Negative electrode active material, solid battery, and method for manufacturing negative electrode active material

Abstract

The present disclosure provides a negative electrode active material in which the pore volume is less likely to decrease, a solid-state battery, and a method for manufacturing the negative electrode active material. The negative electrode active material of the present disclosure includes a plurality of Si-based particles. The Si-based particles include a porous Si particle and a metal oxide film formed on at least a part of a surface of the porous Si particle. The metal oxide film includes at least one of a Mg element and an Al element.

Description

Technical Field

[0001] This disclosure relates to a negative electrode active material, a solid-state battery, and a method for manufacturing the negative electrode active material. Background Technology

[0002] It is known that Si-based active materials have a large theoretical capacity, which is effective for achieving high energy density in batteries. On the other hand, it is known that Si-based active materials tend to expand during charging.

[0003] Japanese Patent Application Publication No. 2023-167083 discloses an active material (hereinafter also referred to as "porous Si particles"). The porous Si particles contain Si and have specific voids (hereinafter also referred to as "pores") inside the primary particles. Summary of the Invention

[0004] Because porous Si particles have low strength, their pores are easily flattened. In other words, the number of pores in porous Si particles is easily reduced. If the negative electrode active material layer of a solid-state battery contains porous Si particles with flattened pores, it may be difficult to suppress the expansion and contraction of the negative electrode active material layer caused by charging or discharging.

[0005] This disclosure is made in view of the above circumstances.

[0006] The problem to be solved by one embodiment of this disclosure is to provide a negative electrode active material and a solid-state battery with a pore size that is not easily reduced.

[0007] Another embodiment of this disclosure aims to solve the problem of providing a method for manufacturing a negative electrode active material that can produce a negative electrode active material with a small amount of pores.

[0008] The means for solving the above-mentioned problems include the following implementation methods.

[0009] <1> A negative electrode active material containing multiple Si-based particles.

[0010] The Si-based particles comprise porous Si particles and a metal oxide film formed on at least a portion of the surface of the porous Si particles.

[0011] The metal oxide film contains at least one of Mg and Al elements.

[0012] <2> According to the above <1> The negative electrode active material, wherein the metal oxide film is composed of MgO.

[0013] <3> According to the above <1> or <2> The negative electrode active material has a metal oxide film thickness of 3 nm to 8 nm.

[0014] <4> A solid-state battery includes a positive electrode, a negative electrode, and a solid electrolyte layer disposed between the positive electrode and the negative electrode.

[0015] The negative electrode includes the above-mentioned... <1> The aforementioned negative electrode active material.

[0016] <5> A method for manufacturing a negative electrode active material includes the following steps:

[0017] Prepare multiple porous Si particles; and

[0018] Metal silicides are deposited on the porous Si particles by physical vapor deposition in a non-oxidizing atmosphere, thereby forming a metal oxide film on at least a portion of the surface of the porous Si particles.

[0019] The metal silicide contains at least one of Mg and Al elements.

[0020] According to one embodiment of this disclosure, a negative electrode active material with a low porosity and a solid-state battery are provided. According to other embodiments of this disclosure, a method for manufacturing a negative electrode active material capable of producing a negative electrode active material with a low porosity is provided. Attached Figure Description

[0021] Hereinafter, the features, advantages, and technical and industrial significance of exemplary embodiments of the present invention will be described with reference to the accompanying drawings, wherein the same reference numerals denote the same elements.

[0022] Figure 1 This is a cross-sectional view of porous Si particles according to an embodiment of the present disclosure.

[0023] Figure 2 This is a cross-sectional view of Si-based particles according to an embodiment of this disclosure.

[0024] Figure 3 This is a cross-sectional view of a solid-state battery according to an embodiment of the present disclosure.

[0025] Figure 4 This is a coordinate graph representing the X-ray diffraction (XRD) pattern of Si-based particles in Example 1. Detailed Implementation

[0026] In this disclosure, the numerical range indicated by "~" refers to the range of values ​​before and after "~" as the minimum and maximum values, respectively. In this disclosure, a combination of two or more preferred methods is a more preferred method.

[0027] (1) Negative electrode active material

[0028] The negative electrode active material disclosed herein comprises a plurality of Si-based particles. The Si-based particles comprise porous Si particles and a metal oxide film formed on at least a portion of the surface of the porous Si particles. The metal oxide film comprises at least one element selected from Mg and Al.

[0029] "Porous Si particles" refers to Si particles with multiple fine pores. "Si particles" refers to particles whose main component is Si. Specifically, the proportion of Si in the total elemental composition of the Si particle can be, for example, 50 mol% or more, 70 mol% or more, or 90 mol% or more. Si particles can contain elements different from Si (e.g., Li, Sn, Fe, Co, Ni, Ti, Cr, B, and P). Si particles can contain impurities (e.g., oxides). The elemental Si contained in the Si particles can be amorphous Si or crystalline Si. The crystalline phase contained in the Si particles is not particularly limited.

[0030] The negative electrode active material disclosed herein has the above-described structure, thus the amount of fine pores is not easily reduced.

[0031] This effect is speculated to be based on the following reasons, but is not limited to these.

[0032] In this disclosure, the Si-based particles contain a metal oxide film. Therefore, the strength of the Si-based particles is higher than that of the Si-based particles without a metal oxide film. As a result, it is presumed that the porosity of the negative electrode active material of this disclosure is less likely to be reduced by pressing or other factors during the fabrication of the electrode body of a solid-state battery.

[0033] The negative electrode active material disclosed herein comprises multiple Si-based particles. The negative electrode active material disclosed herein may be composed of multiple Si-based particles, or may contain particles different from Si-based particles.

[0034] Si-based particles have multiple micropores. The amount of micropores in Si-based particles is not particularly limited and can be 0.10 cc / g to 1.00 cc / g, 0.70 cc / g to 0.90 cc / g, or 0.75 cc / g to 0.83 cc / g. The method for determining the amount of micropores in Si-based particles is the same as that described in the examples.

[0035] The particle thickness variation rate of multiple Si-based particles is greater than 70%, which can be 75% to 100% or 80% to 90%. There is a tendency that the higher the particle thickness variation rate (%), the less likely the porosity of the Si-based particles will be reduced due to pressing or other factors during the fabrication of the electrode body of the solid-state battery. The method for measuring the particle thickness variation rate is the same as that described in the examples.

[0036] The particle size (D) of Si-based particles 50The particle size can range from 10 μm to 1000 μm, from 20 μm to 100 μm, or from 20 μm to 50 μm. 50 ")" represents the particle size (median particle size) at which the cumulative value of the particle size distribution of the volume reference obtained by laser diffraction and scattering is 50%.

[0037] The metal oxide film is formed on at least a portion of the surface of the porous Si particles. From the viewpoint of improving the strength of Si-based particles, the metal oxide film is preferably formed on the entire surface of the porous Si particles.

[0038] The thickness of the metal oxide film is not particularly limited and can range from 1 nm to 100 nm. Preferably, the thickness of the metal oxide film is 3 nm to 8 nm. Therefore, the amount of pores in the negative electrode active material of this disclosure is less likely to decrease, and the battery performance of the solid-state battery can be improved. From the viewpoint of making it even less likely to decrease the amount of pores, the thickness of the metal oxide film can be 5 nm to 8 nm.

[0039] The metal oxide film contains at least one of Mg and Al elements, preferably Mg. The metal oxide film is preferably composed of MgO. By using MgO as the metal oxide, the porosity of the negative electrode active material of this disclosure is less likely to be reduced.

[0040] The amount of metal (i.e., at least one of Mg and Al) relative to the Si-based particles is not particularly limited, and can be 1% to 50% by mass, 10% to 40% by mass, or 20% to 30% by mass. The method for determining the amount of metal relative to the Si-based particles is the same as that described in the examples.

[0041] The following is for reference Figure 1 and Figure 2 An example of porous Si particles and an example of Si-based particles disclosed herein will be described. For example... Figure 1 As shown, the porous Si particles 100 of the embodiments of this disclosure have a plurality of fine pores P. For example... Figure 2 As shown, the Si-based particles 300 of the embodiments of this disclosure comprise porous Si particles 100 and a metal oxide film 200. The metal oxide film 200 is formed on a portion of the surface of the porous Si particles 100. The metal oxide film 200 comprises at least one of Mg and Al elements.

[0042] (2) Solid-state batteries

[0043] The solid-state battery disclosed herein includes a positive electrode, a negative electrode, and a solid electrolyte layer disposed between the positive electrode and the negative electrode. The negative electrode has a negative electrode active material layer. The negative electrode active material layer contains the negative electrode active material of this disclosure.

[0044] The solid-state battery of this disclosure has the above-described structure, thus the negative electrode active material layer expands and contracts during charging or discharging. Therefore, even with alternating and repeated charging and discharging, the battery performance of the solid-state battery of this disclosure is not easily degraded.

[0045] The solid-state battery disclosed herein can have at least one power generation unit. The power generation unit has a negative electrode, a solid electrolyte layer, and a positive electrode. In the case of an all-solid-state battery having multiple power generation units, the multiple power generation units can be connected in parallel or in series.

[0046] (2.1) Negative electrode

[0047] The negative electrode of this disclosure may have a negative current collector and a negative active material layer formed on at least one main surface of the negative current collector. The negative active material layer contains the negative active material of this disclosure and may further contain at least one of a solid electrolyte, a conductive material, and a binder, as needed.

[0048] Examples of solid electrolytes include sulfide solid electrolytes, oxide solid electrolytes, nitride solid electrolytes, and halide solid electrolytes. Sulfide solid electrolytes may contain Li and S elements. Preferably, sulfide solid electrolytes contain at least one of P, Ge, Sn, and Si elements. Sulfide solid electrolytes may contain at least one of O and halogen elements (e.g., F, Cl, Br, and I elements). Examples of sulfide solid electrolytes included in a solid electrolyte layer include Li₂S-P₂S₅ and Li₂S-P₂S₅-Z. m S n (m and n are positive numbers, and Z is any one of Ge, Zn, and Ga) and Li2S-SiS2-Li x MO y (x and y are positive numbers, and M is any one of P, Si, Ge, B, Al, Ga, and In), etc. The description "Li2S-P2S5" indicates a material formed using a raw material composition containing Li2S and P2S5, and the same applies to other descriptions. The solid electrolyte can be a known solid electrolyte. The solid electrolyte can be glass, glass-ceramic, or a crystalline material. Glass is obtained by amorphous treatment of the raw material composition (e.g., a mixture of Li2S and P2S5). Examples of amorphous treatment include mechanical grinding. Glass-ceramic is obtained by heat treatment of glass. Crystalline materials are obtained, for example, by solid-state reaction treatment of the raw material composition. The proportion of solid electrolyte in the negative electrode active material layer can be 20% to 80% by mass.

[0049] Examples of conductive materials include carbon materials, metal particles, and conductive polymers. Examples of carbon materials include acetylene black (AB), Ketjen black (KB), carbon fibers, carbon nanotubes (CNTs), and carbon nanofibers (VGCF). The volume ratio of carbon materials to the negative electrode active material layer can be 5% to 10% by volume.

[0050] Examples of adhesives include fluoride-based adhesives (such as polyvinylidene fluoride (PVDF), polyimide-based adhesives, and rubber-based adhesives. The volume ratio of the adhesive to the negative electrode active material layer can be 2% to 5% by volume.

[0051] The negative current collector is the layer that collects electricity from the negative electrode active material layer. Examples of negative current collectors include aluminum, SUS (Steel Use Stainless), copper, nickel, and carbon. Examples of shapes for negative current collectors include foil.

[0052] (2.2) Solid electrolyte layer

[0053] The solid electrolyte layer contains a solid electrolyte and may further contain a binder if necessary. The thickness of the solid electrolyte layer can range from 0.1 μm to 300 μm.

[0054] As a solid electrolyte, the same solid electrolyte as that exemplified as a solid electrolyte that may be included in the negative electrode active material layer can be cited. The content of solid electrolyte in the solid electrolyte layer can be 70% to 100% by mass, or 90% to 100% by mass. As a binder, the same binder as that exemplified as a binder that may be included in the negative electrode active material layer can be cited.

[0055] (2.3) Positive electrode

[0056] The positive electrode comprises a positive electrode active material. The positive electrode may have a positive electrode current collector and a layer of positive electrode active material formed on at least one of the main surfaces of the positive electrode current collector. The positive electrode active material layer comprises the positive electrode active material and may further comprise, as needed, at least one of a solid electrolyte, a conductive material, and a binder.

[0057] Examples of positive electrode active materials include oxide active materials. Examples of oxide active materials include layered rock salt active materials (e.g., LiNi). 1 / 3 Co 1 / 3 Mn 1 / 3 O2 and LiCoO2, etc.), spinel-type active materials (such as LiMn2O4 and Li4Ti5O4, etc.). 12 (etc.) and olivine-type active substances (such as LiFePO4 and LiMnPO4, etc.).

[0058] As a solid electrolyte, the same solid electrolyte as that exemplified as a solid electrolyte that may be included in the negative electrode active material layer can be cited. The proportion of solid electrolyte in the positive electrode active material layer is not particularly limited and can be 20% to 80% by mass. As a conductive material, the same material as that exemplified as a conductive material that may be included in the negative electrode active material layer can be cited. The volume ratio of the conductive material to the positive electrode active material layer can be 5% to 10% by volume. As a binder, the same binder as that exemplified as a binder that may be included in the negative electrode active material layer can be cited. The volume ratio of the binder to the positive electrode active material layer can be 5% to 10% by volume.

[0059] The positive current collector is the layer that collects electricity from the positive electrode active material layer. Examples of positive current collectors include SUS, aluminum, nickel, iron, titanium, and carbon. Examples of the shape of the positive current collector include foil.

[0060] (2.4) Exterior body

[0061] The solid-state battery disclosed herein typically includes an outer casing. The outer casing houses the positive electrode, a solid electrolyte layer, and a negative electrode. There are no particular limitations on the outer casing; for example, laminated outer casings may be cited.

[0062] (2.5) Implementation Method

[0063] The following is for reference Figure 3 An example of the battery disclosed herein will be described. The battery 1 of this embodiment has a power generation unit 1U and an outer casing 40. The power generation unit 1U has a negative electrode 10, a solid electrolyte layer 20, and a positive electrode 30 stacked sequentially. The negative electrode 10 has a negative electrode active material layer 11 and a negative electrode current collector 12. The positive electrode 30 has a positive electrode active material layer 31 and a positive electrode current collector 32. The outer casing 40 houses the power generation unit 1U. The negative electrode active material layer 11 contains the negative electrode active material of this disclosure.

[0064] (3) Method for manufacturing negative electrode active material

[0065] The method for manufacturing the negative electrode active material disclosed herein includes: preparing a plurality of porous Si particles (hereinafter also referred to as the "preparation step"); and depositing a metal silicide onto the porous Si particles by physical vapor deposition in a non-oxidizing atmosphere to form a metal oxide film on at least a portion of the surface of the porous Si particles (hereinafter also referred to as the "coating step"). The metal silicide comprises at least one of Mg and Al elements. The preparation step and the coating step are performed sequentially.

[0066] (3.1) Preparation process

[0067] In the preparation process, multiple porous Si particles are prepared. The method for preparing multiple porous Si particles can be a known method. For example, a method for manufacturing porous Si particles can be to sinter the Si particles (material: SiO₂) in a non-oxidizing atmosphere. X (0<X2≤2))) and a Mg source (e.g. Mg2Si), the vaporized Mg source is attached to the surface of Si particles, and then the magnesium oxide of the Si particles with the Mg source is dissolved with an acid (e.g., hydrochloric acid and nitric acid) to produce porous Si particles. In this method, it is believed that the porous Si particles are generated through the reaction of the following formula (a).

[0068] Formula (a): SiO X +XMg→XMg2O+Porous Si

[0069] The Si particles in the raw material can be perfectly spherical. "Perfectly spherical" means that, with the long side length of the primary particle (porous Si particle) denoted as 'a' and the short side length as 'b', the ratio of b to a (b / a) is 0.9 to 1.0. 'a' and 'b' are determined by measuring the cross-sectional images of the porous Si particles.

[0070] (3.2) Coating process

[0071] In the coating process, metal silicide is deposited on the porous Si particles by physical vapor deposition in a non-oxidizing atmosphere (e.g., an atmosphere below 10 Pa), forming a metal oxide film on at least a portion of the surface of the porous Si particles. This yields Si-based particles.

[0072] The metal silicide contains at least one of Mg and Al elements, selected according to the type of metal oxide film. Examples of metal silicides include magnesium silicide (Mg₂Si), aluminum silicide (Al-Si), and aluminum silicide nitride (AlSiN). The feeding ratio of the metal silicide to the porous Si particles can be 100% to 1000% by mass, or 200% to 300% by mass.

[0073] Examples of physical vapor deposition methods include vacuum evaporation, ion plating, sputtering, and pulsed laser deposition (PLD). Vacuum evaporation is a typical physical vapor deposition method. When vacuum evaporation is used, the thickness of the metal oxide film and the amount of metal relative to the Si particles can be adjusted by changing the firing temperature (hereinafter referred to as "firing temperature") of the metal silicide and porous Si particles. The firing temperature simply needs to be the temperature at which the metal silicide vaporizes and the porous Si particles do not melt. The firing temperature can be 600℃~750℃, 650℃~750℃, or 700℃~750℃. If the firing temperature is 700℃~750℃, multiple Si particles with a metal content of 20%~30% relative to the Si particles and a film thickness of 5nm~8nm can be obtained.

[0074] The present disclosure will be described in more detail below through embodiments, but the invention of the present disclosure is not limited to these embodiments.

[0075] [1] Examples and Comparative Examples

[0076] [1.1] Example 1

[0077] [1.1.1] Preparation process

[0078] Prepare silica (composition: SiO2, particle size: 25 μm, manufacturer: High Purity Chemical Research Institute Co., Ltd.) and magnesium silicide (composition: Mg2Si, manufacturer: High Purity Chemical Research Institute Co., Ltd.). Place 1.0 part by mass of silica in the upper part of a sintering vessel and 2.6 parts by mass of magnesium silicide in the lower part of the vessel. Separate the two using a mesh (500 mesh, material: SUS304). Place the sintering vessel in a vacuum sintering furnace. Sinter the silica and magnesium silicide at 700°C for 6 hours under an atmosphere below 10 Pa. This yields a porous Si precursor. The porous Si precursor is then placed in hydrochloric acid (concentration: 1 mol / L) and stirred for 1 hour. Subsequently, the solid and liquid are separated by vacuum filtration. The separated solid is washed with ethanol. This yields porous Si particles.

[0079] [1.1.2] Coating process

[0080] Magnesium silicide (composition: Mg2Si, manufacturer: High Purity Chemical Research Institute Co., Ltd.) was prepared. 1.0 parts by mass of porous Si particles were placed in the upper part of a sintering container, and 2.6 parts by mass of magnesium silicide were placed in the lower part of the container. The two were separated using a mesh (500 mesh, material: SUS304). The sintering container was placed in a vacuum sintering furnace. The porous Si particles and magnesium silicide were sintered at 750°C for 2 hours under an atmosphere below 10 Pa. This yielded Si-based particles (anode active material).

[0081] [1.2] Example 2

[0082] Except for changing the firing temperature of the coating process to 650°C, Si-based particles (negative electrode active material) were obtained in the same manner as in Example 1.

[0083] [1.3] Comparative Example 1

[0084] Except that no coating process was performed, porous Si particles (hereinafter also referred to as "Si-based particles") (anode active material) were obtained in the same manner as in Example 1.

[0085] [2] Determination method

[0086] [2.1] X-ray crystal structure analysis

[0087] The XRD pattern of Si-based particles was determined using an X-ray diffraction apparatus. The XRD pattern of the Si-based particles from Example 1 is shown below. Figure 4 It can be seen that the Si-based particles in Examples 1 and 2 contain a MgO phase (i.e., a metal oxide film). It can be seen that the Si-based particles in Comparative Example 1 do not contain a MgO phase (i.e., a metal oxide film).

[0088] [2.2] Mg content (mass%)

[0089] The amount of Mg (mass%) in Si-based particles was determined by high-frequency inductively coupled plasma atomic emission spectrometry (ICP-AES).

[0090] [2.3] Thickness of metal oxide films (nm)

[0091] The thickness of the metal oxide film (MgO film) was determined by scanning electron microscopy (SEM) based on Si-based particles.

[0092] [2.4] Fine pore volume (cc / g)

[0093] The porosity of Si-based particles in their unpressed state was determined by measuring the pore distribution using a nitrogen adsorption method.

[0094] [2.5] Granule thickness change rate (%)

[0095] 50 mg of Si-based particles were added to an alumina cylinder (diameter: 11.28 mm). A uniaxial press was used, employing pins (material: SKD11) to press the Si-based particles from both above and below. The pressing load was 1 ton / cm². 2 Thus, Si-based particle granules were obtained. The thickness of the Si-based particle granules (hereinafter also referred to as "particle thickness (1 ton / cm)") was measured using a micrometer. 2 )”).

[0096] Subsequently, a single-axis press was used in the same manner, employing pins (material: SKD11) to press the Si-based particle granules from above and below. The pressing load was 6 tons / cm². 2 Next, the thickness of the Si-based particles was measured using a micrometer (hereinafter also referred to as "particle thickness (6 t / cm)"). 2 )”).

[0097] The particle thickness variation rate (%) is calculated using the following formula (1). A high particle thickness variation rate (%) quantitatively indicates that the porosity of Si-based particles is not easily reduced. The permissible particle thickness variation rate (%) is greater than 70%.

[0098] Formula (1): Granule thickness change rate (%) = Granule thickness (6 tons / cm) 2 ) / Particle thickness (1 ton / cm) 2 )

[0099] [2.6] Battery performance

[0100] As shown below, evaluation cells were fabricated using Si-based particles, and the battery performance (%) was determined.

[0101] [2.6.1] Negative electrode

[0102] A negative electrode active material (Si-based particles), a sulfide solid electrolyte (15LiBr-10LiI-75(0.75Li2S-0.25P2S5)), and an organic solvent (diisobutyl ketone) were mixed and stirred using a homogenizer to obtain a mixture. The volume ratio (Si-based particles: sulfide solid electrolyte) was 65:35. A conductive additive (VGCF) and a binder diluted to 5% by mass (PVDF) were added to the mixture, and the mixture was stirred using a homogenizer. This yielded a negative electrode slurry. The volume ratio of the conductive additive to the negative electrode slurry was 5% by volume. The volume ratio of the binder to the negative electrode slurry was 2% by volume. The obtained negative electrode slurry was coated onto a negative electrode current collector (nickel foil) using a casting coater and then dried. This yielded a negative electrode. The negative electrode has a negative electrode current collector and a layer of negative electrode active material formed on one main surface of the negative electrode current collector.

[0103] [2.6.2] Solid electrolyte layer

[0104] A sulfide solid electrolyte (15LiBr-10LiI-75(0.75Li2S-0.25P2S5)), a binder (PVDF) diluted to 5% by mass, and a dispersion medium (a mixture of heptane and dibutyl ether) are mixed to obtain a slurry for a solid electrolyte layer. The solid content of the slurry for the solid electrolyte layer is 50% by mass. The solid electrolyte layer slurry is coated onto a substrate (aluminum foil) and dried. This yields a solid electrolyte layer with a substrate. The solid electrolyte layer with a substrate has a substrate and a solid electrolyte layer formed on one main surface of the substrate.

[0105] [2.6.3] Positive electrode

[0106] The positive electrode active material (LiNi) 1 / 3 Co 1 / 3 Mn 1 / 3 A mixture of O2, a dispersion medium (butyl butyrate), a binder (5% by mass butyl butyrate solution of PVDF-based binder), a sulfide solid electrolyte (15LiBr-10LiI-75(0.75Li2S-0.25P2S5)), and a conductive material (VGCF) is obtained. This yields a positive electrode slurry. The positive electrode slurry is coated onto a positive current collector (aluminum foil) and dried. This yields a positive electrode. The positive electrode has a positive current collector and a layer of positive active material formed on one main surface of the positive current collector.

[0107] [2.6.4] Evaluation Unit

[0108] A solid electrolyte layer is laminated onto the positive electrode in a manner that brings the solid electrolyte layer into contact with the positive electrode active material layer, and then pressed. A substrate (aluminum foil) for the solid electrolyte layer is peeled off, and a negative electrode is laminated in a manner that brings the solid electrolyte layer into contact with the negative electrode active material layer, and then pressed. This yields an evaluation cell.

[0109] [2.6.5] Determination

[0110] The evaluation unit is charged in a constant current mode using a current equivalent to a 10-hour rate (0.1C) until the voltage reaches 4.55V. After reaching 4.55V, the evaluation unit is charged in a constant voltage mode. After switching to constant voltage mode, charging ends when the charging current decays to a current equivalent to 0.01C.

[0111] The evaluation cell was discharged in a constant-current manner using a current equivalent to 0.1C. Discharge ended when the voltage reached 3.0V. The initial charge-discharge efficiency was calculated by dividing the discharge capacity by the charge capacity. The relative value of the initial charge-discharge efficiency when the initial charge-discharge efficiency of Comparative Example 1 was set to 100% was taken as "Battery Performance (%)". A high battery performance (%) indicates a large cell capacity and excellent battery performance.

[0112] (Table 1)

[0113]

[0114] In Table 1, "pore size before pressing" refers to the pore size of Si-based particles in their unpressed state.

[0115] The negative electrode active materials of Examples 1 and 2 contain Si-based particles. The Si-based particles comprise porous Si particles and a metal oxide film formed on the surface of the porous Si particles. The metal oxide film contains Mg. Therefore, the particle thickness variation rate is greater than 70%. As a result, the negative electrode active materials of Examples 1 and 2 are "negative electrode active materials with minimal reduction in porosity".

Claims

1. A negative electrode active material, It contains multiple Si-based particles. The Si-based particles comprise porous Si particles and a metal oxide film formed on at least a portion of the surface of the porous Si particles. The metal oxide film contains at least one of Mg and Al elements.

2. The negative electrode active material according to claim 1, The metal oxide film is composed of MgO.

3. The negative electrode active material according to claim 1, The thickness of the metal oxide film is 3nm to 8nm.

4. A solid-state battery comprising a positive electrode, a negative electrode, and a solid electrolyte layer disposed between the positive electrode and the negative electrode. The negative electrode has a negative electrode active material layer. The negative electrode active material layer comprises the negative electrode active material as described in claim 1.

5. A method for manufacturing a negative electrode active material, comprising the following steps: Prepare multiple porous Si particles; and Metal silicides are deposited on the porous Si particles by physical vapor deposition in a non-oxidizing atmosphere, thereby forming a metal oxide film on at least a portion of the surface of the porous Si particles. The metal silicide contains at least one of Mg and Al elements.

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