in the thickness direction, and by setting the average intra-particle porosity of the graphite particles contained in the upper half of the layer to 13% or less and smaller than the average intra-particle porosity of the graphite particles contained in the lower half of the layer. [Explanation of symbols]

[0083] 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 40 Negative electrode current collector 42 Negative electrode composite layer 42a Upper half area 42b Lower half area

Claims

1. a negative electrode mixture layer having a negative electrode active material including graphite particles and a Si-containing material; The Si-containing material includes a Si-containing material A having a carbon phase and silicon particles dispersed within the carbon phase; the content of the graphite particles is 80% by mass or more and 95% by mass or less with respect to the total mass of the negative electrode active material, a total content of the Si-containing material is 5% by mass or more and 20% by mass or less with respect to a total mass of the negative electrode active material, the content of the Si-containing material A is 1% by mass or more and 19% by mass or less with respect to the total mass of the negative electrode active material, the carbon phase of the Si-containing material A comprises crystalline carbon, a negative electrode for a secondary battery, wherein, when the negative electrode mixture layer is divided into two equal halves in a thickness direction, the graphite particles contained in an upper half region have an average intra-particle porosity of 13% or less and are smaller than the average intra-particle porosity of the graphite particles contained in a lower half region.

2. 2. The negative electrode for a secondary battery according to claim 1, wherein the graphite particles contained in the upper half region have an average intra-particle porosity of 2.5% or more and 13% or less, and the graphite particles contained in the lower half region have an average intra-particle porosity of 8% or more and 20% or less.

3. 3. 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.

4. 4. The negative electrode for a secondary battery according to claim 3, wherein a mass ratio (A / B) of said Si-containing material A to said Si-containing material B is 0.2 or more and 20 or less.

5. A negative electrode for a secondary battery according to claim 1, wherein the silicon particles have an average particle size of 400 nm or less after charging and discharging.

6. 5. The negative electrode for a secondary battery according to claim 3, 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.

7. The silicate phase of the Si-containing material B has the general formula Li ２ｚ SiO （２＋ｚ） 7. The negative electrode for a secondary battery according to claim 3, comprising a lithium silicate represented by (0<z<2).

8. 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 7.

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