RU2851659C1 - Method for producing solid electrolyte of lgps type - Google Patents

Abstract

FIELD: industrial chemistry.

SUBSTANCE: invention relates to the creation of solid electrolytes which can be used in rechargeable lithium-ion batteries. Method for producing a solid electrolyte containing Li, Sn, P and S, comprises mixing Li₃PS₄ crystals with Li₄SnS₄ crystals and heating the resulting precursor at a temperature of 300-700°C. The average particle diameter (D50) of the Li₃PS₄ and Li₄SnS₄ crystals is from 0.1 to 5 mcm.

EFFECT: solid electrolyte containing Li, Sn, P and S, having a small particle diameter and high ionic conductivity, without grinding.

6 cl, 5 dwg, 1 tbl, 7 ex

Description

AREA OF TECHNOLOGY

[0001]

The present invention relates to a method for producing a solid electrolyte based on LGPS. Solid electrolytes based on LGPS refer to solid electrolytes with a specific crystal structure containing Li, P, and S, but the present invention relates to a solid electrolyte containing Li, Sn, P, and S.

LEVEL OF TECHNOLOGY

[0002]

Recently, demand for lithium-ion rechargeable batteries has increased in applications such as portable information terminals, portable electronic equipment, electric vehicles, hybrid electric vehicles, and stationary energy storage systems. However, lithium-ion rechargeable batteries currently use a flammable organic solvent as the electrolyte solution, requiring a robust outer casing to prevent organic solvent leakage. Furthermore, in the case of portable personal computers, for example, a design must be employed that prevents the risk of electrolyte leakage. This limits the design of these devices.

[0003]

Moreover, the application range of such batteries has expanded to include mobile applications such as vehicles and aircraft, while stationary lithium-ion rechargeable batteries require high capacity. Under these circumstances, safety is becoming a higher priority than before, with efforts focused on developing a fully solid-state lithium-ion rechargeable battery that does not use any toxic substances, including organic solvents.

[0004]

For example, the use of oxide, phosphate compound, organic polymer, sulfide, etc. as a solid electrolyte in a fully solid-state lithium-ion rechargeable battery has been studied.

Among such solid electrolytes, sulfide is characterized by high ionic conductivity, is relatively soft, and readily forms an interface between solids. Sulfide is stable with respect to active materials and has been developed as a practical solid electrolyte.

[0005]

Among sulfide solid electrolytes, there is a solid electrolyte based on LGPS, which has a specific crystalline structure (Patent Document 1). Among sulfide solid electrolytes, LGPS has relatively high ionic conductivity and can operate stably in a range from low temperatures of -30°C to high temperatures of 100°C, making it a promising candidate for practical application.

[0006]

The development of solid electrolyte compositions based on LGPS, without the use of expensive Ge, is progressing well. Since the solid electrolyte based on LGPS, containing Sn, exhibits good ionic conductivity and is low in cost, its practical application is highly anticipated (non-patent documents 1 and 2).

[0007]

To address the challenges of achieving high energy density and high power output in batteries using solid electrolytes, it is necessary to reduce the interface resistance between the solid electrolyte layer and the electrode. Since it is assumed that the interface resistance can be reduced by using microparticles of solid electrolyte, a solid electrolyte with a small particle diameter is required.

Examples of common methods for producing solid electrolyte microparticles include milling methods in which a ball mill, jet mill, or the like is used after synthesizing large particles _. However, such milling methods are not suitable for increasing particle size due to process-related issues. Furthermore, _Li10SnP2S12 , which is one of the Sn-containing solid electrolytes based on LGPS, is known to decompose when exposed to milling energy or the like (Non-patent document 3). For this reason _, it is desirable to obtain a solid electrolyte with a small particle diameter without milling or the like.

STATE OF THE ART DOCUMENTS

PATENT DOCUMENTS

[0008]

Patent Document 1: WO 2018/173939

NON-PATENT DOCUMENTS

[0009]

Non-Patent Document 1: Journal of Power Sources 396 (2018) 824–830

Non-Patent Document 2: J. Am. Chem. Soc. 2013, 135, 15694–15697

Non-Patent Document 3: Nano Energy Volume 67, January 2020, 104252

ESSENCE OF THE INVENTION

PROBLEMS SOLVED BY THE PRESENT INVENTION

[0010]

Under the above circumstances, the problem solved by the present invention is to provide a method for producing a solid electrolyte based on LGPS, which contains Sn and has a small particle diameter, without carrying out grinding, etc.

WAYS TO SOLVE THESE PROBLEMS

[0011]

The present inventors tirelessly conducted research to solve the above-described problem and unexpectedly discovered that by mixing _Li3PS4 crystals having a specific average particle diameter with crystals consisting of Li, Sn _, and S subjected to calcination, fine particles of an Sn-containing LGPS-based solid electrolyte can be obtained without grinding, etc.

[0012]

In particular, the present invention is described below.

1. A method for producing a solid electrolyte based on LGPS, comprising:

a step in which Li ₃ PS ₄ crystals with an average particle diameter (D50) of 0.1 to 5 μm are mixed with crystals consisting of Li, Sn and S, the average diameter of which is from 0.1 to 5 μm, to obtain a precursor; and

a stage in which said precursor is subjected to heat treatment at a temperature of 300 to 700°C.

2. The method according to claim 1, wherein the obtained solid electrolyte based on LGPS has an average particle diameter (D50) of 0.5 to 5 μm.

3. The method according to claim 1 or 2, wherein the solid electrolyte based on LGPS has peaks at at least 2θ=20.18°±0.50°, 20.44°±0.50°, 26.96°±0.50° and 29.58°±0.50° in X-ray diffraction (CuKα: λ=1.5405 Å).

4. The method according to any one of claims 1 to 3, wherein the Li ₃ PS ₄ crystals are β-Li ₃ PS ₄ .

5. The method according to any one of paragraphs 1-4, in which the heat treatment step is carried out in an inert gas atmosphere.

6. The method according to any one of claims 1 to 5, wherein the solid electrolyte based on LGPS comprises an octahedron O consisting of Li and S, a tetrahedron T ₁ consisting of S and at least one element selected from the group consisting of P and Sn, and a tetrahedron T ₂ consisting of P and S, and has a predominantly crystalline structure in which the tetrahedron T ₁ and the octahedron O have a common edge, and the tetrahedron T ₂ and the octahedron O have a common vertex.

7. The method according to any one of claims 1 to 6, wherein the crystals consisting of Li, Sn and S have the composition Li ₄ SnS ₄ .

BENEFICIAL EFFECT OF THE INVENTION

[0013]

The present invention provides a method for producing a solid electrolyte based on LGPS having a small particle diameter. Furthermore, the present invention provides a molded article obtained by heating and molding the solid electrolyte based on LGPS, and a fully solid-state battery comprising the solid electrolyte based on LGPS.

BRIEF DESCRIPTION OF GRAPHIC MATERIALS

[0014]

Fig. 1 is a schematic diagram showing the crystal structure of a solid electrolyte based on LGPS according to one embodiment of the present invention.

Fig. 2 is a schematic cross-sectional view of a solid-state battery according to one embodiment of the present invention.

Fig. 3 is a graph showing the results of X-ray diffraction measurements of the solid electrolytes based on LGPS obtained in Examples 1-4, Comparative Examples 1-2, and Reference Example 1.

Fig. 4 is a graph showing the results of measuring the ionic conductivity of the solid electrolytes based on LGPS obtained in Examples 1-4, Comparative Examples 1-2, and Reference Example 1.

Fig. 5 is a graph showing the results of Raman spectroscopy of the solid electrolytes based on LGPS obtained in Examples 1-4, Comparative Examples 1-2, and Reference Example 1.

EMBODIMENT FOR CARRYING OUT THE PRESENT INVENTION

[0015]

The method for producing a solid electrolyte based on LGPS according to the present invention is described in detail below. It should be noted that the materials, compositions, etc. described below do not limit the present invention and can be modified in various ways within the scope of the present invention.

[0016]

Method for producing solid electrolyte based on LGPS

According to one embodiment of the present invention, the proposed method for producing a solid electrolyte based on LGPS includes: a stage in which Li ₃ PS ₄ crystals with an average particle diameter (D50) of 0.1 to 5 μm are mixed with crystals consisting of Li, Sn and S, the average diameter of which is from 0.1 to 5 μm, to obtain a precursor; and a stage in which said precursor is subjected to heat treatment at a temperature of 300 to 700°C.

The average particle diameter (D50) of the Li ₃ PS ₄ crystals is preferably from 0.1 to 4 μm, and more preferably from 0.1 to 2.5 μm.

The average particle diameter (D50) of the crystals consisting of Li, Sn and S (preferably crystals with the composition Li ₄ SnS ₄ ) is preferably from 0.1 to 3 μm and more preferably from 0.1 to 1.5 μm.

The average particle diameter (D50) of the obtained solid electrolyte based on LGPS is preferably from 0.5 to 5 μm, more preferably from 0.1 to 4 μm, and particularly preferably from 0.1 to 2.5 μm.

According to the present invention, the average particle diameter (D50) of each of the above-described raw materials and product can be measured using the method described below in the Examples section.

[0017]

In X-ray diffraction (CuKα: λ=1.5405 Å), the LGPS-based solid electrolyte preferably has peaks at at least 2θ=20.18°±0.50°, 20.44°±0.50°, 26.96°±0.50°, and 29.58°±0.50°. More preferably, said electrolyte has peaks at 2θ=17.38±0.50°, 20.18°±0.50°, 20.44°±0.50°, 23.56±0.50°, 26.96°±0.50°, 29.07±0.50°, 29.58°±0.50° and 31.71±0.50°.

In addition, with respect to the LGPS-based solid electrolyte, when the diffraction intensity of the peak at 2θ=29.58°±0.50° is denoted as I _A , and the diffraction intensity of the peak at 2θ=27.33°±0.50° is denoted as I _B , I _B /I _A is preferably less than 0.50. More preferably, I _B /I _A is less than 0.40. This is because the peak of the LGPS crystals corresponds to I _A , and the crystalline phase having low ionic conductivity corresponds to I _B .

In addition, it is preferable that the solid electrolyte based on LGPS contains an octahedron O consisting of Li and S, a tetrahedron T ₁ consisting of S and at least one element selected from the group consisting of P and Sn, and a tetrahedron T ₂ consisting of P and S, and has a predominantly crystalline structure in which the tetrahedron T ₁ and the octahedron O have a common edge, and the tetrahedron T ₂ and the octahedron O have a common vertex, as shown in Fig. 1.

[0018]

As a standard method for producing a solid electrolyte based on LGPS, after synthesizing an ionic conductor using _Li2S , _P2S5 , and _MxSy (e.g., _{GeS2 )} as starting materials _, a mechanical milling method using a vibration mill or a planetary ball mill (WO 2011/118801) or a melting-quenching method described in WO 2014/196442 is used. However, in the case of the mechanical milling method, it is difficult to scale up to the industrial production level, and the implementation of the melting-quenching method without exposure to the atmosphere is significantly limited due to the control of the atmosphere composition. It should be noted that the solid electrolyte based on LGPS and the starting materials for producing it have the property that they interact in an atmosphere with moisture or oxygen, which leads to a change in their quality. In this regard, according to the production method proposed in the present invention, there is no need for an amorphization step. _Li3PS4 crystal microparticles and crystal microparticles composed of Li, Sn _, and S are used as raw materials and mixed in the presence of a solid phase or solvent to obtain a precursor, and then heat-treated to thereby obtain _an LGPS-based solid electrolyte having a small particle diameter. In addition, it is important to use _Li3PS4 crystals as raw materials because the volatilization/decomposition of the precursor during heat treatment can be suppressed. If _P2S5 is present in the precursor (it can be measured by Raman spectroscopy), _the formation of by-products and the amount of unreacted raw materials increase during the heat treatment step due to the high volatility/decomposition tendency _{of P2S5} , and for this reason, a stable and high-quality LGPS-based solid electrolyte cannot be easily obtained.

[0019]

Li ₃ PS ₄ crystals

The Li ₃ PS ₄ crystals that can be used in the present invention may be of the α, β or γ type, but β-Li ₃ PS ₄ is more preferred. This is because the said compound is in a relatively stable state in the LGPS synthesis system.

As the Li ₃ PS ₄ crystals according to the present invention, a commercially available product can be used, but, for example, Li ₃ PS ₄ crystals can be synthesized from Li ₂ S and P ₂ S ₅ in an inert gas atmosphere (e.g., argon). For example, Li ₂ S and P ₂ S ₅ are weighed so that the molar ratio of Li ₂ S: P ₂ S ₅ is 1.5: 1, after which Li ₂ S and P ₂ S ₅ are added in this order to a solvent such as tetrahydrofuran and stirred at room temperature for about 5 to 15 hours. Li ₂ S is additionally added to the resulting mixture so that the composition of all the starting materials, including the substances described above, corresponds to the molar ratio of Li ₂ S: P ₂ S ₅ = 3: 1, and stirring is carried out at room temperature for about 5 to 15 hours to obtain a suspension. The solvent is removed from the resulting suspension under reduced pressure at a temperature of approximately 25 to 70°C. The mixture is then dried in vacuum at a temperature of approximately 120 to 240°C for 1 to 6 hours, thereby obtaining β-Li ₃ PS ₄ .

The _Li2S crystals can be either synthesized or commercially available. The lower the moisture content, the better, as mixing with moisture degrades the other starting materials and the precursor. A moisture content of 300 ppm or less is preferred, and 50 ppm or less is particularly preferred.

[0020]

Obtaining microparticles of Li ₃ PS ₄ crystals

One of the characteristics of the present invention is the use of Li ₃ PS ₄ crystals as starting materials, the average diameter (D50) of the particles of which is from 0.1 to 5 μm.

The method for producing the microparticles of Li ₃ PS ₄ crystals used in the present invention is not particularly limited, provided that the average particle diameter (D50) of said Li ₃ PS ₄ crystals is from 0.1 to 5 μm. For example, in an inert gas atmosphere (e.g., argon), Li ₃ PS ₄ is added to a solvent such as acetonitrile and stirred at room temperature to obtain a suspension. The solvent is removed from the resulting suspension under reduced pressure at a temperature of about 25 to 70°C. Thereafter, the resulting powder is dried under vacuum at a temperature of about 120 to 240°C for about 1 to 6 hours to remove the coordinating solvent. Thereafter, the mixture is cooled to room temperature, thereby obtaining Li ₃ PS ₄ powder in the form of microparticles.

[0021]

Crystals consisting of Li, Sn and S

The crystals used in the present invention, consisting of Li, Sn and S, are not particularly limited, and particularly preferred examples thereof include crystals having the composition _Li4SnS4 . It should be noted that it is sufficient when the specific crystals described _above have the composition described above, and said crystals may consist of a single-crystal structure or may consist of a combination of different crystal structures. For _example , crystals having the composition _Li4SnS4 may consist of _a combination _{of Li4Sn2S6} and _Li2S with different crystal structures.

Since the present invention preferably uses crystals with the composition Li ₄ SnS ₄ , a commercially available product can be used, but, for example, such crystals can be synthesized from Li ₂ S crystals and SnS ₂ crystals in an inert gas atmosphere (e.g., argon). For example, Li ₂ S and SnS ₂ are weighed so that the molar ratio of Li ₂ S:SnS ₂ is 2:1, and these substances are mixed using an agate mortar. Then, the resulting mixture is calcined at a temperature of 350 to 700 °C for 1 to 12 hours in an inert gas atmosphere (e.g., argon) to obtain crystals having the composition Li ₄ SnS ₄ .

It should be noted that even if some of the above-described starting materials used are amorphous, this does not pose a problem.

For all crystalline starting materials, it is important to have a small particle diameter, with the particle diameter preferably being between 10 nm and 10 μm, more preferably between 10 nm and 5 μm, and even more preferably between 100 nm and 1 μm. It should be noted that particle diameter can be measured using an SEM (scanning electron microscope), a laser scattering particle size distribution instrument, or a similar device. Using small particles during thermal treatment facilitates the reaction and can reduce the formation of byproducts.

[0022]

Obtaining microparticles of crystals consisting of Li, Sn and S

One of the characteristics of the present invention is the use of crystals consisting of Li, Sn and S as starting materials, the average diameter (D50) of the particles of which is from 0.1 to 5 μm.

The method for producing microparticles of crystals consisting of Li, Sn and S, which can be used in the present invention, has no particular limitations, provided that said crystals have an average particle diameter (D50) of from 0.1 to 5 μm. For example, in an inert gas atmosphere (e.g., argon), Li ₄ SnS ₄ is added to methanol and stirred at room temperature for about 3 to 8 hours. The resulting suspension is filtered using a membrane filter to obtain a homogeneous solution of Li ₄ SnS ₄ . The resulting homogeneous solution of Li ₄ SnS ₄ is mixed with acetonitrile to obtain a homogeneous methanol-acetonitrile solution. Methanol is removed from the resulting homogeneous solution at a temperature of about 30 to 70°C under reduced pressure to precipitate Li ₄ SnS ₄ in acetonitrile. Thereafter, the specified conditions are maintained (under reduced pressure, about 30 to 70°C) and acetonitrile is removed. To remove the coordinating solvent, the resulting powder is dried under vacuum at a temperature of approximately 120 to 240°C for approximately 1 to 6 hours. The powder is then cooled to room temperature, thereby obtaining Li ₄ SnS ₄ powder in the form of microparticles.

[0023]

Precursor synthesis stage

In the precursor synthesis stage, Li ₃ PS ₄ crystals in the form of microparticles are mixed with microparticles of crystals consisting of Li, Sn, and S to form the precursor. The molar ratio of these substances can be adjusted to ensure a specific ratio of the elements making up the crystal structure described above. For example, in the case of Li ₁₀ SnP ₂ S ₁₂ , mixing is carried out at a molar ratio of Li ₃ PS ₄ : Li ₄ SnS ₄ = 2:1.

The mixing method can be carried out in the presence of a solid phase or a solvent. It should be noted that the mixing method using a solvent is suitable for large-scale synthesis, since uniform mixing can be ensured. When using a solvent, it is preferable to use a solvent that does not react with the starting materials or the obtained precursor. Examples of the solvent include an ether-based solvent, an ester-based solvent, a hydrocarbon-based solvent, and a nitrile-based solvent. Specific examples of the solvent include tetrahydrofuran, cyclopentyl methyl ether, diisopropyl ether, diethyl ether, dimethyl ether, dioxane, methyl acetate, ethyl acetate, butyl acetate, and acetonitrile. To prevent deterioration of the composition of the starting materials, it is preferable to remove oxygen and moisture contained in the solvent in advance. In particular, with respect to the moisture content, it is preferably 100 ppm or less, and more preferably 50 ppm or less. Mixing is preferably carried out in an inert gas atmosphere. Nitrogen, helium, argon, or the like can be used as the inert gas. By removing oxygen and moisture from the inert gas, deterioration of the starting material composition can be prevented. The oxygen and moisture concentrations in the inert gas are each preferably 1000 ppm or less, more preferably 100 ppm or less, and particularly preferably 10 ppm or less.

[0024]

During mixing, the substance may be in a uniformly dispersed suspension, but it is more preferable for some of the starting materials (regardless of type) to be dissolved. If a suspension is present, it is preferably broken up by stirring to break up aggregated particles. A homogenizer or ultrasonic disperser can also be used.

When mixing in the presence of a solid phase, mixing can be performed using a mortar, a pounder, a ball mill, or similar devices. These methods typically prevent the crystals from becoming amorphous. Regarding mixing, the synthesis is preferably performed under vacuum or an inert gas atmosphere, and the synthesis conditions are the same as those used when using a solvent.

Heating is not required for mixing temperature, but if a solvent is used, heating can be used to increase solubility or the rate of dissolution of the substances. When heating is used, it is sufficient to do so at a temperature corresponding to the boiling point of the solvent or below. However, heating can also be performed under pressure using an autoclave or similar device. It should be noted that when mixing at high temperatures, the reaction proceeds before the starting materials are sufficiently mixed, and byproducts easily form. For this reason, mixing is preferably performed at close to room temperature.

[0025]

As for mixing time, it's sufficient when the time required to obtain a homogeneous mixture can be achieved. The specified time often depends on the production scale, but, for example, a homogeneous mixture can be achieved by mixing for anywhere from 0.1 to 24 hours.

[0026]

When using a solvent, the precursor is obtained by removing the solvent. The solvent is removed by hot air drying or vacuum drying, with the optimal drying temperature varying depending on the type of solvent. The time for solvent removal can be reduced by using a temperature significantly above the boiling point. The temperature for solvent removal is preferably from 60 to 280°C, and more preferably from 100 to 250°C. When removing the solvent under reduced pressure, as in the case of vacuum drying or a similar method, the temperature during solvent removal can be lowered, thereby reducing the required time. In addition, the time required for solvent removal can also be reduced by passing an inert gas, such as nitrogen or argon, in which the moisture content is relatively low. It should be noted that the thermal treatment step described below and the solvent removal can be performed simultaneously.

[0027]

Heat treatment stage

In the production method according to the present invention, the precursor obtained in the precursor synthesis step is subjected to heat treatment, thereby obtaining a solid electrolyte based on LGPS having a small particle diameter. The heat treatment temperature is from 300 to 700°C, preferably from 350 to 650°C, and particularly preferably from 450 to 600°C. If the temperature is below the specified range, the formation of the desired crystals is difficult, and if the temperature is above the specified range, crystals other than the desired crystals are formed.

[0028]

The heat treatment time varies slightly depending on the heat treatment temperature, but crystallization generally proceeds sufficiently if the heat treatment time is between 0.1 and 24 hours. It is not preferable to conduct heat treatment at high temperatures for extended periods exceeding the above range, as there is a risk of altering the quality of the LGPS-based solid electrolyte.

Heat treatment can be carried out under vacuum or in an inert gas atmosphere, but is preferably carried out under an inert gas atmosphere. Nitrogen, helium, argon, or similar gases can be used as inert gases, with argon being preferred. Oxygen and moisture content are preferably low, and heat treatment conditions are the same as those used during mixing in the precursor synthesis stage.

[0029]

The LGPS-based solid electrolyte according to the present invention, obtained by the method described above, can be molded to the desired molded article using various methods and can be used for various applications, including the all-solid-state batteries described below. The molding method is not particularly limited. For example, a method similar to the method for molding the corresponding layers constituting the all-solid-state battery described below for the all-solid-state battery can be used.

[0030]

All-solid-state battery

The LGPS-based solid electrolyte according to the present invention can be used, for example, as a solid electrolyte for all-solid-state batteries. Furthermore, according to another embodiment of the present invention, an all-solid-state battery is provided containing the above-described solid electrolyte for all-solid-state batteries.

[0031]

In this regard, an "all-solid-state battery" is an all-solid-state lithium-ion rechargeable battery. Fig. 2 shows a schematic cross-sectional view of an all-solid-state battery according to one embodiment of the present invention. All-solid-state battery 10 has a structure in which a solid electrolyte layer 2 is located between a positive electrode layer 1 and a negative electrode layer 3. All-solid-state battery 10 can be used in a variety of devices, including mobile phones, personal computers, and automobiles.

The LGPS-based solid electrolyte according to the present invention may be present as a solid electrolyte in at least one layer selected from positive electrode layer 1, negative electrode layer 3, and solid electrolyte layer 2. In the case where the LGPS-based solid electrolyte according to the present invention is contained in positive electrode layer 1 or in negative electrode layer 3, the LGPS-based solid electrolyte according to the present invention is used in combination with a generally known positive electrode active material or negative electrode active material for lithium-ion rechargeable batteries. The quantitative content of the LGPS-based solid electrolyte according to the present invention, which is contained in positive electrode layer 1 or in negative electrode layer 3, is not particularly limited.

In the case where the LGPS-based solid electrolyte according to the present invention is contained in the solid electrolyte layer 2, the solid electrolyte layer 2 may be composed only of the LGPS-based solid electrolyte according to the present invention, and, if necessary, a solid oxide electrolyte (for example, _Li7La3Zr2O12 ), a solid sulfide electrolyte (for example, _{Li2SP2S5 )} , other complex hydride solid electrolytes (for example, _LiBH4 and _3LiBH4 -LiI) _, etc. _may be _suitably used in combination _.

[0032]

A fully solid-state battery is obtained by molding and layer-by-layer deposition of the layers described above, and the molding method and the layer-by-layer deposition method of the corresponding layers are not particularly limited.

Examples of such methods include: a method in which a solid electrolyte and/or an active electrode material are dispersed in a solvent to form a suspension-like mixture, which is applied using a knife device, a centrifugal method, or the like, and subjected to rolling to form a film; a gas-phase method in which film formation and layer-by-layer application are carried out using a vacuum deposition method, an ion plating method, a sputtering method, a laser burning method, or the like; and a pressing method in which powder is obtained by hot pressing or cold pressing (without heating) and layer-by-layer application is carried out.

[0033]

Since the LGPS-based solid electrolyte according to the present invention is relatively soft, it is particularly preferable to produce a fully solid-state battery by forming the corresponding layers using a pressing method and applying the layers layer by layer. Pressing methods include hot pressing, which involves heating, and cold pressing, which does not use heating. However, even cold pressing can achieve sufficient layer formation.

It should be noted that the present invention includes a molded article obtained by heating and molding a solid electrolyte based on LGPS according to the present invention. The molded article is suitably used as an all-solid-state battery. Furthermore, the present invention includes a method for producing an all-solid-state battery, comprising the step of heating and molding a solid electrolyte based on LGPS according to the present invention.

EXAMPLES

[0034]

Hereinafter, embodiments of the present invention will be described in more detail using examples, but the said embodiments are not limited to the given examples.

[0035]

Method for measuring average particle diameter

The average particle diameters of Li ₃ PS ₄ and crystals with the composition Li ₄ SnS ₄ used as starting materials and the average particle diameters of the obtained solid electrolytes were measured in the manner described below.

A small amount of each of the above-described raw materials or solid electrolytes was taken, the resulting powder was added to acetonitrile as a dispersion medium, and dispersed for 3 minutes using an ultrasonic homogenizer (UH-50, manufactured by SMT Co., Ltd.). Next, the particle size distribution was measured using a laser scattering/diffraction-based particle size distribution measuring device (Misrotrac MT3000EXII, manufactured by Nikkiso Co., Ltd.) to determine the average particle diameter (D50).

[0036]

(Example 1)

Method for producing β-Li ₃ PS ₄

In a glove box under an argon atmosphere _{, Li2S} (manufactured by Sigma-Aldrich, purity: 99.8%) and _P2S5 (manufactured by Sigma-Aldrich, purity: 99%) were weighed so that the molar ratio of _Li2S : _P2S5 was 1.5:1. Then, _Li2S and _P2S5 were added to tetrahydrofuran (manufactured by Wako Pure Chemical Industries, Ltd., high dehydrated grade) in this order so that the concentration of ( _Li2S + _P2S5 ) was ₁₀ % by mass, and stirring was carried out at room temperature for ₁₂ hours. The mixture gradually dissolved, and _a nearly uniform solution containing a small amount of insoluble matter was obtained.

An additional amount of Li ₂ S was added to the resulting solution such that the composition of all starting materials, including the substances described above, corresponded to a molar ratio of Li ₂ S:P ₂ S ₅ = 3:1, and the mixture was stirred at room temperature for 12 hours to obtain a suspension. The solvent was removed from the resulting suspension under reduced pressure at a temperature of 50°C. The mixture was then dried under vacuum at 180°C for 4 hours, thereby obtaining β-Li ₃ PS ₄ . This sequence of operations was performed in a glovebox under an argon atmosphere.

The obtained β-Li ₃ PS ₄ was characterized by Raman spectroscopy as described below, and the peak at 420 cm ^-1 corresponding to PS ₄ ^3- was confirmed. It should be noted that all the sulfide starting material used was crystalline.

The stage of obtaining β-Li ₃ PS ₄ microparticles

In a glove box under an argon atmosphere, β- _Li3PS4 synthesized in the above step was added to acetonitrile (manufactured by Wako Pure Chemical Industries, Ltd., high dehydrated grade ₎ so that its concentration became 6% by mass, and stirring was performed at room temperature to obtain a suspension.

Acetonitrile was removed from the resulting suspension under reduced pressure at 50°C. The resulting powder was then dried under vacuum at 180°C for 4 hours to remove the coordinating solvent. The solvent was removed while the suspension was stirred. The powder was then cooled to room temperature, yielding β-Li ₃ PS ₄ powder microparticles. The average diameter (D50) of the β-Li ₃ PS ₄ powder microparticles was 2.1 μm.

[0037]

Method for obtaining crystals having the composition Li ₄ SnS ₄

_Li2S (produced by Sigma-Aldrich, purity: 99.8%) and _SnS2 (Kojundo Chemical Laboratory Co., Ltd., 99.9%) were used as the starting materials. These materials were weighed in a glove box under an argon atmosphere to ensure a stoichiometric amount of crystals with the composition _{of Li4SnS4} and stirred for 15 minutes using an agate mortar. 1.0 g of the obtained powder and 15 zirconia balls (cf.: 10 mm) were placed in a 45 ml zirconia vessel, and the vessel was hermetically sealed. The zirconia vessel was attached to a planetary ball mill (manufactured by Fritsch Japan Co., Ltd.) and ball milling was performed at 500 rpm for 10 hours. The precursor powder obtained by ball milling was calcined at 450°C for 8 hours, thereby obtaining crystals having the composition Li ₄ SnS ₄ .

The step of obtaining microparticles of crystals with the composition Li ₄ SnS ₄ In a glove box under an argon atmosphere, 1 g of crystals with the composition Li ₄ SnS ₄ , synthesized in the above-described step of obtaining, was weighed. Then, the weighed crystals with the composition Li ₄ SnS ₄ were added to 10 ml of methanol (manufactured by Wako Pure Chemical Industries, Ltd., grade "highly dehydrated", boiling point: 64 °C) and stirred at room temperature for 6 hours. The mixture gradually dissolved, and a suspension containing an insoluble substance was obtained. The resulting suspension was filtered through a membrane filter (PTFE, pore size: 1.0 μm) to obtain a homogeneous solution of crystals with the composition Li ₄ SnS ₄ .

A homogeneous solution of the crystals with the composition _Li4SnS4 obtained as described above was mixed with 200 ml of acetonitrile (manufactured by Wako Pure Chemical Industries, Ltd., high dehydrated grade, boiling point: 82°C) to obtain _a methanol-acetonitrile homogeneous solution.

Methanol was removed from the resulting homogeneous solution under reduced pressure at 50°C to precipitate crystals of Li ₄ SnS ₄ composition in acetonitrile. The specified conditions (under reduced pressure, 50°C) were then maintained, and the acetonitrile was removed to obtain a powder. Solvent removal was carried out while simultaneously stirring the homogeneous solution.

To remove the coordinating solvent _, the powder obtained as described above was dried under vacuum at 180°C for 4 hours. The powder was then cooled to room temperature, thereby obtaining a powder of _Li4SnS4 crystals in the form of microparticles. The average diameter ( _D50 ) of the _Li4SnS4 crystal powder microparticles was 1.1 μm.

[0038]

Synthesis of Li ₁₀ SnP ₂ S ₁₂

In a glove box under an argon atmosphere, the above-described β-Li ₃ PS ₄ crystal microparticles and Li ₄ SnS ₄ crystal microparticles were weighed so that the p-Li ₃ PS ₄ : Li ₄ SnS ₄ molar ratio was 2:1, and mixed using an agate mortar. The mixture was calcined under an argon atmosphere at 550°C for 8 hours, thereby obtaining Li ₁₀ SnP ₂ S ₁₂ crystals (a solid electrolyte based on LGPS). The average particle diameter (D50) of the resulting solid electrolyte was 2.1 μm.

[0039]

(Example 2)

Li ₁₀ SnP ₂ S ₁₂ crystals were obtained by a method similar to that described in Example 1, except that "calcination in an argon atmosphere at 550°C for 8 hours" in Example 1 was replaced by "annealing in an argon atmosphere at 550°C for 2 hours." The average particle diameter (D50) of the resulting solid electrolyte was 2.0 μm.

[0040]

(Example 3)

In a glove box under an argon atmosphere, β-Li ₃ PS ₄ from the microparticle preparation step and crystals with the composition Li ₄ SnS ₄ from the microparticle preparation step obtained in Example 1 were weighed so that the molar ratio of β-Li ₃ PS ₄ : Li ₄ SnS ₄ was 2:1. The weighed mixed powder was added to acetonitrile (manufactured by Wako Pure Chemical Industries, Ltd., grade "highly dehydrated") so that its concentration was 5% by weight, and stirred at room temperature to obtain a suspension. Under reduced pressure, acetonitrile was removed from the resulting suspension at a temperature of 50°C. After this, to remove the coordinating solvent, the resulting powder was dried under vacuum at 180°C for 4 hours. The solvent was removed while simultaneously stirring the suspension. The powder was then cooled to room temperature to obtain a precursor powder. This precursor was calcined in an argon atmosphere at 550°C for 8 hours, thereby producing Li ₁₀ SnP ₂ S ₁₂ crystals. The average particle diameter (D50) of the resulting solid electrolyte was 3.6 μm.

[0041]

(Example 4)

In _a glove box under an argon atmosphere _, β- _Li3PS4 from the microparticle preparation stage and crystals with the composition _Li4SnS4 from the microparticle preparation stage obtained in Example 1 were weighed so that the molar ratio of β- _Li3PS4 : _Li4SnS4 was _2.7 : ₁ . 600 mg of the weighed precursor powder and 80 g of 3 mm diameter zirconium dioxide spheres were placed in a 50 ^cm3 Duran bottle and hermetically sealed. All of the above weighing, adding, and sealing operations were carried out in a glove box under an argon atmosphere, with all devices used being dehydrated with a desiccant before use.

Using a ball mill stand (manufactured by Asahi-rika Co., Ltd., type AV-1), the bottle was rotated at normal temperature at 60 rpm for 1 hour to achieve stirring, and then the powder was collected. This powder was calcined in an argon atmosphere at ₅₅₀ °C for 2 hours to obtain _{Li9.81Sn0.81P2.19S12} crystals. The average particle diameter ( _D50 ) _of the obtained solid electrolyte was 2.3 μm.

[0042]

(Comparative example 1)

In a glove box under an argon atmosphere, β-Li ₃ PS ₄ after the step of obtaining microparticles and crystals with the composition Li ₄ SnSb ₄ before the step of obtaining microparticles in Example 1 were weighed so that the molar ratio of β-Li ₃ PS ₄ : Li ₄ SnS ₄ was 2:1, and stirring was carried out using an agate mortar. The mixture was calcined under an argon atmosphere at a temperature of 550 °C for 8 hours, thereby obtaining Li ₁₀ SnP ₂ S ₁₂ crystals. The average diameter (D50) of the powder particles of the crystals with the composition Li ₄ SnSb ₄ , which were not subjected to the step of obtaining microparticles, was 11 μm. In addition, the average diameter (D50) of the particles of the obtained solid electrolyte was 11 μm.

[0043]

(Comparative example 2)

In a glove box under an argon atmosphere, β-Li ₃ PS ₄ before the step of obtaining microparticles and crystals with the composition Li ₄ SnS ₄ before the step of obtaining microparticles in Example 1 were weighed so that the molar ratio of β-Li ₃ PS ₄ : Li ₄ SnS ₄ was 2:1, and stirring was carried out using an agate mortar. The mixture was calcined under an argon atmosphere at a temperature of 550 °C for 8 hours, thereby obtaining Li ₁₀ SnP ₂ S ₁₂ crystals. The average diameter (D50) of β-Li ₃ PS ₄ powder particles, which were not subjected to the step of obtaining microparticles, was 8.0 μm. In addition, the average diameter (D50) of the particles of the obtained solid electrolyte was 6.8 μm. It should be noted that Comparative Example 2 corresponds to the working example in Patent Document 1 (WO 2018/173939) described in the [BACKGROUND ART] section.

[0044]

(Reference example 1)

In the synthesis of Li ₁₀ SnP ₂ S _12, the starting materials were weighed so that the molar ratio of Li ₂ S: P ₂ S ₅ : SnS ₂ was 5:1:1, and mixed using an agate mortar. The resulting mixture was then placed in a 45 mL zirconia vessel, then zirconia balls ("YTZ" manufactured by Nikkato Corporation, ϕ: 10 mm, quantity: 15) were placed into it, and the vessel was completely sealed. The vessel was attached to a planetary ball mill ("P-7" manufactured by Fritsch) and mechanically ground at a rotation speed of 800 rpm for 10 hours to obtain the precursor powder. The said precursor was calcined in an argon atmosphere at 475°C for 8 hours, thereby obtaining Li ₁₀ SnP ₂ S ₁₂ crystals. The average particle diameter (D50) of the resulting solid electrolyte was 30 μm.

[0045]

[0046]

X-ray diffraction measurement

The LGPS-based solid electrolyte powders obtained in Examples 1-4, Comparative Examples 1-2, and Reference Example 1 were subjected to X-ray diffraction measurements in an argon atmosphere at room temperature (25°C) (X'Pert3 Powder instrument manufactured by PANalytical, CuKα: λ=1.5405 Å).

The X-ray diffraction measurement results of the solid electrolytes based on LGPS obtained in Examples 1-4, Comparative Examples 1-2 and Reference Example 1 are shown in Fig. 3.

As shown in Fig. 3, in Examples 1 to 4, diffraction peaks were observed at least at 2θ=20.18°±0.50°, 20.44°±0.50°, 26.96°±0.50°, and 29.58°±0.40°, and the diffraction curve was consistent with the diffraction curve of Li ₁₀ SnP ₂ S ₁₂ from the ICSD (Inorganic Crystal Structure Database) database.

[0047]

Lithium-ion conductivity measurement

Each of the LGPS-based solid electrolytes obtained in Examples 1-4, Comparative Examples 1-2, and Reference Example 1 was subjected to uniaxial molding (240 MPa) to obtain a disk with a thickness of approximately 1 mm and a diameter of 8 mm. The AC impedance was measured by a four-terminal method using an In (indium) electrode at room temperature (25°C) and at 10°C intervals in the temperature range from 30°C to 100°C and down to -20°C (SI1260 IMPEDANCE/GAIN-PHASE ANALYZER, manufactured by Solartron), and then the lithium-ion conductivity was calculated.

Specifically, the sample was placed in a thermostatic bath set at 25°C and held for 30 minutes, after which the lithium-ion conductivity was measured. The temperature of the thermostatic bath was then increased from 30°C to 100°C in 10°C intervals and held at each temperature for 25 minutes, after which the ionic conductivity was measured. After completing the measurement at 100°C, the temperature of the thermostatic bath was decreased from 90°C to 30°C in 10°C intervals and held at each temperature for 40 minutes, after which the lithium-ion conductivity was measured. The lithium-ion conductivity of the sample was then measured after being held in a thermostatic bath set at 25°C for 40 minutes. The thermostatic bath temperature was then lowered from 20°C to -20°C in 10°C increments and maintained at each temperature for 40 minutes. The lithium-ion conductivity was then measured. The frequency range of measurements was from 0.1 Hz to 1 MHz, and the amplitude was 50 mV. The results of the lithium-ion conductivity measurements during the temperature reduction are shown in Fig. 4.

In Examples 1-4, solid electrolytes were successfully obtained whose average particle diameter (D50) was 4 μm or less, while the ionic conductivity was maintained in the range from 1 mS/cm to 3 mS/cm.

[0048]

Raman spectroscopy

(1) Obtaining a sample

The sample for measurement was obtained using an airtight cuvette with a quartz glass top (ϕ: 60 mm, thickness: 1 mm) as an optical window. The sample was bonded to the quartz glass in a glovebox under an argon atmosphere. The cuvette was then sealed and removed from the glovebox, and measurements were performed using Raman spectroscopy.

(2) Measurement conditions

Using a NRS-5100 laser Raman spectrometer (manufactured by JASCO Corporation), the measurement was performed at an excitation wavelength of 532.15 nm for an exposure time of 5 seconds.

[0049]

The Raman spectroscopy results of the LGPS-based solid electrolytes obtained in Examples 1-4, Comparative Examples 1-2, and Reference Example 1 are shown in Fig. 5. For each sample in Examples 1-4 and Comparative Examples _1-2 , the peak at 420 cm ^{-1 corresponding} to _PS43- was confirmed. The Raman spectra of the _Li10SnP2S12 crystals of Examples _1-4 matched the spectrum of _Li10SnP2S12 obtained _by the standard synthesis method in Reference _Example 1.

EXPLANATION OF LETTER OR NUMERIC DESIGNATIONS

[0050]

1 layer of positive electrode

2 layer of solid electrolyte

3 layer of negative electrode

10 all-solid-state battery

Claims (8)

1. A method for producing a solid electrolyte containing Li, Sn, P and S, comprising mixing Li ₃ PS ₄ crystals with an average particle diameter (D50) of 0.1 to 5 μm with Li ₄ SnS ₄ crystals with an average particle diameter (D50) of 0.1 to 5 μm to obtain a precursor, and heating the said precursor at a temperature of 300 to 700°C. 2. The method according to claim 1, wherein the obtained solid electrolyte containing Li, Sn, P and S has an average particle diameter (D50) of 0.5 to 5 μm. 3. The method according to claim 1 or 2, wherein the solid electrolyte containing Li, Sn, P and S has peaks at at least 2θ=20.18°±0.50°, 20.44°±0.50°, 26.96°±0.50° and 29.58°±0.50° in X-ray diffraction (CuKα: λ=1.5405 Å). 4. The method according to any one of claims 1 to 3, wherein the Li ₃ PS ₄ crystals are β-Li ₃ PS ₄ crystals. 5. The method according to any one of paragraphs 1-4, in which the heating is carried out in an inert gas atmosphere. 6. The method according to any one of claims 1 to 5, wherein the solid electrolyte containing Li, Sn, P and S comprises an octahedron O consisting of Li and S, a tetrahedron T ₁ consisting of S and at least one element selected from the group consisting of P and Sn, and a tetrahedron T ₂ consisting of P and S, and has a predominantly crystalline structure in which the tetrahedron T ₁ and the octahedron O have a common edge, and the tetrahedron T ₂ and the octahedron O have a common vertex.