Composite particles for negative electrode active material, method for producing composite particles for negative electrode active material, and non-aqueous electrolyte secondary battery

Abstract

A negative electrode active material composite particle according to one embodiment is a composite particle for a negative electrode active material, the composite particle including: a lithium silicate phase; silicon particles dispersed in the lithium silicate phase; and a resin baked body. The resin baked body containing at least one element selected from F and N. The composite particle has a porosity of 10% or less, for example, and the content of the resin baked body is 1-20% by mass with respect to the mass of the composite particle.

Description

[Technical Field]

[0001] The present disclosure relates to composite particles for a negative electrode active material, a method for producing composite particles for a negative electrode active material, and a non-aqueous electrolyte secondary battery in which the composite particles are used as a negative electrode active material. [Background technology]

[0002] Silicon-containing materials are known to be able to store more lithium ions per unit volume than carbon materials such as graphite. Therefore, using silicon materials as anode active materials can increase the capacity of batteries. Silicon materials function as anode active materials with high charge / discharge capacities, but they suffer from the drawback of large expansion and contraction during charging and discharging, making them prone to particle cracking. When cracks occur in the anode active material particles, the number of particles isolated from the conductive path in the anode increases, preventing these particles from contributing to charging and discharging, leading to a decrease in battery capacity. Furthermore, when cracks occur in silicon material particles, the silicon in the particles comes into direct contact with the electrolyte, accelerating the decomposition reaction of the electrolyte and resulting in a decrease in battery capacity.

[0003] In light of this situation, various studies have been conducted on silicon materials used as negative electrode active materials. For example, Patent Documents 1 and 2 propose a method of forming a carbon coating on the particle surface of a silicon material to increase electronic conductivity, thereby making it easier to ensure a conductive path even when particles crack, and suppressing capacity loss. The method in Patent Document 1 involves mixing silicon particles, graphite, and a carbon precursor, and then carbonizing the carbon precursor. The method in Patent Document 2 involves forming a carbon coating on the particle surface of a silicon compound by chemical vapor deposition.

[0004] Patent Document 3 discloses a method for reducing expansion and contraction during charge and discharge by compounding silicon alloy powder with carbon in order to suppress cracking of active material particles and improve the cycle characteristics of a battery. Furthermore, Patent Document 4 discloses a method for improving cycle characteristics by using a lithium silicate phase as a matrix phase compounded with silicon particles to suppress side reactions between the active material and the electrolyte during charge and discharge. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Laid-Open No. 2000-272911 [Patent Document 2] Japanese Patent Application Laid-Open No. 2000-215887 [Patent Document 3] Patent No. 5094013 [Patent Document 4] International Publication No. 2018 / 101072 Summary of the Invention [Problem to be solved by the invention]

[0006] However, the methods disclosed in Patent Documents 1 and 2 are ineffective in preventing cracking of active material particles, and are only effective in suppressing the decomposition reaction of the electrolyte solution. Similarly, the method disclosed in Patent Document 3 is unable to fully suppress cracking of active material particles or the decomposition reaction of the electrolyte solution. Furthermore, the silicon material disclosed in Patent Document 4 has many voids within the particles, which can lead to a reaction between the silicon in the particles and the electrolyte solution or cracking of the active material particles, leaving room for improvement in cycle characteristics.

[0007] An object of the present disclosure is to provide composite particles for a negative electrode active material that contribute to improving the cycle characteristics of a battery. [Means for solving the problem]

[0008] The composite particle for a negative electrode active material according to one embodiment of the present disclosure is a composite particle for a negative electrode active material comprising a lithium silicate phase, silicon particles dispersed in the lithium silicate phase, and a sintered resin body, wherein the sintered resin body contains at least one element selected from F and N.

[0009] A non-aqueous electrolyte secondary battery according to one embodiment of the present disclosure includes a negative electrode containing the above-described composite particles for a negative electrode active material, a positive electrode, and a non-aqueous electrolyte.

[0010] A method for producing composite particles for a negative electrode active material according to one embodiment of the present disclosure includes the steps of: mixing precursor particles containing a lithium silicate phase and silicon particles dispersed in the lithium silicate phase with a resin containing at least one element selected from F and N to prepare a mixture; and firing the mixture. [Effects of the Invention]

[0011] By applying the composite particles according to one embodiment of the present disclosure to the negative electrode active material, the cycle characteristics of the battery can be improved. [Brief explanation of the drawings]

[0012] [Figure 1] 1 is a cross-sectional view of a nonaqueous electrolyte secondary battery according to an embodiment; [Figure 2] FIG. 1 is a cross-sectional view of a composite particle according to an embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0013] Composite particles containing a lithium silicate phase coated on silicon particles are useful for suppressing the reaction between silicon particles and the electrolyte solution and improving cycle characteristics. However, batteries using composite particles containing a lithium silicate phase have poor cycle characteristics due to the large amount of voids present within the particles. The inventors conducted extensive research to solve this problem and discovered that incorporating a sintered resin containing at least one element selected from F and N into composite particles containing a lithium silicate phase improves cycle characteristics. The sintered resin is believed to fill the internal voids of the lithium silicate phase, reinforcing the silicate phase and suppressing particle cracking during charge and discharge. By using such composite particles as the negative electrode active material, a nonaqueous electrolyte secondary battery with excellent cycle characteristics can be obtained.

[0014] The composite particles according to the present disclosure can be produced by a simple method of mixing precursor particles containing a lithium silicate phase and silicon particles with a resin containing at least one element selected from F and N, and then calcining the mixture. Furthermore, the calcination of the mixture can be carried out under atmospheric pressure, and special high-temperature and high-pressure conditions are not required. Examples of suitable resins include polyimide and polytetrafluoroethylene (PTFE). As shown in the examples below, composite particles using resins that do not contain F or N, such as acrylic resins and polyvinyl alcohol, did not show any improvement in cycle characteristics. In other words, calcined resins containing F or N specifically modify the composite particles and contribute to improving cycle characteristics.

[0015] Hereinafter, with reference to the drawings, an example of an embodiment of the composite particle for a negative electrode active material according to the present disclosure and a nonaqueous electrolyte secondary battery in which the composite particle is used as a negative electrode active material will be described in detail. Note that the present disclosure also includes selective combinations of multiple embodiments and modified examples described below.

[0016] In the following, a cylindrical battery in which a wound electrode assembly 14 is housed in a cylindrical outer can 16 with a bottom is exemplified, but the outer can of the battery is not limited to a cylindrical outer can and may be, for example, a prismatic outer can (prismatic battery) or a coin-shaped outer can (coin battery), or may be an outer can made of a laminate sheet including a metal layer and a resin layer (laminated battery).The electrode assembly may also be a laminated electrode assembly in which multiple positive electrodes and multiple negative electrodes are alternately stacked with separators interposed therebetween.

[0017] FIG. 1 is a schematic diagram showing a cross section of a nonaqueous electrolyte secondary battery 10 according to an embodiment. As shown in FIG. 1, the nonaqueous electrolyte secondary battery 10 includes a wound electrode assembly 14, a nonaqueous electrolyte, and an outer can 16 that accommodates the electrode assembly 14 and the nonaqueous electrolyte. The electrode assembly 14 has a positive electrode 11, a negative electrode 12, and a separator 13, and has a wound structure in which the positive electrode 11 and the negative electrode 12 are spirally wound with the separator 13 interposed therebetween. The outer can 16 is a cylindrical metal container that is open on one axial side and has a bottom, and the opening of the outer can 16 is closed by a sealing member 17. For ease of explanation, the sealing member 17 side of the battery is referred to as the top, and the bottom side of the outer can 16 is referred to as the bottom.

[0018] 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 include esters, ethers, nitriles, amides, and mixed solvents of two or more of these. The non-aqueous solvent may contain a halogen-substituted compound in which at least a portion of the hydrogen atoms in these solvents are substituted with a halogen element such as fluorine. Examples of the non-aqueous solvent include ethylene carbonate (EC), ethyl methyl carbonate (EMC), dimethyl carbonate (DMC), and mixed solvents thereof. The electrolyte salt may be, for example, a lithium salt such as LiPF6. The non-aqueous electrolyte is not limited to a liquid electrolyte, but may also be a solid electrolyte.

[0019] The positive electrode 11, negative electrode 12, and separator 13 that make up the electrode assembly 14 are all long, strip-like bodies that are spirally wound and alternately stacked in the radial direction of the electrode assembly 14. The negative electrode 12 is formed to be slightly larger than the positive electrode 11 in order to prevent lithium deposition. That is, the negative electrode 12 is formed to be longer than the positive electrode 11 in the longitudinal direction and width direction (short direction). The separator 13 is formed to be at least slightly larger than the positive electrode 11, and two separators 13 are arranged to sandwich the positive electrode 11. The electrode assembly 14 has a positive electrode lead 20 connected to the positive electrode 11 by welding or the like, and a negative electrode lead 21 connected to the negative electrode 12 by welding or the like.

[0020] Insulating plates 18 and 19 are disposed above and below the electrode body 14. In the example shown in Fig. 1, the positive electrode lead 20 passes through a through-hole in the insulating plate 18 and extends toward the sealing body 17, and the negative electrode lead 21 passes outside the insulating plate 19 and extends toward the bottom side of the outer can 16. The positive electrode lead 20 is connected to the underside of an internal terminal plate 23 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 internal terminal plate 23, serves as the positive electrode terminal. The negative electrode lead 21 is connected to the inner bottom surface of the outer can 16 by welding or the like, and the outer can 16 serves as the negative electrode terminal.

[0021] As described above, the outer can 16 is a cylindrical metal container with a bottom and an opening on one axial side. A gasket 28 is provided between the outer can 16 and the sealing body 17 to ensure sealing of the battery interior and insulation between the outer can 16 and the sealing body 17. The outer can 16 has a grooved portion 22 that supports the sealing body 17, with part of the side surface protruding inward. The grooved portion 22 is preferably formed in an annular shape along the circumferential direction of the outer can 16, and supports the sealing body 17 on its top surface. The sealing body 17 is fixed to the top of the outer can 16 by the grooved portion 22 and the open end of the outer can 16 that is crimped to the sealing body 17.

[0022] The sealing body 17 has a structure in which, in order from the electrode body 14 side, an internal terminal plate 23, a lower valve body 24, an insulating member 25, an upper valve body 26, and a cap 27 are stacked. Each member constituting the sealing body 17 has, for example, a disk or ring shape, and each member except for the insulating member 25 is electrically connected to one another. The lower valve body 24 and the upper valve body 26 are connected at their respective centers, and the insulating member 25 is interposed between their respective peripheral edges. If an abnormality occurs in the battery and the internal pressure increases, 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. If the internal pressure further increases, the upper valve body 26 ruptures, and gas is discharged from the opening of the cap 27.

[0023] The positive electrode 11, negative electrode 12, and separator 13 that constitute the nonaqueous electrolyte secondary battery 10, and in particular the negative electrode active material that constitutes the negative electrode 12, will be described in detail below.

[0024] [Positive electrode] The positive electrode 11 has a positive electrode core and a positive electrode mixture layer provided on the surface of the positive electrode core. The positive electrode core can be a foil of a metal, such as aluminum or an aluminum alloy, that is stable within the potential range of the positive electrode 11, or a film with such a metal disposed on the surface. The positive electrode mixture layer contains a positive electrode active material, a conductive agent, and a binder, and is preferably provided on both sides of the positive electrode core except for the exposed portion of the core to which the positive electrode lead is connected. The thickness of the positive electrode mixture layer is, for example, 50 μm to 150 μm on one side of the positive electrode core. The positive electrode 11 can be produced by applying a positive electrode mixture slurry containing a positive electrode active material, a conductive agent, a binder, etc. to the surface of the positive electrode core, drying the coating, and then compressing it to form a positive electrode mixture layer on both sides of the positive electrode core.

[0025] The positive electrode active material is composed mainly of a lithium transition metal composite oxide. Elements other than Li contained in the lithium transition metal composite oxide include Ni, Co, Mn, Al, B, Mg, Ti, V, Cr, Fe, Cu, Zn, Ga, Sr, Zr, Nb, In, Sn, Ta, W, Si, and P. An example of a suitable lithium transition metal composite oxide is a composite oxide containing at least one of Ni, Co, and Mn. Specific examples include a lithium transition metal composite oxide containing Ni, Co, and Mn, and a lithium transition metal composite oxide containing Ni, Co, and Al.

[0026] Examples of conductive agents contained in the positive electrode mixture layer include carbon materials such as carbon black, acetylene black, ketjen black, and graphite. Examples of binders contained in the positive electrode mixture layer include fluororesins such as polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVdF), polyacrylonitrile (PAN), polyimide resins, acrylic resins, and polyolefin resins. These resins may be used in combination with cellulose derivatives such as carboxymethyl cellulose (CMC) or its salts, polyethylene oxide (PEO), and the like.

[0027] [Negative electrode] The negative electrode 12 has a negative electrode core and a negative electrode mixture layer provided on the surface of the negative electrode core. The negative electrode core can be made of a foil of a metal that is stable within the potential range of the negative electrode, such as copper or a copper alloy, or a film with such a metal disposed on the surface. The negative electrode mixture layer is preferably provided on both sides of the negative electrode core, excluding the core exposed portion where the negative electrode lead is connected. The thickness of the negative electrode mixture layer is, for example, 50 μm to 150 μm on one side of the negative electrode core. The negative electrode 12 can be produced by applying a negative electrode mixture slurry containing a negative electrode active material and a binder to the surface of the negative electrode core, drying the coating, and then compressing it to form a negative electrode mixture layer on both sides of the negative electrode core.

[0028] FIG. 2 is a diagram schematically illustrating the cross section of a composite particle 30. The composite particle 30 is contained in the negative electrode mixture layer. The composite particle 30 is a composite containing silicon (Si). When the battery is charged, the Si in the particle alloys with Li and absorbs Li, and when the battery is discharged, the absorbed Li is released. The composite particle 30 can absorb more Li per unit volume than carbon materials such as graphite, and therefore functions as a high-capacity negative electrode active material. The negative electrode mixture layer may contain a carbon-based active material together with the composite particle 30. The combined use of the composite particle 30, which is a Si-based active material, and the carbon-based active material makes it easy to realize a nonaqueous electrolyte secondary battery with high capacity and excellent cycle characteristics.

[0029] Graphite is preferably used as the carbon-based active material. Graphite may be, for example, natural graphite such as spherical graphite or flake graphite, artificial graphite such as massive artificial graphite or graphitized mesophase carbon microbeads, or a mixture thereof. The volume-based median diameter (D50) of graphite is, for example, 10 μm to 30 μm, preferably 18 μm to 24 μm. D50 refers to the particle size at which the cumulative frequency of the smallest particle size in the volume-based particle size distribution is 50%, and is also called the median diameter. The particle size distribution can be measured using a laser diffraction particle size distribution analyzer (e.g., MT3000II, manufactured by Microtrack Bell Corporation) using water as a dispersion medium.

[0030] The binder contained in the negative electrode mixture layer can be, as in the case of the positive electrode 11, fluororesin, PAN, polyimide resin, acrylic resin, polyolefin resin, etc., but is preferably styrene butadiene rubber (SBR), polyacrylic acid (PAA) or a salt thereof, CMC or a salt thereof, polyvinyl alcohol (PVA), etc. Among these, PAA or a salt thereof is preferably used. PAA has good compatibility with the composite particles 30, and the addition of PAA contributes to improving cycle characteristics. As the binder in the negative electrode mixture layer, it is preferable to use a combination of SBR and PAA or a salt thereof, and more preferably a combination of SBR, PAA or a salt thereof, and CMC or a salt thereof. The content of the binder is, for example, 0.1 to 5 mass % or 0.5 to 3 mass % relative to the mass of the negative electrode mixture layer.

[0031] As shown in FIG. 2 , composite particle 30 is a particle including a silicate phase 31, Si particles 32 dispersed in the silicate phase 31, and a sintered resin body 33. The sintered resin body 33 contains at least one element selected from F and N. As will be described in detail later, composite particle 30 is produced by mixing a predetermined resin with precursor particles including silicate phase 31 and Si particles 32, and sintering the mixture. Sintered resin body 33 is a sintered product of the resin mixed with the precursor particles, and is present within composite particle 30, similar to Si particles 32, and can be said to be dispersed in silicate phase 31, which is the matrix phase constituting composite particle 30.

[0032] It was found that particle cracking is specifically suppressed by introducing the sintered resin 33 into the composite particles 30. Therefore, when the composite particles 30 are used as the negative electrode active material, the cycle characteristics of the battery are significantly improved compared to when a silicon-based active material not containing the sintered resin 33 is used. As shown in the examples described later, introducing a sintered resin that does not contain F or N into the particles does not improve the cycle characteristics, so it is believed that the sintered resin 33 that contains F or N specifically modifies the particles.

[0033] The content of the composite particles 30 is, for example, 0.1% by mass or more, more preferably 0.5 to 30% by mass, or 1 to 15% by mass, or 2 to 10% by mass, relative to the mass of the negative electrode mixture layer. If the content of the composite particles 30 is within this range, it becomes easier to achieve both high capacity and good cycle characteristics. The main component of the negative electrode mixture layer is the negative electrode active material, and the content of the negative electrode active material is preferably 90 to 99% by mass, or 92 to 98% by mass, relative to the mass of the negative electrode mixture layer. The negative electrode mixture layer contains, for example, 65 to 95% by mass of a carbon-based active material.

[0034] The composite particle 30 has a particle structure in which fine Si particles 32 are dispersed in a silicate phase 31. The fine Si particles 32 are dispersed approximately uniformly in a lithium silicate matrix, and the cross section of the composite particle 30 has a sea-island structure in which the silicate phase 31 forms a sea and the Si particles 32 form islands. It is preferable that a conductive layer made of a highly conductive material is formed on the particle surface of the composite particle 30. An example of a suitable conductive layer is a carbon coating made of a carbon material. The thickness of the carbon coating is preferably 1 to 200 nm, more preferably 5 to 100 nm, in consideration of ensuring conductivity and diffusibility of Li ions into the particle interior.

[0035] The silicate phase 31 is composed of an aggregate of particles that are finer than the Si particles 32. Cracking of the composite particles 30 during charge and discharge occurs mainly when cracks occur in the silicate phase 31 due to volume changes in the Si particles 32. It is believed that in the composite particles 30, the effect of the sintered resin 33 makes it difficult for cracks to occur in the silicate phase 31, thereby suppressing particle cracking. From the viewpoint of achieving both battery capacity and cycle characteristics, the content of the Si particles 32 is preferably 35 to 75 mass% with respect to the total mass of the composite particles 30.

[0036] The silicate phase 31 has the general formula Li 2z SiO (2+z)It is preferably composed of a compound represented by (0 < z < 2). That is, the lithium silicate phase does not contain Li4SiO4 (Z = 2). Li4SiO4 is an unstable compound and reacts with water to exhibit alkalinity, which may alter Si and cause a decrease in charge-discharge capacity. From the viewpoints of stability, ease of production, Li ion conductivity, etc., it is preferable that the silicate phase 31 is mainly composed of Li2SiO3 (Z = 1) or Li2Si2O5 (Z = 1 / 2).

[0037] The carbon coating constituting the conductive layer is composed of, for example, carbon black, acetylene black, ketjen black, graphite, and a mixture of two or more of these. Examples of the method for carbon-coating the particle surface of the composite particles 30 include the CVD method using acetylene, methane, etc., and the method of mixing coal pitch, petroleum pitch, phenolic resin, etc. with the composite particles 30 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.

[0038] As described above, the resin fired body 33 is a fired product of a resin containing at least one element selected from F and N. The resin constituting the resin fired body 33 may be any resin containing F or N. Examples of suitable resins include fluorine-based resins such as PTFE, PVdF, perfluoroalkoxy alkane (PFA), perfluoroethylene propene copolymer (FEP), ethylene tetrafluoroethylene copolymer (ETFE), polychlorotrifluoroethylene (PCTFE), and N-containing resins such as polyamide, aramid, and polyimide. The resin fired body 33 has the same main skeleton as the resin before firing. The resin is partially decomposed by firing, and the mass decreases by about 5 to 20% in differential thermal analysis, but the main skeleton of the resin does not change.

[0039] The content of the sintered resin 33 is preferably 0.5 to 25 mass%, more preferably 1 to 20 mass%, and particularly preferably 1 to 15 mass%, or 2 to 10 mass%, relative to the mass of the composite particle 30. If the content of the sintered resin 33 is within this range, it becomes easier to achieve both high capacity and good cycle characteristics. Although internal voids exist in the composite particle 30, the voids are fewer than those in conventional particles. In the cross section of the composite particle 30, for example, the area occupied by the sintered resin 33 is larger than the area occupied by the voids. In this case, cracking of the composite particle 30 can be more effectively suppressed.

[0040] The sintered resin bodies 33 are not concentrated in a particular part of the silicate phase 31, but are distributed over a wide area within the silicate phase 31. The sintered resin bodies 33 are present in a fibrous or particulate form in the cross section of the composite particle 30. The sintered resin bodies 33 also cover at least a portion of the Si particles 32. In this case, the sintered resin bodies 33 efficiently absorb the volume change of the Si particles 32, and particle cracking can be more effectively suppressed. A portion of the sintered resin bodies 33 may be in contact with the particle surface of the Si particles 32 and may be interposed between the silicate phase 31 and the Si particles 32.

[0041] The porosity of the composite particle 30 is, for example, 10% or less, preferably 7% or less, more preferably 5% or less, and even more preferably 3% or less. The porosity of a particle means the proportion of voids in the cross-section of the composite particle 30 relative to the area of ​​the composite particle 30. The porosity can be determined by observing the cross-section of the particle using a scanning electron microscope (SEM).

[0042] The D50 of the composite particles 30 is, for example, 5 μm to 25 μm, and is preferably smaller than the D50 of graphite. The D50 of the composite particles 30 is preferably 5 μm to 20 μm, and more preferably 5 μm to 15 μm. If the particle size of the composite particles 30 is within this range, it becomes easier to achieve both high capacity and good cycle characteristics.

[0043] The composite particles 30 are produced by mixing precursor particles containing a silicate phase 31 and Si particles 32 dispersed in the silicate phase 31 with a resin containing at least one element selected from F and N to produce a mixture, and then firing the mixture. The precursor particles are produced by mixing lithium silicate particles that will become the silicate phase 31 with raw material Si that will become the Si particles 32, and pulverizing the mixture in an inert gas atmosphere. A ball mill can be used to pulverize the mixture, for example, at 200 rpm for 50 hours. The lithium silicate particles are produced by mixing silicon dioxide and lithium carbonate, followed by firing the mixture in air.

[0044] By mixing the precursor particles and resin, the resin penetrates into the voids in the precursor particles and is incorporated into the precursor particles. A ball mill can be used to mix the resin, and mixing is performed, for example, at 200 rpm for 1 hour. As mentioned above, the resin used may be a fluororesin such as PTFE or PVdF, or a resin containing N such as polyimide. The mixture of precursor particles and resin is fired at a temperature of 300 to 1000°C in an inert gas atmosphere. Suitable firing temperatures include 400 to 800°C or 500 to 700°C. The firing time is, for example, 2 to 6 hours or 3 to 5 hours. Through this firing, the resin incorporated into the precursor particles becomes a fired resin body 33.

[0045] The mixture may be fired under pressurized conditions, but is preferably fired under atmospheric pressure to reduce process costs. In other words, special high-temperature and high-pressure conditions are not required for the production of composite particles 30. Composite particles 30 can be produced by a simple method in which precursor particles containing silicate phase 31 and Si particles 32 are mixed with a resin containing at least one element selected from F and N, and then the mixture is fired. After firing the mixture, a pulverization treatment may be performed, if necessary. In this manner, composite particles 30 containing silicate phase 31, Si particles 32, and a fired resin body 33 are obtained.

[0046] As described above, it is preferable to form a carbon coating on the surface of the composite particles 30. For example, the composite particles 30 are mixed with a carbon raw material such as coal pitch, and then heat-treated to form a carbon coating that covers the surface of the composite particles 30. An example of a suitable firing temperature is 500 to 700°C, which is set to a temperature lower than the firing temperature of the mixture.

[0047] [Separator] A porous sheet having ion permeability and insulating properties is used for the separator 13. Specific examples of the porous sheet include a microporous thin film, a woven fabric, and a nonwoven fabric. Suitable materials for the separator 13 include polyolefins such as polyethylene, polypropylene, and copolymers of ethylene and α-olefins, and cellulose. The separator 13 may have either a single-layer structure or a laminated structure. A heat-resistant layer containing inorganic particles, or a heat-resistant layer made of a highly heat-resistant resin such as an aramid resin, polyimide, or polyamideimide, may be formed on the surface of the separator 13. [Example]

[0048] The present disclosure will be further described below with reference to examples, but the present disclosure is not limited to these examples.

[0049] Example 1 [Preparation of composite particles] Silicon dioxide and lithium carbonate were mixed to a Si / Li atomic ratio of 1.05. The mixture was then calcined in air at 950°C for 10 hours to obtain lithium silicate with the formula Li2O·2.1SiO2 (x = 2.1). The resulting lithium silicate was then crushed to a D50 of 10 μm. The 10 μm lithium silicate was mixed with raw silicon (3N, D50:10 μm) in a 50:50 mass ratio. The mixture was then loaded into a 500 mL stainless steel pot (500 mL volume) of a planetary ball mill (Fritsch, P-5). The pot was then filled with 24 20 mm diameter stainless steel balls, closed with a lid, and crushed for 50 hours at 200 rpm in an inert gas atmosphere to produce precursor particles for the composite particles.

[0050] Next, a predetermined amount of polyimide (PI) was added to the pot containing the precursor particles under an inert gas atmosphere and mixed at 200 rpm for 1 hour. The amount of polyimide added was adjusted so that the content of the sintered resin was 1% by mass relative to the composite particles. This mixing process allowed the resin to be incorporated into the voids of the precursor particles. The precursor particles mixed with the resin were removed from the pot and fired at 600°C for 4 hours in an inert gas atmosphere to obtain composite particles containing a silicate phase, silicon particles dispersed in the silicate phase, and the sintered resin. The composite particles were pulverized, passed through a 40 μm mesh, and then mixed with coal pitch (MCP250, manufactured by JFE Chemical Corporation). The mixture was fired at 600°C for 1 hour in an inert gas atmosphere to form a conductive layer made of conductive carbon on the surface of the composite particles. The amount of conductive carbon coating was 5% by mass relative to the total mass of the composite particles and conductive carbon. Then, using a sieve, composite particles A1 (D50: 10 μm) with a conductive layer were obtained.

[0051] [Preparation of negative electrode] As the negative electrode active material, graphite with a D50 of 22 μm was used together with composite particles A1. Graphite, composite particles A1, a dispersion of polyacrylic acid (PAA), carboxymethyl cellulose (CMC), and styrene butadiene rubber (SBR) were mixed in a solids mass ratio of 92.2:4.8:1:1:1, and the mixture was kneaded in ion-exchanged water to prepare a negative electrode mixture slurry. This negative electrode mixture slurry was applied to both sides of a negative electrode core made of copper foil by a doctor blade method, and the coating was dried and then compressed using a rolling roller to produce a negative electrode with a negative electrode mixture layer formed on both sides of the negative electrode core.

[0052] [Preparation of positive electrode] Lithium nickel-cobalt aluminate was used as the positive electrode active material. The positive electrode active material, acetylene black, and polyvinylidene fluoride were mixed in a mass ratio of 95:2.5:2.5, and N-methyl-2-pyrrolidone (NMP) was added. The mixture was then stirred using a mixer (TK Hibismix, manufactured by Primix Corporation) to prepare a positive electrode mixture slurry. This positive electrode mixture slurry was applied to both sides of a positive electrode core made of aluminum foil using a doctor blade method. The coating was dried and then compressed using a rolling roller to produce a positive electrode with a positive electrode mixture layer formed on both sides of the positive electrode core.

[0053] [Preparation of non-aqueous electrolyte] A non-aqueous electrolyte solution was prepared by dissolving LiPF6 at a concentration of 1 mol / L in a mixed solvent of ethylene carbonate (EC) and diethyl carbonate (DEC) mixed at a volume ratio of 3:7 (25°C).

[0054] [Fabrication of non-aqueous electrolyte secondary battery] Leads were attached to the positive and negative electrodes, respectively, and the positive and negative electrodes were spirally wound with a separator interposed therebetween so that the leads were positioned at the outermost periphery to produce a wound electrode assembly. This electrode assembly was inserted into an exterior case made of aluminum laminate film and vacuum dried at 105°C for 2 hours. A nonaqueous electrolyte solution was then poured into the exterior case, and the opening of the exterior case was sealed to obtain a battery.

[0055] <Example 2> Composite particles A2 and a battery were produced in the same manner as in Example 1, except that the amount of polyimide added to the precursor particles was changed so that the content of the baked resin was 5% by mass.

[0056] Example 3 Composite particles A3 and a battery were produced in the same manner as in Example 1, except that in the production of composite particles, the resin mixed with the precursor particles was changed from polyimide (PI) to polytetrafluoroethylene (PTFE).

[0057] Example 4 Composite particles A4 and a battery were produced in the same manner as in Example 2, except that in the production of composite particles, the resin mixed with the precursor particles was changed from polyimide (PI) to PTFE.

[0058] <Example 5> Composite particles A5 and a battery were produced in the same manner as in Example 2, except that in the production of composite particles, the resin mixed with the precursor particles was changed from polyimide (PI) to polyvinylidene fluoride (PVdF).

[0059] Example 6 Composite particles A6 and a battery were produced in the same manner as in Example 1, except that the amount of PTFE added to the precursor particles was changed so that the content of the baked resin was 8% by mass.

[0060] Example 7 Composite particles A7 and a battery were produced in the same manner as in Example 1, except that the amount of PTFE added to the precursor particles was changed so that the content of the baked resin was 20 mass %.

[0061] <Comparative Example 1> Composite particles B1 and a battery were produced in the same manner as in Example 2, except that polyimide (PI) was not mixed with the precursor particles in the production of the composite particles.

[0062] <Comparative Example 2> Composite particles B2 and a battery were produced in the same manner as in Example 2, except that in the production of composite particles, the resin mixed with the precursor particles was changed from polyimide (PI) to polyvinyl alcohol (PVA).

[0063] <Comparative Example 3> Composite particles B3 and a battery were produced in the same manner as in Example 2, except that in the production of composite particles, the resin mixed with the precursor particles was changed from polyimide (PI) to polyacrylic acid (PAA).

[0064] The charge-discharge cycle characteristics of each battery in the examples and comparative examples were evaluated by the following method. The evaluation results are shown in Table 1 together with the type of resin mixed with the precursor particles and the content of the baked resin.

[0065] [Evaluation of cycle characteristics] A cycle test was carried out on each battery under the following charge / discharge conditions. The ratio of the discharge capacity at the 200th cycle to the discharge capacity at the 1st cycle was calculated as the capacity retention rate. <Charging> The battery was charged at a constant current of 1 It (800 mA) until the battery voltage reached 4.2 V, and then charged at a constant voltage of 4.2 V until the current reached 1 / 20 It (40 mA). <Discharge> The battery was discharged at a constant current of 1 It (800 mA) until the battery voltage reached 2.75 V. The rest period between charge and discharge was 10 minutes.

[0066] [Table 1]

[0067] As shown in Table 1, all of the batteries of the examples have higher capacity retention rates and superior charge-discharge cycle characteristics compared to the batteries of the comparative examples. The results shown in Table 1 show that by introducing a sintered resin containing at least one element selected from F and N into composite particles containing a lithium silicate phase, the composite particles are specifically modified, resulting in significantly improved cycle characteristics.

[0068] Among the examples, when F-containing PTFE and PVdF were used as the resin mixed with the precursor particles (Examples 3 to 7), and when the content of the baked resin in the composite particles was 1 to 8 mass% (Examples 1 to 6), the effect of improving the cycle characteristics was more remarkable. Among them, the batteries of Examples 4 to 6 had particularly excellent cycle characteristics.

[0069] When the precursor particles were not mixed with the resin, i.e., when the composite particles did not contain a sintered resin, the capacity retention rate decreased to 60% (Comparative Example 1). Furthermore, even when a mixture of precursor particles with a resin that does not contain F or N, such as PVA or PAA, was used, no improvement in the capacity retention rate was obtained, and when PVA was mixed, the capacity retention rate was lower than when no resin was mixed (Comparative Examples 2 and 3). [Explanation of symbols]

[0070] 10 non-aqueous electrolyte secondary battery, 11 positive electrode, 12 negative electrode, 13 separator, 14 electrode body, 16 outer can, 17 sealing body, 18, 19 insulating plate, 20 positive electrode lead, 21 negative electrode lead, 22 grooved portion, 23 internal terminal plate, 24 lower valve body, 25 insulating member, 26 upper valve body, 27 cap, 28 gasket, 30 composite particle, 31 silicate phase, 32 Si particle, 33 resin sintered body

Claims

1. Composite particles comprising a lithium silicate phase, silicon particles dispersed in the lithium silicate phase, and a resin sintered body, the composite particles having a volume-based median diameter of 5 to 25 μm, the content of the silicon particles is 35 to 75 mass% relative to the mass of the composite particles; the content of the baked resin is 0.5 to 25% by mass relative to the mass of the composite particles, The baked resin composite particles contain at least one element selected from F and N.

2. 2. The composite particle for a negative electrode active material according to claim 1, wherein the content of said baked resin is 1 to 20% by mass with respect to the mass of said composite particle.

3. 3. The composite particle for a negative electrode active material according to claim 1, wherein the baked resin body covers at least a part of the silicon particle.

4. 4. The composite particle for a negative electrode active material according to claim 1, wherein the composite particle has a porosity of 10% or less.

5. A non-aqueous electrolyte secondary battery comprising a negative electrode containing the composite particles for a negative electrode active material according to any one of claims 1 to 4, a positive electrode, and a non-aqueous electrolyte.

6. 6. The nonaqueous electrolyte secondary battery according to claim 5, wherein the content of the composite particles is 0.5 to 30% by mass relative to the mass of the negative electrode mixture layer.

7. The nonaqueous electrolyte secondary battery according to claim 6 , wherein the mixture layer contains polyacrylic acid.

8. a step of preparing a mixture by mixing precursor particles containing a lithium silicate phase and silicon particles dispersed in the lithium silicate phase with a resin containing at least one element selected from F and N; a step of baking the mixture to form the resin into a baked resin body; A method for producing composite particles having a volume-based median diameter of 5 to 25 μm, comprising: a content of the silicon particles is 35 to 75 mass % relative to a mass of the composite particles, and a content of the sintered resin is 0.5 to 25 mass % relative to a mass of the composite particles.

9. 9. The method for producing composite particles for a negative electrode active material according to claim 8, wherein the mixture is fired at a temperature of 300 to 1000° C. in an inert gas atmosphere.

10. The method for producing composite particles for a negative electrode active material according to claim 8 or 9, wherein the mixture is fired under atmospheric pressure.

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