Composite active material and solid battery

Abstract

A main object of the present disclosure is to provide a composite active material capable of improving the initial charge / discharge efficiency in a solid battery.SOLUTION: The present disclosure solves the above problem by providing a composite active material comprising: an electrode active material comprising a Si element; and a coating layer coating a surface of the electrode active material, wherein the coating layer comprises a hydride solid electrolyte.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present disclosure relates to composite active materials and solid-state batteries. [Background technology]

[0002] In recent years, there has been active development of batteries. For example, in the automotive industry, development of batteries for use in electric vehicles (BEVs), plug-in hybrid vehicles (PHEVs), and hybrid electric vehicles (HEVs) is progressing.

[0003] For example, Patent Document 1 discloses an active material containing Si and having voids inside the primary particles as an electrode active material used in batteries. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Publication No. 2023-167083 Summary of the Invention [Problem to be solved by the invention]

[0005] From the viewpoint of improving battery performance, a solid-state battery with good initial charge-discharge efficiency is desired. The present disclosure has been made in view of the above-described circumstances, and has as its main object to provide a composite active material that can improve the initial charge-discharge efficiency of a solid-state battery. [Means for solving the problem]

[0006] [1] an electrode active material containing Si element; A composite active material having a coating layer that coats the surface of the electrode active material, The coating layer is a composite active material containing a hydride solid electrolyte.

[0007] [2] The composite active material according to [1], wherein the hydride solid electrolyte contains a halide salt.

[0008] [3] The composite active material according to [1] or [2], wherein the hydride solid electrolyte has a hexagonal crystal structure.

[0009] [4] The BET specific surface area of ​​the above composite active material is 15m 2 / g or more, 45m 2 / g or less of the composite active material according to any one of [1] to [3].

[0010] [5] The composite active material according to any one of [1] to [4], wherein the electrode active material contains carbon.

[0011] [6] The composite active material according to [5], wherein the electrode active material is composed of a core portion containing the Si element and a carbon coating portion that coats the core portion.

[0012] [7] The composite active material according to [6], wherein the core portion contains porous carbon and silicon filled inside the porous carbon.

[0013] [8] The composite active material according to any one of [5] to [7], wherein the carbon content in the composite active material is 30% by weight or more and 60% by weight or less.

[0014] [9] The composite active material according to any one of [1] to [8], wherein the coating layer has a thickness of 0.4 nm or more and 5.0 nm or less.

[0015]

[10] A solid-state battery having a positive electrode active material layer, a negative electrode active material layer, and a solid electrolyte layer disposed between the positive electrode active material layer and the negative electrode active material layer, The negative electrode active material layer comprises the composite active material according to any one of [1] to [9]. [Effects of the Invention]

[0016] The present disclosure provides an effect of improving the initial charge-discharge efficiency of a solid-state battery. [Brief explanation of the drawings]

[0017] [Figure 1] 1 is a schematic cross-sectional view illustrating a composite active material and an electrode active material according to the present disclosure. [Figure 2] FIG. 2 is a diagram illustrating the crystal structure of a hydride solid electrolyte according to the present disclosure. [Figure 3] FIG. 1 is a schematic cross-sectional view illustrating a solid-state battery according to the present disclosure. [Figure 4] 1 is a graph summarizing the results of BET specific surface area measurements and SEM-EDS analysis for Examples 1 to 3 and Comparative Example 1. [Figure 5] FIG. 1 is a diagram showing the results of XRD measurement. [Figure 6] 1 is a graph showing the results of charge / discharge tests for Examples 1 to 3 and Comparative Example 1. [Figure 7] FIG. 1 is a schematic cross-sectional view illustrating an electrode active material (Si—C-based active material) according to the present disclosure. [Figure 8] 1 is a graph showing the results of charge-discharge tests for Examples 6 to 8 and Comparative Examples 3 and 4. DETAILED DESCRIPTION OF THE INVENTION

[0018] The composite active material and the solid-state battery according to the present disclosure will be described in detail below. Note that the drawings shown below are schematic diagrams, and the size and shape of each part are appropriately exaggerated for ease of understanding.

[0019] A. Composite active material FIG. 1(a) is a schematic cross-sectional view illustrating an example of a composite active material according to the present disclosure. In particular, FIG. 1(a) illustrates a composite active material in which the electrode active material is solid particles. The composite active material 10 shown in FIG. 1(a) includes an electrode active material 1 containing elemental Si and a coating layer 2 that coats the surface of the electrode active material 1. The coating layer 2 contains a hydride solid electrolyte. Herein, in the present specification, when the electrode active material is a porous particle, or when the electrode active material is a porous particle of Si (particles containing porous Si), the electrode active material may be referred to as porous Si (p-Si), and the composite active material may be referred to as composite porous Si (composite p-Si).

[0020] According to the present disclosure, the surface of the electrode active material is coated with a coating layer containing a hydride solid electrolyte, thereby improving the initial charge / discharge efficiency of the solid-state battery.

[0021] FIG. 1(b) is a schematic cross-sectional view illustrating an example of an electrode active material (solid particle) according to the present disclosure. As shown in FIG. 1(b), fine depressions H are usually formed on the surface of the electrode active material 1. When the electrode active material is a porous particle, it is thought that more depressions (pores) H are formed due to internal voids (the BET specific surface area increases). If such depressions H increase the contact area with the solid electrolyte (e.g., a sulfide solid electrolyte), decomposition of the solid electrolyte may be accelerated. As a result, carrier ions such as Li ions are consumed due to the decomposition of the solid electrolyte, which may reduce the initial charge / discharge efficiency of the solid-state battery.

[0022] In contrast, in the composite active material of the present disclosure, the surface of the electrode active material is coated with a coating layer containing a hydride solid electrolyte. The coating layer functions as a barrier layer, preventing contact between the electrode active material and the solid electrolyte in the electrode and suppressing decomposition (reductive decomposition) of the solid electrolyte. Generally, hydride solid electrolytes have good stability (reduction resistance), so decomposition of the hydride solid electrolyte itself is suppressed. Furthermore, the hydride solid electrolyte fills the fine depressions and voids in the electrode active material, reducing the BET surface area of ​​the composite active material. This prevents an increase in the contact area with the solid electrolyte in the electrode and further suppresses decomposition of the solid electrolyte. For these reasons, Li-ion consumption due to decomposition of the solid electrolyte can be suppressed, and a decrease in the initial charge / discharge efficiency of the solid-state battery can be suppressed.

[0023] 1. Electrode active material The electrode active material in the present disclosure contains elemental Si.

[0024] The electrode active material is a so-called Si-based active material. Examples of Si-based active materials include simple Si, Si alloys, and Si oxides. Examples of Si alloys include Si-Al alloys, Si-Sn alloys, Si-In alloys, Si-Ag alloys, Si-Pb alloys, Si-Sb alloys, Si-Bi alloys, Si-Mg alloys, Si-Ca alloys, Si-Ge alloys, and Si-Pb alloys. The Si alloy may be a binary alloy or a multi-component alloy of three or more components. Examples of Si oxides include SiO.

[0025] The electrode active material may also be a so-called Si-C-based active material containing Si element and carbon (C element). The Si-C-based active material is typically a composite active material in which Si and C components exist independently. In the Si-C-based active material, Si may exist on at least one of the surface and the interior of the carbon (C) carrier. The inclusion of carbon in the electrode active material can improve the electronic conductivity of the electrode active material. As a result, the resistance (initial resistance) during the initial charge / discharge can be suppressed.

[0026] The electrode active material in the present disclosure may be solid particles without voids inside (particularly inside primary particles), or may be porous particles with voids inside (particularly inside primary particles). When the electrode active material is porous particles (e.g., porous Si), the voids can suppress expansion and contraction of the active material due to charging and discharging of the battery, contributing to good cycle characteristics of the solid battery. The electrode active material may be solid particles containing Si element, or porous particles containing Si element. Examples of solid particles containing Si element include solid particles of Si. Examples of porous particles containing Si element include particles containing porous Si and particles containing porous carbon and Si supported on the porous carbon. The electrode active material may also have a carbon coating portion, as described below, on the surface of the solid particles or porous particles.

[0027] As shown in FIG. 7, the electrode active material 1 (Si-C based active material) preferably has at least a core portion 1A containing Si element. On the other hand, the electrode active material 1 (Si-C based active material) may have a carbon coating portion 1B that covers the core portion 1A, or may not have a carbon coating portion. The core portion 1A preferably contains porous carbon α and silicon β filled inside the porous carbon α (pores of the porous carbon). Note that the silicon β may be present on the surface of the porous carbon α so as to coat the porous carbon α.

[0028] The average size (average pore diameter) of the pores in the porous carbon is not particularly limited, but may be, for example, 100 nm or less, 80 nm or less, 50 nm or less, 30 nm or less, or 10 nm or less. On the other hand, the average pore diameter may be, for example, 0.1 nm or more, 0.5 nm or more, 1 nm or more, or 2 nm or more. The pore volume of the porous carbon is, for example, 0.1 cm. 3 / g or more, and 0.5cm 3 / g or more, and 3On the other hand, the pore volume may be, for example, 10 cm 3 / g or less, and 5cm 3 The porous carbon is usually in the form of particles.

[0029] The core portion, in which porous carbon is filled with silicon, can be produced by a vapor deposition method such as CVD. For example, silicon can be deposited in the pores of the porous carbon by exposing the porous carbon to silane gas in a high-temperature environment. The conditions for CVD can be adjusted appropriately depending on the desired core portion and electrode active material.

[0030] The carbon content (porous carbon content) in the core is not particularly limited, but may be, for example, 30% by weight or more, 35% by weight or more, or 40% by weight or more, while the carbon content in the core is, for example, 60% by weight or less, 50% by weight or less, or 45% by weight or less.

[0031] As shown in FIG. 7, the electrode active material 1 (Si—C-based active material) may have a carbon coating portion 1B that coats a core portion 1A. The carbon coating portion may contain only C element, or may contain unavoidable elements such as O element and H element. The carbon coating portion may coat a portion of the surface of the core portion, or may coat the entire surface. In the former case, the coverage of the carbon coating portion is, for example, 40% or more, or may be 50% or more. The coverage of the carbon coating portion is, for example, 90% or less, or may be 80% or less, or may be 70% or less.

[0032] The thickness of the carbon coating portion is not particularly limited, but is, for example, 1.0 nm or more and 5.0 nm or less. The method for forming the carbon coating portion is not particularly limited, but examples thereof include vapor deposition methods such as CVD (Chemical Vapor Deposition).

[0033] The porosity of the porous particles is, for example, 4% or more, and may be 10% or more. The porosity may be, for example, 40% or less, and may be 20% or less. The porosity of the electrode active material can be calculated, for example, by removing the coating layer from the composite active material by washing with water, drying the material, and then observing the cross-section of the electrode active material with a scanning electron microscope (SEM). For example, the silicon portion and the void portion are clearly distinguished from the SEM image using image analysis software, and the image is binarized. The areas of the silicon portion and the void portion are determined, and the porosity (%) is calculated using the following formula. When the electrode active material is a Si alloy, the porosity can be calculated by using the silicon portion area in the formula below as the metal portion area. Porosity (%) = 100 × (area of ​​void part) / ((area of ​​silicon part) + (area of ​​void part))

[0034] The average size of the voids in the porous particles (average pore diameter) is not particularly limited, but is, for example, 10 nm or more and 100 nm or less. The pore volume is, for example, 0.10 cm 3 / g or more, 0.50cm 3 / g or less.

[0035] Here, the BET specific surface area of ​​the electrode active material is, for example, 3 m 2 / g or more, and 10m 2 / g or more, and 2 / g or more, 2 / g, and 25m 2 / g or more, and 2 / g or more, and 2 / g or more, and 2 / g or more, and 2 On the other hand, the BET specific surface area of ​​the electrode active material may be, for example, 100 m 2 The BET specific surface area of ​​the electrode active material can be calculated by, for example, removing the coating layer from the composite active material by washing with water or the like, drying the resulting electrode active material, and then subjecting the resulting electrode active material to the BET method using a pore size distribution measurement device.

[0036] The electrode active material may have a diamond-type crystalline phase, a clathrate I crystalline phase, or a clathrate II crystalline phase. In the clathrate I or II crystalline phase, a polyhedron (cage) containing pentagons or hexagons is formed by multiple Si elements. This polyhedron has a space inside that can encapsulate Li ions, thereby suppressing volume change during charge and discharge.

[0037] The electrode active material is usually in the form of particles. The electrode active material may be in the form of primary particles or secondary particles formed by aggregation of primary particles. The average particle diameter D of the electrode active material 50 is not particularly limited, but is, for example, 1 nm or more, may be 10 nm or more, or may be 100 nm or more. 50 is, for example, 50 μm or less, and may be 20 μm or less. 50 refers to the cumulative 50% particle size in the volume-based particle size distribution measured by a laser diffraction particle size distribution analyzer. The electrode active material (porous Si), which is a porous particle, can be prepared by the method described in the examples below.

[0038] 2.Coating layer The coating layer coats the surface of the electrode active material and contains a hydride solid electrolyte.

[0039] The hydride solid electrolyte of the present disclosure has, for example, a metal cation such as a Li ion and a complex ion (anion) containing H. The complex ion may be, for example, [M x H y ] z- M is the central element in the complex ion, and examples thereof include nonmetallic elements such as B and N, and metallic elements such as Al. x, y, and z are numbers determined arbitrarily depending on the valence of the central element. Examples of complex ions include [BH4] - , [NH2] - , [AlH4] - , and [AlH6] 3- Examples include:

[0040] The hydride solid electrolyte may also contain a halide salt (halide). Examples of halides include lithium halides such as LiF, LiCl, LiBr, and LiI. The halide salt preferably forms a solid solution with the hydride solid electrolyte. Forming a solid solution with the halide salt makes it easier for the hydride solid electrolyte to maintain the hexagonal crystal structure described below, even in low-temperature environments such as room temperature. The proportion of the halide is not particularly limited, but is, for example, 0.1 mol or more and 0.5 mol or less per mol of the hydride solid electrolyte. The presence of a halide in the coating layer can be confirmed, for example, by SEM-EDX (SEM-EDS; scanning electron microscope / energy dispersive X-ray spectroscopy). The proportion of halogen elements calculated by the SEM-EDX measurement (the proportion of halogen elements derived from the halogen element in the coating layer) is, for example, 1.0 wt % or more, or may be 2.0 wt % or more, or may be 3.0 wt % or more. On the other hand, the proportion of halogen elements is, for example, 8.0% by weight or less, may be 7.0% by weight or less, or may be 6.0% by weight or less.

[0041] Furthermore, the hydride solid electrolyte of the present disclosure preferably has a predetermined crystal structure. Here, the crystal structure of the hydride solid electrolyte will be described with reference to FIG. 2. In FIG. 2, LiBH4 is shown as the hydride solid electrolyte. The hydride solid electrolyte has an orthorhombic crystal structure (low-temperature phase) as shown in FIG. 2(a) in a low-temperature environment such as room temperature, but has a hexagonal crystal structure (high-temperature phase) as shown in FIG. 2(b) in a high-temperature environment such as 120°C or higher. The hydride solid electrolyte of the present disclosure may have either an orthorhombic crystal structure or a hexagonal crystal structure, with the latter being preferred because this improves ionic conductivity. In particular, the hydride solid electrolyte preferably has a hexagonal crystal structure as the main phase. "Having it as the main phase" means that the peak belonging to the hexagonal crystal structure has the largest diffraction intensity among the peaks observed in X-ray diffraction measurement. The crystal structure of the hydride solid electrolyte can be confirmed by X-ray diffraction measurement (XRD measurement) using CuKα radiation.

[0042] In X-ray diffraction measurements using CuKα radiation, peak positions belonging to the orthorhombic crystal structure include, for example, 2θ=25.63°, 26.94°, 27.73°, and 29.90°. In X-ray diffraction measurements using CuKα radiation, peak positions belonging to the hexagonal crystal structure include, for example, 2θ=23.99°, 25.58°, 27.29°, 35.18°, 41.64°, 45.88°, and 49.58°. The peak positions may vary within a range of ±0.50°, ±0.30°, or ±0.10°.

[0043] The ionic conductivity of the hydride solid electrolyte at 25°C is not particularly limited, but is preferably high. The ionic conductivity is, for example, 1.0 × 10 -8 S / cm or more, 1.0×10 -3 S / cm or less.

[0044] The coating layer may contain only one type of hydride solid electrolyte, or may contain two or more types.

[0045] The coating layer in the present disclosure may also contain at least one of a conductive material and a binder. Examples of conductive materials include carbon materials, metal particles, and conductive polymers. Examples of carbon materials include particulate carbon materials such as acetylene black (AB) and ketjen black (KB), and fibrous carbon materials such as carbon fibers, carbon nanotubes (CNT), and carbon nanofibers (CNF). Examples of binders include rubber-based binders and fluoride-based binders. When the coating layer contains at least one of a conductive material and a binder, the proportion of the hydride solid electrolyte in the coating layer is, for example, 90% by weight or more and 98% by weight or less. On the other hand, the coating layer does not necessarily contain a conductive material or a binder.

[0046] The thickness (average thickness) of the coating layer is not particularly limited, but is, for example, 5.0 nm or less, or may be 3.0 nm or less, or 1.5 nm or less, or 1.0 nm or less, while the thickness of the coating layer is, for example, 0.1 nm or more, or may be 0.4 nm or more, or may be 0.5 nm or more.

[0047] Furthermore, in the composite active material of the present disclosure, the coverage by the coating layer is not particularly limited, but may be, for example, 20% or more, 25% or more, or 30% or more. On the other hand, the coverage may be, for example, 85% or less, 80% or less, 70% or less, 60% or less, 50% or less, or 40% or less. Particularly when the electrode active material is a porous particle, if the coverage is too high, the BET specific surface area of ​​the composite active material may become too small, and the effect of suppressing volume change may not be fully exerted. The coverage can be calculated by observation with a scanning electron microscope (SEM).

[0048] 3.Composite active material The composite active material of the present disclosure is typically used in batteries. In particular, the composite active material is preferably used in solid-state batteries. The composite active material may be used as either a positive electrode active material or a negative electrode active material in the battery, with the latter being preferred because a battery with a higher capacity can be obtained.

[0049] The composite active material is usually in the form of particles. The average particle diameter (D 50 ) is the average particle diameter (D 50 ), but is not particularly limited thereto, and is, for example, 1 μm or more and 50 μm or less.

[0050] Here, the BET specific surface area of ​​the composite active material is usually smaller than that of the electrode active material described above. This is because the hydrogenated solid electrolyte fills the grooves on the surface of the electrode active material and the voids inside. The BET specific surface area of ​​the composite active material is, for example, 50 m 2 / g or less, and 45m 2 / g or less, and 2 / g or less, and 2 On the other hand, the BET specific surface area of ​​the composite active material may be, for example, 3 m 2 / g or more, and 10m 2 / g or more, and 2 / g or more, 2 / g or more. When the BET specific surface area of ​​the composite active material is X and the BET specific surface area of ​​the electrode active material is Y, X / Y is, for example, 0.5 or more, or may be 0.6 or more, or may be 0.7 or more. On the other hand, X / Y is, for example, 0.9 or less, or may be 0.8 or less.

[0051] Furthermore, when the electrode active material is a porous particle, the average pore diameter and pore volume of the composite active material are usually smaller than those of the above-mentioned electrode active material (porous particle). This is because the pores of the porous particle are filled with a hydrogenated solid electrolyte. The average pore diameter of the composite active material is, for example, 80 nm or less. The pore volume of the composite active material is, for example, 0.05 cm 3 / g or more, and 0.10 cm 3 / g or more, and 3 On the other hand, the pore volume may be, for example, 0.40 cm 3 / g or less, and 0.30 cm 3 / g or less.

[0052] The composite active material can be produced by subjecting a mixture containing an electrode active material and a hydride solid electrolyte to a compressive shear treatment to form the coating layer. Alternatively, the hydride solid electrolyte and a halide salt may be mixed in advance to form a solid solution. Examples of compressive shear treatment include placing the mixture in a container and mixing it with a crushing medium such as a blade, beads, or balls, and applying compressive shear energy to the mixture present between the container wall and the mixture.

[0053] B. Solid state battery Fig. 3 is a schematic cross-sectional view illustrating a solid-state battery according to the present disclosure. The solid-state battery 20 shown in Fig. 3 includes a positive electrode active material layer 21, a negative electrode active material layer 22, a solid electrolyte layer 23 disposed between the positive electrode active material layer 21 and the negative electrode active material layer 22, a positive electrode current collector 24 that collects electrons from the positive electrode active material layer 21, and a negative electrode current collector 25 that collects electrons from the negative electrode active material layer 22. In particular, in the solid-state battery 20 according to the present disclosure, the negative electrode active material layer 22 contains the composite active material described above in "A. Composite active material."

[0054] According to the present disclosure, since the negative electrode active material layer contains the above-described composite active material, a solid-state battery with good initial charge-discharge efficiency is obtained.

[0055] 1.Negative electrode active material layer The negative electrode active material layer contains the composite active material described above. The negative electrode active material layer may also contain at least one of a conductive material, a binder, and an electrolyte, as necessary.

[0056] The conductive material and binder are the same as those described in "A. Composite Active Material." Examples of electrolytes include solid electrolytes. Examples of solid electrolytes include inorganic solid electrolytes such as sulfide solid electrolytes, oxide solid electrolytes, nitride solid electrolytes, halide solid electrolytes, and complex hydrides. Among these, sulfide solid electrolytes are particularly preferred because of their high ionic conductivity. Sulfide solid electrolytes typically contain sulfur (S) as the main anion element. Oxide solid electrolytes, nitride solid electrolytes, and halide solid electrolytes typically contain oxygen (O), nitrogen (N), and halogen (X), respectively, as the main anion element.

[0057] The thickness of the negative electrode active material layer is not particularly limited, but is, for example, 0.1 μm or more and 1000 μm or less. The negative electrode active material layer can be formed, for example, by a coating method. In the coating method, a slurry containing at least the above-mentioned composite active material is applied to a negative electrode current collector and then dried to form the negative electrode active material layer.

[0058] 2. Positive electrode active material layer The positive electrode active material layer contains at least a positive electrode active material. The positive electrode active material layer may also contain at least one of a conductive material, a binder, and an electrolyte, as necessary. The conductive material, binder, and electrolyte are the same as those described in "1. Negative electrode active material layer."

[0059] The positive electrode active material may be, for example, an oxide active material, such as LiCoO2 or LiNi. 0.33 Co 0.33 Mn 0.33 O2 and LiNi 0.8 Co 0.15 Al 0.05 O2, etc., rock salt layered active materials, LiMn2O4 and Li4Ti5O 12 and olivine type active materials such as LiFePO4. The positive electrode active material is, for example, in the form of particles.

[0060] The positive electrode active material layer can be formed by the above-mentioned coating method. The thickness of the positive electrode active material layer is, for example, 0.1 μm or more and 1000 μm or less.

[0061] 3.Solid electrolyte layer The solid electrolyte layer is a layer disposed between the positive electrode active material layer and the negative electrode active material layer, and contains at least a solid electrolyte. It may also contain a binder, if necessary. The binder is the same as that described in "1. Negative electrode active material layer."

[0062] Examples of the solid electrolyte include the inorganic solid electrolytes described in "1. Negative electrode active material layer." Other examples of the solid electrolyte include organic solid electrolytes such as polymer electrolytes and gel electrolytes. The solid electrolyte layer may contain a liquid electrolyte (electrolytic solution) as the electrolyte. The thickness of the solid electrolyte layer is, for example, 1 μm or more and 500 μm or less.

[0063] 4. Other configurations The solid-state battery of the present disclosure typically includes a positive electrode current collector that collects current from the positive electrode active material layer and a negative electrode current collector that collects current from the negative electrode active material layer. Examples of materials for the positive electrode current collector include stainless steel, aluminum, nickel, iron, titanium, and carbon. Examples of materials for the negative electrode current collector include stainless steel, copper, nickel, and carbon.

[0064] The solid-state battery according to the present disclosure may further include a restraining jig that applies a restraining pressure to the positive electrode active material layer, the solid electrolyte layer, and the negative electrode active material layer in the thickness direction. The restraining pressure is, for example, 0.1 MPa or more, or may be 1 MPa or more, or may be 5 MPa or more. Meanwhile, the restraining pressure is, for example, 100 MPa or less, or may be 50 MPa or less, or may be 20 MPa or less.

[0065] 5. Solid state battery The type of solid-state battery in the present disclosure is typically a lithium-ion battery. The solid-state battery in the present disclosure may be a semi-solid-state battery or an all-solid-state battery. The solid-state battery in the present disclosure may be a primary battery or a secondary battery, with secondary batteries being preferred. This is because they can be repeatedly charged and discharged, making them useful, for example, as automotive batteries.

[0066] Examples of applications of solid-state batteries include power sources for vehicles such as hybrid electric vehicles (HEVs), plug-in hybrid electric vehicles (PHEVs), electric vehicles (BEVs), gasoline-powered vehicles, and diesel-powered vehicles. In particular, they are preferably used as driving power sources for hybrid electric vehicles (HEVs), plug-in hybrid electric vehicles (PHEVs), or electric vehicles (BEVs). Solid-state batteries may also be used as power sources for mobile objects other than vehicles (for example, railways, ships, and aircraft), and may also be used as power sources for electrical appliances such as information processing devices.

[0067] The present disclosure is not limited to the above-described embodiments. The above-described embodiments are merely examples, and any configuration that is substantially identical to the technical idea described in the claims of the present disclosure and that provides similar effects is included within the technical scope of the present disclosure. [Example]

[0068] [Example 1] (Preparation of composite active material) First, an electrode active material (porous Si particles: p-Si) containing Si element and having voids inside was prepared as follows. Si particles (average particle diameter: 0.5 μm) and Na particles were mixed in a molar ratio of 1:1 and heated at 700 °C to synthesize NaSi (Zintl compound). The Zintl compound was then heated at 340 °C to remove Na. Additional heating at 430 °C was then performed to further remove Na. This produced an electrode active material.

[0069] Next, the above-mentioned electrode active material was coated with a coating layer in the following manner. LiBH4 and LiI were weighed out in a 3:1 molar ratio and mixed in an agate mortar for 5 minutes. Tetrahydrofuran (THF) was added to the resulting mixture to obtain a solution. The amount of THF added was five times the weight of the porous Si particles. The porous Si particles were added to the solution and mixed on a hot plate at 100 °C. After the THF had fully evaporated, the powder was collected and further dried in vacuum at 120 °C for 12 hours. This resulted in a composite active material in which a coating layer containing a hydride solid electrolyte (3LiBH4-LiI) containing a halide salt was formed on the surface of the porous Si. The ratio of the hydride solid electrolyte to the total of the hydride solid electrolyte and porous Si was 1.7 wt%. The ratio of the hydride solid electrolyte to the total surface area of ​​the electrode active material was determined based on the BET specific surface area of ​​the electrode active material described below, assuming that a coating layer of the same thickness (target thickness: 0.1 nm) would be formed on all particles.

[0070] (Preparation of evaluation battery) The composite active material, sulfide solid electrolyte (LiI-Li2S-P2S5; sulfide glass solid electrolyte), conductive material (carbon nanotubes), binder (styrene butadiene rubber (SBR)), and dispersion medium (diisobutyl ketone) were mixed to obtain a negative electrode slurry. The volume ratio of the composite active material, sulfide solid electrolyte, binder, and conductive material in the negative electrode slurry was 51.2:41.9:6.6:0.3. The negative electrode slurry was applied to a negative electrode current collector (Cu foil) and dried to obtain a negative electrode having a negative electrode active material layer and a negative electrode current collector.

[0071] In addition, the positive electrode active material (LiNi 0.8 Co 0.15 Al 0.05A positive electrode slurry was prepared by mixing a composite active material (LiI-LiS-P2S5; crystalline sulfide solid electrolyte), a binder (styrene butadiene rubber (SBR)), a conductive material (vapor-grown carbon fiber (VGCF)), and a dispersion medium (1,2,3,4-tetrahydronaphthalene). The volume ratio of the composite active material, sulfide solid electrolyte, binder, and conductive material in the positive electrode slurry was 59.7:32.1:5.4:2.8. The positive electrode slurry was applied to a positive electrode current collector (Al foil) and dried to obtain a positive electrode with a positive electrode active material layer and a positive electrode current collector. The size of the positive electrode was adjusted to be smaller than that of the negative electrode.

[0072] In addition, a sulfide solid electrolyte (LiI-Li2S-P2S5; sulfide glass solid electrolyte), a binder (acrylate butadiene rubber: ABR), and a dispersion medium (n-heptane, butyl butyrate) were mixed to obtain a slurry. The slurry was applied to a substrate (Al foil) and dried to obtain a transfer member with a solid electrolyte layer. The size of the solid electrolyte layer was the same as that of the negative electrode.

[0073] The negative electrode and the transfer member were stacked and pressed together so that the negative electrode active material layer and the solid electrolyte layer faced each other. The base material was then peeled off to transfer the solid electrolyte layer. The positive electrode was then stacked and pressed together so that the solid electrolyte layer and the positive electrode active material layer faced each other. A terminal was then attached, and the electrodes were constrained at a pressure of 20 MPa relative to the electrode surface area. This resulted in a test battery (all-solid-state battery).

[0074] [Examples 2 to 3] Composite active materials and evaluation batteries were fabricated in the same manner as in Example 1, except that the proportion of the hydride solid electrolyte was changed as shown in Table 1. The proportion of the hydride solid electrolyte was set so that the target thickness of the coating layer was 0.4 nm and 1.0 nm, respectively.

[0075] [Comparative Example 1] A test battery was fabricated in the same manner as in Example 1, except that the porous Si particles were used as the negative electrode active material.

[0076] [Examples 4 to 5] Composite active materials and evaluation batteries were fabricated in the same manner as in Example 1, except that solid Si particles (Si particles described in Example 1) having a diamond-type crystalline phase were used instead of porous Si particles, and the proportion of hydride solid electrolyte was adjusted so that the target thickness of the coating layer was 0.4 nm and 1.0 nm, respectively. Here, in Examples 4 and 5, the target thickness of the coating layer is the same as in Examples 2 and 3, respectively, but the proportion of hydride solid electrolyte is different from that in Examples 2 and 3, as shown in Table 1. This is because the electrode active materials in Examples 4 and 5 are solid particles and have smaller BET specific surface areas than the electrode active materials in Examples 2 and 3.

[0077] Comparative Example 2 A test battery was fabricated in the same manner as in Example 1, except that solid Si particles were used as the negative electrode active material.

[0078] [Rating 1] The negative electrode active materials prepared in Examples 1 to 5 and Comparative Examples 1 and 2 were evaluated as follows.

[0079] (BET specific surface area measurement) For the negative electrode active materials prepared in Examples 1 to 5 and Comparative Examples 1 and 2, the BET specific surface area was calculated by the BET method using a pore size distribution analyzer. The results are shown in Table 1. In Examples 1 to 3, the p-Si was coated in the same manner as in Comparative Example 1, so the specific surface area of ​​Comparative Example 1 can be considered to be the specific surface area of ​​the electrode active material in the negative electrode active material (composite active material) of Examples 1 to 3. Similarly, the specific surface area of ​​Comparative Example 2 can be considered to be the specific surface area of ​​the electrode active material in the negative electrode active material (composite active material) of Examples 4 and 5.

[0080] (SEM-EDS analysis) The composite active materials prepared in Examples 1 to 5 were subjected to SEM-EDS analysis to measure the proportion of halogen element (I element) in the composite active materials. The results are shown in Table 1. The relationship between the BET specific surface area and the proportion of I element in Examples 1 to 3 and Comparative Example 1 is summarized in Figure 4.

[0081] [Table 1]

[0082] As shown in Table 1 and FIG. 4, it was confirmed that the proportion of I element increased by increasing the proportion of the hydride solid electrolyte. Furthermore, in all Examples, the BET specific surface area was smaller than that of the Comparative Example (electrode active material without a coating layer). Regarding Examples 1 to 3 and Comparative Example 1, the pore volume of each negative electrode active material was measured using a pore distribution analyzer, although not shown in the table. Comparative Example 1 had a pore volume of approximately 0.3 cm. 3 / g, Example 1 is about 0.25 cm 3 / g, and Example 2 is about 0.18 cm 3 / g, and Example 3 is about 0.16 cm 3 / g. It is presumed that the pores of the porous particles were filled with the hydride solid electrolyte, resulting in a decrease in the specific surface area and pore volume.

[0083] (XRD analysis) In Reference Example 1, a hydride solid electrolyte having a hexagonal crystal structure was prepared. The hydride solid electrolyte having a hexagonal crystal structure was prepared by mixing LiBH4 and LiI weighed out to a molar ratio of 3:1 in an agate mortar, followed by heat treatment (in a vacuum environment, at 120°C for 12 hours). In Reference Example 2, a hydride solid electrolyte having an orthorhombic crystal structure was prepared. The hydride solid electrolyte having an orthorhombic crystal structure was prepared in the same manner as in Reference Example 1, except that lithium halide (LiI) was not doped. The results of XRD measurements for Reference Examples 1 and 2 are shown in FIG. 5(a). Note that peaks characteristic of a hexagonal crystal structure are marked in FIG. 5(a). The results of XRD measurements for the negative electrode active materials prepared in Reference Example 1, Example 1, and Comparative Example 1 are shown in FIG. 5(b).

[0084] As shown in FIG. 5(a), in Reference Example 1, a peak characteristic of a hexagonal crystal structure was observed as the main peak. On the other hand, for the hydride solid electrolyte of Reference Example 2, although peaks thought to be impurities were also observed, no peak characteristic of a hexagonal crystal was observed. This confirmed that doping with halide (lithium halide) makes it easier to maintain the hexagonal crystal structure. Furthermore, as shown in FIG. 5(b), no hexagonal peak was observed in Comparative Example 1 (porous Si), but the above-mentioned hexagonal peak was observed in Example 1 (composite porous Si). Note that for Example 1, although the hexagonal peak is difficult to observe in the figure due to the intensity of other peaks, it is confirmed that a hexagonal peak is present numerically.

[0085] [Rating 2] The all-solid-state batteries produced in Examples 1 to 5 and Comparative Examples 1 and 2 were evaluated as follows.

[0086] (Calculation of initial charge / discharge efficiency) The test batteries fabricated in Examples 1 to 5 and Comparative Examples 1 and 2 were constrained under a pressure of 20 MPa. The constrained test batteries were activated by CCCV charging and CCCV discharging. The CCCV charging conditions were 1 / 10C, ​​1 / 100C, and an upper limit voltage of 4.05V. The CCCV discharging conditions were 1 / 3C, 1 / 100C, and a lower limit voltage of 2.5V. The activated test batteries were subjected to an initial charge (CCCV charge) to measure the initial charge capacity (specific capacity). Thereafter, an initial discharge (CCCV discharge) was performed to measure the initial discharge amount (specific capacity). The initial charge / discharge efficiency ((initial discharge capacity / initial charge capacity) × 100) was calculated from the obtained initial charge capacity and initial discharge capacity. The initial charge conditions were 1 / 3C, 1 / 100C, and an upper limit voltage of 4.05V, and the initial discharge conditions were 1 / 3C, 1 / 100C, and a lower limit voltage of 2.5V. The charge / discharge efficiency was calculated as an average value of n=4. The results are shown together with the specific surface area in Table 2. Examples 1 to 3 and Comparative Example 1 are also shown in FIG.

[0087] [Table 2]

[0088] As shown in Table 2, the initial charge-discharge efficiency was improved by forming a coating layer in both cases where the electrode active material was a porous particle (Examples 1 to 3 and Comparative Example 1) and where it was a solid particle (Examples 4 to 5 and Comparative Example 2). Furthermore, as shown in FIG. 6(a), the initial charge capacity (left side of the bar graph) did not change significantly between Comparative Example 1 and Examples 1 to 3, but the initial discharge capacity (right side of the bar graph) was significantly higher in the Examples than in the Comparative Examples. This suggests that the consumption of Li ions due to decomposition of the solid electrolyte was suppressed in the Examples. Furthermore, as shown in Table 2 and FIG. 6(b), the initial charge-discharge efficiency increased as the specific surface area decreased, suggesting that the contact area between the electrode active material and the solid electrolyte could be reduced, thereby suppressing decomposition of the solid electrolyte.

[0089] [Example 6] Silicon (Si) was filled into the porous carbon base material by CVD to form a core. A carbon coating was then formed on the surface of the core by CVD. This resulted in the production of an electrode active material having a core and a carbon coating. Specifically, the porous carbon was placed in a tubular furnace, heated to 500°C using Ar flow, and CVD was performed for 30 minutes while flowing silane (SiH). Prior to heating, the atmosphere in the tubular furnace was replaced with Ar gas. This resulted in the filling of the porous carbon with Si. Subsequently, in the same reaction vessel, the temperature was raised to 650°C using Ar flow, and CVD was performed for 20 minutes while flowing acetylene (C2H2) and Ar. This resulted in the formation of a carbon coating on the surface of the core. Note that, when preparing an electrode active material by CVD, N2 gas can be used instead of Ar gas as an inert gas. A coating layer was formed on the resulting electrode active material in the same manner as in Example 1, resulting in a composite active material. The proportion of the hydride solid electrolyte was set so that the target thickness of the coating layer was 0.4 nm. Except for using the above composite active material, an evaluation battery (all-solid-state battery) was fabricated in the same manner as in Example 1. Note that, since the fabricated electrode active material has silicon filled inside the porous carbon, it is believed that the BET specific surface area is smaller than that of Comparative Example 1, in which the electrode active material is porous Si particles.

[0090] [Examples 7 to 8] Test batteries were fabricated in the same manner as in Example 6, except that the proportion of the hydride solid electrolyte was changed to prepare the composite active material so that the target thickness of the coating layer was 1.0 nm (Example 7) or 5.0 nm (Example 8).

[0091] Comparative Example 3 A test battery was fabricated in the same manner as in Example 6, except that solid Si particles without a coating layer were used as the negative electrode active material.

[0092] Comparative Example 4 An electrode active material having a core portion and a carbon coating portion was produced in the same manner as in Example 6. A battery for evaluation was produced in the same manner as in Example 6, except that this electrode active material was used as the negative electrode active material.

[0093] [Rating 3] (Measurement of carbon content) The carbon content of the negative electrode active materials prepared in Examples 6 to 8 and Comparative Examples 3 and 4 was measured by XPS analysis. The results are shown in Table 3. As shown in Table 3, the presence of carbon was also confirmed in Comparative Example 3, which did not have a carbon coating portion. This is presumed to be carbon derived from functional groups that are inevitably contained.

[0094] (Calculation of initial resistance value) The initial resistance values ​​of the all-solid-state batteries fabricated in Examples 6 to 8 and Comparative Examples 3 and 4 were calculated as follows. The evaluation batteries were activated under the same conditions as in Evaluation 2 above. Then, CCCV charging (1 / 10C, ​​1 / 100C, upper limit voltage 4.05V) was performed up to SOC 50%. The voltage change (ΔV) when a current value of 6C rate was applied was obtained, and the resistance value (initial resistance value) was calculated according to Ohm's law. The resistance value of Comparative Example 3 was set to 100 and evaluated relatively. The results are shown in Table 3 and FIG. 8.

[0095] [Table 3]

[0096] As shown in Table 3 and FIG. 8, it was shown that the initial resistance can be suppressed by using a Si-C composite active material (Si-C) as the electrode active material. It was also confirmed that the initial resistance can be further suppressed by coating the surface of the Si-C with a coating layer containing a hydride solid electrolyte. Together with the results of Tables 1 and 2 above, it was shown that a solid-state battery with good charge / discharge efficiency and suppressed initial resistance can be obtained by coating the surface of the Si-C composite active material with a coating layer containing a hydride solid electrolyte. [Explanation of symbols]

[0097] 1 …electrode active material 2 …Covering layer 10 …complex living substances 20 … Solid-state batteries

Claims

1. an electrode active material containing Si element; A composite active material having a coating layer that coats the surface of the electrode active material, The coating layer comprises a composite active material containing a hydride solid electrolyte.

2. The composite active material of claim 1 , wherein the hydride solid electrolyte contains a halide salt.

3. 2. The composite active material of claim 1, wherein the hydride solid electrolyte has a hexagonal crystal structure.

4. The BET specific surface area of ​​the composite active material is 15 m ２ / g or more, 45m ２ The composite active material according to claim 1 , wherein the Mo content is 0.1 / g or less.

5. The composite active material according to claim 1 , wherein the electrode active material contains carbon.

6. 6. The composite active material according to claim 5, wherein the electrode active material is composed of a core portion containing the Si element and a carbon coating portion coating the core portion.

7. The composite active material according to claim 6 , wherein the core portion contains porous carbon and silicon filled inside the porous carbon.

8. 6. The composite active material according to claim 5, wherein the carbon content in the composite active material is 30% by weight or more and 60% by weight or less.

9. 2. The composite active material according to claim 1, wherein the coating layer has a thickness of 0.4 nm or more and 5.0 nm or less.

10. A solid-state battery having a positive electrode active material layer, a negative electrode active material layer, and a solid electrolyte layer disposed between the positive electrode active material layer and the negative electrode active material layer, A solid-state battery, wherein the negative electrode active material layer contains the composite active material according to any one of claims 1 to 9.

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