Solid Electrolyte, Method for Producing the Same, Electrode Binder, Electrode Layer, and Battery

Abstract

To provide a solid electrolyte by which satisfactory battery characteristics can be obtained.SOLUTION: A solid electrolyte comprises a lithium element, a phosphorus element, a sulfur element and an X element (X is at least one halogen element). The solid electrolyte is 35% or less in the compressibility when an applied load is 30 kPa in measurement of a powder layer shear force. The solid electrolyte is suitably manufactured by: a sintering step of sintering a material composition containing a lithium element, a phosphorus element, a sulfur element and an X element (X is at least one halogen element) in an inert gas atmosphere to obtain a sintered product; and a pulverizing step of performing pulverization of the sintered product two or more times.SELECTED DRAWING: None

Description

Technical Field

[0001] The present invention relates to a solid electrolyte and a method for producing the same. The present invention also relates to an electrode binder, an electrode layer, and a battery including the solid electrolyte.

Background Art

[0002] Since a solid battery does not use a flammable organic solvent, it is possible to simplify the safety device, and moreover, it can be excellent in manufacturing cost and productivity, and it also has a feature that it can be stacked in series in the cell to increase the voltage.

[0003] As a conventional technique related to a solid electrolyte used in a solid battery, for example, Patent Document 1 describes a method for producing a sulfide solid electrolyte in which a sulfide solid electrolyte material is crushed by a mechanical milling method to obtain flat particles, and then the flat particles are crushed by a mechanical milling method to obtain spherical particles. It is described that a battery using the solid electrolyte obtained by this method has a high capacity retention rate.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] By the way, in a solid battery, due to the fact that the electrolyte is solid, it is not easy to sufficiently increase the contact between the active material and the solid electrolyte and the contact between the solid electrolytes. When these contacts are not sufficient, the improvement of battery characteristics is hindered. Therefore, an object of the present invention is to provide a solid electrolyte capable of obtaining good battery characteristics.

Means for Solving the Problems

[0006] The present invention contains a lithium (Li) element, a phosphorus (P) element, a sulfur (S) element, and an X element (X is at least one halogen element), and provides a solid electrolyte having a compression ratio of 35% or less when the applied load in powder layer shear force measurement is 30 kPa.

[0007] The present invention also includes a firing step of firing a raw material composition containing a lithium (Li) element, a phosphorus (P) element, a sulfur (S) element, and an X element (X is at least one halogen element) in an inert gas atmosphere to obtain a fired product, and a pulverization step of pulverizing the fired product a plurality of times, and provides a method for manufacturing a solid electrolyte having the steps.

Advantages of the Invention

[0008] According to the present invention, a solid electrolyte capable of obtaining good battery characteristics is provided.

Embodiments for Carrying Out the Invention

[0009] Hereinafter, the present invention will be described based on its preferred embodiments. The solid electrolyte of the present invention contains at least a lithium (Li) element, a phosphorus (P) element, a sulfur (S) element, and an X element.

[0010] The X element contained in the solid electrolyte of the present invention is a halogen element. More specifically, at least one element selected from chlorine (Cl) element, bromine (Br) element, and iodine (I) element is used. The halogen (X) element may be one of these elements, or a combination of two or more. As long as these elements are included, the solid electrolyte of the present invention may be any of a crystalline material, glass ceramics, and glass. From the viewpoint of enhancing the lithium ion conductivity of the solid electrolyte of the present invention, it is preferable that the solid electrolyte contains at least a chlorine (Cl) element or a bromine (Br) element as the halogen (X) element, and it is more preferable that the solid electrolyte contains both a bromine (Br) element and a chlorine (Cl) element.

[0011] Examples of the solid electrolyte containing the above-described elements include, but are not limited to, Li2S-P2S5-LiX (where X is at least one halogen element).

[0012] In particular, from the viewpoint of enhancing the lithium ion conductivity of the solid electrolyte, it is preferable that the solid electrolyte containing the above-described elements contains a compound represented by the composition formula Li a PS b X c (where X is at least one halogen element. a represents a number of 3.0 or more and 6.0 or less. b represents a number of 3.5 or more and 4.8 or less. c represents a number of 0.1 or more and 3.0 or less).

[0013] In the above composition formula, a representing the molar ratio of the lithium (Li) element is preferably, for example, a number of 3.0 or more and 6.0 or less, more preferably a number of 3.2 or more and 5.8 or less, and still more preferably a number of 3.4 or more and 5.4 or less. Note that a may be less than 5.4. In the above composition formula, b representing the molar ratio of the sulfur (S) element is preferably, for example, a number of 3.5 or more and 4.8 or less, more preferably a number of 3.8 or more and 4.6 or less, and still more preferably a number of 4.0 or more and 4.4 or less. Note that b may be less than 4.4. In the above composition formula, c is preferably, for example, a number of 0.1 or more and 3.0 or less, more preferably a number of 0.2 or more and 2.5 or less, and still more preferably a number of 0.4 or more and 2.0 or less. Compounds in which a, b, and c are within this range have a sufficiently high lithium ion conductivity. In the solid electrolyte of the present invention, there may be a case where only one kind of the compound represented by the above composition formula is contained, or there may be a case where two or more kinds of compounds are contained.

[0014] In the present invention, the charged amount is Li a PS b X cThe compound obtained in such a manner may contain elements other than lithium (Li), phosphorus (P), sulfur (S), and halogen (X) elements. For example, it may be possible to replace a part of the lithium (Li) element with other alkali metal elements, replace a part of the phosphorus (P) element with other pnictogen elements, or replace a part of the sulfur (S) element with other chalcogen elements.

[0015] It is preferable that the solid electrolyte of the present invention contains a crystal phase having a particularly argyrodite-type crystal structure from the viewpoint of enhancing the lithium ion conductivity of the solid electrolyte. The argyrodite-type crystal structure is a crystal structure possessed by a group of compounds derived from a mineral represented by the chemical formula: Ag8GeS6. Whether or not the solid electrolyte of the present invention has a crystal phase of an argyrodite-type crystal structure can be confirmed by measurement using XRD or the like. For example, in the diffraction pattern measured by XRD using CuKα1 line, the crystal phase of the argyrodite-type crystal structure shows diffraction peaks characteristic of 2θ = 15.3° ± 1.0°, 17.7° ± 1.0°, 25.2° ± 1.0°, 30.0° ± 1.0°, 30.9° ± 1.0°, and 44.3° ± 1.0°. Also, depending on the types of elements constituting the solid electrolyte, in addition to the above diffraction peaks, diffraction peaks characteristic of 2θ = 47.2° ± 1.0°, 51.7° ± 1.0°, 58.3° ± 1.0°, 60.7° ± 1.0°, 61.5° ± 1.0°, 70.4° ± 1.0°, and 72.6° ± 1.0° may be shown. For the identification of the diffraction peaks derived from the argyrodite-type crystal structure, for example, the data of PDF number 00-034-0688 can be used.

[0016] The solid electrolyte of the present invention preferably consists of powder as an aggregate of particles. One of the features of the solid electrolyte of the present invention is that the powder has high fluidity. The solid electrolyte of the present invention made of powder with high fluidity is such that voids are less likely to occur between the particles constituting the powder. As a result, in the solid electrolyte of the present invention, the contact between the particles of the solid electrolyte and the contact between the particles of the solid electrolyte and the particles of the active material become good, and the lithium ion conductivity increases. Due to this, the internal resistance of the battery containing the solid electrolyte of the present invention decreases. The decrease in the internal resistance contributes to the improvement of the rate characteristics of the battery.

[0017] In the field of powder engineering, various parameters are known as measures of the fluidity of powders. For example, parameters such as compressibility, porosity, internal friction angle, adhesion force, angle of repose, and angle of collapse are known. As a result of the inventors' intensive study, among these various parameters, it has been found that it is effective to evaluate the fluidity by the compressibility, which most reflects the fluidity of the solid electrolyte of the present invention. Specifically, in the present invention, the compressibility in powder layer shear force measurement, measured for the powder of the solid electrolyte, is preferably 35% or less. This compressibility is the value when the applied load is 30 kPa. When the compressibility of the powder of the solid electrolyte is below this value, the powder has very good fluidity, and voids are less likely to occur between the particles even in the natural state (i.e., the state without applying an external force), and the opportunity for contact between the particles increases. From the viewpoint of making this advantage more prominent, the compressibility of the powder of the solid electrolyte is preferably 32% or less, more preferably 28% or less, and even more preferably 25% or less. The closer the value of the compressibility is to zero, the more preferable it is.

[0018] The compression ratio is defined as the reduction rate of the volume of the powder from the state without applying a load to the state with the set load applied. The compression ratio serves as an indicator representing the amount of voids between particles and the strength of the strong agglomerated structure of the powder. The compression ratio can be measured from the volume (static bulk height) measured without load after filling a cell with a predetermined amount of powder using a powder layer shear force measuring device and the volume (bulk height) when a set load of 30 kPa is applied. Specifically, when the volume measured without load is denoted as V0 and the volume when 30 kPa is applied is denoted as V1, the compression ratio C is expressed by the following formula. C(%)=(V0 - V1) / V0×100

[0019] The reason for setting the applied load to a relatively low load of 30 kPa is to accurately and reproducibly reflect the degree of the existence of voids between particles. If the applied load is set too high, not only will the voids between particles due to compression disappear, but also the particles themselves will collapse, and the value of the compression ratio will not accurately reflect the fluidity of the powder.

[0020] In this specification, a powder rheometer FT4 (manufactured by freemantechnology) is used for measuring the compression ratio. However, the use of equivalent devices from other companies is not prohibited. A sample is placed in a measuring container with an inner diameter of 25 mm so as to be a specified amount, and the compression ratio is measured with the set value of the applied load set to 30 kPa. At this time, the holding time from when the load is applied until the measurement starts is set to about 300 seconds (the device automatically starts the measurement when the fluctuation of the applied pressure becomes small).

[0021] The powder of the solid electrolyte having the fluidity described above is preferably manufactured, for example, according to the method described later.

[0022] The fluidity of the powder is also affected by the particle size. From the viewpoint of enhancing the fluidity of the powder of the solid electrolyte, the particle size of the solid electrolyte is the volume cumulative particle size D at 50% by volume in the cumulative volume according to the laser diffraction scattering particle size distribution measurement method. 50It is represented by 50 and is preferably, for example, 10 μm or less, more preferably 7.0 μm or less, particularly preferably 5.0 μm or less, still more preferably 2.0 μm or less, and even more preferably 1.0 μm or less. On the other hand, the volume cumulative particle size D

[0023] of the solid electrolyte 50 is measured using an automatic sample feeder for a laser diffraction particle size distribution measuring device (Microtrac SDC manufactured by Microtrac Bell Co., Ltd.). The powder of the solid electrolyte is put into a water-soluble solvent, irradiated with 40 W of ultrasonic waves for 60 seconds at a flow rate of 40%, and then the particle size distribution is measured using a laser diffraction particle size distribution measuring machine "MT3000II" manufactured by Microtrac Bell Co., Ltd. The volume cumulative particle size D 50 is determined from the chart of the obtained volume-based particle size distribution.

[0024] The powder of the solid electrolyte preferably has a relatively small specific surface area. The specific surface area is a parameter that reflects the surface shape of the particles. Powders with a large specific surface area may have an adverse effect on the fluidity of the powder, for example, if the surface of the particles is uneven. In addition, powders with a large specific surface area tend to have a small particle size of the particles constituting the powder and a high frictional force between the particles, so they may also have an adverse effect on the fluidity of the powder. From the above viewpoints, the powder of the solid electrolyte preferably has a BET specific surface area of 30 m 2 / g or less, more preferably 20 m 2 / g or less, and even more preferably 13 m 2 / g or less. Also, the powder of the solid electrolyte preferably has a BET specific surface area of 3.0 m 2 / g or more and 5.0 m2 It is more preferably 10 m / g or more, and even more preferably 10 m / g or more. 2 It is even more preferably 10 m / g or more.

[0025] The BET specific surface area of the solid electrolyte powder is measured, for example, by the following method. Using a pretreatment apparatus "BELPREP-vacII" manufactured by MicrotracBEL Corporation, heat in a vacuum at 120 °C for 1 hour. Then, using a specific surface area measuring apparatus "BELSORP-miniII" manufactured by MicrotracBEL Corporation, calculate by the BET (Brunauer-Emmett-Teller) method from the nitrogen gas adsorption amount at liquid nitrogen temperature (77 K) to obtain the specific surface area.

[0026] The above-described compressibility, particle size, and specific surface area can be adjusted by appropriately pulverizing the solid electrolyte. Generally, pulverization is roughly classified into dry pulverization and wet pulverization. Among them, wet pulverization can more accurately control the particle size and the like. In wet pulverization, a slurry in which the solid electrolyte to be pulverized is dispersed in a solvent is used. Water or an organic solvent is used as the solvent for wet pulverization. From the viewpoint of the stability of the solid electrolyte, an organic solvent is preferably used for wet pulverization of the solid electrolyte. After wet pulverization, an operation for separating the solid electrolyte from the organic solvent is required. A small amount of the organic solvent may remain in the solid electrolyte after separating the organic solvent. The present inventor has found that this residual organic solvent is one of the factors affecting the fluidity of the solid electrolyte. Therefore, it is desirable to appropriately control the amount of the organic solvent contained in the solid electrolyte from the viewpoint of improving the fluidity of the solid electrolyte powder.

[0027] From the above viewpoint, the solid electrolyte may further contain an organic solvent. The amount of the organic solvent contained in the solid electrolyte is preferably 0.01% by mass or more and 10.0% by mass or less, more preferably 0.03% by mass or more and 7.0% by mass or less, still more preferably 0.05% by mass or more and 5.0% by mass or less, and even more preferably 0.07% by mass or more and 3.0% by mass or less.

[0028] As the organic solvent, from the viewpoint of enhancing the fluidity of the solid electrolyte powder, for example, aromatic organic solvents such as toluene, xylene, benzene, and solvent naphtha, and aliphatic organic solvents such as heptane, decane, normal hexane, cyclohexane, and mineral spirit can be mentioned. These organic solvents can be used alone or in combination of two or more. When two or more organic solvents are contained in the solid electrolyte, the amount of the above-mentioned organic solvent refers to the total amount of all organic solvents.

[0029] In the present invention, the amount of the organic solvent contained in the solid electrolyte is measured by the thermogravimetric method. Specifically, the solid electrolyte is heated in a muffle furnace to heat-remove the organic solvent. The heating is carried out at 180°C for 20 minutes. The heating atmosphere is an inert gas atmosphere such as nitrogen or argon. When the mass before heating is W1 and the mass after heating is W2, (W1 - W2) / W1 × 100 is defined as the content of the organic solvent. It is appropriate that the amount of the solid electrolyte used is about 5 g.

[0030] The solid electrolyte of the present invention has lithium ion conductivity in a solid state. The solid electrolyte of the present invention preferably has a lithium ion conductivity of 0.5 mS / cm or more, more preferably 1.0 mS / cm or more, and still more preferably 1.5 mS / cm or more at room temperature, that is, 25°C. The lithium ion conductivity can be measured using the method described in the examples below.

[0031] The solid electrolyte of the present invention can preferably be manufactured by the method described below. As raw materials, a lithium source compound, a phosphorus source compound, a sulfur source compound, and a halogen source compound are used. As the lithium source compound, for example, lithium sulfide (Li2S) can be used. As the phosphorus source compound, for example, diphosphorus pentasulfide (P2S5) can be used. As the sulfur source compound, when the lithium source compound and / or the phosphorus source compound is a sulfide, the sulfide can be used as the sulfur source compound. As the halogen source compound, lithium halide LiX can be used. These raw materials are mixed so that the lithium element, phosphorus element, sulfur element, and halogen element have a predetermined molar ratio. Then, by firing the raw material composition obtained by the mixing in an inert gas atmosphere, a fired product containing a crystal phase represented by Li a PS b X c and having an argyrodite-type crystal structure is obtained.

[0032] In the conventional technology regarding the production of solid electrolytes, the above firing was performed in a hydrogen sulfide atmosphere. However, in this production method, it is preferable to perform the firing in an inert gas atmosphere from the viewpoint of successfully obtaining a highly fluid solid electrolyte powder. In this specification, "performing firing in an inert gas atmosphere" means using only an inert gas as the firing atmosphere and the absence of non-inert gases such as hydrogen sulfide in the atmosphere. Therefore, when a non-inert gas is present in the firing atmosphere or when a substance that generates a non-inert gas during firing is contained in the raw material composition, it does not correspond to "performing firing in an inert gas atmosphere". As the inert gas, noble gases such as argon or nitrogen can be used.

[0033] The fired product thus obtained is subjected to a predetermined pulverization process. In this manufacturing method, it is preferable to perform the pulverization of the fired product multiple times from the viewpoint of successfully obtaining a powder of a solid electrolyte with high fluidity. In particular, when the fired product is crystalline, it is not easy to obtain a powder of a solid electrolyte having a desired fluidity by a single pulverization, so it is desirable to perform the pulverization multiple times. Further, by performing the pulverization multiple times, the surface of the particles of the solid electrolyte can be brought into a chemical state with increased fluidity.

[0034] When pulverizing the fired product, at least two dry pulverizations can be performed, or at least two wet pulverizations can be performed, or at least one dry pulverization and at least one wet pulverization can be performed. When performing a combination of dry pulverization and wet pulverization, the dry pulverization may be performed first and then the wet pulverization, or the wet pulverization may be performed first and then the dry pulverization.

[0035] For dry pulverization, for example, a jet mill, a ball mill, a rod mill, a vibration ball mill, a planetary mill, a disk mill, etc. can be used. On the other hand, for wet pulverization, various media mills can be used. As the media mill, a ball mill, a bead mill, a paint shaker, a homogenizer, etc. can be used.

[0036] In particular, from the viewpoint of obtaining a powder of a solid electrolyte having a desired fluidity, it is preferable to perform at least two wet pulverizations using a pulverization medium, for example, wet pulverization using a bead mill or a ball mill. In this case, from the viewpoint of obtaining a powder of a solid electrolyte having a desired fluidity, it is preferable that the size (diameter) of the pulverization medium used in the subsequent wet pulverization is smaller. For example, when the fired product is pulverized by performing wet pulverization twice, the diameter of the pulverization media used for the first wet pulverization is preferably set to 2.0 mm or more and 20 mm or less, and more preferably set to 5.0 mm or more and 15 mm or less. The diameter of the pulverization media used for the second wet pulverization is preferably set to 0.03 mm or more and 5.0 mm or less, and more preferably set to 0.1 mm or more and 2.0 mm or less, on the condition that it is smaller than the diameter of the pulverization media used for the first wet pulverization. The material of the pulverization media used for wet pulverization may be the same or different between the first and the second time. Examples of the material of the pulverization media include various ceramics such as alumina and zirconia.

[0037] In this manufacturing method, it is also preferable to first perform at least one dry pulverization and then perform at least two wet pulverizations. By pulverizing the fired product in such an order and number of times, it is possible to more successfully obtain a powder of the solid electrolyte having desired fluidity. In this case, for example, a hammer mill can be used for dry pulverization, and a bead mill or a ball mill can be used for wet pulverization.

[0038] As described above, in this manufacturing method, from the viewpoint of obtaining a powder of the solid electrolyte having desired fluidity, it is advantageous to perform the step of obtaining the fired product of the solid electrolyte in an inert gas atmosphere and to perform pulverization of the fired product a plurality of times, and it is even more advantageous to perform wet pulverization of the fired product a plurality of times.

[0039] After wet pulverization, in order to separate the powder of the solid electrolyte and the organic solvent, for example, it is preferable to subject a slurry containing the powder of the solid electrolyte and the organic solvent to a solid-liquid separation treatment such as natural filtration, centrifugal separation, pressure filtration, or vacuum filtration. Alternatively, it may be subjected to hot air drying or vacuum drying without performing those operations. The type of the organic solvent contained in the slurry is as described above. Among these operations, from the viewpoint of easily adjusting the amount of the organic solvent remaining in the solid electrolyte powder and enhancing the fluidity of the solid electrolyte powder, it is preferable to adopt a method of performing drying under reduced pressure after solid-liquid separation by filtration under reduced pressure. Adopting this method is also advantageous in that the apparatus is relatively simple and the drying time can be shortened. The pressure during drying under reduced pressure is preferably 10,000 Pa or less, particularly 5,000 Pa or less in terms of absolute pressure. There is no particular limitation on the lower limit value of the pressure. The temperature is preferably 50°C or higher and 200°C or lower, particularly 70°C or higher and 160°C or lower.

[0040] The solid electrolyte obtained by the above method can be used as a material constituting the solid electrolyte layer, the positive electrode layer, or the negative electrode layer. Specifically, the solid electrolyte of the present invention can be used in a battery having a positive electrode layer, a negative electrode layer, and a solid electrolyte layer between the positive electrode layer and the negative electrode layer. That is, the solid electrolyte can be used in a so-called solid battery. More specifically, it can be used in a lithium solid battery. The lithium solid battery may be a primary battery or a secondary battery. There is no particular limitation on the shape of the battery, and for example, shapes such as a laminate type, a cylindrical type, and a rectangular type can be adopted. The "solid battery" includes not only a solid battery that does not contain any liquid substance or gel substance as an electrolyte, but also, for example, an embodiment containing a liquid substance or gel substance with 50% by mass or less, 30% by mass or less, or 10% by mass or less as an electrolyte.

[0041] When the solid electrolyte of the present invention is included in the solid electrolyte layer, the solid electrolyte layer can be produced, for example, by dropping a slurry composed of the solid electrolyte, a binder, and a solvent onto a substrate and scraping it with a doctor blade or the like, a method of cutting with an air knife after bringing the substrate into contact with the slurry, a method of forming a coating film by a screen printing method or the like, and then removing the solvent through heat drying. Alternatively, it can also be produced by pressing the powdery solid electrolyte into a compact by pressing or the like and then appropriately processing it. The thickness of the solid electrolyte layer is preferably typically 5 μm or more and 300 μm or less, and more preferably 10 μm or more and 100 μm or less, in view of the balance between short-circuit prevention and volumetric capacity density.

[0042] The solid electrolyte of the present invention is used together with an active material to form an electrode binder. The proportion of the solid electrolyte in the electrode binder is typically 10% by mass or more and 50% by mass or less. The electrode binder may contain other materials such as a conductive aid and a binder as needed. By mixing the electrode binder and a solvent to prepare a paste, applying it onto a current collector such as an aluminum foil, and drying it, an electrode layer such as a positive electrode layer and / or a negative electrode layer can be formed.

[0043] As the positive electrode material constituting the positive electrode layer, a positive electrode material used as a positive electrode active material of a lithium-ion battery can be appropriately used. For example, a positive electrode active material containing lithium, specifically, a spinel-type lithium transition metal oxide and a lithium metal oxide having a layered structure can be mentioned. By using a high-voltage positive electrode material as the positive electrode material, the energy density can be improved. The positive electrode material may contain a conductive material in addition to the positive electrode active material, or may contain other materials.

[0044] As the negative electrode material constituting the negative electrode layer, a negative electrode material used as a negative electrode active material of a lithium-ion battery can be appropriately used. Since the solid electrolyte of the present invention is electrochemically stable, carbon-based materials such as graphite, artificial graphite, natural graphite, and non-graphitizable carbon (hard carbon), which are materials that charge and discharge at a potential lower than that of lithium metal or comparable to that of lithium metal (about 0.1 V vs. Li + / Li), can be used as the negative electrode material. Thereby, the energy density of the solid battery can be greatly improved. Also, silicon or tin, which is promising as a high-capacity material, can be used as the active material. In a battery using a general electrolyte solution, the electrolyte solution reacts with the active material during charge and discharge, and corrosion occurs on the surface of the active material, resulting in significant deterioration of the battery characteristics. In contrast, when the solid electrolyte of the present invention is used instead of the electrolyte solution and silicon or tin is used as the negative electrode active material, the above-described corrosion reaction does not occur, so that the durability of the battery can be improved. Regarding the negative electrode material, a conductive material may be included in addition to the negative electrode active material, or other materials may be included.

Examples

[0045] Hereinafter, the present invention will be described in more detail with reference to examples. However, the scope of the present invention is not limited to such examples. Unless otherwise specified, "%" means "% by mass".

[0046] [Example 1] Li 5.4 PS 4.4 Cl 0.8 Br 0.8 Li2S powder, P2S5 powder, LiCl powder, and LiBr powder were weighed so that the total amount would be 75 g to have the composition of. These powders were pulverized and mixed using a ball mill to obtain a mixed composition. The mixed composition was fired to obtain a fired product. The firing was carried out using a tubular electric furnace. During firing, nitrogen gas with a purity of 100% was circulated in the electric furnace. The firing temperature was set at 500°C. The obtained fired product was crushed using a mortar and pestle. Subsequently, it was roughly pulverized using a wet ball mill (zirconia beads with a diameter of 10 mm) with toluene. The roughly pulverized fired product was finely pulverized using a wet bead mill (a bead mill manufactured by Asizawa Fine Tech Co., Ltd., model number: DMS110) with toluene. α-alumina beads with a diameter of 0.3 mm were used for the fine pulverization by the wet bead mill. After the finely pulverized fired product was subjected to solid-liquid separation, it was dried by heating to 150°C in a container depressurized to 3000 Pa (absolute pressure) by vacuum pumping to remove toluene. The dried fired product was sieved through a sieve with an opening of 53 μm to obtain the powder of the target solid electrolyte. It was confirmed that the obtained solid electrolyte had an argyrodite-type crystal structure.

[0047] [Comparative Example 1] In Example 1, the firing atmosphere was changed to hydrogen sulfide gas with a purity of 100%. Also, the firing temperature was changed to 500°C. A powder of the solid electrolyte was obtained in the same manner as in Example 1 except for these.

[0048] [Comparative Example 2] In Example 1, elemental sulfur was contained in the mixed composition to be fired so that sulfur vaporized during firing. Except for this, the powder of the solid electrolyte was obtained in the same manner as in Example 1.

[0049] 〔Evaluation〕 For the solid electrolytes obtained in the examples and comparative examples, the compression ratio, particle size D 50 , BET specific surface area, and solvent content were measured by the methods described above. Also, a solid battery was fabricated by the following method, and the discharge capacity retention rate of the battery was measured by the following method. The above results are shown in Table 1 below.

[0050] 〔Fabrication of Solid Battery〕 A negative electrode mixture powder was prepared by mixing carbon powder and solid electrolyte powder in a mass ratio of 64:36 in a mortar. 0.50 g of the prepared electrode mixture powder was uniaxially press-molded at 20 MPa to obtain a negative electrode mixture pellet with a diameter of 13 mm. The obtained negative electrode mixture pellet was crushed in a mortar and passed through a sieve with an opening of 53 μm to obtain a negative electrode mixture powder. A positive electrode mixture powder was prepared by mixing lithium nickel manganese cobalt oxide powder, solid electrolyte powder, and a conductive aid in a ratio of 60:37:3 in a mortar. As the solid electrolyte powder, the powders of the solid electrolytes obtained in the examples and comparative examples were used. 0.30 g of the prepared electrode mixture powder was uniaxially press-molded at 20 MPa to obtain a positive electrode mixture pellet with a diameter of 13 mm. The obtained positive electrode mixture pellet was crushed in a mortar and passed through a sieve with an opening of 53 μm to obtain a positive electrode mixture powder. 0.050 g of the solid electrolyte powder was poured into a PP cylinder (inner opening diameter 10.5 mm) with open top and bottom through the upper opening while the lower opening of the cylinder was closed with an electrode (made of SUS). Next, the upper opening of the cylinder was clamped with electrodes and uniaxially press-molded at 4 MPa to fabricate a solid electrolyte layer composed of a pellet with a diameter of 10.5 mm. The upper electrode was removed once, 0.010 g of the positive electrode active material powder was inserted into the cylinder, and then the upper electrode was attached again. The cylinder was turned upside down, the electrode that had come to the upper side newly was removed once, 0.011 g of the negative electrode active material powder was inserted, the upper opening of the cylinder was clamped with the electrode, and uniaxial press molding was performed once each at 9 MPa, 19 MPa, and 28 MPa to crimp the negative electrode active material powder, the solid electrolyte layer, and the positive electrode active material powder. Next, the space between the upper electrode and the lower electrode was clamped with a C-clamp and constrained at 4 N·m to fabricate a solid-state battery (solid lithium secondary battery). The fabrication of the solid-state battery was carried out in a glove box replaced with Ar having an average dew point of -80°C.

[0051] 〔Measurement of Discharge Capacity Retention Rate〕 Regarding the obtained solid-state battery, charging was performed with the C-rate set to 0.1C, and then discharging was performed with the C-rate set to 0.1C. At this time, the charge cut-off voltage was 4.5V and the discharge cut-off voltage was 2.5V. This was repeated 3 times, and the discharge capacity at the third time was taken as the discharge capacity at 0.1C. Next, charging was performed with the C-rate set to 0.2C and the charge cut-off voltage set to 4.5V, and then discharging was performed with the C-rate changed in the order of 0.2C, 0.5C, 1C, 2C, 3C, 4C, 5C and the discharge cut-off voltage set to 2.5V, and the discharge capacity at 5C, which was the 10th time, was obtained. Then, the discharge capacity at 5C was divided by the discharge capacity at 0.1C and multiplied by 100 to calculate the discharge capacity retention rate.

[0052]

Table 1

[0053] As is clear from the results shown in Table 1, it can be seen that the battery using the solid electrolyte of Example 1 with a low compression ratio has a higher discharge capacity retention rate at a high rate than the battery using the solid electrolyte of the comparative example with a high compression ratio.

Claims

1. A solid electrolyte containing a lithium (Li) element, a phosphorus (P) element, a sulfur (S) element, and an X element (X is at least one halogen element), wherein the compression ratio is 35% or less when the applied load in powder layer shear force measurement is 30 kPa.

2. The volume-cumulative particle size D at a cumulative volume of 50% by the laser diffraction scattering particle size distribution measurement method ５０ is 0.1 μm or more and 10 μm or less, and the solid electrolyte according to claim 1.

3. The specific surface area by the BET method is 30 m ２ / g or less, the solid electrolyte according to claim 1 or 2.

4. The solid electrolyte according to any one of Claims 1 to 3, wherein the content of the organic solvent is 0.01% by mass or more and 10.0% by mass or less.

5. The solid electrolyte according to any one of Claims 1 to 4, which is crystalline.

6. The solid electrolyte according to any one of Claims 1 to 5, containing a crystal phase having an argyrodite-type crystal structure.

7. The solid electrolyte according to any one of Claims 1 to 6, wherein the X element contains at least chlorine.

8. A method for producing the solid electrolyte according to Claim 1, comprising: a firing step of firing a raw material composition containing a lithium (Li) element, a phosphorus (P) element, a sulfur (S) element, and an X element (X is at least one halogen element) in an inert gas atmosphere to obtain a fired product; a grinding step of grinding the fired product a plurality of times.

9. The production method according to Claim 8, wherein the grinding step is a step of performing at least two wet grindings.

10. The production method according to Claim 9, wherein the grinding step is performed so that the amount of the organic solvent contained in the solid electrolyte is 0.01% by mass or more and 10.0% by mass or less.

11. An electrode binder containing the solid electrolyte according to any one of Claims 1 to 7 and an active material.

12. An electrode layer containing the solid electrolyte according to any one of Claims 1 to 7 or the electrode binder according to Claim 7.

13. A battery having a positive electrode layer, a negative electrode layer, and a solid electrolyte layer between the positive electrode layer and the negative electrode layer, the battery containing the solid electrolyte according to any one of Claims 1 to 7.

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