Anode for solid-state battery, solid-state battery, and method for manufacturing anode for solid-state battery

Abstract

To provide a negative electrode for a solid-state battery, in which an ion transfer resistance is suppressed.SOLUTION: A negative electrode 12 for a solid-state battery, of the present disclosure, comprises a negative electrode active material layer 11 including a negative electrode active material 30 and a solid electrolyte 20, wherein an average aspect ratio of the negative electrode active material 30 in the negative electrode active material layer 11 is larger than 0.5, and an average elastic modulus of the negative electrode active material 30 is 370 MPa or less. Also, the solid-state battery 100 of the present disclosure comprises: a positive electrode 16; the negative electrode 12; and a solid electrolyte layer 13 provided between the positive electrode 16 and the negative electrode 12. The negative electrode 12 is the negative electrode 12 for the solid-state battery.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present disclosure relates to a negative electrode for a solid state battery, a solid state battery, and a method for manufacturing a negative electrode for a solid state battery. [Background technology]

[0002] In recent years, research and development of all-solid-state batteries using solid electrolytes has been actively conducted. Patent Document 1 discloses an all-solid-state battery equipped with a negative electrode containing a high content of graphite particles, from 70% by mass to 90% by mass, in the negative electrode mixture layer.

[0003] Patent Document 2 discloses an all-solid-state battery in which the hardness of the graphite contained in the negative electrode active material layer is 0.36 GPa or more. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Publication No. 2019-16484 [Patent Document 2] International Publication No. 2014 / 016907 Summary of the Invention [Problem to be solved by the invention]

[0005] The present disclosure provides a negative electrode for a solid-state battery with reduced ion transport resistance. [Means for solving the problem]

[0006] The negative electrode for a solid battery according to the present disclosure comprises: a negative electrode active material layer including a negative electrode active material and a solid electrolyte; the average aspect ratio of the negative electrode active material in the negative electrode active material layer is greater than 0.5, The negative electrode active material has an average elastic modulus of 370 MPa or less. [Effects of the Invention]

[0007] The present disclosure provides a negative electrode for a solid-state battery with reduced ion transport resistance. [Brief explanation of the drawings]

[0008] [Figure 1] FIG. 1 is a schematic cross-sectional view showing how lithium ions and electrons are transported and diffused in the negative electrode active material layer during charging of an all-solid-state lithium ion secondary battery. [Figure 2] FIG. 2 is a cross-sectional view showing a schematic configuration of the negative electrode for an all-solid-state lithium ion secondary battery in the first embodiment. [Figure 3] FIG. 3 is a cross-sectional view showing a schematic configuration of an all-solid-state lithium-ion secondary battery in the second embodiment. [Figure 4A] FIG. 4A is an explanatory diagram showing how to determine the aspect ratio of the negative electrode active material in the first embodiment. [Figure 4B] FIG. 4B is an explanatory diagram showing how to determine the orientation angle of the negative electrode active material in Embodiment 1. As shown in FIG. [Figure 5] FIG. 5 is an explanatory diagram showing the mechanism by which springback occurs in the negative electrode active material layer in the first embodiment. [Figure 6A] FIG. 6A is an FE-SEM image of a negative electrode active material layer including the negative electrode active material of Comparative Example 1. [Figure 6B] FIG. 6B is an image after binarization processing of the FE-SEM image shown in FIG. 4A. [Figure 7] FIG. 7 is a cross-sectional view showing a schematic configuration of a symmetric cell used for measuring ion transport resistance. [Figure 8] FIG. 8 is a graph showing a Cole-Cole plot obtained by measuring the impedance of the symmetric cell shown in FIG. [Figure 9] FIG. 9 is a diagram showing an equivalent circuit of the symmetric cell shown in FIG. 7 in the impedance measurement shown in FIG. [Figure 10] FIG. 10 is a graph showing the relationship between the pressing pressure and the Warburg open circuit resistance Wo-R for the symmetric cell of Comparative Example 1 and the symmetric cell of Comparative Example 4. [Figure 11]FIG. 11 is a graph showing the relationship between the confining pressure and the Warburg open circuit resistance Wo-R for the symmetric cell of Comparative Example 1 and the symmetric cell of Comparative Example 4. [Figure 12A] FIG. 12A is a graph showing the results of a charge rate test at 25° C. for the batteries of Comparative Example 5 and Example 5. [Figure 12B] FIG. 12B is a graph showing the results of a charge rate test at 60° C. for the batteries of Comparative Example 5 and Example 5. [Figure 13A] FIG. 13A is a graph showing the results of a charge rate test at 25° C. for the batteries of Examples 6 to 9. [Figure 13B] FIG. 13B is a graph showing the relationship between the volume ratio of the negative electrode active material and the capacity retention rate for the batteries of Examples 6 to 9. DETAILED DESCRIPTION OF THE INVENTION

[0009] (Findings that formed the basis of this disclosure) A lithium-ion secondary battery is composed of a positive electrode, a negative electrode, and an electrolyte disposed between them. The electrolyte is a non-aqueous liquid or solid. However, because widely used electrolyte solutions are flammable, lithium-ion batteries using electrolyte solutions must be equipped with a system to ensure safety. On the other hand, solid electrolytes are non-flammable, making it possible to simplify such systems. Therefore, various lithium-ion secondary batteries using solid electrolytes (hereinafter referred to as all-solid-state lithium-ion secondary batteries) have been proposed.

[0010] There is a major difference between lithium-ion secondary batteries that use electrolytes and all-solid-state lithium-ion secondary batteries in the method of forming lithium-ion conduction paths in the electrodes. In lithium-ion secondary batteries that use electrolytes, the lithium-ion conduction paths are formed by infiltrating the electrolyte into the gaps between the active materials after electrode formation. On the other hand, in all-solid-state lithium-ion secondary batteries, the lithium-ion conduction paths are formed by kneading the active material, solid electrolyte, and binder and then pressure-molding them.

[0011] There is also a significant difference in the mechanism of lithium ion transport from the electrolyte to the active material between liquid electrolyte lithium ion secondary batteries and all-solid-state lithium ion secondary batteries. In liquid electrolyte lithium ion secondary batteries, after the desolvation reaction of lithium ions, lithium ions are transported through the organic SEI layer formed on the surface of the electrode. On the other hand, in all-solid-state lithium ion secondary batteries, lithium ions are transported by being pushed out one after another from the solid electrolyte to the active material in a domino effect.

[0012] Due to the two differences mentioned above, all-solid-state lithium-ion secondary batteries have technical issues that differ from those of lithium-ion secondary batteries that use electrolytes, and measures to address these issues are necessary.

[0013] The charging operation of an all-solid-state lithium-ion secondary battery is as follows. Lithium stored in the positive electrode active material in the positive electrode active material layer releases electrons and becomes ionized (i.e., oxidized), and then moves from the positive electrode active material layer to the solid electrolyte layer, using the connected solid electrolyte sections in the positive electrode active material layer as a pathway. The lithium ions that move from the solid electrolyte layer to the negative electrode active material layer then reach the negative electrode active material, using the connected solid electrolyte sections in the negative electrode active material layer as a pathway. The lithium ions that reach the negative electrode active material receive electrons from the negative electrode active material (i.e., are reduced). In this way, lithium diffuses from the solid electrolyte into the negative electrode active material and is stored in the negative electrode active material layer.

[0014] In the lithium ion conduction mechanism during the charging operation of an all-solid-state lithium ion secondary battery, it is known that the transport and diffusion of lithium ions in the negative electrode active material layer has a significant impact on the charge rate performance. Figure 1 shows how lithium ions and electrons are transported and diffused in the negative electrode active material layer during the charging operation of an all-solid-state lithium ion secondary battery. As shown in Figure 1, a negative electrode 52 includes a negative electrode current collector 50 and a negative electrode active material layer 51. The negative electrode active material layer 51 includes a negative electrode active material 70 and a solid electrolyte 60. A solid electrolyte layer 53 is disposed between the negative electrode 52 and the positive electrode (not shown). In Figure 1, Li + represents lithium ion, and e- indicates an electron, respectively.

[0015] In a typical negative electrode active material layer 51 as shown in Fig. 1, there exist both an electron conduction path formed by contact between particles of the negative electrode active material 70 and an ion conduction path formed by interconnected particles of the solid electrolyte 60. The main factors that have a significant impact on the charge rate performance of an all-solid-state lithium-ion secondary battery are the resistance to lithium ion transport (hereinafter referred to as ion transport resistance) and the resistance to lithium diffusion from the solid electrolyte 60 to the negative electrode active material 70 (hereinafter referred to as reaction resistance). In Fig. 1, the ion transport resistance is represented by the dotted line indicated by reference numeral 55, and the reaction resistance is represented by the solid line indicated by reference numeral 56.

[0016] Patent Document 1 aims to increase the capacity of an all-solid-state battery by reducing the content of solid electrolyte, which transports lithium ions but does not have a power storage function, and increasing the content of graphite particles, which have a power storage function, in the negative electrode mixture layer. Furthermore, Patent Document 1 mentions that by increasing the specific surface area of ​​the graphite particles by roughening the surface, the physical contact area between the graphite particles and the solid electrolyte in the negative electrode mixture layer can be increased, thereby reducing contact resistance, i.e., reaction resistance.

[0017] Patent Document 2 mentions that in the negative electrode active material layer, by setting the micro-scale hardness of graphite measured by nanoindentation method within a predetermined range, the relative proportion of the edge planes of the graphite when confined at a predetermined confining pressure is maintained. In other words, Patent Document 2 aims to reduce reaction resistance by suppressing the reduction in the edge planes present on the graphite surface.

[0018] Meanwhile, after extensive research, the present inventors have discovered that in order to achieve high capacity and high charge rate performance in all-solid-state lithium-ion secondary batteries, measures to reduce ion transport resistance rather than reaction resistance are necessary. The ion transport resistance increases as the degree of tortuosity of the ion conduction path shown in Figure 1 increases. In other words, to reduce ion transport resistance, it is important to minimize the degree of tortuosity of the ion conduction path. In contrast, for example, as in Patent Document 1, increasing the blending ratio of graphite particles, which are the negative electrode active material, to increase capacity reduces the proportion of solid electrolyte responsible for transporting lithium ions in the negative electrode active material layer. As a result, the degree of tortuosity of the ion conduction path increases, and ion transport resistance, rather than reaction resistance, becomes dominant in charge rate performance.

[0019] Based on the above findings, the present inventors arrived at the negative electrode for a solid battery of the present disclosure, which has reduced ion transport resistance.

[0020] (Summary of one aspect of the present disclosure) The negative electrode for a solid battery according to the first aspect of the present disclosure comprises: a negative electrode active material layer including a negative electrode active material and a solid electrolyte; the average aspect ratio of the negative electrode active material in the negative electrode active material layer is greater than 0.5, The negative electrode active material has an average elastic modulus of 370 MPa or less.

[0021] According to the above configuration, the ion transport resistance in the negative electrode active material layer can be reduced.

[0022] In the second aspect of the present disclosure, for example, in the solid state battery negative electrode according to the first aspect, the average elastic modulus may be 59 MPa or more and 370 MPa or less. With this configuration, it is possible to prevent the generation of microcracks in the negative electrode active material layer due to volume expansion of the negative electrode active material layer that occurs when pressure is released after pressure molding, that is, so-called springback.

[0023] In the third aspect of the present disclosure, for example, in the negative electrode for a solid battery according to the first or second aspect, the average aspect ratio may be greater than 0.5 and less than or equal to 0.8. With this configuration, the ion transport resistance in the negative electrode active material layer can be further reduced.

[0024] In a fourth aspect of the present disclosure, for example, in the solid battery negative electrode according to any one of the first to third aspects, the negative electrode active material layer may have a porosity of 30% or less. With this configuration, a solid battery with improved charge rate performance can be achieved.

[0025] In a fifth aspect of the present disclosure, for example, in the solid battery negative electrode according to any one of the first to fourth aspects, the volumetric blending ratio of the negative electrode active material to the total volume of materials contained in the negative electrode active material layer may be 50% or more and less than 70%. With this configuration, a significant decrease in the charge rate performance of the solid battery can be suppressed.

[0026] In a sixth aspect of the present disclosure, for example, in the solid state battery negative electrode according to any one of the first to fifth aspects, the negative electrode active material may contain graphite. With this configuration, it is possible to easily control the degree of curvature of the ion conduction path in the negative electrode active material layer.

[0027] In a seventh aspect of the present disclosure, for example, in the negative electrode for a solid state battery according to any one of the first to sixth aspects, the solid electrolyte may contain a sulfide solid electrolyte. With this configuration, an all-solid-state lithium-ion secondary battery with improved charge-discharge characteristics can be achieved.

[0028] In an eighth aspect of the present disclosure, for example, in the negative electrode for a solid state battery according to the seventh aspect, the sulfide solid electrolyte may include at least one of a LiS-P2S5-based glass ceramic electrolyte and an argyrodite-type sulfide solid electrolyte. This configuration makes it possible to achieve a solid state battery with improved charge-discharge characteristics.

[0029] A solid state battery according to a ninth aspect of the present disclosure includes: A positive electrode and a negative electrode; a solid electrolyte layer provided between the positive electrode and the negative electrode; Equipped with The negative electrode is a negative electrode for a solid battery according to any one of the first to eighth aspects.

[0030] According to the above configuration, a solid-state battery can achieve high capacity and high charge rate performance.

[0031] A method for producing a negative electrode for a solid battery according to a tenth aspect of the present disclosure includes: mixing a negative electrode active material and a solid electrolyte to prepare a negative electrode mixture; pressure-molding the negative electrode mixture to obtain a negative electrode active material layer; Including, The negative electrode mixture is pressure-molded so that the average aspect ratio of the negative electrode active material in the negative electrode active material layer is greater than 0.5; The negative electrode active material has an average elastic modulus of 370 MPa or less.

[0032] According to the above configuration, the ion transport resistance in the negative electrode active material layer can be reduced.

[0033] Hereinafter, embodiments of the present disclosure will be described with reference to the drawings.

[0034] (Embodiment 1) FIG. 2 is a cross-sectional view showing a schematic configuration of the negative electrode for an all-solid-state lithium ion secondary battery in the first embodiment.

[0035] [Negative electrode for all-solid-state lithium-ion secondary battery 12] The negative electrode 12 for an all-solid-state lithium-ion secondary battery in the first embodiment includes a negative electrode current collector 10 and a negative electrode active material layer 11. The negative electrode active material layer 11 is in contact with the negative electrode current collector 10. The negative electrode active material layer 11 includes a solid electrolyte 20 and a negative electrode active material 30. The particles of the solid electrolyte 20 and the particles of the negative electrode active material 30 are mixed and compressed to form the negative electrode active material layer 11.

[0036] [Negative electrode current collector 10] The negative electrode current collector 10 is made of a conductive material, such as a metal, a conductive oxide, a conductive nitride, a conductive carbide, a conductive boride, or a conductive resin.

[0037] [Negative electrode active material layer 11] The negative electrode active material layer 11 is a layer in which the negative electrode active material 30 and the solid electrolyte 20 are mixed and dispersed at a predetermined volumetric blending ratio. As shown in Fig. 1 , the negative electrode active material layer 11 contains both electron conduction paths formed by contact between particles of the negative electrode active material 30 and ion conduction paths formed by connections between particles of the solid electrolyte 20.

[0038] The porosity of the negative electrode active material layer 11 may be 30% or less. With the above configuration, an all-solid-state lithium ion secondary battery with improved charge rate performance can be achieved. The porosity of the negative electrode active material layer 11 may be 15% or less. It is desirable that the porosity of the negative electrode active material layer 11 is as small as possible. A method for calculating the porosity of the negative electrode active material layer 11 will be described later.

[0039] The volumetric blending ratio of the negative electrode active material 30 to the total volume of the materials contained in the negative electrode active material layer 11 may be 50% or more and less than 70%. If the volumetric blending ratio of the negative electrode active material 30 exceeds 70%, the charge rate performance of the all-solid-state lithium-ion secondary battery will be significantly reduced. If the volumetric blending ratio of the negative electrode active material 30 is 50% or more and less than 70%, it is possible to prevent a significant reduction in the charge rate performance of the all-solid-state lithium-ion secondary battery. The volumetric blending ratio of the negative electrode active material 30 may be 50% or more and less than 60%. When the negative electrode active material layer 11 contains only the solid electrolyte 20 and the negative electrode active material 30, the volumetric blending ratio of the negative electrode active material 30 is the ratio to the total volume of the solid electrolyte 20 and the negative electrode active material 30.

[0040] The ion transport resistance of the negative electrode active material layer 11 is 17 Ω·cm 2 May be less than 16 Ω cm 2According to the above configuration, an all-solid-state lithium ion secondary battery with reduced ion transport resistance can be achieved.

[0041] In this specification, the ion transport resistance and other measurements are measured at room temperature (20±15°C). Ion transport resistance (Ω·cm 2 ) can be converted into resistivity (Ω·cm). The resistivity can be calculated by dividing the ion transport resistance by the thickness of the negative electrode active material layer 11.

[0042] The negative electrode active material layer 11 may contain a conductive additive, a binder, and the like, as needed.

[0043] The conductive additive is not particularly limited as long as it is an electron-conductive material. Examples of the conductive additive include carbon materials, metals, and conductive polymers. Examples of carbon materials include graphite such as natural graphite (e.g., block graphite, flake graphite) or artificial graphite, acetylene black, carbon black, ketjen black, carbon whiskers, needle coke, and carbon fibers. Examples of metals include copper, nickel, aluminum, silver, and gold. These materials may be used alone or in combination. The conductive additive contributes to reducing the electronic resistance of the negative electrode active material layer 11.

[0044] The binder is not particularly limited as long as it serves to bind the active material particles and the conductive additive particles together. Examples of binders include fluorine-containing resins such as polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVdF), and fluorine rubber, thermoplastic resins such as polypropylene and polyethylene, ethylene propylene diene monomer (EPDM) rubber, sulfonated EPDM rubber, and natural butyl rubber (NBR). These materials may be used alone or in combination. The binder may be, for example, a cellulose-based or styrene butadiene rubber (SBR) water dispersion. The binder has the effect of maintaining the shape of the negative electrode active material layer 11.

[0045] Examples of solvents for dispersing the negative electrode active material 30, solid electrolyte 20, conductive agent, and binder include N-methylpyrrolidone, dimethylformamide, dimethylacetamide, methyl ethyl ketone, cyclohexanone, methyl acetate, methyl acrylate, diethylenetriamine, N,N-dimethylaminopropylamine, ethylene oxide, and tetrahydrofuran. For example, a dispersant and / or a thickener may be added to the solvent. Examples of thickeners include carboxymethylcellulose (CMC) and methylcellulose.

[0046] The thickness of the negative electrode active material layer 11 may be 5 μm or more and 200 μm or less. In a commonly used wet coating process, such as an applicator or die coater, the lower limit for controlling the thickness of the coating film is 10 μm. From this perspective, 5 μm is one guideline for the lower limit of the film thickness after drying, although this depends on the proportion of solid components in the coating slurry. By appropriately adjusting the thickness of the negative electrode active material 11, electrode cracking during drying can be prevented and yield can be improved. When a high-capacity material such as silicon is used as the material for the negative electrode active material 30, the thickness of the negative electrode active material layer 11 can be reduced to 10 μm or less.

[0047] (Negative electrode active material 30) The negative electrode active material 30 is a material that has the property of absorbing and releasing lithium ions.

[0048] The average aspect ratio of the negative electrode active material 30 in the negative electrode active material layer 11 may be greater than 0.5, or may be greater than 0.5 and 0.8 or less.

[0049] As shown in FIG. 2 , lithium ions that reach the negative electrode active material layer 11 from the positive electrode active material layer (not shown) via the solid electrolyte layer (not shown) migrate through the negative electrode active material layer 11 along ion conduction paths formed by interconnected particles of the solid electrolyte 20, and are accumulated in the negative electrode active material 30. The greater the degree of deformation of the negative electrode active material 30 in the pressure direction, the greater the degree of tortuosity of the ion conduction path. That is, the degree of tortuosity of the ion conduction path tends to increase depending on the degree of deformation of the negative electrode active material 30. When the average aspect ratio of the negative electrode active material 30 in the negative electrode active material layer 11 after pressure molding is greater than 0.5, the degree of tortuosity of the ion conduction path is reduced, thereby reducing the ion transport resistance in the negative electrode active material layer 11. This allows the all-solid-state lithium-ion secondary battery to achieve high capacity and high charge rate performance.

[0050] FIG. 4A is an explanatory diagram showing how to determine the aspect ratio of the negative electrode active material 30. The aspect ratio of the negative electrode active material 30 is the ratio of the minor axis diameter to the major axis diameter of the negative electrode active material 30 in the negative electrode active material layer 11 after pressure molding, and is expressed as minor axis diameter / major axis diameter. As shown in FIG. 4A , the minor axis diameter of the negative electrode active material 30 is defined as the distance between a pair of parallel lines that sandwich the outline of the negative electrode active material 30, where the distance between the pair of parallel lines is the smallest. When the outline of the negative electrode active material 30 is sandwiched between another pair of parallel lines perpendicular to the pair of parallel lines that define the minor axis diameter, the distance between the other pair of parallel lines that is the largest is defined as the major axis diameter of the negative electrode active material 30. The closer the aspect ratio is to 1, the higher the sphericity of the negative electrode active material 30. The method for calculating the average aspect ratio of the negative electrode active material 30 in the negative electrode active material layer 11 will be described later.

[0051] The average orientation angle of the negative electrode active material 30 in the negative electrode active material layer 11 may be 27 degrees or more.

[0052] The degree of curvature of the ion conduction path increases as the orientation angle of the negative electrode active material 30 with respect to the pressure direction approaches 0 degrees. That is, the degree of curvature of the ion conduction path also depends on the orientation angle of the negative electrode active material 30. When the average orientation angle of the negative electrode active material 30 in the negative electrode active material layer 11 after pressure molding is 27 degrees or more, the degree of curvature of the ion conduction path is reduced, and therefore the ion transport resistance in the negative electrode active material layer 11 is reduced. This makes it possible to achieve high capacity and high charge rate performance in an all-solid-state lithium-ion secondary battery.

[0053] 4B is an explanatory diagram showing how to determine the orientation angle of the negative electrode active material 30. The arrow in FIG. 4B indicates the pressure direction. As shown in FIG. 4B, the orientation angle of the negative electrode active material 30 is the angle formed by a line segment corresponding to the major axis diameter of the negative electrode active material 30 in the negative electrode active material layer 11 after pressure molding, and a plane perpendicular to the pressure direction (thickness direction of the negative electrode active material layer 11). A method for calculating the average orientation angle of the negative electrode active material 30 in the negative electrode active material layer 11 will be described later.

[0054] The average elastic modulus of the negative electrode active material 30 may be 370 MPa or less, or may be 59 MPa or more and 370 MPa or less. With the above configuration, it is possible to prevent microcracks from occurring in the negative electrode active material layer 11 due to spring back. A method for calculating the average elastic modulus of the negative electrode active material 30 will be described later.

[0055] FIG. 5 is an explanatory diagram showing the mechanism by which springback occurs in the negative electrode active material layer. The table in FIG. 5 shows the procedure for producing the negative electrode active material layer from top to bottom. First, 2 Once pressed at a pressure of 6tf / cm 2 After releasing the pressure, the clamping jig was used to reduce the pressure to 1.53 tf / cm 2The negative electrode active material is constrained with a pressure of 1000 kJ / cm. The arrows in the table in Figure 5 indicate the direction of pressure application. In the table in Figure 5, natural spherical graphite is shown as an example of an anode active material with poor mechanical properties. Mesocarbon microbeads (MCMB) are shown as an example of an anode active material with better mechanical properties than natural spherical graphite. In all-solid-state lithium-ion secondary batteries, it is important to reduce the voids between the solid electrolyte particles by pressing under high pressure to densify the anode active material layer. However, if the mechanical properties of the anode active material, such as the particle hardness, are too high, as in the case of MCMB shown in Figure 5, springback occurs when the pressure is released, causing cracks in the anode active material layer and breaking the ion conduction path. Such breaks are difficult to repair even with subsequent application of pressure by constraining using a restraining jig. Note that Figure 5 does not necessarily indicate that springback occurs when the anode active material is MCMB.

[0056] On the other hand, in the negative electrode 12 of this embodiment, the average elastic modulus of the negative electrode active material 30 is low, at 370 MPa or less, so that it is possible to avoid cracks occurring in the negative electrode active material layer due to springback when pressure is released, and thus to prevent breaks in the ion conduction paths.

[0057] The specific surface area of ​​the negative electrode active material 30 is 3.5 m 2 / g. During charging, electrons are usually donated to the negative electrode active material 30 by a reduction reaction in an all-solid-state lithium ion secondary battery. If these electrons are donated to the solid electrolyte 20 instead of the lithium ions, the solid electrolyte 20 undergoes a reduction decomposition reaction, reducing the charging efficiency of the all-solid-state lithium ion secondary battery. If the specific surface area of ​​the negative electrode active material 30 is 3.5 m 2 If the specific surface area of ​​the negative electrode active material 30 is less than 2.5 m / g, the reduction decomposition reaction of the solid electrolyte 20 in the negative electrode active material layer 11 can be suppressed. 2 The lower limit of the specific surface area of ​​the negative electrode active material 30 is not particularly limited, and may be, for example, 1.5 m 2 / g. The method for measuring the specific surface area of ​​the negative electrode active material 30 will be described later.

[0058] The median diameter of the negative electrode active material 30 may be 5 μm or more and 20 μm or less. The "median diameter" refers to the particle size when the cumulative volume in the volume-based particle size distribution is equal to 50%. The volume-based particle size distribution is measured, for example, by a laser diffraction measurement device. When the median diameter of the negative electrode active material 30 is in this range, the thickness of the negative electrode active material layer 11 can be made sufficiently thin.

[0059] The negative electrode active material 30 may be made of metals, semi-metals, oxides, nitrides, or carbon. Examples of metals or semi-metals include lithium, silicon, amorphous silicon, aluminum, silver, tin, antimony, and alloys thereof. Examples of oxides include Li4Ti5O 12 , Li2SrTi6O 14 , TiO2, Nb2O5, SnO2, Ta2O5, WO2, WO3, Fe2O3, CoO, MoO2, SiO, SnBPO6, and mixtures thereof. Nitrides include LiCoN, Li3FeN2, Li7MnN4, and mixtures thereof. Carbons include natural spherical graphite, which is made by folding natural flake graphite into spherical leaves using a hybridization device; MCMB with high sphericity; artificial graphite made from coal coke or petroleum coke; hard carbon; soft carbon; carbon nanotubes; and mixtures thereof. The negative electrode active material 30 can be one or a combination of two or more selected from these negative electrode active materials.

[0060] The negative electrode active material 30 may contain graphite such as natural spherical graphite or artificial graphite. Graphite such as natural spherical graphite or artificial graphite has easy control over its shape and mechanical properties such as hardness. This configuration allows for easy control of the degree of curvature of the ion conduction path in the negative electrode active material layer 11. The negative electrode active material 30 may be graphite.

[0061] When the negative electrode active material 30 is graphite, the graphite may be natural spheroidized graphite, MCMB, or a mixture thereof. The MCMB may be a crushed product obtained by crushing the MCMB.

[0062] (Solid electrolyte 20) An inorganic solid electrolyte, a polymer solid electrolyte, or a mixture thereof can be used as the solid electrolyte 20. Inorganic solid electrolytes include sulfide solid electrolytes and oxide solid electrolytes.

[0063] The solid electrolyte 20 may contain a sulfide solid electrolyte. According to the above configuration, an all-solid-state lithium ion secondary battery with improved charge / discharge characteristics can be achieved.

[0064] The sulfide solid electrolyte contained in the solid electrolyte 20 may include a Li2S-P2S5-based glass ceramic electrolyte. This configuration allows for the realization of an all-solid-state lithium-ion secondary battery with improved charge-discharge characteristics. The Li2S-P2S5-based glass ceramic electrolyte is a glass-ceramic sulfide solid electrolyte. Examples of Li2S-P2S5-based glass ceramic electrolytes include Li2S-P2S5, Li2S-P2S5-LiI, Li2S-P2S5-Li2O-LiI, Li2S-SiS2, Li2S-SiS2-LiI, Li2S-SiS2-LiBr, Li2S-SiS2-LiCl, Li2S-SiS2-B2S3-LiI, Li2S-SiS2-P2S5-LiI, Li2S-B2S3, Li2S-P2S5-GeS, Li2S-P2S5-ZnS, and Li These include 2S-P2S5-GaS, Li2S-GeS2, Li2S-SiS2-Li3PO4, Li2S-SiS2-LiPO, Li2S-SiS2-LiSiO, Li2S-SiS2-LiGeO, Li2S-SiS2-LiBO, Li2S-SiS2-LiAlO, Li2S-SiS2-LiGaO, Li2S-SiS2-LiInO, Li4GeS4-Li3PS3, Li4SiS4-Li3PS4, and Li3PS4-Li2S.

[0065] The sulfide solid electrolyte contained in the solid electrolyte 20 may include an argyrodite-type sulfide solid electrolyte. According to the above configuration, an all-solid-state lithium ion secondary battery with improved charge / discharge characteristics can be achieved. The argyrodite-type sulfide solid electrolyte is a sulfide solid electrolyte having an argyrodite-type crystalline phase with high ionic conductivity. An example of an argyrodite-type sulfide solid electrolyte is Li6PS5Cl.

[0066] The solid electrolyte 20 may contain only a sulfide solid electrolyte. In other words, the solid electrolyte 20 may consist essentially of a sulfide solid electrolyte. "Containing only a sulfide solid electrolyte" means that, with the exception of inevitable impurities, no materials other than the sulfide solid electrolyte are intentionally added. For example, the raw materials for the sulfide solid electrolyte and by-products generated during the production of the sulfide solid electrolyte are included in the inevitable impurities.

[0067] Examples of oxide solid electrolytes included in solid electrolyte 20 include LiPON, LiAlTi(PO4)3, LiAlGeTi(PO4)3, LiLaTiO, LiLaZrO, Li3PO4, Li2SiO2, Li3SiO4, Li3VO4, Li4SiO4-Zn2SiO4, Li4GeO4-Li2GeZnO4, Li2GeZnO4-Zn2GeO4, and Li4GeO4-Li3VO4.

[0068] Examples of the polymer solid electrolyte contained in the solid electrolyte 20 include fluororesin, polyethylene oxide, polyacrylonitrile, polyacrylate, derivatives thereof, and copolymers thereof.

[0069] The shape of the solid electrolyte 20 is not particularly limited, and may be needle-like, spherical, oval-spherical, scale-like, etc. The shape of the solid electrolyte 20 may also be particulate.

[0070] When the solid electrolyte 20 is in a particulate (e.g., spherical) shape, the median diameter of the solid electrolyte 20 may be smaller than the median diameter of the negative electrode active material 30. This allows the negative electrode active material 30 and the solid electrolyte 20 to form a better dispersed state in the negative electrode active material layer 11.

[0071] The median diameter of the solid electrolyte 20 may be set to correspond to the median diameter of the negative electrode active material 30. When the median diameter of the negative electrode active material 30 is 5 μm or more and 20 μm or less, the median diameter of the solid electrolyte 20 may be 0.5 μm or more and 2 μm or less. With the above configuration, the porosity of the negative electrode active material layer 11 can be reduced.

[0072] Next, a method for manufacturing the negative electrode 12 for an all-solid-state lithium-ion secondary battery will be described. The method for manufacturing the negative electrode 12 for an all-solid-state lithium-ion secondary battery includes mixing the negative electrode active material 30 and the solid electrolyte 20 to prepare a negative electrode mixture, and pressure-molding the negative electrode mixture to obtain the negative electrode active material layer 11. The negative electrode mixture is pressure-molded so that the average aspect ratio of the negative electrode active material 30 in the negative electrode active material layer 11 is greater than 0.5. The negative electrode active material 30 used has an average elastic modulus of 370 MPa or less.

[0073] <Method for Calculating Porosity of Negative Electrode Active Material Layer 11> The porosity of the negative electrode active material layer 11 in the first embodiment is calculated, for example, by the following method.

[0074] First, the pore volume distribution of the negative electrode active material layer 11 is measured using a mercury porosimeter. The porosimeter used is an Autopore III9410 manufactured by Shimadzu Corporation. From the obtained pore volume distribution, the distribution of pores with a pore diameter of 15 μm or less is extracted (excluding the distribution of pores with a pore diameter exceeding 15 μm), and the cumulative pore volume (Vp) is calculated. Note that pores with a pore diameter exceeding 15 μm are not included in the cumulative pore volume because they originate from the surface irregularities of the negative electrode active material layer 11. The obtained cumulative pore volume Vp is divided by the apparent volume (Va) of the active material layer, and the porosity of the negative electrode active material layer 11 can be calculated using the following equation (1). Va is calculated from the projected area (S) of the negative electrode active material layer 11 and the thickness (T) of the negative electrode active material layer 11 (Va = ST). The thickness (T) of the negative electrode active material layer 11 is measured using a contact-type thickness measuring device.

[0075] Porosity (%)=(Vp / Va)×100 (1)

[0076] <Method of Calculating Average Aspect Ratio of Negative Electrode Active Material 30 in Negative Electrode Active Material Layer 11> The average aspect ratio of the negative electrode active material 30 in the negative electrode active material layer 11 in the first embodiment is calculated, for example, by the following method.

[0077] First, the negative electrode active material layer 11 after pressure molding is cross-sectionally processed by a cross-section polisher (CP) (registered trademark) method, and the polished surface is observed with a field emission scanning electron microscope (FE-SEM). The captured FE-SEM image is subjected to binarization processing using image processing software to distinguish between the negative electrode active material 30 and the solid electrolyte 20, and the outline of the negative electrode active material 30 is extracted.

[0078] Next, the aspect ratio of each negative electrode active material 30 is obtained from the binarized image. As shown in FIG. 4A, the aspect ratio of the negative electrode active material 30 is calculated as the ratio of the minor axis diameter to the major axis diameter of the negative electrode active material 30. In this embodiment, one binarized image contains 100 to 200 contour-extracted negative electrode active material pieces 30. The average aspect ratio is calculated from the aspect ratios of these 100 to 200 negative electrode active material pieces 30. Note that while one FE-SEM image and its binarized image are two-dimensional information, three-dimensional information can be restored by repeatedly performing cross-sectional processing using the CP method and cross-sectional observation.

[0079] Fig. 6A is an example of an FE-SEM image of a negative electrode active material layer. The FE-SEM image of Fig. 6A shows a cross section of the negative electrode active material layer, and contains natural flake graphite of Comparative Example 1, which will be described later, as the negative electrode active material. Fig. 6B is an image after binarization of the FE-SEM image shown in Fig. 6A. The binarization image shown in Fig. 6B contains 107 particles of negative electrode active material.

[0080] <Method for Calculating Average Orientation Angle of Negative Electrode Active Material 30 in Negative Electrode Active Material Layer 11> From the binarized FE-SEM image as shown in Fig. 6B, the orientation angle as well as the aspect ratio of the negative electrode active material 30 can be obtained. As shown in Fig. 4B, the orientation angle of the negative electrode active material 30 is determined as the angle between a line segment corresponding to the major axis diameter of the negative electrode active material 30 and a plane perpendicular to the pressure direction. In this embodiment, as in the case of the average aspect ratio, the average orientation angle is calculated from the orientation angles of 100 to 200 negative electrode active material particles 30 included in the binarized FE-SEM image.

[0081] <Method for Calculating Average Elastic Modulus of Negative Electrode Active Material 30> The average elastic modulus of the negative electrode active material 30 in the first embodiment is determined based on the Japanese Industrial Standard JIS Z 8844:2019 "Method for measuring breaking strength and deformation strength of microparticles," which is used in the fields of food processing and pharmaceuticals. The average elastic modulus of the negative electrode active material 30 is calculated based on the 10% deformation strength of the negative electrode active material 30 as microparticles measured using a microcompression tester "MCT-510" manufactured by Shimadzu Corporation.

[0082] First, the median diameter of the negative electrode active material 30 is determined using a laser diffraction / scattering particle size distribution analyzer. Next, seven negative electrode active materials 30 with a size close to the determined median diameter are selected. A microcompression test is performed on the seven selected negative electrode active materials 30 using a cone-shaped flat indenter (Φ50 μm) with a test force of 49 mN, a load rate of 1.0141 mN / sec, and a load holding period of 5 seconds. The average value of the 10% deformation strength is calculated for the five negative electrode active materials 30 excluding the maximum and minimum values. Since the deformation rate is 10%, the elastic modulus, which corresponds to the spring constant of one particle of the negative electrode active material 30, is calculated as 10 times the 10% deformation strength.

[0083] In this embodiment, the average elastic modulus is calculated by measuring the 10% deformation strength of the raw material particles of the negative electrode active material 30. However, for example, it is also possible to calculate the average elastic modulus of the negative electrode active material 30 by measuring the 10% deformation strength of the negative electrode active material 30 extracted from the negative electrode active material layer 11 after pressure molding. If the deformation is between 10% and 30%, approximately half of the negative electrode active material 30 contained in the negative electrode active material layer 11 will not collapse due to pressure molding, regardless of whether it has a secondary structure or a primary structure. The negative electrode active material 30 that does not collapse will recover to its original shape when the pressure molding pressure is released, and its mechanical properties will not change. Therefore, it can be considered that there is no significant difference between the average elastic modulus calculated from the negative electrode active material 30 extracted from the negative electrode active material layer 11 after pressure molding and the average elastic modulus of the raw material particles of the negative electrode active material 30.

[0084] Note that Patent Document 2 focuses on the hardness of graphite as a negative electrode active material on the submicron scale, such as the edge plane and basal plane. Therefore, Patent Document 2 measures the hardness of graphite by nanoindentation. The present disclosure focuses not on the hardness of the negative electrode active material 30 on the submicron scale, but on the mechanical properties of the negative electrode active material 30 as a single particle, and therefore does not use nanoindentation.

[0085] <Method for measuring the specific surface area of ​​the negative electrode active material 30> The specific surface area of ​​the negative electrode active material 30 in the first embodiment can be measured by, for example, mercury intrusion porosimetry. The specific surface area of ​​the negative electrode active material 30 can also be obtained by converting data of an adsorption isotherm obtained by a gas adsorption method using argon gas using the Brunauer-Emmett-Teller (BET) method.

[0086] <Method of Calculating Average Circularity and Average Aspect Ratio of Negative Electrode Active Material 30> The circularity and aspect ratio of the raw material particles of the negative electrode active material 30 can be obtained by particle shape analysis using, for example, a particle shape analyzer manufactured by Malvern Panalytical. Fine particles of the negative electrode active material 30 with an equivalent circle diameter of less than 0.5 μm are excluded from the analysis data because they are below the minimum particle size at which their shape can be recognized. The circularity and aspect ratio are measured for 20,000 to 30,000 raw material particles of the negative electrode active material 30 with an equivalent circle diameter of 0.5 μm or more. The average values ​​of the measured circularity and aspect ratio are taken as the average circularity and average aspect ratio of the raw material particles of the negative electrode active material 30.

[0087] (Embodiment 2) The following describes the second embodiment. Explanations that overlap with the first embodiment will be omitted where appropriate.

[0088] FIG. 3 is a cross-sectional view showing a schematic configuration of an all-solid-state lithium-ion secondary battery 100 according to the second embodiment.

[0089] The all-solid-state lithium ion secondary battery 100 can be configured as a battery of various shapes, such as a coin type, a cylindrical type, a square type, a sheet type, a button type, a flat type, or a laminated type.

[0090] The all-solid-state lithium-ion secondary battery 100 in the second embodiment includes a positive electrode 16 , a solid electrolyte layer 13 , and a negative electrode 12 .

[0091] The solid electrolyte layer 13 is disposed between the positive electrode 16 and the negative electrode 12 .

[0092] The negative electrode 12 is the all-solid-state lithium-ion secondary battery negative electrode 12 in Embodiment 1. With the above configuration, the all-solid-state lithium-ion secondary battery 100 can achieve high capacity and high charge rate performance.

[0093] [Positive electrode 16] Positive electrode 16 in the second embodiment includes a positive electrode current collector 15 and a positive electrode active material layer 14. Positive electrode active material layer 14 includes a solid electrolyte and a positive electrode active material.

[0094] [Positive electrode current collector 15] The positive electrode current collector 15 is made of an electronic conductor. As the material for the positive electrode current collector 15, the materials described for the negative electrode current collector 10 in the first embodiment can be used as appropriate.

[0095] [Cathode active material layer 14] The positive electrode active material layer 14 is a layer in which a positive electrode active material and a solid electrolyte are mixed and dispersed at a predetermined volumetric blending ratio.

[0096] The volumetric blending ratio of the positive electrode active material to the positive electrode active material layer 14 may be 60% or more and 90% or less.

[0097] The positive electrode active material layer 14 may contain a conductive additive, a binder, etc. as needed. The conductive additive and binder described for the negative electrode active material layer 11 of the first embodiment can be used as appropriate.

[0098] For the same reasons as in the case of the negative electrode active material layer 11, the thickness of the positive electrode active material layer 14 may be 5 μm or more and 200 μm or less.

[0099] (Cathode active material) The positive electrode active material is a material that has the property of absorbing and releasing lithium ions.

[0100] Positive electrode active materials include lithium-containing transition metal oxides, vanadium oxides, chromium oxides, and lithium-containing transition metal sulfides. Examples of lithium-containing transition metal oxides include LiCoO2, LiNiO2, LiMnO2, LiMn2O4, LiNiCoMnO2, LiNiCoO2, LiCoMnO2, LiNiMnO2, LiNiCoMnO4, LiMnNiO4, LiMnCoO4, LiNiCoAlO2, LiNiPO4, LiCoPO4, LiMnPO4, LiFePO4, Li2NiSiO4, Li2CoSiO4, Li2MnSiO4, Li2FeSiO4, LiNiBO3, LiCoBO3, LiMnBO3, and LiFeBO3. Examples of lithium-containing transition metal sulfides include LiTiS2, Li2TiS3, and Li3NbS4. The positive electrode active material can be selected from these positive electrode active materials alone or in combination.

[0101] The positive electrode active material layer 14 may contain Li(NiCoMn)O2 (hereinafter referred to as NCM) as the positive electrode active material. The positive electrode active material layer 14 may contain NCM in which Ni:Co:Mn=5:2:3 as the positive electrode active material. Hereinafter, NCM in which Ni:Co:Mn=5:2:3 is referred to as NCM523.

[0102] The positive electrode active material may have a median diameter of 1 μm or more and 10 μm or less. When the positive electrode active material is a secondary particle granulated by sintering and agglomerating primary particles of about 0.1 μm to 1 μm, the upper limit of the positive electrode active material may be 10 μm.

[0103] (solid electrolyte) An inorganic solid electrolyte or a polymer solid electrolyte can be used as the solid electrolyte contained in the positive electrode active material layer 14. As the inorganic solid electrolyte or the polymer solid electrolyte, those described for the negative electrode active material layer 11 in the first embodiment can be used as appropriate.

[0104] The positive electrode active material layer 14 may contain a sulfide solid electrolyte as a solid electrolyte. As the sulfide solid electrolyte, the sulfide solid electrolyte described for the negative electrode active material layer 11 in the first embodiment can be used as appropriate.

[0105] The shape of the solid electrolyte contained in the positive electrode active material layer 14 is not particularly limited, and may be needle-like, spherical, oval-spherical, scale-like, etc. The shape of the solid electrolyte contained in the positive electrode active material layer 14 may be particulate.

[0106] When the solid electrolyte contained in the positive electrode active material layer 14 is particulate (for example, spherical), the median diameter of the solid electrolyte contained in the positive electrode active material layer 14 may be smaller than the median diameter of the positive electrode active material. This allows the positive electrode active material and the solid electrolyte to form a better dispersed state in the positive electrode active material layer 14.

[0107] The median diameter of the solid electrolyte contained in the positive electrode active material layer 14 may be set to correspond to the median diameter of the positive electrode active material. When the median diameter of the positive electrode active material is 1 μm or more and 10 μm or less, the median diameter of the solid electrolyte contained in the positive electrode active material layer 14 may be 0.1 μm or more and 1 μm or less. This configuration can reduce the porosity of the positive electrode active material layer 14.

[0108] [Solid electrolyte layer 13] The solid electrolyte layer 13 is a layer containing a solid electrolyte. An inorganic solid electrolyte or a polymer solid electrolyte can be used as the solid electrolyte contained in the solid electrolyte layer 13. As the inorganic solid electrolyte or the polymer solid electrolyte, those described for the negative electrode active material layer 11 of the first embodiment can be used as appropriate.

[0109] The shape of the solid electrolyte contained in solid electrolyte layer 13 is not particularly limited and may be needle-like, spherical, oval-spherical, scale-like, etc. The shape of the solid electrolyte contained in solid electrolyte layer 13 may be particulate.

[0110] When the solid electrolyte contained in solid electrolyte layer 13 is in the form of particles (for example, spheres), the median diameter of the solid electrolyte may be 0.1 μm or more and 10 μm or less. When the median diameter of the solid electrolyte particles is in this range, pinholes are less likely to occur in solid electrolyte layer 13, and it is easier to form solid electrolyte layer 13 with a uniform thickness.

[0111] Solid electrolyte layer 13 may contain a conductive additive, a binder, etc. as needed. The conductive additive and binder described for negative electrode active material layer 11 in the first embodiment can be used as appropriate.

[0112] The thickness of solid electrolyte layer 13 may be 15 μm or more and 60 μm or less. In this case, the number of solid electrolyte particles contained in solid electrolyte layer 13 in the thickness direction may be three or more. [Example]

[0113] Hereinafter, the present disclosure will be described in detail using comparative examples and examples.

[0114] [Evaluation of raw material particles for negative electrode active material] For Comparative Examples 1 to 4 and Examples 1 to 4, the median diameter, specific surface area, average circularity, average aspect ratio, and average elastic modulus of the raw material particles of the negative electrode active material were determined by the calculation and measurement methods described above.

[0115] Comparative Example 1 The negative electrode active material used was natural spherical graphite, which was obtained by folding natural flake graphite into spherical leaves using a hybridization device. This natural spherical graphite is referred to as natural spherical graphite A. The average circularity of natural spherical graphite A was 0.904, and the average aspect ratio was 0.655. The median diameter of natural spherical graphite A was 10.6 μm. The average 10% deformation strength of natural spherical graphite A was 5.55 MPa. That is, the average elastic modulus of natural spherical graphite A was 55.5 MPa. Furthermore, the porosity calculated using the above formula (1) for the negative electrode active material layer containing natural spherical graphite A as the negative electrode active material was 6.3%.

[0116] Comparative Example 2 As the negative electrode active material, natural spherical graphite was used, which was obtained by subjecting natural flake graphite, a different ore from that of the natural spherical graphite A of Comparative Example 1, to a spheroidizing treatment. This natural spherical graphite is referred to as natural spherical graphite B. The natural spherical graphite B had an average circularity of 0.918 and an average aspect ratio of 0.691. The natural spherical graphite B had a median diameter of 18.4 μm. The natural spherical graphite B had an average 10% deformation strength of 3.05 MPa. That is, the natural spherical graphite B had an average elastic modulus of 30.5 MPa.

[0117] Comparative Example 3 MCMB was used as the negative electrode active material. This MCMB is referred to as uncrushed MCMB A. MCMB is a primary particle formed by concentrically growing graphene layers. It differs from natural spherical graphite, which is a secondary particle formed by folding natural flake graphite primary particles and then laminating them into spherical leaves. The average circularity of uncrushed MCMB A was 0.960, and the average aspect ratio was 0.836. The median diameter of uncrushed MCMB A was 11.6 μm. The average 10% deformation strength of uncrushed MCMB A was 37.9 MPa. In other words, the average elastic modulus of uncrushed MCMB A was 379 MPa.

[0118] Comparative Example 4 As the negative electrode active material, MCMB was used, which had improved particle hardness without significantly changing the particle shape or particle size compared to the uncrushed MCMB A of Comparative Example 3. This MCMB is referred to as uncrushed MCMB B. The median diameter of uncrushed MCMB B was 11.0 μm, similar to that of Comparative Example 3. Meanwhile, the average value of the 10% deformation strength of uncrushed MCMB B was 88.9 MPa. That is, the average elastic modulus of uncrushed MCMB B was 889 MPa. Furthermore, the porosity calculated by the above formula (1) for the negative electrode active material layer containing uncrushed MCMB B as the negative electrode active material was 9.1%.

[0119] Example 1 As the negative electrode active material, natural flake graphite from the same ore as natural spherical graphite A in Comparative Example 1 was used, but the granulation by spheroidization treatment was more advanced than for natural spherical graphite A, resulting in a natural spherical graphite with improved sphericity. This natural spherical graphite is referred to as natural spherical graphite C. The natural spherical graphite C had an average circularity of 0.932 and an average aspect ratio of 0.703. The natural spherical graphite C had a median diameter of 15.8 μm. The natural spherical graphite C had an average 10% deformation strength of 7.37 MPa. That is, the natural spherical graphite C had an average elastic modulus of 73.7 MPa. Furthermore, the porosity calculated by the above formula (1) for a negative electrode active material layer containing natural spherical graphite C as the negative electrode active material was 7.4%.

[0120] Example 2 As the negative electrode active material, natural spheroidized graphite was used, which was obtained by subjecting natural flake graphite, a different ore from that of natural spheroidized graphite A in Comparative Example 1, to a spheroidization treatment and thereby achieving improved sphericity compared to natural spheroidized graphite A. This natural spheroidized graphite is referred to as natural spheroidized graphite D. The natural spheroidized graphite D had an average circularity of 0.935 and an average aspect ratio of 0.686. The natural spheroidized graphite D had a median diameter of 11.4 μm. The natural spheroidized graphite D had an average 10% deformation strength of 5.96 MPa. That is, the natural spheroidized graphite D had an average elastic modulus of 59.6 MPa.

[0121] Example 3 The negative electrode active material used was MCMB obtained by further growing and granulating the uncrushed MCMB A of Comparative Example 3, and then crushing the MCMB. This crushed MCMB is referred to as crushed MCMB C. The average circularity of crushed MCMB C was 0.903, and the average aspect ratio was 0.702. The median diameter of crushed MCMB C was 12.3 μm. The average 10% deformation strength of crushed MCMB C was 17.9 MPa. That is, the average elastic modulus of crushed MCMB C was 179 MPa. Furthermore, the porosity calculated by the above formula (1) for the negative electrode active material layer containing crushed MCMB C as the negative electrode active material was 7.2%.

[0122] Example 4 As the negative electrode active material, MCMBs crushed even finer than crushed MCMB product C of Example 3 were used. This crushed MCMB was referred to as crushed MCMB product D. The average circularity of crushed MCMB product D was 0.924, and the average aspect ratio was 0.741. The median diameter of crushed MCMB product D was 8.1 μm. The average 10% deformation strength of crushed MCMB product D was 36.7 MPa. That is, the average elastic modulus of crushed MCMB product D was 367 MPa.

[0123] [Evaluation of the negative electrode active material layer and the negative electrode active material in the negative electrode active material layer] <Method for measuring ion transport resistance of negative electrode active material layer> The ion transport resistance of the negative electrode active material layer is measured, for example, by the following method.

[0124] Fig. 7 is a cross-sectional view showing the schematic configuration of an evaluation cell used to measure ion transport resistance. First, an evaluation cell having negative electrodes as shown in Fig. 7 is prepared. This evaluation cell is a symmetric cell 90 in which a negative electrode active material layer 11 and a negative electrode current collector 10 having the same weight per unit area are stacked on both sides of a solid electrolyte layer 13.

[0125] Next, AC impedance measurement was performed on the symmetric cell 90 using a VMP300 manufactured by BioLogic, with the voltage amplitude set to 10 mV and the frequency range set to 7 MHz to 100 mHz. FIG. 8 is a graph showing a Cole-Cole plot obtained by the impedance measurement of the symmetric cell 90. FIG. 9 is a diagram showing an equivalent circuit of the symmetric cell shown in FIG. 7. The Warburg open circuit resistance Wo-R was calculated by fitting the graph of FIG. 8 with the equivalent circuit shown in FIG. 9. The calculated resistance Wo-R represents the ion transport resistance of two negative electrode active material layers 11. Therefore, half of the Warburg open circuit resistance Wo-R corresponds to the ion transport resistance of one negative electrode active material layer 11.

[0126] According to the above-mentioned measurement method, the ion transport resistance of the negative electrode active material layer containing the negative electrode active material was measured for Comparative Examples 1 to 4 and Examples 1 to 4.

[0127] First, symmetric cells 90 were fabricated for Comparative Examples 1 to 4 and Examples 1 to 4. An argyrodite-type sulfide solid electrolyte was used as the solid electrolyte contained in the negative electrode active material layer 11. The volumetric blending ratio of the negative electrode active material and the sulfide solid electrolyte to the total volume of the materials contained in the negative electrode active material layer 11 was 50%:50%. The weight per unit area of ​​the negative electrode active material layer 11 was set to 11.4 mg.

[0128] Here, the procedure for fabricating the symmetrical cell 90 will be described in detail. 2 100 mg of sulfide solid electrolyte powder was placed in a hollow Macor with holes, and the density was 1 tf / cm 2 The solid electrolyte layer 13 was pressed at a pressure of 1 tf / cm for 1 minute to form the primary molding of the solid electrolyte layer 13. Next, 11.4 mg of a powdered negative electrode mixture having a volumetric blending ratio of 50%:50% was placed under the primary molding of the solid electrolyte layer 13, and 2 The lower negative electrode active material layer 11 was primarily formed by pressing at a pressure of 1 tf / cm for 1 minute. Next, 11.4 mg of a powdered negative electrode mixture was placed on the upper side of the solid electrolyte layer 13, and the pressure was 1 tf / cm 2The mixture was pressed for 1 minute at a pressure of 6 tf / cm to form the upper negative electrode active material layer 11. Next, current collectors 10 were placed on the upper side of the upper negative electrode active material layer 11 and on the lower side of the lower negative electrode active material layer 11, respectively, and the mixture was pressed at a pressure of 6 tf / cm 2 After the final molding, the mixture was pressed at a pressure of 6 tf / cm for 1 minute. 2 Release the pressure and use the restraint jig to set the pressure to 1.53 tf / cm 2 was detained under pressure.

[0129] Using the symmetrical cells 90 fabricated in Comparative Examples 1 to 4 and Examples 1 to 4, the ion transport resistance of each negative electrode active material layer 11 was measured by an AC impedance method.

[0130] Next, for each of the symmetrical cells 90 of Comparative Examples 1 to 4 and Examples 1 to 4, the restraining jig was dismantled and a 1 cm 2 The pellet-shaped symmetric cells 90 were taken out. Each of the taken-out pellet-shaped symmetric cells 90 was subjected to cross-section processing by the CP method to obtain FE-SEM images. From the obtained FE-SEM images after binarization processing, the average aspect ratio and average orientation angle of the negative electrode active material in the negative electrode active material layer 11 were determined for Comparative Examples 1 to 4 and Examples 1 to 4 using the calculation method described above. Note that for Comparative Example 3, the electrode cracked due to springback, so the aspect ratio and orientation angle of the negative electrode active material could not be measured.

[0131] Furthermore, for Comparative Examples 1 to 4 and Examples 1 to 4, the cumulative irreversible capacity of the negative electrode active material layer 11 was measured according to the following procedure.

[0132] First, for Comparative Examples 1 to 4 and Examples 1 to 4, half cells for evaluating the negative electrode were prepared using a lithium-indium alloy as the counter electrode. The volumetric blending ratio of the negative electrode active material and sulfide solid electrolyte to the total volume of the materials contained in the negative electrode active material layer was 50%:50%, the same as the blending ratio in the control cell described above. Stainless steel foil was used as the negative electrode current collector. The prepared half cells of Comparative Examples 1 to 4 and Examples 1 to 4 were repeatedly charged and discharged three times. The total difference between the charge capacity and discharge capacity for the three cycles was calculated as the cumulative irreversible capacity.

[0133] The results obtained from the above measurements are shown in Table 1.

[0134] [Table 1]

[0135] The ion transport resistance (Ω cm 2 ) can be converted into resistivity (Ω·cm). Here, the thicknesses of the negative electrode active material layers in Comparative Examples 1 to 4 and Examples 1 to 4 are as follows: Comparative example 1: 61.0μm Comparative Example 2: No data Comparative Example 3: No data Comparative Example 4: 62.45 μm Example 1: 60.70 μm Example 2: No data Example 3: 60.70 μm Example 4: No data For each of Comparative Examples 1 to 4 and Examples 1 to 4, the resistivity can be calculated by dividing the ion transport resistance shown in Table 1 by the thickness of the negative electrode active material layer.

[0136] Natural spherical graphite is a secondary particle formed by folding and spheroidizing primary particles of natural flake graphite. Therefore, the hardness of a single particle is generally less than that of a positive electrode active material or a solid electrolyte. Unlike lithium-ion secondary batteries that use an electrolyte solution, all-solid-state lithium-ion secondary batteries involve mixing a negative electrode active material and a solid electrolyte and then press-molding the mixture under high pressure to form an ion conduction path. Therefore, in all-solid-state lithium-ion secondary batteries, if the negative electrode active material in the negative electrode active material layer is significantly deformed and oriented by press-molding, the degree of curvature of the ion conduction path increases, resulting in an increase in ion transport resistance.

[0137] For this reason, when natural spherical graphite is used as a negative electrode active material for all-solid-state lithium-ion secondary batteries, it is important to increase the sphericity of the raw material particles and harden their mechanical properties. Specifically, the sphericity of the raw material particles can be increased by optimizing the material, shape, and size of the primary particles of natural flake graphite or by improving the sphericity treatment method.

[0138] In Example 1, the same natural flake graphite ore as in Comparative Example 1 was used, but by further granulation by spheroidization treatment than in Comparative Example 1, the sphericity and mechanical properties of the raw material particles were improved. In fact, the negative electrode active material layer after pressure molding in Example 1 had improved average aspect ratio and average orientation angle compared to the negative electrode active material layer after pressure molding in Comparative Example 1. Therefore, the negative electrode active material layer after pressure molding in Example 1 had an ion transport resistance of 17·94 Ω·cm in Comparative Example 1. 2 to 15.34 Ω·cm 2 It was possible to reduce it to

[0139] In Comparative Example 2 and Example 2, natural flake graphite from a different ore was used than in Comparative Example 1 and Example 1. In Comparative Example 2 and Example 2, the sphericity was further improved compared to that of natural spherical graphite A in Comparative Example 1. However, the raw material particles of Comparative Example 2 had a smaller average modulus of elasticity and inferior mechanical properties compared to the raw material particles of Comparative Example 1. Therefore, in Example 2, the average modulus of elasticity of the raw material particles was increased by reducing the median diameter without significantly changing the sphericity from Comparative Example 2. The raw material particles of Example 2 had improved average circularity, average aspect ratio, and average modulus of elasticity compared to the raw material particles of Comparative Example 1. Furthermore, the raw material particles of Example 2 had an improved average modulus of elasticity compared to the raw material particles of Comparative Example 2. In fact, the negative electrode active material layer after pressure molding in Example 2 had improved average aspect ratio and average orientation angle compared to the negative electrode active material layer after pressure molding in Comparative Example 2. Therefore, the ion transport resistance of the negative electrode active material layer after pressure molding in Example 2 was 19.01 Ω·cm, which was 19.01 Ω·cm in Comparative Example 2. 2 to 15.68 Ω·cm 2 It was possible to reduce it to

[0140] In Comparative Examples 3 and 4, uncrushed MCMB was used. MCMB is a primary particle, and therefore has higher mechanical properties as a particle compared to natural spherical graphite, which is a secondary particle. MCMB also has high sphericity, as can be seen from the average circularity exceeding 0.950. Therefore, in Comparative Example 4, the average aspect ratio and average orientation angle in the negative electrode active material layer were significantly improved compared to Comparative Examples 1 and 2, which used natural spherical graphite. On the other hand, in Comparative Examples 2 and 4, no improvement was observed in the ion transport resistance of the negative electrode active material layer. This is because, as shown in FIG. 5, the average aspect ratio and average orientation angle in the negative electrode active material layer were significantly improved compared to Comparative Examples 1 and 2, which used natural spherical graphite. 2 After pressure molding at 1.53tf / cm 2 This is because when the pressure was released before the negative electrode active material layer was restrained by springback, microcracks were generated in the negative electrode active material layer.

[0141] [Verification experiment of the effect of springback] Next, an experiment was carried out to verify the influence of springback that occurs when pressure is released after pressure molding of the negative electrode active material layer.

[0142] For Comparative Examples 1 and 4, the symmetrical cell 90 was fabricated according to the above-described procedure, and the luminance was 6 tf / cm 2 Laminates were prepared in a state prior to the final molding, in which they were pressed for one minute at a pressure of 1000 kJ / cm2. Each laminate was pressed using a hydraulic press under varying pressures in the order of (a) to (m) below. For each of the conditions (a) to (m), the Warburg open circuit resistance (Wo-R) was calculated using the method for measuring the ion transport resistance of the negative electrode active material layer described above. The results are shown in Figure 10. (a) 1tf / cm 2 When pressure is applied (b) Released state (c) 2tf / cm 2 When pressure is applied (d) Released state (e) 3tf / cm 2 When pressure is applied (f) Released state (g)4tf / cm 2 When pressure is applied (h) Released state (i) 5tf / cm 2 When pressure is applied (j) release state (k)6tf / cm 2 When pressure is applied (l) Released state (m)6tf / cm 2 When pressure is applied

[0143] FIG. 10 is a graph showing the relationship between press pressure and Warburg open circuit resistance (Wo-R) for the symmetric cell of Comparative Example 1 and the symmetric cell of Comparative Example 4. The horizontal axis represents the press pressure in the order (a) to (m) above. The vertical axis represents the Warburg open circuit resistance. As shown in FIG. 10, in the laminate of Comparative Example 1, which had a negative electrode active material layer using natural spherical graphite A, almost no increase in the Warburg open circuit resistance (i.e., ion transport resistance) was observed even in the released state after pressing. On the other hand, in the laminate of Comparative Example 4, which had a negative electrode active material layer using uncrushed MCMB D, it was found that the Warburg open circuit resistance (Wo-R) significantly increased in the released state after pressing. This is due to the occurrence of springback after pressing in the laminate of Comparative Example 4, which resulted in cracks in the negative electrode active material layer. In fact, when the negative electrode active material layer in the released state was observed, no noticeable cracks were found in the laminate of Comparative Example 1, and the shape of the negative electrode active material layer was maintained. On the other hand, in the laminate of Comparative Example 4, cracks occurred everywhere, and it was confirmed that the shape of the layer could not be maintained.

[0144] Next, for each of the laminates of Comparative Example 1 and Comparative Example 4, 2 The confining pressure was gradually increased from 0.01 to 0.1, and the change in the Warburg open circuit resistance (Wo-R) was observed. The results are shown in Figure 11.

[0145] FIG. 11 is a graph showing the relationship between the confining pressure and the Warburg open circuit resistance Wo-R. The horizontal axis represents the confining pressure, and the vertical axis represents the Warburg open circuit resistance Wo-R. The uncrushed MCMB D contained in the laminate of Comparative Example 4 has better sphericity and mechanical properties than the natural spheroidized graphite A contained in the laminate of Comparative Example 1. Nevertheless, the laminate of Comparative Example 1 has a sphericity of 3 tf / cm 2 It was confirmed that at a confinement pressure lower than 3 tf / cm, cracks in the negative electrode active material layer due to springback caused the Warburg open circuit resistance (Wo-R) to increase. 2It was confirmed that at a confining pressure exceeding 3 tf / cm, the Warburg open circuit resistance value Wo-R of the laminate of Comparative Example 4 was smaller than that of the laminate of Comparative Example 1. This is because cracks generated in the negative electrode active material layer due to springback were formed at a confining pressure of 3 tf / cm 2 However, when the restraint jig was used, the repair was performed with a restraint pressure of more than 3 tf / cm 2 Applying such a large pressure is not practical from the viewpoint of practical application. Therefore, it is important to avoid springback of the negative electrode active material layer by controlling the mechanical properties of the negative electrode active material, rather than the restraining pressure.

[0146] We repeatedly investigated the correlation between the mechanical properties of the raw material particles of the negative electrode active material and the occurrence of springback. As a result, we found that if the average elastic modulus of the raw material particles of the negative electrode active material is 370 MPa or less, it is possible to prevent cracks from occurring in the negative electrode active material layer due to springback.

[0147] MCMB forms a strong structure by growing graphene layers concentrically. Therefore, crushing MCMB results in anisotropy. Anisotropic MCMBs are easily deformed. That is, by subjecting MCMB to a crushing treatment, its mechanical properties can be adjusted. For example, as shown in Table 1, the average elastic modulus of uncrushed MCMB A of Comparative Example 3 is very large, at 379 MPa. Example 3 is crushed MCMB C, which is obtained by further growing and granulating the uncrushed MCMB A of Comparative Example 3. Example 4 is crushed MCMB D, which is crushed even more finely than crushed MCMB C of Example 3. As shown in Table 1, in Examples 3 and 4, crushing the MCMB reduces the average elastic modulus to 179 MPa and 367 MPa, respectively. As a result, springback is avoided in Examples 3 and 4, and the ion transport resistance of the negative electrode active material layer is reduced compared to Comparative Example 3.

[0148] [Charge rate test] Next, a charge rate test was carried out using the battery.

[0149] Comparative Example 5 A battery having a negative electrode active material layer using natural spherical graphite A of Comparative Example 1 was fabricated as Comparative Example 5.

[0150] Example 5 A battery having a negative electrode active material layer using the crushed MCMB product C of Example 3 was fabricated as Example 5.

[0151] First, in both Comparative Example 5 and Example 5, a positive electrode and a negative electrode were fabricated according to the following procedure.

[0152] The volumetric ratio of the negative electrode active material and the sulfide solid electrolyte to the total volume of the materials contained in the negative electrode active material layer was 50%:50%, the same as the ratio in the control cell described above. Stainless steel foil was used as the negative electrode current collector.

[0153] NCM523 was used as the positive electrode active material in the positive electrode active material layer. The same argyrodite-type sulfide solid electrolyte as used in the negative electrode active material layer was used as the solid electrolyte in the positive electrode active material layer. NCM523, the sulfide solid electrolyte, a binder, a thickener, and a conductive additive were mixed in an organic solvent at a predetermined ratio and dispersed to prepare a positive electrode slurry. The resulting positive electrode slurry was applied to a stainless steel foil positive electrode current collector and vacuum dried to evaporate the organic solvent, producing a positive electrode.

[0154] The solid electrolyte contained in the solid electrolyte layer was the same argyrodite-type sulfide solid electrolyte as that used in the negative electrode active material layer. The weight of the solid electrolyte layer was the same as that of the control cell described above, 1 cm 2 The capacity ratio of the positive electrode to the negative electrode was set to 2.365 mAh, and the negative electrode active material layer was set to 1 cm to 1.2 cm . 2 The hit weight has been adjusted.

[0155] Batteries of Comparative Example 5 and Example 5 were fabricated using the above-described positive and negative electrodes.

[0156] FIG. 12A is a graph showing the results of a charge rate test at 25°C for the batteries of Comparative Example 5 and Example 5. FIG. 12B is a graph showing the results of a charge rate test at 60°C for the batteries of Comparative Example 5 and Example 5. The horizontal axis represents the charge rate as a percentage of time. The vertical axis represents the capacity retention rate relative to the rated capacity. The rated capacity is the capacity obtained when the battery is charged at a charge rate of 0.1 C and a cutoff voltage of 4.2 V under a 25°C environment. As shown in FIGS. 12A and 12B, Example 5, in which the sphericity and mechanical properties of the negative electrode active material were improved, exhibited improved charge rate performance compared to Comparative Example 5. This indicates that controlling the sphericity of the negative electrode active material suppresses deformation and orientation of the negative electrode active material due to pressure molding, and controlling the mechanical properties of the negative electrode active material prevents springback of the negative electrode active material layer. Thus, it was found that controlling the sphericity and mechanical properties of the negative electrode active material can reduce the ion transport resistance of the negative electrode active material layer.

[0157] [Charge rate test with varying volumetric blend ratio] Next, for the battery of Example 5, a charge rate test was carried out by changing the volumetric blending ratio of the negative electrode active material and the sulfide solid electrolyte to the total volume of the materials contained in the negative electrode active material layer.

[0158] Example 6 The volumetric blending ratio of the negative electrode active material and the sulfide solid electrolyte to the total volume of the materials contained in the negative electrode active material layer was 50%:50%.

[0159] Example 7 The volumetric blending ratio of the negative electrode active material and the sulfide solid electrolyte to the total volume of the materials contained in the negative electrode active material layer was 60%:40%.

[0160] Example 8 The volumetric blending ratio of the negative electrode active material and the sulfide solid electrolyte to the total volume of the materials contained in the negative electrode active material layer was 70%:30%.

[0161] Example 9 The volumetric blending ratio of the negative electrode active material and the sulfide solid electrolyte to the total volume of the materials contained in the negative electrode active material layer was 80%:20%.

[0162] Following the procedure described above, a battery was fabricated, and after molding, it was held at 1.53 tf / cm 2 The charge rate test was carried out at 25° C. The test results are shown in FIGS. 13A and 13B.

[0163] FIG. 13A is a graph showing the results of a charge rate test at 25°C for the batteries of Examples 6 to 9. The horizontal axis shows the charge rate as a percentage of time. The vertical axis shows the capacity retention rate based on the rated capacity. FIG. 13B is a graph showing the relationship between the volume ratio of the negative electrode active material and the capacity retention rate for the batteries of Examples 6 to 9. The horizontal axis shows the volume ratio of the negative electrode active material. The vertical axis shows the capacity retention rate at 2C charging. As shown in FIG. 13A, it was found that the charge rate performance was higher when the volume ratio of the negative electrode active material to the negative electrode active material layer was smaller. Furthermore, as shown in FIG. 13B, a sharp drop in charge rate performance was observed when the volume ratio of the negative electrode active material was between 70% and 80%. [Industrial Applicability]

[0164] The negative electrode for an all-solid-state lithium ion secondary battery and the all-solid-state lithium ion secondary battery disclosed herein are useful for energy storage devices such as in-vehicle lithium ion secondary batteries. [Explanation of symbols]

[0165] 10 Negative electrode current collector 11 Negative electrode active material layer 12 Negative electrode 13 Solid electrolyte layer 14 Cathode active material layer 15 Positive electrode current collector 16 positive electrode 20 Solid electrolyte 30 Negative electrode active material 51 Negative electrode current collector 52 Negative electrode active material layer 53 Negative electrode 55 Ion transport resistance 56 Reaction Resistance 60 Solid electrolyte 70 Anode active material 90 symmetrical cells 100 All-solid-state lithium-ion secondary battery

Claims

1. a negative electrode active material layer including a negative electrode active material and a solid electrolyte; an average aspect ratio (minor axis diameter / major axis diameter) of the negative electrode active material in the negative electrode active material layer is greater than 0.5; the negative electrode active material has an average elastic modulus of 370 MPa or less; Negative electrode for solid-state batteries.

2. The average elastic modulus is 59 MPa or more and 370 MPa or less. The negative electrode for a solid battery according to claim 1 .

3. the average aspect ratio (minor axis diameter / major axis diameter) is greater than 0.5 and less than or equal to 0.8; The negative electrode for a solid battery according to claim 1 or 2.

4. the porosity of the negative electrode active material layer is 30% or less; The negative electrode for a solid battery according to claim 1 .

5. a volumetric blending ratio of the negative electrode active material to the total volume of materials contained in the negative electrode active material layer is 50% or more and less than 70%; The negative electrode for a solid battery according to claim 1 .

6. The negative electrode active material includes graphite. The negative electrode for a solid battery according to claim 1 .

7. The solid electrolyte includes a sulfide solid electrolyte. The negative electrode for a solid battery according to claim 1 .

8. The sulfide solid electrolyte is Li 2 S-P 2 S 5 and at least one of a glass ceramic electrolyte and an argyrodite-type sulfide solid electrolyte. The negative electrode for a solid battery according to claim 7 .

9. A positive electrode and a negative electrode; a solid electrolyte layer provided between the positive electrode and the negative electrode; Equipped with The negative electrode is the negative electrode for a solid battery according to any one of claims 1 to 8. solid state battery.

10. mixing a negative electrode active material and a solid electrolyte to prepare a negative electrode mixture; pressure-molding the negative electrode mixture to obtain a negative electrode active material layer; Including, The negative electrode mixture is pressure-molded so that the average aspect ratio (minor axis diameter / major axis diameter) of the negative electrode active material in the negative electrode active material layer is greater than 0.5; The negative electrode active material has an average elastic modulus of 370 MPa or less. A method for manufacturing a negative electrode for a solid-state battery.

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