Negative electrode-forming material for all-solid-state lithium ion secondary battery, and all-solid-state lithium ion secondary battery

Abstract

The technical problem of the present invention is to control the size of an amorphous silicon block, which is spontaneously and unevenly generated, as uniformly as possible when charging and discharging LiB using silicon as a negative electrode active material. The present invention is a material for forming a negative electrode for an all-solid-state lithium ion secondary battery, the material having a negative electrode active material layer on a negative electrode current collector, island-shaped protrusions formed from a composition for forming an all-solid-state lithium ion secondary battery negative electrode, which contains silicon crystals having an average particle diameter of 0.5-5.0 [mu] m, are formed in a pattern at intervals, and are formed on the bottom surfaces of grooves formed between the island-shaped protrusions. A connection layer formed from the composition for forming a negative electrode for an all-solid-state lithium ion secondary battery is continuously formed with the island-like protrusions, and after charging and discharging of a negative electrode using the material for forming a negative electrode, an active material is fused to form a compact block having a uniform shape.

Description

Technical Field

[0001] The present invention relates to a material for forming a negative electrode for an all-solid-state lithium-ion secondary battery. Further, the present invention relates to an all-solid-state lithium-ion secondary battery using a negative electrode obtained from the forming material. Background Art

[0002] Lithium-ion secondary batteries (hereinafter sometimes simply referred to as "LiBs") have a high energy density among secondary batteries and are thus widely popularized. Among lithium-ion secondary batteries, a non-aqueous electrolyte system LiB is well known, which is configured such that a positive electrode containing an active material such as lithium cobalt oxide (LiCoO2) or lithium iron phosphate (LiFePO4) and a negative electrode containing an active material capable of occluding / releasing lithium ions are disposed with a separator therebetween, and a non-aqueous electrolyte obtained by dissolving an electrolyte composed of a lithium salt such as LiBF4 in an organic solvent such as ethylene carbonate is filled as an electrolyte. Charging and discharging of the non-aqueous electrolyte system LiB are performed as follows: Lithium ions in the secondary battery move between the positive electrode and the negative electrode via the non-aqueous electrolyte, and the lithium ions are inserted into / extracted from the active materials of the positive electrode and the negative electrode. However, there is a risk of electrolyte leakage in the above non-aqueous electrolyte system LiB, and improvement is desired in terms of safety.

[0003] With the expansion of the use of batteries, large batteries such as automotive batteries and stationary batteries have attracted attention. In large batteries, ensuring safety becomes more important than in small batteries. For batteries using the above electrolyte, by replacing the electrolyte with an inorganic solid electrolyte, it is easier to ensure safety even when the battery is enlarged compared to a LiB using an electrolyte. Thus, a safe and highly reliable all-solid-state lithium-ion secondary battery (all-solid-state LiB) has been proposed.

[0004] Further, as a negative electrode active material of a LiB, carbon has been conventionally used. From the viewpoint of increasing the capacity, using silicon as a negative electrode active material is being studied. The theoretical capacity density of silicon as a negative electrode active material is 4200 mAh / g (in terms of volume capacity density, it is 2370 mAh / cm 3 ), which is about 11 times higher in terms of the capacity / weight ratio and about 3 times higher in terms of the capacity / volume ratio than carbon, and a significant increase in the capacity of the secondary battery is expected.

[0005] However, the volume change of silicon during charging and discharging is very large, about 400%, so deterioration such as pulverization and isolation of silicon occurs during repeated charging and discharging. As a result, the charge-discharge efficiency and cycle life characteristics are lacking, and in particular, in large batteries for electric vehicles assuming long-term use, the practical application of a LiB using silicon as a negative electrode active material is considered difficult.

[0006] Particularly, in conventional lithium ion batteries with liquid-based ion conductors such as electrolyte solutions and ionic liquids, when silicon is used as the negative electrode active material, the following technical problems exist.

[0007] (1) Due to the expansion / contraction of silicon during charge / discharge, the electrode structure collapses and the battery performance deteriorates.

[0008] (2) The deterioration of battery performance is caused by the following: due to expansion / contraction, silicon particles are pulverized, and decomposition products of the liquid-based ion conductor adhere to the surface of these pulverized particles. As a result, the ionic conductivity at the interface between the negative electrode active material and the ion conductor decreases, and the path of electronic conductivity is cut off.

[0009] (3) In order to maintain electronic conductivity even in the case of pulverization, a large amount of an electronic conductivity imparting agent such as carbon needs to be added. However, carbon also acts as a negative electrode active material, so there are drawbacks such as competition between carbon and silicon, an increase in irreversible capacity due to carbon, and localization of lithium.

[0010] (4) Decomposition products of the liquid-based ion conductor adhere to the interface between silicon, which is the negative electrode active material, and the ion conductor. Therefore, the resistance at this interface becomes high, the current density during charge / discharge cannot be increased, and it is difficult to improve the battery performance. Moreover, the formation of these decomposition products consumes the electrochemical capacity, so the irreversible capacity increases, leading to a decrease in the electrochemical capacity of the negative electrode active material.

[0011] (5) Due to the occurrence of such phenomena, silicon microparticles become isolated or detached from the negative electrode current collector and cannot participate in the next charge / discharge. If charge / discharge is further performed in this state, the amount of lithium intercalated into the silicon particles that remain in contact with the negative electrode current collector increases. Therefore, the change in expansion / contraction of these silicon particles becomes larger, and the pulverization is further aggravated. Repeating this process, the number of silicon particles participating in charge / discharge decreases, and the charge / discharge capacity decreases sharply. That is, due to electrode collapse, the cycle characteristics deteriorate.

[0012] On the other hand, the technical problems of silicon negative electrode materials in all-solid-state batteries using solid electrolytes can be considered as follows.

[0013] (1) During charging, that is, when the intercalation reaction of lithium into silicon occurs, expansion of the active material occurs. At this time, there is no mechanism to relieve this volume change in the electrode layer where the solid electrolyte and the active material are closely packed. Therefore, stress is generated in the solid electrolyte layer formed on the electrode layer and even in the positive electrode layer serving as the counter electrode, resulting in damage to the solid electrolyte layer, short circuit between the negative electrode layer and the positive electrode layer, etc., which impairs the function of the battery.

[0014] (2) Moreover, during discharging, i.e., when the insertion / extraction reaction of lithium from the negative electrode active material occurs, volume shrinkage of the active material takes place. At this time, a phenomenon occurs where the distance between the surface of the active material and the solid electrolyte becomes farther, so the ion conduction path to the active material is cut off. Also, the contact between the carbon-based material added as an electron conductivity imparting agent to the negative electrode current collector and the electrode layer and the active material is dissociated due to the shrinkage of the active material. Therefore, the electron conductivity path is also cut off. For these reasons, the silicon particles are detached from the electrochemical reaction system and thus isolated, resulting in a sharp deterioration of the electrochemical capacity. In addition, even when there is no problem of expansion during charging, battery performance deterioration occurs due to the shrinkage of the active material during discharging.

[0015] Therefore, for a lithium ion secondary battery using silicon as the negative electrode active material, even if high capacity density is expected, its practical application is difficult. From the viewpoint of suppressing electrode breakdown accompanied by the expansion and contraction of silicon during charge and discharge, various technical proposals exist. In Patent Document 1 (Japanese Patent Laid-Open No. 2003-109590), a negative electrode material is disclosed in which volume change is alleviated by doping silicon with phosphorus, boron, or aluminum. In Patent Document 2 (Japanese Patent Laid-Open No. 2005-11699), a battery structure is proposed in which the volume change of the negative electrode is absorbed by controlling the density of the negative electrode and the size of the gap inside the battery, reducing the influence of the volume change. However, the secondary batteries described in these patent documents use non-aqueous electrolytes, which are not preferable from the viewpoint of safety as described above.

[0016] In addition, in Patent Document 3 (Japanese Patent Laid-Open No. 2021-68706), silicon is exemplified as one of the negative electrode materials of all-solid-state LiB, but it is not a usage example, so there is no description of technical problems such as the refinement and isolation of silicon caused by repeated charge and discharge, nor is there any disclosure of a solution thereto. Moreover, although it is described as all-solid-state, an ionic liquid is used, and it is not a strictly all-solid-state battery, and the risk of liquid leakage cannot be eliminated.

[0017] The capacity reduction caused by the refinement of silicon is a technical problem in both non-aqueous electrolyte-based LiB and all-solid-state LiB, but the reasons are different. In addition, a major cause of the capacity reduction in all-solid-state LiB is poor contact between the solid electrolyte or the electron conductivity imparting agent, the negative electrode current collector, and silicon as the electrode active material. Therefore, in the manufacturing process of all-solid-state batteries, there is a process of applying a large molding pressure. Also, during battery operation, in order to maintain contact conduction, a pressure called constraint pressure is sometimes applied. Thus, in all-solid-state LiB, compared with non-aqueous electrolyte-based LiB, the capacity reduction caused by the refinement of silicon is less likely to occur.

[0018] In addition, in all-solid-state LiBs, generally, the negative electrode active material layer is formed by mixing fine carbon particles such as acetylene black, which serve as an electron conductivity imparting agent for imparting electron conductivity, and metal fine particles; and / or a solid electrolyte or the like that serves as an ion conductivity imparting agent. When charging and discharging are performed under a constrained pressure, when silicon expands and contracts, the solid electrolyte embeds into the gaps between silicon particles and the cracks in the particles, and the refinement of silicon gradually intensifies, resulting in a decrease in capacity. In addition, by adding a large amount of the electron conductivity imparting agent and the ion conductivity imparting agent, the relative volume of silicon in the negative electrode decreases, which is not preferable from the viewpoint of increasing the capacity.

[0019] In Non-Patent Document 1, an all-solid-state LiB is proposed, in which silicon crystal particles having an average particle diameter of 0.8 to 3.9 μm are used as the negative electrode active material. It is speculated that by using silicon crystals of a specific particle diameter, moderate voids are generated in the negative electrode active material layer, and the stress during volume change caused by the expansion and contraction of the negative electrode active material is alleviated through these voids, so that the refinement and isolation of the negative electrode active material do not occur.

[0020] Prior Art Documents

[0021] Patent Documents

[0022] Patent Document 1: Japanese Patent Application Laid-Open No. 2003-109590

[0023] Patent Document 2: Japanese Patent Application Laid-Open No. 2005-11699

[0024] Patent Document 3: Japanese Patent Application Laid-Open No. 2021-68706

[0025] Non-Patent Documents

[0026] Non-Patent Document 1: Nobuya Machida et al. (2022) "Fabrication of Coated Silicon Electrodes and Application to Negative Electrodes for All-Solid-State Batteries" Proceedings of the 2022 Autumn Meeting of the Powder and Powder Metallurgy Association 2-60A Summary of the Invention

[0027] Problems to be Solved by the Invention

[0028] As described above, from the viewpoints of safety and increasing the capacity, there is a high expectation for realizing an all-solid-state LiB using silicon as the negative electrode active material, but in the previously proposed technologies, many technical problems remain. That is, an object of the present invention is to provide an all-solid-state lithium ion secondary battery in which there is no refinement or isolation of the negative electrode active material (silicon), and the cycle characteristics are better.

[0029] In order to solve the above technical problems, the present inventor focused on the technology described in the non-patent document 1 and manufactured an all-solid-state LiB. Among them, silicon crystal particles with an average particle diameter of 0.8 to 1.6 μm were used as the negative electrode active material, and an attempt was made to evaluate its characteristics. As a result, an improvement in the cycle characteristics was confirmed. Therefore, further research was carried out, and the structure of the negative electrode active material layer was carefully examined. When a battery having a negative electrode active material layer using the silicon crystal particles of the above specific particle diameter was assembled and charged and discharged, it was confirmed that a part or all of the silicon crystal particles were amorphous, and as shown in the SEM image in Figure 7 it was divided into blocks and densified in each block. The negative electrode active material layer after the above block formation is dense and in close contact with the negative electrode current collector, and thus exhibits excellent electronic conductivity. In addition, it is considered that gaps are generated between the blocks, and through these gaps, the stress during volume change caused by the expansion and contraction of each block is alleviated, and the refinement and isolation of silicon as the negative electrode active material are suppressed.

[0030] However, the sizes of the blocks spontaneously generated during charge and discharge and the gaps between the blocks are not uniform. Therefore, the volume change caused by the expansion and contraction of the blocks with non-uniform sizes may not be completely alleviated by the gaps with non-uniform widths. That is, when large blocks are adjacent to each other with a narrow gap, the blocks may contact and break during charge and discharge, resulting in refinement and isolation.

[0031] In addition, there are many groove portions reaching the negative electrode current collector in the groove portions formed between the blocks. Therefore, the solid electrolyte may invade the interface between the blocks and the negative electrode current collector, generating gaps and promoting the isolation of silicon.

[0032] Thus, in the above technology, performance deviations may occur in the obtained all-solid-state LiBs in terms of discharge performance, durability, etc., which becomes a problem in industrial implementation.

[0033] Therefore, the present inventor thought that the above problems could be solved in the following way, and thus completed the present invention, that is, when charging and discharging an all-solid-state LiB using silicon as the negative electrode active material, the sizes of the amorphous silicon blocks spontaneously and non-uniformly generated are controlled as uniformly as possible, and gaps are not easily generated between the blocks and the negative electrode current collector at the groove portions formed between the blocks.

[0034] Solutions for solving the problems

[0035] In order to solve the above technical problems, the present invention includes the following main ideas.

[0036] (1) A material for forming a negative electrode of an all-solid-state lithium-ion secondary battery. In this case, a negative electrode active material layer is provided on a negative electrode current collector. Regarding the negative electrode active material layer, island-shaped protrusions formed from a composition for forming a negative electrode of an all-solid-state lithium-ion secondary battery containing silicon crystals with an average particle size of 0.5 to 5.0 μm are formed in a pattern with intervals therebetween, and on the bottom surface of the groove portion formed between the above-mentioned island-shaped protrusions, a connection layer formed from the composition for forming a negative electrode of an all-solid-state lithium-ion secondary battery is continuously formed with the island-shaped protrusions.

[0037] (2) The material for forming a negative electrode of an all-solid-state lithium-ion secondary battery according to (1), wherein a negative electrode active material layer is provided on the current collector. Regarding the negative electrode active material layer, the width of the island-shaped protrusions is in the range of 10 μm to 100 μm, the height is in the range of 5 to 100 μm, the interval of the island-shaped protrusions is 10 to 50% relative to the width of the island-shaped protrusions, the thickness of the connection layer is in the range of 1 to 50% relative to the height of the island-shaped protrusions, and the pattern is formed in a manner not exceeding 15 μm.

[0038] (3) The material for forming a negative electrode of an all-solid-state lithium-ion secondary battery according to (1), wherein a negative electrode active material layer is provided on the negative electrode current collector. Regarding the negative electrode active material layer, the pattern is formed in such a way that the shape of the island-shaped protrusions is cylindrical, elliptical cylindrical, multi-prismatic, frustum of a cone-shaped, elliptical frustum of a cone-shaped, multi-pyramid frustum-shaped, conical, elliptical conical or multi-pyramidal.

[0039] (4) The material for forming a negative electrode of an all-solid-state lithium-ion secondary battery according to (1), wherein the silicon crystal filling rate is 35 to 55% by volume.

[0040] (5) The material for forming a negative electrode of an all-solid-state lithium-ion secondary battery according to (1), wherein the negative electrode active material layer contains an electron conductivity-imparting agent of 5 parts by mass or less relative to 100 parts by mass of the silicon crystals.

[0041] (6) An all-solid-state lithium-ion secondary battery, characterized in that it is an all-solid-state lithium-ion secondary battery having a positive electrode, a negative electrode and a solid electrolyte layer, and the negative electrode is composed of the material for forming a negative electrode of an all-solid-state lithium-ion secondary battery according to any one of (1) to (5).

[0042] (7) The all-solid-state lithium-ion secondary battery according to (6), wherein the solid electrolyte layer also exists in the groove portion.

[0043] Advantages of the Invention

[0044] In the present invention, when manufacturing an all-solid-state LiB, a negative electrode active material layer is formed on a negative electrode current collector to obtain a material for forming an all-solid-state LiB negative electrode, which is a precursor of an all-solid-state LiB negative electrode. Regarding the negative electrode active material layer, silicon crystals with an average particle size of 0.5 to 5.0 μm are used as the negative electrode active material, and island-shaped protrusions formed from a negative electrode-forming composition containing the negative electrode active material are formed in a pattern. Moreover, on the bottom surface of the groove portion formed between the above-mentioned island-shaped protrusions, a layer formed from the composition for forming an all-solid-state lithium ion secondary battery negative electrode (hereinafter referred to as "connection layer") is continuously formed with the island-shaped protrusions. When using this material for forming an all-solid-state LiB negative electrode to assemble an all-solid-state lithium ion secondary battery and perform charge and discharge, part or all of the silicon crystal particles as the negative electrode active material are amorphousized and densified. At the same time, the shape and size of the island-shaped protrusions are roughly maintained, and the negative electrode active materials are fused into a massive block. This massive block has a small volume change rate during charge and discharge of the all-solid-state LiB and good cycle characteristics. In addition, since the above-mentioned blocks are in close contact with the negative electrode current collector and the solid electrolyte, high electronic conductivity and ionic conductivity can be achieved even without using an electron conductivity-imparting agent and an ionic conductivity-imparting agent, and the current density during charge and discharge can be increased. Moreover, the formation of large cracks is suppressed in the above-mentioned dense block, so that the phenomenon of the solid electrolyte invading into the silicon block during charge and discharge is not likely to occur, and the capacity reduction caused by the refinement and isolation of silicon can be avoided. Furthermore, by previously forming the negative electrode active material layer in a pattern, the size of the amorphous silicon blocks formed after charge and discharge can be made uniform, and in addition, the intervals between the blocks can also be made uniform. A solid electrolyte can exist in the grooves (gaps) between the blocks, which has the function of absorbing the volume expansion during charge and discharge, and it is not easy to cause the breakdown of the negative electrode due to volume change, which has become a problem due to repeated use. In addition, by forming a connection layer on the bottom surface of the groove portion formed between the island-shaped protrusions, the phenomenon of the solid electrolyte invading into the interface between the silicon and the negative electrode current collector can be effectively prevented, and the isolation of silicon can be prevented, thereby further improving the cycle characteristics of the all-solid-state LiB.

[0045] As a result, an all-solid-state lithium ion secondary battery with good cycle characteristics and a higher current density during charge and discharge can be obtained in a state with extremely little deviation in performance between products. BRIEF DESCRIPTION OF THE DRAWINGS

[0046] Figure 1 It is a schematic cross-sectional view showing one embodiment of the material for forming an all-solid-state LiB negative electrode of the present invention.

[0047] Figure 2 It is a schematic top view showing one embodiment of the material for forming an all-solid-state LiB negative electrode of the present invention.

[0048] Figure 3It is a top view showing another embodiment of the material for forming the all-solid-state LiB negative electrode of the present invention.

[0049] Figure 4 It is a top view showing another embodiment of the material for forming the all-solid-state LiB negative electrode of the present invention.

[0050] Figure 5 It is a top view showing another embodiment of the material for forming the all-solid-state LiB negative electrode of the present invention.

[0051] Figure 6 It is a schematic cross-sectional view showing one embodiment of the state before charge and discharge of the all-solid-state LiB of the present invention.

[0052] Figure 7 It is an SEM image showing the state after the active material layer is agglomerated during charge and discharge in an all-solid-state LiB in which the all-solid-state LiB negative electrode active material layer is formed on the entire surface of the negative electrode current collector. Detailed Embodiments

[0053] Hereinafter, embodiments of the present invention will be described. First, the negative electrode active material particles for an all-solid-state lithium secondary battery will be described, then the composition for forming a negative electrode for an all-solid-state lithium secondary battery containing the particles will be described, and further, the material for forming a negative electrode for an all-solid-state lithium secondary battery obtained by using the composition for forming a negative electrode for an all-solid-state lithium secondary battery and the all-solid-state lithium secondary battery containing the negative electrode for an all-solid-state lithium secondary battery will be described.

[0054] In this specification, an all-solid-state battery refers to a battery that does not contain a liquid substance such as a non-aqueous electrolyte or an ionic liquid as an electrolyte.

[0055] The material for forming a negative electrode for an all-solid-state lithium secondary battery refers to a laminate obtained by coating a composition for forming a negative electrode for an all-solid-state lithium secondary battery on a negative electrode current collector and drying it, and has a negative electrode active material before charge and discharge. When charging and discharging, the silicon crystal particles as the negative electrode active material are amorphousized, and are fused into a block and densified.

[0056] (Negative Electrode Active Material Particles for All-Solid-State Lithium Secondary Battery)

[0057] The negative electrode active material particles for an all-solid-state LiB used in the present invention are composed of silicon crystals having an average particle diameter of 0.5 to 5.0 μm. When using this negative electrode active material to assemble a battery and performing charge and discharge, part or all of the silicon crystal particles are amorphousized, and are fused and densified. Therefore, a negative electrode with less capacity reduction caused by the refinement and isolation of silicon is obtained, and the cycle characteristics are improved.

[0058] Silicon crystals refer to both polycrystalline silicon and single-crystalline silicon. Therefore, the negative electrode active material particles of the present invention can be polycrystalline silicon particles, single-crystalline silicon particles, or a mixture thereof. The polycrystalline silicon particles can be obtained by crushing and classifying a polycrystalline silicon rod obtained by the so-called Siemens method. The single-crystalline silicon particles can be obtained by pulverizing and classifying a single-crystalline silicon obtained by the so-called Czochralski method. In addition, they can also be obtained by crushing and classifying metallurgical silicon obtained by the reduction method of silica. Silicon crystals show distinct peaks by X-ray diffraction.

[0059] From the viewpoint of promoting the agglomeration and densification of the negative electrode active material layer, the average particle diameter of the silicon crystal particles is preferably 0.8 to 2.5 μm, more preferably 1.0 to 1.6 μm. It should be noted that the average particle diameter refers to the 50% cumulative diameter (D50) in the particle size distribution measurement result based on the laser scattering method.

[0060] From the viewpoint of promoting the agglomeration and densification of the negative electrode active material layer, it is more preferable for the silicon crystal particles to have both the above-mentioned average particle diameter and the following characteristics.

[0061] The ratio of the arithmetic standard deviation of the particle diameter to the arithmetic mean (standard deviation / mean) can preferably be greater than 0.53, more preferably greater than 0.55, and particularly preferably greater than 0.57.

[0062] The specific surface area of the particles is in the range of 3 to 50 m 2 / g, more preferably 10 to 25 m 2 / g. The specific surface area is obtained by gas adsorption measurement based on the constant volume method.

[0063] When obtained by pulverization, the particle shape is irregular, but other shapes such as spherical shape can be adopted without particular limitation.

[0064] In order to improve the adhesion between the silicon particles and the binder component described later, the silicon particles can also be surface-treated.

[0065] The purity of the silicon crystal is not particularly limited. In the present invention, the purity of the silicon crystal is preferably 90% by mass or more. Specifically, it is preferable to use silicon with an oxygen concentration of less than 5.0% by mass, a nitrogen concentration of less than 1.0% by mass, and a halogen element concentration of less than 0.1% by mass. Oxygen, nitrogen, and halogen elements will combine with Li, thus becoming an irreversible capacity and reducing the characteristics as a battery. Therefore, it is preferably controlled within the above range.

[0066] In addition, as the silicon crystal, silicon with a carbon concentration of 0.5 to 5.0% by mass and an oxygen concentration of 0.5 to 5.0% by mass on the surface can also be used. High-purity silicon has low conductivity, and in most cases, an electron conductivity imparting agent such as carbon is used in combination. However, as described above, since the amount of the negative electrode active material is relatively reduced due to the addition of the electron conductivity imparting agent, it is not preferable. Therefore, it has been found that by attaching carbon and oxygen as impurities to the surface of high-purity silicon, the polarity of the particle surface is increased, and electrons can easily move near the particles, so that the electron conductivity imparting agent can be replaced.

[0067] Such surface carbon and surface oxygen mostly originate from the binder and the dispersion medium, but are not limited thereto.

[0068] In addition, metal impurities that also act as electron conductivity imparting agents like carbon can be attached to the surface of silicon. Examples of the above-mentioned metal include aluminum and zirconium. Preferably, the surface aluminum concentration is set to 0.1 to 1.0% by mass, and the surface zirconium concentration is set to 0.1 to 1.0% by mass. Aluminum and zirconium mostly originate from alumina and zirconia used during the pulverization of silicon, but are not limited thereto.

[0069] Moreover, considering the possibility of improving the chemical stability against sulfide-based solid electrolytes or the possibility of improving the diffusivity of lithium, doped silicon can be used in the present invention. As the dopant, an element having a larger atomic radius than silicon in single crystal form is preferably used. Examples of such elements include P, Ge, Sn, Sb, etc. When the above elements are doped and alloyed with silicon, the interatomic distance is larger than that of pure silicon, and smooth insertion and extraction of Li can be expected.

[0070] (Composition for forming negative electrode of all-solid-state lithium-ion secondary battery)

[0071] The above negative electrode active material is mixed with the components constituting the negative electrode to prepare a composition for forming a negative electrode, and a negative electrode active material layer is formed on the negative electrode current collector, thereby obtaining a negative electrode. When the above negative electrode active material is used to assemble a battery and charge and discharge are performed, part or all of the silicon crystal particles are amorphousized, and they fuse into a block and are densified in each block. The block-shaped negative electrode active material layer is dense and adheres tightly to the negative electrode current collector, so it exhibits excellent electron conductivity. Therefore, when using the negative electrode active material particles of the present invention, the amount of the electron conductivity imparting agent used can be reduced, and in addition, the electron conductivity imparting agent can be omitted according to circumstances.

[0072] That is, the composition for forming an all-solid-state LiB negative electrode contains the above-described active material particles for a negative electrode, and the content of the electron conductivity-imparting agent is preferably 5 parts by mass or less with respect to 100 parts by mass of the negative electrode active material particles. According to the present invention, since the proportion of the electron conductivity-imparting agent is reduced, the relative amount of the active material particles can be increased, which helps to improve the capacity. The content of the electron conductivity-imparting agent in the composition for forming an all-solid-state LiB negative electrode is preferably 3% by mass or less, more preferably 1% by mass or less, and still more preferably substantially free of the electron conductivity-imparting agent.

[0073] In order to improve the ionic conductivity, the composition for forming an all-solid-state LiB negative electrode may contain an ionic conductivity-imparting agent. As the above-described ionic conductivity-imparting agent, a solid electrolyte described later can be used. If the blending amount of the ionic conductivity-imparting agent is too large, the relative amount of the active material particles is reduced and the cycle characteristics are also reduced. Therefore, the content of the ionic conductivity-imparting agent in the composition for forming an all-solid-state LiB negative electrode is preferably 30% by mass or less, more preferably 20% by mass or less, and still more preferably substantially free of the ionic conductivity-imparting agent.

[0074] The composition for forming an all-solid-state LiB negative electrode may contain a binder, a plasticizer, and the like. The total of these components is preferably 20 parts by mass or less, more preferably 15 parts by mass or less, with respect to 100 parts by mass of the negative electrode active material particles. If the blending amounts of the binder and the plasticizer are too large, the amount of the active material in the negative electrode active material layer is relatively reduced, which is not preferable in terms of increasing the battery capacity.

[0075] Examples of the binder include thermosetting resins such as thermosetting polyimide, phenolic resin, epoxy resin, urea resin, melamine resin, unsaturated polyester resin, and polyurethane; cellulose derivatives such as carboxymethyl cellulose, hydroxypropyl methyl cellulose, hydroxypropyl cellulose, ethyl cellulose, hydroxyethyl cellulose, and hydroxymethyl cellulose, or water-soluble polymers such as polyvinyl alcohol; polycarbonate resins such as polypropylene carbonate; polyvinylidene fluoride; styrene-butadiene copolymers (so-called SBR rubbers), styrene-propylene copolymers, and styrene-ethylene-propylene copolymers (so-called SES series, SEPS series).

[0076] In addition, when forming island-shaped convex portions formed of the all-solid-state LiB negative electrode forming composition described later on the negative electrode current collector, the all-solid-state LiB negative electrode forming composition may contain a dispersion medium for coating. As the dispersion medium, it can be appropriately selected from alcohols, aldehydes, ketones, ethers, esters, amides, imides, aliphatic hydrocarbons, alicyclic hydrocarbons, aromatic hydrocarbons, heterocycles, etc. For example, methanol, ethanol, n-propanol, isopropanol, n-butanol, isobutanol, pentanol, hexanol, heptanol, octanol, 2-ethylhexanol, benzyl alcohol, ethylene glycol monomethyl ether, diethylene glycol monomethyl ether, triethylene glycol monomethyl ether, ethylene glycol monoisopropyl ether, diethylene glycol monoisopropyl ether, triethylene glycol monoisopropyl ether, ethylene glycol monobutyl ether, diethylene glycol butyl ether, triethylene glycol monobutyl ether, propylene glycol monomethyl ether, dipropylene glycol monomethyl ether, ethyl acetate, n-propyl acetate, isopropyl acetate, n-butyl acetate, isobutyl acetate, hexyl acetate, ethylene glycol monomethyl ether acetate, diethylene glycol monomethyl ether acetate, triethylene glycol monomethyl ether acetate, ethylene glycol monoisopropyl ether acetate, diethylene glycol monoisopropyl ether acetate, triethylene glycol monoisopropyl ether acetate, ethylene glycol monobutyl ether acetate, diethylene glycol monobutyl ether acetate, triethylene glycol monobutyl ether acetate, propylene glycol monomethyl ether acetate, dipropylene glycol monomethyl ether acetate, hexane, nonane, decane, isodecane, dodecane, isododecane, turpentine, naphthenic solvent, isoparaffinic solvent, cyclohexane, methylcyclohexane, dimethylcyclohexane, ethylcyclohexane, alkylcyclohexane, toluene, xylene, aromatic high-boiling solvent, acetone, methyl ethyl ketone, methyl amyl ketone, methyl isobutyl ketone, cyclohexanone, terpineol, dihydroterpineol, dihydroterpineol acetate, NMP (N-methylpyrrolidone), anisole, diethyl ether, dipropyl ether, dibutyl ether, etc. They can be used alone or in combination. The amount of the dispersion medium used can be appropriately determined so that the viscosity of the paste containing the all-solid-state LiB negative electrode forming composition becomes the optimum viscosity in the method for forming the negative electrode active material layer. The above dispersion medium is removed by drying after coating the paste containing the all-solid-state LiB negative electrode forming composition.

[0077] (Material for forming negative electrode of all-solid-state lithium ion secondary battery)

[0078] A negative electrode active material layer formed of the above all-solid-state LiB negative electrode forming composition is formed on the negative electrode current collector, whereby a material for forming an all-solid-state LiB negative electrode is obtained. The material for forming the negative electrode is one of the components constituting the battery and functions as a negative electrode itself. When a LiB is assembled and charged and discharged, the silicon crystal becomes amorphous, so the crystal structure is different from that of the negative electrode in the battery after charge and discharge.

[0079] The filling rate of silicon as the active material in the above-mentioned negative electrode active material layer is preferably in the range of 35 to 55% by volume, and more preferably in the range of 40 to 50% by volume. When the filling rate of the active material is low, voids exist in the negative electrode active material layer. Therefore, during charge and discharge, lithium ions are occluded in the silicon active material, and the volume expansion during amorphization (lithium-silicon alloying) can be alleviated. The above-mentioned active material filling rate can be adjusted by the particle size or particle size distribution of the silicon particles, the addition amount of the binder, etc. It should be noted that the active material filling rate in the negative electrode active material layer refers to the ratio of the volume of silicon to the volume of the negative electrode.

[0080] As the negative electrode current collector, copper foil, nickel foil, or SUS foil is usually used, and other conductive metal foils can also be used. The negative electrode current collector can also be electrolytic copper with an anti-rust treatment on the surface. The thickness of the negative electrode current collector is not particularly limited. From the viewpoints of miniaturization and operability of the battery, it is usually 3 μm to 100 μm thick. In the case of the roll-to-roll method, a negative electrode current collector with a thickness of 5 μm to 50 μm is preferably used. The shape of the negative electrode current collector can be a non-perforated sheet, or a perforated sheet such as a two-dimensional mesh, a three-dimensional network, or a punched metal. The surface of the negative electrode current collector can also be subjected to known surface treatments, such as mechanical surface processing, etching, chemical conversion treatment, anodic oxidation, wash primer, corona discharge, glow discharge, etc.

[0081] In the negative electrode active material layer containing silicon crystals, during charge and discharge, the silicon crystal particles as the negative electrode active material are amorphized and fused into a massive and dense state. However, the lumping of the negative electrode active material layer occurs spontaneously during charge and discharge. Therefore, the size, shape, and gaps between the respective lumps are likely to become uneven. If the size, shape, and gaps between the respective lumps are uneven, due to the expansion and contraction during charge and discharge, adjacent lumps come into contact with each other, generating a mutual compression force, and the lumps may sometimes break. As a result, the negative electrode active material may sometimes be pulverized and isolated, leading to a decrease in capacity. In addition, there are many grooves reaching the negative electrode current collector in the groove portions formed between the lumps. Since the solid electrolyte invades the interface between the lumps and the negative electrode current collector, gaps are generated, and the peeling of the lumps is aggravated, promoting the isolation of silicon.

[0082] The inventors of the present invention conducted in-depth research on a method for controlling the size, shape, and spacing of self-generated lumps, and as a result, they came up with the idea of pre-forming the negative electrode active material layer of the all-solid-state LiB negative electrode forming material before charge and discharge into the shape of island-shaped protrusions, forming them in a pattern with intervals, and further forming a connection layer that covers the bottom of the grooves formed between the above-mentioned island-shaped protrusions.

[0083] That is, as Figure 1 the cross-sectional view of Figure 2As shown in the top view, the material 10 for forming an all-solid-state LiB negative electrode according to the present embodiment is characterized in that a negative electrode active material layer 2 formed of a composition for forming an all-solid-state LiB negative electrode is provided on a negative electrode current collector 1, and the negative electrode active material layer 2 has the following structure: island-shaped convex portions 11 are formed in a pattern at intervals, and on the bottom surface of a groove portion 12 formed between the island-shaped convex portions, a connection layer 13 formed of the composition for forming a negative electrode of an all-solid-state lithium ion secondary battery is continuously formed with the island-shaped convex portions.

[0084] The shape of the island-shaped convex portion 11 is not particularly limited. In Figure 1 , Figure 2 the case where the island-shaped convex portion 11 is a frustum of a quadrangular pyramid is shown, and it may also be cylindrical ( Figure 3 ), an elliptical cylinder, or may be a triangular prism, a quadrangular prism, a pentagonal prism, a hexagonal prism, etc. of a polygonal prism shape. In addition, it may also be a frustum of a cone, a frustum of an elliptical cone, or a frustum of a triangular pyramid, a frustum of a quadrangular pyramid, a frustum of a pentagonal pyramid, a frustum of a hexagonal pyramid ( Figure 4 ) etc. of a frustum of a polygonal pyramid, or may be a cone, an elliptical cone, a polygonal pyramid, or a combination thereof.

[0085] The island-shaped convex portion 11 may be covered by the solid electrolyte when forming the all-solid-state LiB, and in addition, the solid electrolyte may also penetrate into the groove 12.

[0086] If referring to Figure 1 to describe the dimensions of each part of the negative electrode active material layer 2 formed on the negative electrode current collector, the width (W) of the island-shaped convex portion 11 is preferably in the range of 10 μm to 100 μm, particularly preferably 15 to 50 μm, and further preferably 15 to 30 μm. By setting the width of the island-shaped convex portion 11 within the above range, even through charge and discharge after forming the all-solid-state LiB, a mass that substantially maintains the outer shape of the island-shaped convex portion is formed, and stable performance can be exhibited. In addition, when the width of the island-shaped convex portion 11 is within the above range and the average size of the mass naturally formed by the charge and discharge of the all-solid-state LiB, specifically the width, is formed to be larger than about 15 to 25 μm, cracks sometimes occur in the mass derived from the island-shaped convex portion, but the above cracks do not reach the negative electrode current collector. In addition, compared with the case where the negative electrode active material layer is formed on the entire surface of the negative electrode current collector, the effect of forming the mass more uniformly is not changed.

[0087] It should be noted that according to its technical idea, the size of the above island-shaped convex portion can also be represented by the area of the island-shaped convex portion, and it is preferably set to 100 to 10000 μm 2 , particularly preferably set to 225 to 2500 μm 2 , and further preferably set to 225 to 900 μm 2The range. Determining the size of the island-shaped convex portion by area is particularly effective in the case of a complicated patterned shape.

[0088] In addition, when the height (H) of the island-shaped convex portion 11 is in the range of 5 to 100 μm, particularly 10 to 80 μm, and further 15 to 50 μm, the performance as a negative electrode active material is sufficiently exhibited, and thus it is preferable. The height (H) of the island-shaped convex portion 11 refers to the distance from the surface of the current collector to the highest point of the island-shaped convex portion 11. Moreover, when the interval (P) between the island-shaped convex portions 11 is in the range of 10 to 50% with respect to the width (W) of the island-shaped convex portion 11, particularly 15 to 30%, the influence of expansion / contraction between the island-shaped convex portions during charge and discharge after forming the all-solid-state LiB can be suppressed, which is preferable. Moreover, the thickness (t) of the connection layer 13 only needs to be less than the height of the island-shaped convex portion, and is preferably in the range of 1 to 50% with respect to the height of the island-shaped convex portion, particularly 10 to 30%, and when the pattern is formed so as not to exceed 15 μm, even through charge and discharge after forming the all-solid-state LiB, it is not easy to cause lumping in the above-mentioned part, and reliably prevents the negative electrode current collector from being exposed to the bottom surface of the groove portion 12, and thus it is preferable.

[0089] As Figure 1 shown, the width (W) of the island-shaped convex portion refers to the length measured starting from the standing portion of the island-shaped convex portion. In addition, the above-mentioned length is the distance between opposite sides in the case of a quadrilateral having opposite sides, the diameter in the case of a circle, and the equivalent diameter in the case of a polygon having an odd number of sides. The interval (P) between the island-shaped convex portions 11 refers to the distance between the standing portions of the opposing island-shaped convex portions. The thickness (t) of the connection layer 13 refers to the average thickness of the negative electrode active material layer existing between the standing portions.

[0090] It should be noted that when the length between the sides and the diameter vary depending on the measurement position, it is preferable that each measured value falls within the above range. In the case where the island-shaped convex portion and the connection layer are connected by a smooth curve, the width of the island-shaped convex portion 11 can be measured by taking the portion exceeding 50% of the height of the island-shaped convex portion as the standing portion. When the angle of the standing portion of the island-shaped convex portion is about 60 to 90°, the amount of silicon in each island-shaped convex portion can be ensured relatively much, which is appropriate.

[0091] In order to form a negative electrode active material layer formed from the all-solid-state LiB negative electrode forming composition on the negative electrode current collector, the following method is recommended: The all-solid-state LiB negative electrode forming composition is made into a paste state by a solvent, and after forming a pattern on the negative electrode current collector, the solvent is dried. The coating method is not particularly limited as long as it can form a pattern of the island-shaped convex portion described later. For example, screen printing, 3D printing, etc. can be cited. In addition, the drying only needs to be performed at a temperature at which the solvent used is sufficiently volatilized.

[0092] In addition, the following method can also be cited: after coating the entire surface of the negative electrode current collector with the composition for forming an all-solid-state LiB negative electrode, in a dry or semi-dry state, a mold material such as a mesh is pressed onto the negative electrode active material layer, thereby forming, for example, island-shaped convex portions 11 and groove portions 12 in a mesh pattern as shown in Figure 5 . In this case, the width of the island-shaped convex portions can be adjusted by the mesh interval of the mesh, the interval between the island-shaped convex portions can be adjusted by the thickness of the mesh, and in addition, the depth of the island-shaped convex portions and the thickness of the connection layer can be adjusted by the pressing depth.

[0093] As described above, in the material for forming an all-solid-state LiB negative electrode having the island-shaped convex portions 11 formed by the composition for forming an all-solid-state LiB negative electrode in a predetermined pattern on the negative electrode current collector, when assembling a LiB using this material for forming an all-solid-state LiB negative electrode and performing charge and discharge, although it is difficult to completely control, in the case of substantially maintaining the shape of the island-shaped convex portions 11, the negative electrode active material layer is amorphized and densified. Therefore, the sizes and shapes of the respective blocks derived from the island-shaped convex portions 11 and the groove portions between the respective blocks are homogenized, and even when repeated expansion and contraction caused by charge and discharge occur, contact between adjacent blocks can be suppressed. As a result, pulverization and isolation of the negative electrode active material are not likely to occur, and capacity reduction can be prevented. And, due to the presence of the connection layer 13, even after charge and discharge, the negative electrode current collector 1 does not expose at the bottom of the groove portion 12. If the negative electrode current collector 1 exposes at the bottom of the groove portion 12, the solid electrolyte invades into the interface between the negative electrode current collector 1 and the block through the exposed portion, generating a gap, whereby the negative electrode active material is isolated. However, in the present invention, by providing the connection portion 13, the above isolation can be effectively prevented.

[0094] The material for forming an all-solid-state LiB negative electrode has the above-mentioned negative electrode active material layer 2 on the negative electrode current collector 1, and a solid electrolyte layer can also be further formed on the negative electrode active material layer. The solid electrolyte is not particularly limited, and general sulfide-based solid electrolytes and oxide-based solid electrolytes can be exemplified. Sulfide-based solid electrolytes are advantageous in terms of high lithium ion conductivity. Oxide-based solid electrolytes are chemically stable and are advantageous from the viewpoint of high voltage tolerance. When an oxide-based solid electrolyte is used in the solid electrolyte layer, a general ion conductive material can also be used as needed to improve the lithium ion conductivity.

[0095] The sulfide-based solid electrolyte contains, for example, lithium, phosphorus, and sulfur, and may also contain elements such as O, Al, B, Si, Ge, I, etc. Specifically, amorphous Li3PS4, amorphous 40LiI·60Li3PS4 (mol%), β-Li3PS4, α-Li3PS4, Li7P3S 11 crystals, etc. can be used. A thio-LISICON-based solid electrolyte can also be used.

[0096] Such sulfide-based solid electrolytes can be obtained by known methods. For example, they can also be produced by the following methods: preparing lithium sulfide (Li2S) and phosphorus pentasulfide (P2S5) as starting materials, mixing Li2S and P2S5 in a ratio of about 50:50 to 80:20 in terms of molar ratio, melting and quenching them, or mechanically grinding them; or known methods such as the suspension method, solution method, sol-gel method, etc. which are so-called wet methods.

[0097] The sulfide-based solid electrolytes obtained by the above methods are amorphous. They can be utilized while maintaining this amorphous state, or they can be heat-treated to form crystalline sulfide-based solid electrolytes. By crystallization, an increase in lithium ion conductivity can be expected.

[0098] Examples of oxide-based solid electrolytes include: Li 5+X La3(Zr X , A 2－X )O 12 (wherein A is one or more elements selected from the group consisting of Sc, Ti, C, Y, Nb, Hf, Ta, Al, Si, Ga, Ge, Sn, and X is 1.4 ≤ X ≤ 2), Li 1+ X Al X Ti 2－X (PO4)3 (X is 0 ≤ X ≤ 1), Li 3X La 2 / 3－X TiO3 (X is 0 ≤ X ≤ 2 / 3), etc. They have high ionic conductivity and high electrochemical stability at room temperature.

[0099] From the viewpoint of electrochemical stability, insulating particles such as silica (SiO2) particles, γ-alumina (Al2O3) particles, cerium dioxide (CeO2) particles, zirconium dioxide (ZrO2) particles, etc. can be added to the oxide-based solid electrolytes. In addition, other known metal oxide particles can also be used.

[0100] The Young's modulus (at 25°C) of the above solid electrolytes is preferably 10 to 70 GPa, more preferably 15 to 30 GPa. When the negative electrode active material is densified into a block, the solid electrolyte is easily embedded in the gaps generated between the blocks, and the ionic conductivity can be maintained higher, so it is preferred. Examples of solid electrolytes with a Young's modulus (at 25°C) of 10 to 70 GPa include: amorphous Li3PS4, LiX-Li3PS4 (X = I, Br, Cl) -based glasses, β-Li3PS4, α-Li3PS4, Li7P3S 11 crystals, Li 10 GeP2S 12Solid electrolytes such as LGPS crystal systems represented by, and thiogermanate crystals represented by Li6PS5X (X = I, Br, Cl), etc.

[0101] The film thickness of the solid electrolyte layer is preferably 500 nm to 1000 μm, more preferably 1 μm to 500 μm. If the film thickness is 500 nm or more, a solid battery that does not have defects or cracks and has stable performance can be fabricated. If the film thickness is 1000 μm or less, a solid battery with a sufficiently low resistance can be fabricated.

[0102] When using the above-mentioned all-solid-state LiB negative electrode forming material to assemble a battery and perform charge and discharge, as described above, part or all of the silicon crystal particles as the negative electrode active material are amorphousized, and are densified into a block while substantially maintaining the pattern formed by the island-shaped protrusions, demonstrating the unique effects of the present invention described above.

[0103] Generally, when using silicon as the negative electrode active material, the theoretical maximum value of the charge amount of all-solid-state LiB is about 3600 mAh / g －1 or so, and the practical charge and discharge usage range is about 1000 - 3000 mAh / g －1 range. That is, it can be speculated that the reason why the all-solid-state LiB of the present invention can stably exhibit good battery performance with good cycle characteristics for a long time is also that the blocks generated by charge and discharge are uniformly controlled by the island-shaped protrusions 11. In addition, due to the action of the connection layer 13, it is not easy to form a groove reaching the negative electrode current collector 1, and the volume change of the generated blocks is small.

[0104] It should be noted that in the half-cell constituted in the following examples, the above range corresponds to a state where the potential with respect to a counter electrode such as a metal lithium electrode is +0.02 V to +1.0 V.

[0105] In addition, when using silicon as the negative electrode active material layer without forming the pattern that is a feature of the present invention, the charge and discharge capacity decreases as the current density increases. Therefore, the current density during charge and discharge of all-solid-state LiB is practically used in the range of 0.1 - 0.4 mA / cm －2 range. In contrast, in the present invention, even when the current density is increased to 0.6 mA / cm －2 or more, no significant capacity reduction is shown, and good cycle characteristics are maintained. That is, it can be considered that one of the reasons why the all-solid-state LiB in the present invention can stably exhibit good battery performance with good cycle characteristics for an extremely long time is also that during use, the volume change of the blocks based on the island-shaped protrusions is small, the interface with the solid electrolyte is stable, and in addition, the connection layer 13 also exists after charge and discharge, thereby preventing the solid electrolyte from invading between the negative electrode current collector 1 and the blocks.

[0106] In addition, it is formed by patterning the island-shaped convex portions, and the blocks after charge and discharge are also formed according to the above pattern, so it also has the advantage that the performance deviation between all-solid-state LiB products is extremely small.

[0107] (All-solid-state lithium-ion secondary battery)

[0108] As Figure 6 shown, the all-solid-state lithium-ion secondary battery of the present invention has: a positive electrode current collector 5, a positive electrode active material layer 4, a negative electrode active material layer 2, a negative electrode current collector 1, and a solid electrolyte layer 3. The negative electrode is formed using a material for forming an all-solid-state LiB negative electrode. When using the material for forming an all-solid-state LiB negative electrode to assemble a LiB and perform charge and discharge, a part or all of the silicon crystals contained in the material for forming an all-solid-state LiB negative electrode is / amorphized. In addition, as a result of charge and discharge, a part or all of the silicon forms an alloy with lithium. That is, after assembling the LiB and performing charge and discharge, the crystal structure of the negative electrode active material layer is different from that of the silicon crystal before assembly, and is composed of amorphous silicon in part or in whole. In this specification, the negative electrode active material after charge and discharge may sometimes be abbreviated as "amorphous silicon", but the amorphous silicon may contain silicon crystals and may also be alloyed with lithium.

[0109] This is because when the battery is charged, the silicon particles in the negative electrode layer absorb lithium and thus expand in volume. However, at this time, the silicon particles fuse with each other and become amorphous. The tiny voids existing between the silicon particles are squeezed out, and dense blocks are formed in units of island-shaped convex portions isolated by groove portions (depending on the size of the island-shaped convex portions, grooves that do not reach the negative electrode current collector are generated inside). Thus, even if the expansion rate of the silicon particles themselves due to charging is about 300%, the increase in the thickness of the negative electrode layer itself is suppressed to about 1.5 times.

[0110] During discharge, as lithium is released, silicon attempts to return to its original volume, but the solid electrolyte, which is relatively softer than the silicon blocks, is sucked into the voids around the blocks, and almost no reduction in the thickness of the negative electrode layer itself occurs.

[0111] The solid electrolyte layer 3 is composed of the solid electrolyte.

[0112] In the all-solid-state LiB of the present invention, the constitution other than the negative electrode can adopt the same constitution as that of a known all-solid-state LiB, and there is no particular limitation. The positive electrode is composed of a positive electrode active material layer 4 and a positive electrode current collector 5, and any known positive electrode active material and current collector can be used.

[0113] Examples

[0114] Hereinafter, the present invention will be described in detail by way of examples, but the present invention is not limited by these examples.

[0115] (Average particle diameter of silicon)

[0116] The average particle size / standard deviation of silicon, etc. was determined by LA-950S2 (manufactured by Horiba, Ltd.) based on laser diffraction / scattered light intensity.

[0117] (Specific surface area of silicon)

[0118] The specific surface area of silicon particles was determined by gas adsorption measurement based on the constant volume method using BELSORP-miniX (manufactured by MicrotracBEL Corporation).

[0119] (Active material filling rate of the negative electrode layer before charge-discharge test)

[0120] (1) The film thickness of the silicon coating film was determined from the SEM image, and the volume V0 of the coating film per unit area was calculated.

[0121] (2) The volume V1 of the silicon crystal itself was calculated by dividing the weight of silicon per unit area in the coating film determined from the weight of the entire negative electrode layer by the density of the silicon crystal.

[0122] (3) The active material filling rate of the coating film was calculated as V1 / V0×100 (%).

[0123] (Charge-discharge cycle test)

[0124] The charge-discharge test was carried out using BTS-2004H (manufactured by Nagano Corporation) with a constant current density test having a current density of 0.1 mA / cm². The measurement temperature was set at 25 °C, and only the initial charge amount was set at 3000 mAh / g, －2 the initial discharge amount was set at 2000 mAh / g, －1 and thereafter, the state after the initial discharge was set at 0 mAh / g, －1 the subsequent charge amount was set at 2000 mAh / g, －1 the discharge amount was set at 2000 mAh / g, －1 and the charge-discharge was carried out accordingly. The charge-discharge of 2000 mAh / g was repeated. －1 －1

[0125] The evaluation result of the charge-discharge cycle test was shown as the number of cycles in which the discharge capacity could reach a maintenance rate of 90% or more with respect to the set capacity of 2000 mAh / g in the cycle test. －1

[0126] However, the case where the test ended without the initial charge capacity reaching 3000 mAh / g was shown as "NG". －1

[0127] (Battery materials)

[0128] ​​​​The following materials are used to assemble a half cell, and the structure and characteristics of the negative electrode are evaluated.

[0129] Counter electrode

[0130] Lithium (Li) foil: film thickness 0.1 mm (manufactured by Honjo Metal Co., Ltd.).

[0131] Indium (In) foil: film thickness 0.127 mm (manufactured by Aldrich).

[0132] Negative electrode

[0133] Negative electrode current collector: CF-T7F-35 (manufactured by Fukuda Metal Foil & Powder Co., Ltd.).

[0134] Negative electrode active material layer: A mixture of 90 parts of silicon crystal particles and 10 parts of thermosetting polyimide resin is used. The thermosetting polyimide resin used is DreamBond (trade name) manufactured by I.S.T Co., Ltd.

[0135] Solid electrolyte: a-40LiI·60Li3PS3 (manufactured by mechanical grinding method).

[0136] As the silicon crystal particles, polysilicon manufactured by Tokuyama Corporation is used as the raw material, and is pulverized by a planetary mill, and the following particles are respectively prepared for use.

[0137] [Table 1]

[0138]

[0139] (Evaluation of the shape of the silicon mass after discharge)

[0140] Regarding the evaluation of the silicon mass present in the negative electrode active material layer after discharge, in the evaluation of the cycle characteristics, a plurality of identical half cells are prepared, and the discharged batteries after the first charge-discharge cycle test and the 200th charge-discharge cycle test are cut along a direction perpendicular to the negative electrode surface including the negative electrode current collector. At this time, regarding the cut portion, it is performed twice in such a way that it is orthogonal to any point on the negative electrode surface. After ion milling treatment (CP treatment) of each cut cross-section using a CrossSection Polisher, it is photographed using SEM (scanning microscope), and elemental analysis is performed using EDS (energy dispersive X-ray spectroscopy).

[0141] For the silicon layer present in the SEM image, the width and height (H) of the mass are measured at more than ten places, and the average length is respectively obtained.

[0142] The packing density of silicon element existing in the silicon mass is determined by image analysis. Using the image analysis software "Azokun" (trade name, manufactured by Asahi Kasei Engineering Corporation), the inside of the silicon mass in the SEM-EDS image is analyzed. The image is segmented into 256 levels of grayscale of light and shade, the threshold for binarization is determined as 160 levels of grayscale of concentration, and the part darker than the threshold is judged as silicon. The case where the area ratio of silicon element in the silicon mass is less than 90% or the area ratio of particles smaller than 5 μm in the particle size distribution of silicon is 20% or more is judged as "isolated".

[0143] (Examples 1-7) Comparative Examples

[0144] (Manufacture of negative electrode)

[0145] 360 mg of the above-mentioned polysilicon particles are mixed with a polyimide solution (27.2 wt% NMP solution) in an amount such that the solid content becomes 40 mg, and further NMP (N-methylpyrrolidone) is added to obtain a composition for forming a all-solid-state LiB negative electrode. The composition is stirred for 2 hours (rotation speed 1056 rpm, revolution speed 1600 rpm) and defoamed for 6 minutes (rotation speed 290 rpm, revolution speed 1360 rpm).

[0146] The obtained coating liquid is evenly coated on the entire surface of the negative electrode current collector using a doctor blade (transport speed 1.9 mm / second, doctor blade gap 12.5 μm). After drying at room temperature for more than half a day, it is heated by a heater under vacuum (250 °C, 30 minutes) to cure the polyimide, and a material for forming a negative electrode is obtained. The thickness of the active material layer is in the range of 14-21 μm.

[0147] (Manufacture of half cell)

[0148] The sheet of the material for forming a negative electrode is punched into a 9 mm Φ and placed in an insulating mold, 65 mg of solid electrolyte particles are filled from above the negative electrode sheet, and uniaxial pressing is performed at a molding pressure of 560 MPa. The thickness of the solid electrolyte layer after pressing is 300-400 μm. After temporarily removing the upper punch, a counter electrode obtained by stacking metal foils of In and Li punched into 6 mm Φ in the order of In / Li / In is placed on the upper side of the solid electrolyte layer, and uniaxial pressing is performed again at a pressure of about 50 MPa, thereby manufacturing an all-solid-state half cell. The assembly of the above half cell is carried out in an argon atmosphere in a glove box isolated from external air in order to completely exclude the influence of oxygen, nitrogen, moisture, etc. The above battery evaluation is performed on the obtained half cell. The results of the first charge-discharge cycle test are shown in Table 2, and the results after the charge-discharge cycle test are shown in Table 3.

[0149] For the cross-section of the negative electrode active material layer after charge and discharge in the case where the above-mentioned flat-coated negative electrode layer is formed, the SEM image is shown in Figure 7 . As Figure 7 shown, in the negative electrode active material layer after charge and discharge, irregular grooves are formed between the blocks. In addition, the respective dimensions are shown in the table. From the values of the average value and the maximum value, it can be seen that the blocks are divided into irregular shapes. In addition, it was confirmed that the above-mentioned grooves reach the current collector.

[0150] [Table 2]

[0151]

[0152] 1) Grooves reaching the current collector are formed, so the intervals on the current collector are measured.

[0153] [Table 3]

[0154]

[0155] (Examples 8 to 14) Examples

[0156] In Examples 1 to 7, after drying the negative electrode active material layer at room temperature, the active material layer was heated to 80°C, and a Ni mesh with a square pattern was pressed for 30 minutes to obtain a negative electrode forming material having prismatic island-shaped protrusions. The mesh pattern used was a square with a side length of 55 μm. A pattern with an island-shaped protrusion width of approximately 55 μm, a height of approximately 25 μm, an interval between the island-shaped protrusions of approximately 25 μm, and a connection layer thickness of approximately 5 μm was transferred to the active material layer.

[0157] Using the above-mentioned negative electrode forming material, evaluation half-cells were obtained in the same manner as in Examples 1 to 7. Various evaluations were performed on the obtained cells. The results are shown in Table 4.

[0158] It should be noted that in Examples 9 to 13, in the silicon negative electrode layer after the charge and discharge cycle test (200 cycles), each block corresponding to the island-shaped protrusion and the connection layer were subdivided into smaller blocks by shallow cracks that did not reach the current collector.

[0159] [Table 4]

[0160]

[0161] ※Examples 8 and 14 are comparative examples.

[0162] (Examples 15 to 21) Examples

[0163] In Examples 1 to 7, after drying the negative electrode active material layer at room temperature, a Ni mesh with a square pattern was pressed for 30 minutes and then dried at 80°C to obtain a negative electrode forming material having prismatic island-shaped protrusions. The mesh pattern was a square with a side length of 30 μm. A pattern with an island-shaped protrusion width of approximately 30 μm, a height of approximately 30 μm, a spacing of approximately 20 μm between the island-shaped protrusions, and a connection layer thickness of approximately 8 μm was transferred to the active material layer.

[0164] Using the above negative electrode forming material, an evaluation half-cell was obtained in the same manner as in Examples 1 to 7. Various evaluations were performed on the obtained cell. The results are shown in Table 5.

[0165] It should be noted that in Examples 16 to 20, in each block of the silicon negative electrode layer after the charge-discharge cycle test (200 cycles), the blocks corresponding to the island-shaped protrusions and the connection layer were subdivided into smaller blocks by shallow cracks that did not reach the current collector.

[0166] [Table 5]

[0167]

[0168] ※Examples 15 and 21 are comparative examples.

[0169] (Examples 22 to 28) Examples

[0170] In Examples 1 to 7, after drying the negative electrode active material layer at room temperature, the active material layer was heated to 80°C, and a Ni mesh with a regular hexagon pattern was pressed for 30 minutes to obtain a negative electrode forming material having hexagonal prismatic island-shaped protrusions. The mesh pattern was a regular hexagon with a side length of approximately 20 μm. A pattern with a hexagonal prism shape with a distance between opposite sides of approximately 35 μm for the island-shaped protrusions, a height of approximately 30 μm, a spacing of approximately 20 μm between the island-shaped protrusions, and a connection layer thickness of approximately 8 μm was transferred to the active material layer.

[0171] Using the above negative electrode forming material, an evaluation half-cell was obtained in the same manner as in Examples 1 to 7. The above various evaluations were performed on the obtained cell. The results are shown in Table 6.

[0172] It should be noted that in Examples 23 to 27, in each block of the silicon negative electrode layer after the charge-discharge cycle test (200 cycles), the blocks corresponding to the island-shaped protrusions and the connection layer were subdivided into smaller blocks by shallow cracks that did not reach the current collector.

[0173] [Table 6]

[0174]

[0175] ※Examples 22 and 28 are comparative examples.

[0176] (Examples 29 to 33) Examples

[0177] For the same batteries of Examples 16 to 20 that use a pre-formed pattern negative electrode layer, the current density during charge-discharge testing was set to 1.0 mA / cm －2 , and charge-discharge tests were carried out, and the results are shown in Table 8.

[0178] It should be noted that in Examples 29 to 33, in each block of the silicon negative electrode layer after the charge-discharge cycle test (200 cycles), each block corresponding to the island-shaped convex portion and the connection layer were subdivided into smaller blocks by shallow cracks that did not reach the current collector.

[0179] [Table 7]

[0180]

[0181] Explanation of reference numerals

[0182] 1: Negative electrode current collector; 2: Negative electrode active material layer; 3: Solid electrolyte layer; 4: Positive electrode active material layer; 5: Positive electrode current collector; 10: Material for forming the negative electrode of all-solid-state lithium-ion secondary battery; 11: Negative electrode active material layer (island-shaped convex portion); 12: Groove portion; 13: Connection layer; 20: All-solid-state lithium-ion secondary battery.

Claims

1. A material for forming a negative electrode of an all-solid-state lithium-ion secondary battery, wherein a negative electrode active material layer is provided on a negative electrode current collector. For the negative electrode active material layer, island-shaped protrusions formed of a composition for forming a negative electrode of an all-solid-state lithium-ion secondary battery containing silicon crystals with an average particle size of 0.5 to 5.0 μm are formed in a pattern with intervals therebetween, and a connection layer formed of the composition for forming a negative electrode of an all-solid-state lithium-ion secondary battery is continuously formed on the bottom surface of the groove portion formed between the island-shaped protrusions.

2. The material for forming a negative electrode of an all-solid-state lithium-ion secondary battery according to claim 1, wherein a negative electrode active material layer is provided on the current collector. For the negative electrode active material layer, the width of the island-shaped protrusions is in the range of 10 μm to 100 μm, the height is in the range of 5 to 100 μm, the interval between the island-shaped protrusions is 10 to 50% relative to the width of the island-shaped protrusions, the thickness of the connection layer is in the range of 1 to 50% relative to the height of the island-shaped protrusions, and the pattern is formed to be not more than 15 μm.

3. The material for forming a negative electrode of an all-solid-state lithium-ion secondary battery according to claim 1, wherein a negative electrode active material layer is provided on a negative electrode current collector. For the negative electrode active material layer, the pattern is formed such that the shape of the island-shaped protrusions is cylindrical, elliptical cylindrical, polygonal prismatic, frustum of a cone-shaped, elliptical frustum of a cone-shaped, polygonal frustum of a pyramid-shaped, conical, elliptical conical or polygonal pyramid-shaped.

4. The material for forming a negative electrode of an all-solid-state lithium-ion secondary battery according to claim 1, wherein the silicon crystal filling rate of the composition for forming a negative electrode of an all-solid-state lithium-ion secondary battery is 35 to 55% by volume.

5. The material for forming a negative electrode of an all-solid-state lithium-ion secondary battery according to claim 1, wherein the negative electrode active material layer contains an electron conductivity imparting agent of 5 parts by mass or less relative to 100 parts by mass of the silicon crystals.

6. A all-solid-state lithium-ion secondary battery, characterized in that, An all-solid-state lithium-ion secondary battery having a positive electrode, a negative electrode, and a solid electrolyte layer, wherein the negative electrode is composed of the material for forming a negative electrode of an all-solid-state lithium-ion secondary battery according to any one of claims 1 to 5.

7. The all-solid-state lithium-ion secondary battery according to claim 6, wherein the solid electrolyte layer is also present in the groove portion.

Images