Negative electrode active material composite particles, negative electrode mixture, lithium ion battery, and method for producing negative electrode active material composite particles

Abstract

To provide: negative electrode active material composite particles including silicon particles and a binder resin and having high cycle properties; a negative electrode mixture including such negative electrode active material composite particles; a lithium ion battery including such negative electrode active material composite particles; a production method for such negative electrode active material composite particles; and a production method for such composite particles.SOLUTION: Negative electrode active material composite particles of the present disclosure include surface-modified negative electrode active material particles and a binder resin. In addition, the surface-modified negative electrode active material particles included in the negative electrode active material composite particles of the present disclosure have a silicon particle and an organic group that modifies the surface of the silicon particle. Furthermore, the ratio of the mass of the binder resin included in the negative electrode active material composite particles of the present disclosure to the total mass of the surface-modified negative electrode active material particles and the binder resin is 15 mass% or less.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present disclosure relates to a negative electrode active material composite particle, a negative electrode mixture, a lithium ion battery, and a method for producing a negative electrode active material composite particle. [Background technology]

[0002] In recent years, the development of batteries has been actively pursued. For example, in the automotive industry, development of batteries for use in electric vehicles or hybrid vehicles is progressing. Silicon is known as a negative electrode active material used in batteries.

[0003] Patent Document 1 discloses an anode active material for an all-solid-state battery, which is composed of anode active material particles that are a composite of silicon and carbon and a binder, and is in the form of granulated particles having a median diameter (D50) of 25 μm or less. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Publication No. 2019-021571 Summary of the Invention [Problem to be solved by the invention]

[0005] In composite particles containing silicon particles and a binder resin, improvements in cycle characteristics are desired.

[0006] An object of the present disclosure is to provide anode active material composite particles that contain silicon particles and a binder resin and have high cycle characteristics, anode mixtures that contain such anode active material composite particles, lithium ion batteries that contain such anode active material composite particles, and a method for producing such anode active material composite particles. [Means for solving the problem]

[0007] The present inventors have found that the above problems can be solved by the following means. <Aspect 1> The negative electrode active material includes surface-modified negative electrode active material particles and a binder resin, the surface-modified negative electrode active material particles have silicon particles and organic groups modifying the surfaces of the silicon particles; and the ratio of the mass of the binder resin to the total mass of the surface-modified negative electrode active material particles and the binder resin is 15 mass% or less; Negative active material composite particles. <Aspect 2> 2. The negative electrode active material composite particle according to aspect 1, wherein the organic group is an alkyl group. <Aspect 3> A negative electrode active material composite particle according to aspect 2, wherein the alkyl group has 5 or more and 20 or less carbon atoms. <Aspect 4> Anode active material composite particles according to aspect 2, wherein the surface-modified cathode active material particles have a silicon-carbon bond, the silicon being silicon of the silicon particles, and the carbon being carbon of an alkyl group. <Aspect 5> A negative electrode mixture comprising the negative electrode active material composite particles according to any one of the first to fourth aspects. <Aspect 6> a negative electrode active material layer; and The negative electrode active material layer contains the negative electrode mixture according to aspect 5. Lithium-ion battery. <Aspect 7> (a) modifying the surfaces of silicon particles with organic groups to obtain surface-modified negative electrode active material particles; (b) forming droplets from a slurry containing the surface-modified negative electrode active material particles, a binder resin, and a solvent to obtain slurry droplets; and (c) flash-drying the slurry droplets in heated gas; A method for producing negative electrode active material composite particles, comprising: <Aspect 8> Aspect 8. The method of aspect 7, wherein in step (a), the surfaces of the silicon particles are modified with the organic groups by a hydrosilylation reaction in which the organic groups are added to hydrosilyl groups on the surfaces of the silicon particles. [Effects of the Invention]

[0008] According to the present disclosure, it is possible to provide anode active material composite particles that contain silicon particles and a binder resin and have high cycle characteristics, anode mixtures that contain such anode active material composite particles, lithium ion batteries that contain such anode active material composite particles, and methods for producing such anode active material composite particles. DETAILED DESCRIPTION OF THE INVENTION

[0009] Hereinafter, embodiments of the present disclosure will be described in detail. Note that the present disclosure is not limited to the following embodiments, and various modifications can be made within the scope of the present disclosure.

[0010] 《Negative electrode active material composite particles》 The negative electrode active material composite particles of the present disclosure include surface-modified negative electrode active material particles and a binder resin. The surface-modified negative electrode active material particles contained in the negative electrode active material composite particles of the present disclosure have silicon particles and organic groups that modify the surfaces of the silicon particles. The ratio of the mass of the binder resin contained in the negative electrode active material composite particles of the present disclosure to the total mass of the surface-modified negative electrode active material particles and the binder resin is 15 mass% or less.

[0011] The present inventors believe that one of the reasons for the poor cycle performance of composite particles containing silicon particles and a binder resin is that the silicon particles tend to aggregate within the composite particles. Specifically, without intending to be bound by any theory, it is believed that when the silicon particles aggregate within the composite particles, the silicon particles are unable to make sufficient contact with the binder resin, causing cracks or the like in the composite particles, resulting in poor cycle performance.

[0012] In response to this, the present inventors have discovered that in composite particles containing silicon particles and a binder resin, the cycle characteristics of the composite particles can be improved by modifying the surfaces of the silicon particles with organic groups, particularly alkyl groups, and by reducing the amount of binder resin.

[0013] The reason for this, without intending to be bound by any theory, is presumed to be as follows. That is, by modifying the surfaces of the silicon particles with organic groups, particularly alkyl groups, the dispersibility of the silicon particles in the composite particles is improved, and it is thought that even when the amount of binder resin in the composite particles is reduced below a certain level, the binder resin can be appropriately arranged even inside the composite particles. In this way, when the proportion of binder resin in the composite particles is below a certain level, the resistance of the composite particles as a whole is reduced, and therefore the cycle characteristics are thought to be improved.

[0014] Furthermore, sufficient contact between the silicon particles and the binder resin prevents aggregation of the silicon particles in the composite particles, and appropriate voids are formed between the silicon particles. This allows the voids to mitigate volumetric changes that occur when the silicon particles expand during charging and discharging, thereby preventing cracking of the composite particles and improving cycle characteristics.

[0015] In the present disclosure, whether or not the negative electrode active material composite particles have been properly prepared can be confirmed by the degree of particle size variation. That is, the value of (D90-D10) / D50 can be used as an index of particle size variation, and if this value is less than 4.0, 3.5 or less, 3.0 or less, 2.5 or less, or 2.0 or less, it can be determined that the negative electrode active material composite particles have been properly prepared.

[0016] <Surface modified negative electrode active material particles> The negative electrode active material composite particles of the present disclosure include surface-modified negative electrode active material particles and a binder resin.

[0017] (silicon particles) The surface-modified negative electrode active material particles include silicon particles.

[0018] The composition of the silicon particles is not particularly limited. The proportion of silicon element in all elements contained in the silicon particles may be, for example, 50 mol % or more, 70 mol % or more, or 90 mol % or more.

[0019] The silicon particles may contain other elements in addition to the element Si, such as element Li, etc. Examples of other elements include element Li, element Sn, element Fe, element Co, element Ni, element Ti, element Cr, element B, and element P.

[0020] The silicon particles may contain impurities such as oxides.

[0021] The silicon particles may be amorphous or crystalline, and the crystalline phase contained in the silicon particles is not particularly limited.

[0022] The shape and size of the silicon particles are not particularly limited. The average particle diameter of the silicon particles may be, for example, 30 nm or more, 50 nm or more, 100 nm or more, or 150 nm or more, or 10 μm or less, 5 μm or less, 3 μm or less, 2 μm or less, or 1 μm or less. The average particle diameter can be determined by observation using an electron microscope such as an SEM, and is determined, for example, as the average value of the maximum Feret diameters of a plurality of particles. The number of samples is preferably large, for example, 20 or more, 50 or more, or even 100 or more. The average particle diameter can be appropriately adjusted, for example, by appropriately changing the production conditions of the silicon particles described below or by performing a classification process.

[0023] The silicon particles contained in the surface-modified negative electrode active material particles may be porous silicon particles. The porous silicon particles contain silicon having a plurality of voids. There are no particular limitations on the shape of the voids in the porous silicon particles.

[0024] The porous silicon particles may be particles containing nanoporous silicon, which refers to silicon containing a plurality of pores with diameters on the nanometer order (less than 1000 nm, preferably 100 nm or less).

[0025] (organic group) The surface-modified negative electrode active material particles of the present disclosure have organic groups that modify the surfaces of the silicon particles.

[0026] The organic group may be a group consisting of one or more carbon atoms and one or more atoms selected from the group consisting of H, O, S, N, B, P, Si, and halogen atoms, and is preferably an alkyl group.

[0027] The number of carbon atoms in the alkyl group may be 5 or more and 20 or less. The number of carbon atoms may also be 6 or more, 19 or less, or 18 or less. The alkyl group may have a linear, branched, or cyclic structure, but is preferably a linear alkyl group. Specific examples of such alkyl groups include pentyl, hexyl, heptyl, octyl, nonyl, decyl, undecyl, dodecyl, tridecyl, tetradecyl, pentadecyl, hexadecyl, heptadecyl, octadecyl, nonadecyl, and eicosyl groups.

[0028] The modification of the surfaces of silicon particles with alkyl groups can be confirmed by measuring the carbon and hydrogen amounts on the surfaces of the silicon particles after the surface modification treatment with alkyl groups, and by analyzing the surface functional group state of the particles, etc. Examples of methods for measuring the surface carbon amount include high-frequency combustion infrared absorption spectroscopy, examples of methods for measuring the surface hydrogen amount include inert gas induction infrared absorption spectroscopy, and examples of methods for analyzing the surface functional group state include, but are not limited to, time-of-flight secondary ion mass spectrometry (TOF-SIMS).

[0029] The surface-modified negative electrode active material particles may have a silicon-carbon bond, in which the silicon is silicon of the silicon particles and the carbon is carbon of the alkyl group, i.e., the carbon of the alkyl group may be directly bonded to the silicon on the surface of the silicon particles.

[0030] <Binder resin> The negative electrode active material composite particles of the present disclosure contain a binder resin. Surface-modified negative electrode active material particles The binder is not particularly limited in type. For example, it may be selected from butadiene rubber (BR)-based binders, butylene rubber (IIR)-based binders, acrylate butadiene rubber (ABR)-based binders, styrene butadiene rubber (SBR)-based binders, polyvinylidene fluoride (PVdF)-based binders, polytetrafluoroethylene (PTFE)-based binders, polyimide (PI)-based binders, carboxymethyl cellulose (CMC)-based binders, polyacrylate-based binders, polyacrylic acid ester-based binders, etc. Only one type of binder may be used alone, or two or more types may be used in combination.

[0031] The ratio of the mass of the binder resin contained in the negative electrode active material composite particles of the present disclosure to the total mass of the surface-modified negative electrode active material particles and the binder resin is 15% by mass or less. This ratio may be 14% by mass or less, 13% by mass or less, 12% by mass or less, 11% by mass or less, or 10% by mass or less. Even with such a small amount of binder resin, the negative electrode active material composite particles of the present disclosure provide the resulting battery with high cycle characteristics.

[0032] <<Method for producing negative electrode active material composite particles>> The method of the present disclosure for producing negative electrode active material composite particles includes: (a) modifying the surfaces of silicon particles with organic groups to obtain surface-modified negative electrode active material particles; (b) forming droplets from a slurry containing the surface-modified negative electrode active material particles, a binder resin, and a solvent to obtain slurry droplets; and (c) flash-drying the slurry droplets in a heated gas.

[0033] According to this method, even when drying is performed by spray drying in step (c), for example, the dispersibility of the silicon particles in the composite particles is improved, and the binder resin is present in the center of the composite particles, allowing the silicon particles to be appropriately bound together, thereby producing the composite particles of the present disclosure.

[0034] In step (a), the surface of the silicon particles may be modified with an organic group by a hydrosilylation reaction in which an organic group is added to the hydrosilyl group on the surface of the silicon particles. For example, when the organic group is an alkyl group, a method for carrying out this hydrosilylation reaction includes a method in which hydrogen fluoride-treated silicon particles, an alkene, and optionally a catalyst are mixed in a solvent and reacted.

[0035] One method for obtaining hydrogen fluoride-treated silicon particles is to add a hydrogen fluoride solution to silicon particles dispersed in a dispersion medium, stir the mixture, separate the resulting suspension into solid and liquid, and then dry it. This removes silicon oxide from the surface of the silicon particles and simultaneously generates hydrosilyl groups (-SiH) on the surface of the silicon particles.

[0036] This hydrosilyl group and the double bond of the alkene can undergo a hydrosilylation reaction.

[0037] With regard to alkenes, alkenes with the corresponding carbon numbers can be used with reference to the above description of alkyl groups in the present disclosure.

[0038] 《Negative electrode mixture》 The negative electrode mixture of the present disclosure includes the negative electrode active material composite particles of the present disclosure, and optionally includes an electrolyte, a conductive additive, and a binder.

[0039] In the present disclosure, the term "negative electrode mixture" refers to a composition that can constitute a negative electrode active material layer either as is or by further containing other components. In the present disclosure, the term "negative electrode mixture slurry" refers to a slurry that contains a dispersion medium in addition to the "negative electrode mixture" and that can be applied and dried to form a negative electrode active material layer.

[0040] <Negative electrode active material> The negative electrode active material includes the negative electrode active material composite particles of the present disclosure.

[0041] <Electrolyte> The material of the solid electrolyte is not particularly limited, and any material that can be used as a solid electrolyte for a lithium ion battery can be used. For example, the solid electrolyte may be a sulfide solid electrolyte.

[0042] Examples of sulfide solid electrolytes include, but are not limited to, amorphous sulfide solid electrolytes, crystalline sulfide solid electrolytes, and argyrodite-type solid electrolytes. Specific examples of sulfide solid electrolytes include Li2S-P2S5-based (Li7P3S 11 , Li3PS4, Li8P2S9, etc.), Li2S-SiS2, LiI-Li2S-SiS2, LiI-Li2S-P2S5, LiI-LiBr-Li2S-P2S5, Li2S-P2S5-GeS2 (Li 13 GeP3S 16 , Li 10 GeP2S 12 ), LiI-Li2S-P2O5, LiI-Li3PO4-P2S5, Li 7-x PS 6-x Cl x etc.; or combinations thereof, but are not limited to these.

[0043] The sulfide solid electrolyte may be glass or crystallized glass (glass ceramic).

[0044] When the negative electrode composite material contains a solid electrolyte, the mass ratio of the negative electrode active material composite particles to the solid electrolyte in the negative electrode composite material (mass of the negative electrode active material composite particles: mass of the solid electrolyte) is preferably 85:15 to 30:70, and more preferably 80:20 to 40:60.

[0045] The electrolyte preferably contains a supporting salt and a solvent.

[0046] Examples of supporting salts (lithium salts) for the electrolyte solution having lithium ion conductivity include inorganic lithium salts such as LiPF6, LiBF4, LiClO4, and LiAsF6, and organic lithium salts such as LiCF3SO3, LiN(CF3SO2)2, LiN(C2F5SO2)2, LiN(FSO2)2, and LiC(CF3SO2)3.

[0047] Examples of solvents used in the electrolytic solution include cyclic esters (cyclic carbonates) such as ethylene carbonate (EC), propylene carbonate (PC), and butylene carbonate (BC), and chain esters (chain carbonates) such as dimethyl carbonate (DMC), diethyl carbonate (DEC), and ethyl methyl carbonate (EMC).The electrolytic solution preferably contains two or more solvents.

[0048] <Conductive additive> The conductive additive is not particularly limited, and may be, for example, VGCF (Vapor Grown Carbon Fiber), acetylene black (AB), Ketjen black (KB), carbon nanotubes (CNT), carbon nanofibers (CNF), or the like, but is not limited thereto.

[0049] <binder> The binder is not particularly limited, and may be, for example, but not limited to, a material such as polyvinylidene fluoride (PVdF), butadiene rubber (BR), or styrene butadiene rubber (SBR), or a combination thereof.

[0050] Lithium-ion battery The lithium ion battery of the present disclosure has a negative electrode active material layer. The negative electrode active material layer contains the negative electrode composite of the present disclosure. The lithium ion battery of the present disclosure may have, in this order, a negative electrode current collector layer, a negative electrode active material layer containing the negative electrode composite of the present disclosure, a solid electrolyte layer, a positive electrode active material layer, and a positive electrode current collector layer.

[0051] The lithium ion battery of the present disclosure may be a liquid-based battery containing an electrolytic solution as an electrolyte layer, or may be a solid-state battery having a solid electrolyte layer as an electrolyte layer. In the present disclosure, the term "solid-state battery" refers to a battery that uses at least a solid electrolyte as an electrolyte. Therefore, a solid-state battery may use a combination of a solid electrolyte and a liquid electrolyte as an electrolyte. The solid-state battery of the present disclosure may also be an all-solid-state battery, i.e., a battery that uses only a solid electrolyte as an electrolyte.

[0052] The lithium ion battery may be a primary battery or a secondary battery.

[0053] Examples of the shape of the lithium ion battery include coin type, laminate type, cylindrical type, and square type.

[0054] <Negative electrode current collector layer> The material used for the negative electrode current collector layer is not particularly limited, and any material that can be used as a negative electrode current collector for a battery can be appropriately adopted. For example, copper, a copper alloy, and copper plated or vapor-deposited with nickel, chromium, carbon, or the like may be used, but is not limited to these.

[0055] The shape of the negative electrode current collector layer is not particularly limited, and examples thereof include foil, plate, mesh, etc. Among these, foil is preferred.

[0056] <Negative electrode active material layer> The negative electrode active material layer contains the negative electrode mixture of the present disclosure. For the negative electrode mixture, reference can be made to the above description of the negative electrode mixture of the present disclosure.

[0057] The thickness of the negative electrode active material layer is, for example, 0.1 μm to 1000 μm, preferably 1 μm to 100 μm, and more preferably 30 μm to 100 μm.

[0058] <Electrolyte layer> The electrolyte layer contains at least an electrolyte. In addition to the electrolyte, the electrolyte layer may also contain a binder, etc., as necessary. For the electrolyte and the binder, reference can be made to the above description of the negative electrode mixture of the present disclosure.

[0059] The thickness of the electrolyte layer is, for example, 0.1 to 300 μm, and preferably 0.1 to 100 μm.

[0060] <Cathode active material layer> The positive electrode active material layer is a layer containing a positive electrode active material, and optionally an electrolyte, a conductive additive, a binder, and the like.

[0061] The material of the positive electrode active material is not particularly limited. For example, the positive electrode active material may be lithium cobalt oxide (LiCoO), lithium nickel oxide (LiNiO), lithium manganese oxide (LiMnO), LiCo 1 / 3 Ni 1 / 3 Mn 1 / 3 O2, Li 1+x Mn 2-x-y Heteroelement-substituted Li-Mn spinel with a composition represented by MyO4 (M is one or more metal elements selected from Al, Mg, Co, Fe, Ni, and Zn), lithium titanate (Li x TiO y ), lithium metal phosphate (LiMPO4, M is one or more metals selected from Fe, Mn, Co, and Ni), etc., but are not limited to these.

[0062] The positive electrode active material may have a coating layer. The coating layer is a layer containing a substance that has lithium ion conductivity, low reactivity with the positive electrode active material and solid electrolyte, and can maintain the shape of the coating layer without flowing even when in contact with the active material and solid electrolyte. Specific examples of materials that constitute the coating layer include LiNbO3 and Li4Ti5O12 , Li3PO4, etc., but are not limited to these.

[0063] The positive electrode active material may be, for example, particulate. The average particle size (D50) of the positive electrode active material is not particularly limited, but may be, for example, 10 nm or more, or 100 nm or more. On the other hand, the average particle size (D50) of the positive electrode active material is, for example, 50 μm or less, or may be 20 μm or less. The average particle size (D50) can be calculated, for example, from measurements using a laser diffraction particle size distribution analyzer or a scanning electron microscope (SEM).

[0064] For the electrolyte, conductive additive, and binder, reference can be made to the above descriptions regarding the negative electrode mixture of the present disclosure.

[0065] When the positive electrode active material layer contains a solid electrolyte, the mass ratio of the positive electrode active material to the solid electrolyte in the positive electrode active material layer (mass of positive electrode active material: mass of solid electrolyte) is preferably 85:15 to 30:70, and more preferably 80:20 to 50:50.

[0066] The thickness of the positive electrode active material layer is, for example, 0.1 μm to 1000 μm, preferably 1 μm to 100 μm, and more preferably 30 μm to 100 μm.

[0067] <Positive electrode current collector layer> The material used for the positive electrode current collector layer is not particularly limited, and any material that can be used as a positive electrode current collector for a battery can be appropriately adopted. Examples of the material include, but are not limited to, SUS, nickel, chromium, gold, platinum, aluminum, iron, titanium, zinc, and the like, as well as these metals plated or vapor-deposited with nickel, chromium, carbon, and the like.

[0068] The shape of the positive electrode current collector layer is not particularly limited, and examples thereof include foil, plate, mesh, etc. Among these, foil is preferred. [Example]

[0069] <<Preparation of Surface-Modified Negative Electrode Active Material Particles>> <Adjusting silicon particles> (Comparative Synthesis Example 1) Silicon particles were prepared as a silicon (Si) source. The silicon particles and metallic lithium (Li) were weighed out in a molar ratio of Li / Si = 4.0, and the weighed silicon particles and Li were mixed in a mortar in an argon atmosphere to obtain a lithium silicon (LiSi) alloy. The obtained LiSi alloy was reacted with ethanol in an argon atmosphere to obtain porous silicon particles.

[0070] 10 g of porous silicon particles were dispersed in 100 mL of ethanol. 3.7 mL of a 46 wt% aqueous solution of hydrogen fluoride (HF) was then added dropwise and stirred at room temperature for 3 hours. After stirring, the solution was suction filtered, washed with 50 mL of ethanol, and filtered, a process repeated eight times. The resulting solid was vacuum dried at 100°C for 12 hours to obtain HF-treated porous silicon particles.

[0071] Modification with alkyl groups (Synthesis Example 1) In an argon (Ar)-purged glove box, 2 g of the HF-treated porous silicon particles, 30 g of mesitylene (Nacalai Tesque), 0.18 g of 1-hexene (Fujifilm Wako Pure Chemical Industries, Ltd.), and 10 μL of a platinum(0)-1,3-divinyl-1,1,3,3-tetramethyldisiloxane complex xylene solution (Sigma-Aldrich) were weighed and placed in a sealed reaction vessel. The mixture was stirred at 50 °C for 24 hours to react (hydrosilylation reaction). The stirred solution was suction filtered, washed with 30 mL of mesitylene, and filtered three times, followed by one wash with 30 mL of ethanol and filtration. The resulting solid was vacuum-dried at 100 °C for 12 hours to convert some of the Si-H bonds on the silicon particle surface to Si-CH. 13 That is, the surface of the silicon particles was covered with hexyl groups (-CH 13 ) to prepare surface-modified negative electrode active material particles.

[0072] (Synthesis Example 2) The same procedure as in Synthesis Example 1 was repeated except that the alkene was changed to 0.30 g of 1-decene (Fujifilm Wako Pure Chemical Industries, Ltd.), and some of the Si-H bonds on the silicon particle surface were converted to Si-C 10 H 21 In other words, the surface of the silicon particles was covered with decyl groups (-C 10 H 21 ) to prepare surface-modified negative electrode active material particles.

[0073] (Synthesis Example 3) Surface-modified negative electrode active material particles were prepared in the same manner as in Synthesis Example 1, except that the alkene was changed to 0.55 g of 1-octadecene (Fujifilm Wako Pure Chemical Industries, Ltd.). Some of the Si-H bonds on the silicon particle surface were converted to Si-C. 18 H 37 In other words, the surface of the silicon particles was covered with octadecyl groups (-C 18 H 37 ) to prepare surface-modified negative electrode active material particles.

[0074] <<Preparation of Negative Electrode Active Material Composite Particles>> (PVdF-HFP is used as the binder resin) (Comparative Examples 1 to 3 and Examples 1 to 9) Polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-HFP) (KF8300 manufactured by Kureha Corporation) was dissolved in dimethyl carbonate to a concentration of 1 wt %. The silicon particles of Comparative Synthesis Example 1 and the surface-modified negative electrode active material particles of Synthesis Examples 1 to 3 were added to this solution and stirred to a weight ratio of silicon particles:PVdF-HFP = 94:6, 92:8, or 90:10, respectively. After further dispersion using a homogenizer, the solution was spray-dried using a spray dryer (ADL311S-A manufactured by Yamato Scientific) to obtain the negative electrode active material composite particles of Comparative Examples 1 to 3 and Examples 1 to 9.

[0075] <SBR is used as the binder resin> (Comparative Example 4 and Example 10) Styrene butadiene (SBR) was dissolved in butyl butyrate to a concentration of 1 wt %. Either the silicon particles of Comparative Synthesis Example 1 or the surface-modified negative electrode active material particles of Synthesis Example 3 were stirred into this solution so that the weight ratio of silicon particles to SBR was 92:8, and the mixture was dispersed using a homogenizer. The solution was then spray-dried using a spray dryer (Yamato Scientific, ADL311S-A) to obtain negative electrode active material composite particles for Comparative Example 4 and Example 10.

[0076] The relationship between the alkyl groups modifying the surfaces of the obtained negative electrode active material composite particles, the type of binder resin, and the amount of binder resin is as shown in Table 2.

[0077] "Making a Battery" <Preparation of Positive Electrode Composite> A polypropylene (PP) container was filled with butyl butyrate, a 5 wt% butyl butyrate solution of a PVDF-based binder, and LiNi as the positive electrode active material. 1 / 3 Co 1 / 3 Mn 1 / 3 O2 (average particle size: 6 μm), Li2S-P2S5-based glass ceramic as a sulfide solid electrolyte, and vapor-grown carbon fiber (VGCF) as a conductive additive were added to a container and stirred for 30 seconds using an ultrasonic disperser (UH-50, manufactured by SMT). Next, the container was shaken for 3 minutes using a shaker (TTM-1, manufactured by Shibata Scientific Co., Ltd.) and then stirred for 30 seconds using the ultrasonic disperser. After further shaking for 3 minutes using the shaker, a slurry-like positive electrode composite (positive electrode composite slurry) was obtained.

[0078] <Formation of Positive Electrode Active Material Layer> The obtained slurry was applied to an aluminum (Al) foil (manufactured by Showa Denko) as a positive electrode current collector layer by a blade method using an applicator. The applied slurry was dried on a hot plate at 100°C for 30 minutes. This formed a positive electrode active material layer on the positive electrode current collector layer.

[0079] <Preparation of negative electrode mixture> Dibutyl ether, mesitylene, a 5 wt% mesitylene solution of SBR binder, VGCF as a conductive additive, Li2S-P2S5-based glass ceramic as a solid electrolyte, and the negative electrode active material composite particles of each example were added to a polypropylene (PP) container and stirred for 30 seconds with an ultrasonic disperser (UH-50 manufactured by SMT Co., Ltd.). The container was then shaken for 30 minutes with a shaker (TTM-1 manufactured by Shibata Scientific Co., Ltd.) to obtain a slurry-like negative electrode composite (negative electrode composite slurry).

[0080] <Formation of negative electrode active material layer> The obtained slurry was applied to a nickel (Ni) foil serving as a negative electrode current collector by a blade method using an applicator. The applied slurry was dried on a hot plate at 100°C for 30 minutes. This formed a negative electrode active material layer on the negative electrode current collector layer.

[0081] <Formation of solid electrolyte layer> Heptane, a 5 wt% heptane solution of an SBR binder, and a Li2S-P2S5-based glass ceramic solid electrolyte were added to a polypropylene (PP) container and stirred for 30 seconds using an ultrasonic disperser (UH-50, manufactured by SMT). The container was then shaken for 30 minutes using a shaker (TTM-1, manufactured by Shibata Scientific Co., Ltd.) to obtain a solid electrolyte slurry. The resulting slurry was applied to an Al foil using an applicator by the blade method. The applied slurry was then dried on a hot plate at 100°C for 30 minutes. This resulted in the formation of a solid electrolyte layer. Three solid electrolyte layers were fabricated.

[0082] <Battery assembly> The positive electrode current collector layer, the positive electrode active material layer, and the first solid electrolyte layer were laminated in this order, and the laminate was set in a roll press and pressed in the first pressing step at a pressure of 100 kN / cm and a temperature of 165°C to obtain a positive electrode laminate.

[0083] The negative electrode current collector layer, the negative electrode active material layer, and the second solid electrolyte layer were laminated in this order, and the laminate was set in a roll press and pressed in a second pressing step at a pressure of 60 kN / cm and a temperature of 25°C to obtain a negative electrode laminate.

[0084] The negative electrode stack and the positive electrode stack were fabricated so that the area of ​​the negative electrode stack was larger than the area of ​​the positive electrode stack.

[0085] Furthermore, an aluminum foil serving as a release sheet, an intermediate solid electrolyte layer formed on the aluminum foil, and the negative electrode laminate were laminated so that the solid electrolyte layers were in contact with each other. This laminate was set in a planar uniaxial press and pre-pressed for 10 seconds at a pressure of 100 MPa and a temperature of 25°C. The aluminum foil was peeled off from the intermediate solid electrolyte layer of this laminate, yielding a negative electrode laminate further laminated with an intermediate solid electrolyte layer.

[0086] The positive electrode laminate and the negative electrode laminate, which further had an intermediate solid electrolyte layer laminated thereon, were stacked so that the solid electrolyte layers were in contact with each other. This stack was placed in a planar uniaxial press and pressed at a pressure of 200 MPa and a temperature of 120°C for 1 minute in the third pressing step. This produced an all-solid-state battery.

[0087] "evaluation" <Analysis of the surface properties of silicon particles> The carbon content on the surface of silicon particles in the Synthesis Example and Comparative Synthesis Example was measured by high-frequency combustion infrared absorption spectroscopy (apparatus used: LECO CSLS600). The hydrogen content of silicon particles in each example was measured by inert gas fusion infrared absorption spectroscopy (apparatus used: LECO TCH600).

[0088] Furthermore, the silicon particles of each example were subjected to surface analysis by time-of-flight secondary ion mass spectrometry (TOF-SIMS) (apparatus used: ION-TOF, TOF.SIMS5, primary ion source: bismuth (Bi)).

[0089] <Measurement of particle size distribution of negative electrode active material composite particles> The negative electrode active material composite particles of each example were dispersed in water and measured with a particle size distribution meter.

[0090] <Battery capacity retention rate> The fabricated battery was restrained at a predetermined restraining pressure using a restraining jig, and was charged at a constant current and constant voltage at 1 / 10C to 4.55V, and then discharged at 1C to 3.0V. The battery was then charged at a constant current and constant voltage at 1 / 3C to 4.35V, and discharged at a constant current and constant voltage at 1 / 3C to 3.00V to determine the initial capacity, and then a charge-discharge test was repeated 100 times at 1C. The capacity retention rate was calculated by subtracting the initial capacity from the capacity after 100 charge-discharge tests.

[0091] "result" <Analysis of the surface properties of silicon particles> Table 1 shows the results of the quantitative analysis of the carbon and hydrogen amounts on the surfaces of the silicon particles of the Comparative Synthesis Example and Synthesis Example, as well as the results of the TOF-SIMS analysis.

[0092] [Table 1]

[0093] As shown in Table 1, the silicon particles of the synthesis example, whose surfaces were modified with alkyl groups, had a higher carbon content on the particle surface than the silicon particles of the comparative synthesis example. Furthermore, the m / z value increased as the number of carbon atoms in the alkyl group increased. From these results, it can be assumed that the surfaces of the silicon particles were successfully modified with alkyl groups.

[0094] <Measurement of particle size distribution of negative electrode active material composite particles> Table 2 shows the results of particle size distribution measurement of the negative electrode active material composite particles.

[0095] <Battery capacity retention rate> Table 2 shows the calculation results of the battery capacity retention rate.

[0096] In Table 2, "resin" means binder resin, and "Si" means surface-modified silicon particles in the examples and unmodified silicon particles in the comparative examples.

[0097] [Table 2]

[0098] As shown in Table 2, the particles of the examples, including silicon particles whose surfaces were modified with alkyl groups, had a (D90-D10) / D50 value of 2.0 or less. This suggests that the particles of the examples were prepared as composite particles. In contrast, the particles of Comparative Examples 1 and 2 had a large (D90-D10) / D50, suggesting that they were not properly prepared as composite particles.

[0099] The batteries containing the particles of Examples 1 to 3 and 4 to 6 had higher capacity retention rates, i.e., better cycle characteristics, than the corresponding batteries of Comparative Examples 1 and 2. As described above, this is thought to be because in these examples, the composite particles were appropriately prepared even with a small amount of binder resin. In particular, the high capacity retention rates of the batteries of Examples 1 to 3 are thought to be due to the fact that the small amount of binder resin reduced the resistance of the battery.

[0100] Furthermore, the batteries containing the particles of Examples 7 to 9 also had higher capacity retention rates than the corresponding battery of Comparative Example 3. Thus, even when the amount of binder resin was relatively large and it was assumed that the particles of the Examples and Comparative Examples were all prepared as composite particles, differences in capacity retention rates occurred. This is thought to be because, in the composite particles of the Examples, surface modification with alkyl groups suppresses aggregation of the silicon particles in the composite particles, forming appropriate voids between the silicon particles. This void can mitigate volume changes that occur when the silicon particles expand during charge and discharge, thereby suppressing cracking of the composite particles.

[0101] Furthermore, the battery of Example 10, in which the type of binder resin was changed, also had a larger capacity retention rate than the corresponding battery of Comparative Example 4.

Claims

1. a plurality of surface-modified negative electrode active material particles; and a binder resin that binds the plurality of surface-modified negative electrode active material particles to one another; the surface-modified negative electrode active material particles have silicon particles and organic groups modifying the surfaces of the silicon particles, a ratio of the mass of the binder resin to the total mass of the plurality of surface-modified negative electrode active material particles and the binder resin is 15 mass% or less; the organic group is an alkyl group, and the surface-modified negative electrode active material particles have a silicon-carbon bond, the silicon being silicon of the silicon particles, and the carbon being carbon of the alkyl group; Negative active material composite particles.

2. The negative electrode active material composite particle according to claim 1 , wherein the alkyl group has 5 or more and 20 or less carbon atoms.

3. A negative electrode mixture comprising the negative electrode active material composite particles according to claim 1 or 2.

4. a negative electrode active material layer; and The negative electrode active material layer contains the negative electrode mixture according to claim 3. Lithium-ion battery.

5. (a) modifying the surfaces of the silicon particles with the organic groups to obtain the plurality of surface-modified negative electrode active material particles; (b) forming droplets from a slurry containing the plurality of surface-modified negative electrode active material particles, the binder resin, and a solvent to obtain slurry droplets; and (c) flash-drying the slurry droplets in a heated gas; The method for producing a negative electrode active material composite particle according to claim 1 or 2, comprising:

6. 6. The method according to claim 5, wherein in step (a), the surfaces of the silicon particles are modified with the organic groups by a hydrosilylation reaction in which the organic groups are added to hydrosilyl groups on the surfaces of the silicon particles.

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