Solid electrolyte, all-solid-state battery, solid electrolyte solution, and method for producing solid electrolyte

Abstract

To provide a solid electrolyte that allows for improved lithium ion conductivity.SOLUTION: A solid electrolyte includes lithium, phosphorus, sulfur, and halogen, in which, when the solid electrolyte is measured by TG-MS, a first peak derived from cyclic sulfur appears in a temperature range of 170°C or higher and lower than 250°C, a second peak derived from the cyclic sulfur appears in a temperature range of 250°C or higher and lower than 300°C, and a peak intensity P1 of the first peak is higher than a peak intensity P2 of the second peak.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present application relates to a solid electrolyte, an all-solid-state battery, a solid electrolyte solution, and a method for producing a solid electrolyte. [Background technology]

[0002] Lithium ion secondary batteries containing non-aqueous electrolytes have high voltage and high capacity, and are widely used as power sources for electronic devices such as mobile phones and laptops, as well as electric vehicles. However, because non-aqueous electrolytes are flammable, there have been safety concerns about lithium ion secondary batteries containing non-aqueous electrolytes. Therefore, efforts to improve safety have been made. of To address this issue, development of all-solid-state batteries containing non-flammable solid electrolytes is underway.

[0003] Known solid electrolytes include oxide solid electrolytes and sulfide solid electrolytes. Of these, sulfide solid electrolytes are expected to have high ionic conductivity. Patent Document 1 below discloses a method for producing a sulfide solid electrolyte.

[0004] Patent Document 1 discloses a method for producing an active material coated with a solid electrolyte by mixing an active material with a forming solution in which a solid electrolyte is dissolved in an organic solvent and drying the mixture. The document also describes that a solid electrolyte can be obtained by dissolving raw materials for forming the solid electrolyte in an organic solvent and reacting the raw materials during the drying process of the resulting solution.

[0005] Furthermore, a new method for producing a solid electrolyte that does not contain sulfur atoms is disclosed in Patent Document 2. Specifically, Patent Document 2 discloses a solid electrolyte composition in which a solid electrolyte material that does not contain sulfur atoms and contains a halogen is dispersed in a solvent. The same document also describes that a solid electrolyte can be obtained by removing the solvent from the solid electrolyte composition. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Patent No. 6095218 [Patent Document 2] International Publication No. 2021 / 131716 Summary of the Invention [Problem to be solved by the invention]

[0007] It is known that solid electrolytes can be obtained by solution synthesis, as described in Patent Document 1. However, the solid electrolytes obtained by solution synthesis have the problem of low lithium ion conductivity.

[0008] In view of the above circumstances, a main object of the present disclosure is to provide a solid electrolyte and a solid electrolyte solution that can improve lithium ion conductivity. [Means for solving the problem]

[0009] In one aspect for solving the above-described problems, the present disclosure provides a solid electrolyte containing lithium, phosphorus, sulfur, and a halogen, wherein, when the solid electrolyte is measured using a TG-MS method, a first peak due to cyclic sulfur appears in a temperature range of 170°C or higher and lower than 250°C, and a second peak due to cyclic sulfur appears in a temperature range of 250°C or higher and lower than 300°C, and the peak intensity P1 of the first peak is higher than the peak intensity P2 of the second peak.

[0010] The solid electrolyte may have an argyrodite structure composed of lithium, phosphorus, sulfur, and a halogen. In the solid electrolyte, the peak intensity ratio P1 / P2 may be 1.19 or more and 2.10 or less. In the solid electrolyte, the cyclic sulfur may be S8.

[0011] The present disclosure provides an all-solid-state battery including a positive electrode, a negative electrode, and a solid electrolyte layer disposed between the positive electrode and the negative electrode, wherein at least one of the positive electrode, the negative electrode, and the solid electrolyte layer contains the solid electrolyte.

[0012] In one aspect of the present disclosure to solve the above problems, there is provided a solid electrolyte solution in which a solid electrolyte material is dissolved in a solvent, the solid electrolyte material including lithium, phosphorus, sulfur, and a halogen, the solvent including a first solvent capable of dissolving the solid electrolyte material and a second solvent incapable of dissolving the solid electrolyte material, the second solvent having a solubility parameter of 10.5 (cal / cm) 1 / 2 The present invention provides a solid electrolyte solution having a viscosity of 1000 psig or less and a vapor pressure of 0.5 kPa or less.

[0013] The content of the second solvent in the solvent of the solid electrolyte solution may be 5 wt % or more and 50 wt % or less. Also, in the solid electrolyte solution, the second solvent may have 7 or more and 10 or less carbon atoms.

[0014] The present disclosure provides a method for producing a solid electrolyte, the method comprising: a preparation step of preparing the solid electrolyte solution; and a removal step of removing the solvent from the solid electrolyte solution.

[0015] The removing step of the above-described manufacturing method may include a first heating step of heating the solid electrolyte solution at a temperature of 50°C or higher and 120°C or lower, and a second heating step of heating the solid electrolyte solution at a temperature of 140°C or higher and 200°C or lower after the first heating step. [Effects of the Invention]

[0016] According to the present disclosure, the ionic conductivity of the solid electrolyte can be improved. [Brief explanation of the drawings]

[0017] [Figure 1] 1 is a flowchart of a method for producing a solid electrolyte in an example. [Figure 2] 1 is a diagram in which the XRD spectrum of Comparative Example 1 and the XRD spectrum of Example 3 are superimposed. [Figure 3] 1 is an XRD spectrum of Comparative Example 2. [Figure 4] FIG. 1 is a diagram in which the XRD spectra of Examples 1 to 3 and Comparative Example 1 are superimposed. [Figure 5]1 is a diagram showing the rate of change in the peak intensity of the main peak, the half-width of the main peak, and the peak intensity of the impurity peak for Examples 1 to 3 and Comparative Example 1, when Comparative Example 1 is used as a reference. [Figure 6] FIG. 1 is a diagram in which the XRD spectra of Examples 3 to 5 and Comparative Example 1 are superimposed. [Figure 7] 1 is a diagram showing the rate of change in the peak intensity of the main peak, the half-width of the main peak, and the peak intensity of the impurity peak for Examples 3 to 5 and Comparative Example 1, when Comparative Example 1 is used as a reference. [Figure 8] FIG. 1 shows the results of DTG for Examples 1 to 5 and Comparative Examples 1 and 2. [Figure 9] 1 is a diagram showing the results of TICC in Examples 1 to 5 and Comparative Examples 1 and 2. FIG. [Figure 10] FIG. 1 shows MS spectra of regions 1 to 3 in Comparative Example 1. [Figure 11] FIG. 10 shows MS spectra of regions 1 to 3 in Example 3. [Figure 12] FIG. 1 shows the MS spectrum of cyclic sulfur S8. [Figure 13] FIG. 1 shows the MS spectrum of cyclic sulfur S6. [Figure 14] FIG. 1 shows the results of TICC focusing on m / z=256 in Examples 1 to 5 and Comparative Examples 1 and 2. [Figure 15] FIG. 1 shows the results of TICC focusing on m / z=192 in Examples 1 to 5 and Comparative Examples 1 and 2. DETAILED DESCRIPTION OF THE INVENTION

[0018] [Solid electrolyte] The solid electrolyte of the present disclosure contains lithium, phosphorus, sulfur, and a halogen, and is characterized in that, when measured using a TG-MS method, a first peak due to cyclic sulfur appears in the temperature range of 170°C or higher and lower than 250°C, a second peak due to cyclic sulfur appears in the temperature range of 250°C or higher and lower than 300°C, and the peak intensity P1 of the first peak is higher than the peak intensity P2 of the second peak. This will be explained in detail below.

[0019] <Components of the solid electrolyte> The solid electrolyte is characterized by containing lithium, phosphorus, sulfur, and halogen. The solid electrolyte only needs to contain at least one kind of halogen (fluorine, chlorine, bromine, and iodine). That is, the solid electrolyte may contain one kind of halogen or two or more kinds of halogens. Further, the solid electrolyte may contain components other than lithium, phosphorus, sulfur, and halogen.

[0020] From the perspective of improving lithium ion conductivity, the solid electrolyte may have an argyrodite structure composed of lithium, phosphorus, sulfur, and halogen. Examples of the solid electrolyte having an argyrodite structure include, for example, Li a P b S c X d (X represents halogen, and two or more kinds of halogens may be present.) Here, 5.5 ≦ a ≦ 6.5, b = 1, 4.5 ≦ c ≦ 5.5, 0.5 ≦ d ≦ 1.5 may be set. For example, Li6PS5Cl, Li6PS5Br, etc. can be mentioned.

[0021] <Measurement by TG-MS method> The TG-MS method (Thermalgravity-Mass Spectrometry) is a method of introducing the gas generated from the sample by heating with temperature increase in TG into MS to obtain a mass spectrum. The measurement conditions of the TG-MS method are as follows. The sample amount is set to 10 mg. The heating rate with temperature increase is set to 10 °C / min. Helium is used as the carrier gas, and the gas flow rate is set to 80 ml / min.

[0022] When the solid electrolyte is analyzed under the above measurement conditions, a first peak due to cyclic sulfur appears in the temperature range of 170°C or higher and lower than 250°C, and a second peak due to cyclic sulfur appears in the temperature range of 250°C or higher and lower than 300°C. The solid electrolyte is characterized in that the peak intensity P1 of the first peak is higher than the peak intensity P2 of the second peak. The peak intensities of the first peak and the second peak are peak intensities in a TICC (total ion current chromatogram). The first peak and the second peak refer to the peaks with the greatest peak intensity in a given temperature range.

[0023] Here, the peak intensity ratio P1 / P2, which is the peak intensity P1 of the first peak to the peak intensity P2 of the second peak, may be greater than 1.00. From the viewpoint of improving ionic conductivity, the peak intensity ratio P1 / P2 may be 1.10 or more, 1.19 or more, 2.10 or less, or 1.90 or less. Furthermore, the first peak may appear in a temperature range of 170°C or more and less than 250°C, or may appear in a temperature range of 190°C or more and 230°C or less. The second peak may appear in a temperature range of 250°C or more and less than 300°C, or may appear in a temperature range of 260°C or more and 290°C or less.

[0024] The cyclic sulfur measured in the first and second peaks is the same cyclic sulfur. The reason why the same cyclic sulfur is detected in different temperature ranges is unclear, but the inventors speculate that this is because different reactions occur in each temperature range. The type of cyclic sulfur is not particularly limited, but may be, for example, S6 or S8. The cyclic sulfur may be S8.

[0025] <shape> The shape of the solid electrolyte is not particularly limited, and may be, for example, particulate. The particle size of the solid electrolyte is not particularly limited, and may be, for example, 0.1 μm or more, 1 μm or more, 100 μm or less, 20 μm or less, or 10 μm or less. Here, in this specification, the "particle size" can be determined by observing particles with a scanning electron microscope (SEM), counting the long side of the rectangle circumscribing each particle as the diameter, and dividing by the number of particles to obtain an average value. The number of particles to be measured should be at least 10 Individuals The number of particles to be measured may be 100 or more.

[0026] <Effects> The solid electrolyte of the present disclosure having the above characteristics has few impurities and a highly crystalline argyrodite structure, thereby enabling the solid electrolyte of the present disclosure to have improved ionic conductivity.

[0027] The solid electrolyte of the present disclosure may also be used in, for example, solid-state batteries, including all-solid-state batteries. Here, a solid-state battery refers to a battery that includes a solid electrolyte. An all-solid-state battery refers to a solid-state battery that does not include a liquid-based material.

[0028] [All-solid battery] The all-solid-state battery of the present disclosure includes a positive electrode, a negative electrode, and a solid electrolyte layer disposed between the positive electrode and the negative electrode, and is characterized in that at least one of the positive electrode, the negative electrode, and the solid electrolyte layer contains the solid electrolyte. Because the all-solid-state battery of the present disclosure includes the solid electrolyte, ionic conductivity can be improved. Below, an embodiment of each configuration of the all-solid-state battery of the present disclosure is described. However, the all-solid-state battery of the present disclosure is not limited to this.

[0029] <Positive electrode> The positive electrode includes a positive electrode current collector and a positive electrode active material layer. The positive electrode active material layer is disposed on the solid electrolyte layer side of the positive electrode current collector and is in contact with the solid electrolyte layer.

[0030] The material of the positive electrode current collector is not particularly limited and can be appropriately selected from known materials depending on the purpose. Examples include Cu, Ni, Cr, Au, Pt, Ag, Al, Fe, Ti, Zn, Co, and stainless steel. The thickness of the positive electrode current collector is not particularly limited and can be appropriately set depending on the desired battery performance. For example, it is in the range of 0.1 μm to 1 mm.

[0031] The positive electrode active material layer contains at least a positive electrode active material. The positive electrode active material can be appropriately selected from known positive electrode active materials used in lithium-ion all-solid-state batteries. Examples include lithium cobalt oxide, nickel cobalt lithium aluminum oxide (NCA), nickel cobalt lithium manganese oxide (NCM), and lithium manganese oxide. The particle size of the positive electrode active material is not particularly limited, but is, for example, in the range of 1 μm to 100 μm. The content of the positive electrode active material in the positive electrode active material layer is not particularly limited, but is, for example, in the range of 50 wt % to 99 wt %. In addition, the surface of the positive electrode active material may be covered with a lithium niobate layer, a lithium titanate layer, a lithium phosphate layer, or the like. layer It may be coated with an oxide layer such as

[0032] The positive electrode active material layer may optionally include a solid electrolyte. Examples of the solid electrolyte include the solid electrolyte of the present disclosure. The solid electrolyte may be appropriately selected from known solid electrolytes used in all-solid-state lithium-ion batteries. Examples include oxide solid electrolytes and sulfide solid electrolytes. A sulfide solid electrolyte is preferred. An example of an oxide solid electrolyte is Li7La3Zr2O 12 , Li 7-x La3Zr 1-x Nb x O 12 , Li 7-3x La3Zr2Al x O 12 , Li 3x La 2 / 3-x TiO3, Li 1+x Al x Ti 2-x (PO4)3, Li 1+x Al x Ge 2-x (PO4)3, Li3PO4, or Li 3+xPO 4-x N x (LiPON), etc. Examples of sulfide solid electrolytes include Li3PS4, Li2S-P2S5, Li2S-SiS2, LiI-Li2S-SiS2, LiI-Si2S-P2S5, Li2S-P2S5-LiI-LiBr, LiI-Li2S-P2S5, LiI-Li2S-P2O5, LiI-Li3PO4-P2S5, and Li2S-P2S5-GeS2. The content of the solid electrolyte in the positive electrode active material layer is not particularly limited, but is, for example, in the range of 1 wt % to 50 wt %.

[0033] The positive electrode active material layer may optionally contain a conductive additive. As the conductive additive, a known conductive additive used in a lithium ion all-solid-state battery may be used. Agent Examples of the conductive additive include carbon materials such as acetylene black, ketjen black, and vapor grown carbon fiber (VGCF), and metal materials such as nickel, aluminum, and stainless steel. The content of the conductive additive in the positive electrode active material layer is not particularly limited, but is, for example, in the range of 0.1% by weight to 10% by weight.

[0034] The positive electrode active material layer may optionally contain a binder. The binder can be appropriately selected from known binders used in lithium-ion all-solid-state batteries. Examples include butadiene rubber (BR), butylene rubber (IIR), acrylate butadiene rubber (ABR), styrene-butadiene rubber (SBR), polyvinylidene fluoride (PVdF), and polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-HFP). The content of the binder in the positive electrode active material layer is not particularly limited, but is, for example, in the range of 0.1% by weight to 10% by weight.

[0035] The shape of the positive electrode active material layer is not particularly limited, but a sheet shape is preferable. The thickness of the positive electrode active material layer is not particularly limited and may be appropriately set depending on the desired battery performance. For example, it is in the range of 0.1 μm to 1 mm.

[0036] <Negative electrode> The negative electrode includes a negative electrode current collector and a negative electrode active material layer. The negative electrode active material layer is disposed on the solid electrolyte layer side of the negative electrode current collector and is in contact with the solid electrolyte layer.

[0037] The material of the negative electrode current collector can be appropriately selected from known materials depending on the purpose. Examples include Cu, Ni, Cr, Au, Pt, Ag, Al, Fe, Ti, Zn, Co, and stainless steel. The thickness of the negative electrode current collector is not particularly limited and may be appropriately set depending on the desired battery performance. For example, it is in the range of 0.1 μm to 1 mm.

[0038] The negative electrode active material layer contains at least a negative electrode active material. The negative electrode active material can be appropriately selected from known negative electrode active materials used in lithium-ion all-solid-state batteries. Examples include silicon-based active materials such as Si, Si alloys, and silicon oxide; carbon-based active materials such as graphite and hard carbon; various oxide-based active materials such as lithium titanate; metallic lithium and lithium alloys. The particle size of the negative electrode active material is not particularly limited, but is, for example, in the range of 1 μm to 100 μm. The content of the negative electrode active material in the negative electrode active material layer is not particularly limited, but is, for example, in the range of 30 wt % to 90 wt %.

[0039] The negative electrode active material layer may optionally contain a solid electrolyte. Examples of the solid electrolyte include the solid electrolyte of the present disclosure. The solid electrolyte may be appropriately selected from known solid electrolytes used in lithium ion all-solid-state batteries. Known solid electrolytes have been described above, so a detailed description is omitted here. The content of the solid electrolyte in the negative electrode active material layer is not particularly limited, but is, for example, in the range of 10% by weight to 70% by weight.

[0040] The negative electrode active material layer may optionally contain a conductive additive and a binder. The types of conductive additive and binder contained in the negative electrode active material layer may be the same as those of the conductive additive and binder that can be used in the positive electrode active material layer. The content of the conductive additive in the negative electrode active material layer is not particularly limited, but is, for example, in the range of 0.1% by weight to 20% by weight. The content of the binder in the negative electrode active material layer is not particularly limited, but is, for example, in the range of 0.1% by weight to 10% by weight.

[0041] The shape of the negative electrode active material layer is not particularly limited, but a sheet shape is preferable. The thickness of the negative electrode active material layer is not particularly limited and may be appropriately set depending on the desired battery performance. For example, it is in the range of 0.1 μm to 1 mm.

[0042] <Solid electrolyte layer> The solid electrolyte layer is disposed between the positive electrode (positive electrode composite layer) and the negative electrode (negative electrode composite layer). The solid electrolyte layer contains at least a solid electrolyte. Examples of the solid electrolyte include the solid electrolyte of the present disclosure. The solid electrolyte can be appropriately selected from known solid electrolytes used in lithium-ion all-solid-state batteries. Known solid electrolytes have been described above, so a description thereof will be omitted here. The content of the solid electrolyte in the solid electrolyte layer is not particularly limited, but is, for example, in the range of 50% by weight to 99% by weight.

[0043] The solid electrolyte layer may also contain an optional binder. The type of binder contained in the solid electrolyte layer may be the same as the type of binder that can be used in the positive electrode active material layer. The content of the binder in the solid electrolyte layer is not particularly limited, but is, for example, in the range of 0.1 wt % to 10 wt %.

[0044] The shape of the solid electrolyte layer is not particularly limited, but a sheet shape is preferable. The thickness of the solid electrolyte layer is not particularly limited and may be appropriately set depending on the desired battery performance. For example, it is in the range of 0.1 μm to 1 mm.

[0045] <Manufacturing method for all-solid-state batteries> All-solid-state batteries can be fabricated by known methods. For example, the materials constituting the positive electrode active material layer are mixed in a predetermined solvent, and the resulting slurry is applied to a substrate or a current collector and dried to obtain a positive electrode active material layer. A negative electrode active material layer and a solid electrolyte layer can be fabricated in a similar manner. Subsequently, these electrode layers (and current collector) are stacked in a predetermined order and pressed at a predetermined pressure to obtain a laminate. The solid electrolyte layer may also be obtained by applying a solid electrolyte solution (described below) to a substrate and drying it.

[0046] [Solid electrolyte solution] The solid electrolyte solution of the present disclosure includes a solid electrolyte material dissolved in a solvent, the solid electrolyte material including lithium, phosphorus, sulfur, and a halogen, the solvent including a first solvent capable of dissolving the solid electrolyte and a second solvent incapable of dissolving the solid electrolyte, and the second solvent has a solubility parameter of 10.5 (cal / cm). 1 / 2 The solid electrolyte of the present disclosure can be obtained by removing the solvent from the solid electrolyte solution of the present disclosure.

[0047] <Solid electrolyte material> The solid electrolyte material contains lithium, phosphorus, sulfur, and a halogen. From the viewpoint of improving the crystallinity of the argyrodite structure, the solid electrolyte material may be composed of lithium, phosphorus, sulfur, and a halogen. The proportions of lithium, phosphorus, sulfur, and a halogen in the solid electrolyte material are appropriately set according to the components of the solid electrolyte. Specific materials contained in the solid electrolyte material include, for example, Li2S, P2S5, LiX (X represents a halogen), and Li3PS4. The solid electrolyte material can be produced by appropriately combining these materials.

[0048] The content of the solid electrolyte material is not particularly limited and can be set appropriately. For example, when the solid electrolyte solution is 100 wt%, the content of the solid electrolyte material may be 0.1 wt% or more, 1 wt% or more, 50 wt% or less, or 10 wt% or less. If the content of the solid electrolyte material is less than 0.1 wt%, drying takes a long time. If the content of the solid electrolyte material exceeds 50 wt%, the crystallinity of the resulting solid electrolyte tends to decrease, and impurities tend to be contained in the solid electrolyte. Therefore, the ionic conductivity of the solid electrolyte tends to decrease.

[0049] <Solvent> The solvent used is one that can dissolve the solid electrolyte material. This is because the solid electrolyte solution is a solution in which the solid electrolyte material is uniformly dissolved. The solvent includes a first solvent that can dissolve the solid electrolyte and a second solvent that cannot dissolve the solid electrolyte. From the viewpoint of obtaining a solid electrolyte with high ionic conductivity when the solvent is removed from the solid electrolyte solution, the solvent may be composed of the first solvent and the second solvent.

[0050] The total content of the first solvent and the second solvent in the solvent may be 80% by weight or more, or 90% by weight or more. The solvent may consist of the first solvent and the second solvent. The ratio of the first solvent to the second solvent in the solvent may be in the range of first solvent:second solvent=5:95 to 50:50.

[0051] (First solvent) The first solvent is a solvent capable of dissolving the solid electrolyte material. electrolytic The term "solvent" refers to a solvent that can dissolve all of the materials that make up the solid electrolyte material. For example, when a solid electrolyte material is added to a first solvent and stirred at room temperature (25°C) for 6 to 12 hours at 500 rpm, if 10 mg / 2 ml or more of the solid electrolyte material can be dissolved in the first solvent, the solvent can be said to be capable of dissolving the solid electrolyte material. The solvent may also be one in which the solubility of the solid electrolyte material is 100 mg / 2 ml.

[0052] The first solvent may consist of a single solvent or multiple solvents. When the first solvent consists of multiple solvents, all of the solvents constituting the first solvent do not have to be solvents capable of dissolving the solid electrolyte material. It is sufficient that the first solvent obtained by mixing multiple solvents can dissolve the solid electrolyte material.

[0053] The first solvent may have a lower boiling point than the second solvent. When the first solvent is composed of multiple solvents, all of the solvents constituting the first solvent have a lower boiling point than the second solvent. This allows the first solvent to be removed first when removing the solvents from the solid electrolyte solution, allowing the solid electrolyte to be precipitated under mild conditions. From the viewpoint of further enhancing the effect, the first solvent may have a boiling point of 120°C or less, a boiling point of 100°C or less, or a boiling point of 50°C or more.

[0054] Examples of the first solvent include alcohols, ethers, esters, amines, and amides. The first solvent may be a C1-C5 solvent or a C1-C4 solvent. Examples of alcohols include methanol, ethanol, propanol, and butanol. Examples of ethers include tetrahydrofuran and diethyl ether. Examples of esters include methyl propionate and ethyl propionate. Examples of amines include ethylenediamine and other amines. Examples of amides include N-methylformamide and N,N-dimethylformamide. These solvents may be used alone or in combination. Among these, the first solvent may be a mixed solvent of alcohol and ether, or may be a mixed solvent of tetrahydrofuran (THF) and ethanol (EtOH).

[0055] The content of the first solvent may be 50 wt% or more, 60 wt% or more, 95 wt% or less, 90 wt% or less, or 80 wt% or less, when the entire solvent is taken as 100 wt%.

[0056] <Second solvent> The second solvent is a solvent that cannot dissolve the solid electrolyte material. This is because the solubility parameter of the second solvent is 10.5 (cal / cm). 1 / 2 This means that the second solvent has a vapor pressure of 0.5 kPa or less. In this way, the second solvent has an extremely low solubility for the solid electrolyte material and is a solvent that is difficult to volatilize.

[0057] The solubility parameter is 1cm 3 The square root of the heat of vaporization required to evaporate a liquid (cal / cm 3 ) 1 / 2 As a reference, for example, PAC, 2008, 80, 233. (Glossary of terms related to solubility (IUPAC Recommendations 2008)) on page 264 can be mentioned. The solubility parameters described in this specification are solubility parameters at 25°C.

[0058] Vapor pressure is the pressure of the gas phase of a substance that is in phase equilibrium with the substance in its liquid or solid phase. Vapor pressure is a physical property specific to a substance. Vapor pressures given in this specification refer to vapor pressures at 25°C.

[0059] The solubility parameter of the second solvent is 10.5 (cal / cm) 1 / 2 When the vapor pressure of the second solvent is 0.5 kPa or less, the solubility of the solid electrolyte material in the entire solvent decreases, making it easier to precipitate a crystalline solid electrolyte. Furthermore, when the vapor pressure of the second solvent is 0.5 kPa or less, the solid electrolyte can be precipitated under mild conditions when removing the solvent, making it easier to precipitate a crystalline solid electrolyte. Therefore, when the solid electrolyte solution contains the second solvent, a highly crystalline solid electrolyte is more likely to precipitate, and the inclusion of impurities in the solid electrolyte is suppressed. Therefore, the solid electrolyte obtained from the solid electrolyte solution has improved ionic conductivity.

[0060] The second solvent may consist of a single solvent or multiple solvents. When the second solvent consists of multiple solvents, all of the solvents constituting the second solvent may satisfy the above solubility parameters and vapor pressures.

[0061] The solubility parameter of the second solvent is 9.4 (cal / cm) 1 / 2 The lower limit of the solubility parameter of the second solvent is not particularly limited, but for example, the solubility parameter of the second solvent may be 1.0 (cal / cm) or less. 1 / 2 It may be 5.0 (cal / cm) or more. 1 / 2 Below above 8.8 (cal / cm) 1 / 2 It may be more than that.

[0062] The vapor pressure of the second solvent may be 0.35 kPa or less, 0.25 kPa or less, or 0.1 kPa or less. The lower limit of the vapor pressure of the second solvent is not particularly limited, but may be, for example, 0.01 kPa or more, or 0.05 kPa or more.

[0063] The second solvent may have a higher boiling point than the first solvent. When the second solvent is composed of multiple solvents, all of the solvents constituting the second solvent have a higher boiling point than the first solvent. This allows the first solvent to be removed first when removing the solvent from the solid electrolyte solution, allowing the solid electrolyte to be precipitated under mild conditions. From the viewpoint of further enhancing the effect, the second solvent may have a boiling point of 150°C or higher, a boiling point of 180°C or higher, a boiling point of 200°C or higher, a boiling point of 300°C or lower, or a boiling point of 250°C or lower.

[0064] The second solvent is not particularly limited as long as it satisfies the above-mentioned ranges of solubility parameter and vapor pressure, but for example, a solvent having a carbon number of 7 to 10 may be selected. Specific examples include parachlorotoluene, mesitylene, and tetralin.

[0065] The content of the second solvent may be 5 wt% or more, 10 wt% or more, 20 wt% or more, 50 wt% or less, or 40 wt% or less, when the entire solvent is taken as 100 wt%. This allows the solid electrolyte to be precipitated under mild conditions, thereby obtaining a solid electrolyte with higher crystallinity and improving the ionic conductivity of the solid electrolyte.

[0066] Since the solid electrolyte and the solid electrolyte material are prone to react with water, it is necessary to use a dehydrated solvent.

[0067] <Other ingredients> The solid electrolyte solution may optionally contain a binder. As the binder, the binder that can be used for the solid electrolyte layer described above can be used. The content of the binder can be appropriately set so as to obtain a desired solid electrolyte.

[0068] [Solid electrolyte manufacturing method] The method for producing a solid electrolyte according to the present disclosure includes a preparation step of preparing the solid electrolyte solution, and a removal step of removing the solvent from the solid electrolyte solution.

[0069] <Preparation process> The preparation step is a step of preparing a solid electrolyte solution. The method for preparing the solid electrolyte solution is not particularly limited, and the solid electrolyte solution may be obtained by simply mixing a solid electrolyte material with a solvent, or by dissolving a solid electrolyte material in a first solvent and then adding a second solvent to obtain the solid electrolyte solution. In addition, a predetermined reaction may be allowed to proceed during the process of preparing the solid electrolyte solution.

[0070] For example, the preparation process may include a first step of suspending Li2S and P2S5 in a first solvent (e.g., an ether solvent) to prepare a Li3PS4 suspension, a second step of adding the Li3PS4 suspension to the first solvent (e.g., an alcohol solution) in which Li2S and LiX have been dissolved to prepare a solution in which the solid electrolyte material is dissolved in the first solvent, and a third step of adding the second solvent to the obtained solution. A solid electrolyte solution may be prepared in this manner.

[0071] <Removal process> The removal step is a step of removing the solvent from the solid electrolyte solution. The method for removing the solvent from the solid electrolyte solution is not particularly limited, and the solvent may be removed by heating the solid electrolyte solution, or by heating the solid electrolyte solution under reduced pressure. The heating temperature may be 50°C or higher, 100°C or higher, 150°C or higher, 300°C or lower, 250°C or lower, or 200°C or lower. The heating atmosphere may be an inert atmosphere, a reduced pressure atmosphere, or a vacuum. The reduced pressure atmosphere may be any pressure lower than atmospheric pressure, for example, 0.01 Pa to 10 Pa. The heating time may be 30 minutes or longer, 1 hour or longer, 12 hours or shorter, or 6 hours or shorter.

[0072] The heating step may be divided into two stages. That is, the removal step may include two heating steps. Specifically, the removal step may include a first heating step in which the solid electrolyte solution is heated at a low temperature, and a second heating step in which the solid electrolyte solution is heated at a high temperature after the first heating step. By heating the solid electrolyte solution at a low temperature, a low-boiling point solvent (e.g., the first solvent; the first solvent and the second solvent may also be removed by azeotropy) can be removed from the solid electrolyte solution, and then, by heating the solid electrolyte solution at a high temperature, a high-boiling point solvent (e.g., the second solvent; the first solvent and the second solvent may also be removed by azeotropy) can be removed from the solid electrolyte solution. In this way, by dividing the heating step into two stages, bumping can be suppressed and the solid electrolyte can be precipitated under milder conditions, resulting in a solid electrolyte with high crystallinity. Therefore, the ionic conductivity of the solid electrolyte can be further improved.

[0073] The heating temperature in the first heating step may be any temperature that allows the first solvent to be removed. For example, it may be 50°C or higher, 70°C or higher, or 120°C or lower, or 100°C or lower. The heating atmosphere in the first heating step may be an inert atmosphere, a reduced pressure atmosphere (e.g., 0.01 Pa to 10 Pa), or a vacuum. The heating time in the first heating step may be 10 minutes or longer, 30 minutes or longer, or 2 hours or shorter, or 1 hour or shorter.

[0074] The heating temperature in the second heating step may be any temperature that allows the second solvent to be removed. For example, the heating temperature may be 130°C or higher, 140°C or higher, 150°C or higher, 300°C or lower, 250°C or lower, 200°C or lower, or 180°C or lower. The heating atmosphere in the second heating step may be an inert atmosphere, a reduced pressure atmosphere (e.g., 0.01 Pa to 10 Pa), or a vacuum. The heating time in the second heating step may be 30 minutes or longer, 1 hour or longer, 12 hours or shorter, or 6 hours or shorter.

[0075] The removal step may be carried out on a hot plate or on a substrate such as a metal foil. [Example]

[0076] The present disclosure will now be further described using examples.

[0077] [Preparation of solid electrolyte] The solid electrolytes of Examples 1 to 5 and Comparative Examples 1 and 2 were prepared according to the following procedure: Figure 1 shows a flowchart of the method for preparing the solid electrolyte.

[0078] First, Li2S, P2S5, and dehydrated THF were mixed in an argon glove box with an Ar atmosphere at a dew point of -60°C or less. The molar ratio of Li2S to P2S5 was 3:1. The resulting mixture was stirred overnight to obtain a THF suspension containing Li3PS4.

[0079] Next, Li2S and LiCl were dissolved in ultra-dehydrated ethanol (EtOH) to obtain an EtOH solution. The molar ratio of Li2S to LiCl was 1:1. Next, the THF suspension and the EtOH solution were mixed to obtain a THF-EtOH solution (a solution in which the solid electrolyte material was dissolved in the first solvent).

[0080] In the THF-EtOH solution, the molar ratio of Li2S, P2S5, and LiCl used as the solid electrolyte material was 5:1:2. The total concentration of the solid electrolyte material in the solution was 4.5 mass%.

[0081] Next, the second solvent shown in Table 1 was added to the THF-EtOH solution and stirred to obtain a solid electrolyte solution. The solid electrolyte solution was then heated on a hot plate in an inert atmosphere at 80°C for 30 minutes. It was then further heated in vacuum at 170°C for 2 hours. This resulted in the solid electrolyte powders (Li6PS5Cl) of Examples 1 to 5 and Comparative Examples 1 and 2.

[0082] [XRD measurement] XRD measurements were carried out with care to prevent the solid electrolyte from coming into contact with the air. The XRD measurements were carried out using a continuous method under the conditions of 10-80°, 2θ / CuKα, 10° / min, and a scan step of 0.01°.

[0083] Figure 2 shows an XRD spectrum of Comparative Example 1, in which no second solvent was added, superimposed on the XRD spectrum of Example 3, in which 10 wt% of tetralin was added. Figure 3 also shows the XRD spectrum of Comparative Example 2, in which 10 wt% of NMP was added.

[0084] In the XRD spectrum, the main peak indicating Li6PS5Cl with an argyrodite structure appears around 30°. Comparing this main peak in Figure 2, the peak of Example 1, in which the second solvent was added, is larger than that of Comparative Example 1, in which the second solvent was not added. 3 It was observed that the peak of impurities was high and the crystallinity was improved. 3 The impurity peaks are smaller than those in the first solvent. Therefore, it was found that the impurities were reduced by adding the second solvent.

[0085] In contrast, in Comparative Example 2, in which NMP (N-methylpyrrolidone) was used as the second solvent, the peak of the argyrodite structure could not be confirmed, and it was found that the crystal structure had collapsed, as shown in Figure 3. This is thought to be because NMP has a large solubility parameter and is highly polar, which caused some kind of reaction with the solid electrolyte to proceed.

[0086] Next, we investigated the changes in the main peak (near 30°) and impurity peak (near 26.5°) depending on the type of second solvent. Figure 4 shows a diagram in which the XRD spectra of Examples 1 to 3 and Comparative Example 1 are superimposed. Figure 5 also shows the rate of change in the peak intensity of the main peak, the half-width of the main peak, and the peak intensity of the impurity peak for Examples 1 to 3 and Comparative Example 1, respectively, when Comparative Example 1 is used as the reference.

[0087] As can be seen from Figures 4 and 5, the main peak increased as the vapor pressure of the second solvent decreased (the boiling point increased). The impurity peak also tended to decrease. The half-width of the main peak showed almost the same behavior in Examples 1 to 3, in which the second solvent was added, and was reduced by 5 to 6% compared to Comparative Example 1. These results suggest that the addition of the second solvent increases crystallinity, and that the lower the vapor pressure (the higher the boiling point), the higher the crystallinity and the lower the impurities.

[0088] Next, we investigated the changes in the main peak (around 30°) and impurity peak (around 26.5°) depending on the content of the second solvent. 3 The XRD spectra of Examples 1 to 5 and Comparative Example 1 are shown in a diagram overlapping each other. 3 The peak intensity of the main peak, the half-width of the main peak, and the rate of change in the peak intensity of the impurity peak are shown for Comparative Example 1 to 5, and Comparative Example 1. Figures 6 and 7 also show the results of XRD spectra of solid electrolytes prepared by double concentration. The test example of double concentration is a reference example, and was prepared from a solid electrolyte solution that was double concentrated by partially removing the solvent using a rotary evaporator.

[0089] As can be seen from Figures 6 and 7, as the amount of the second solvent added increased (as the solid electrolyte solution became less polar), the main peak increased and the half-width of the main peak decreased. Therefore, it is thought that as the amount of the second solvent added increased (as the solid electrolyte solution became less polar), the crystallinity increased. On the other hand, with regard to the impurity peak, Example 4, in which 30 wt% of the second solvent was added, had the lowest impurity peak. Furthermore, Examples 3 to 5, in which the second solvent was added, all tended to have low impurity peaks.

[0090] From the above results, the following characteristics are important in order to improve the crystallinity of solid electrolytes and reduce impurities. Essential (1) It is important that the second solvent is a low polar solvent with a low solubility parameter. For example, the solubility parameter of the second solvent is 10.5 (cal / cm). 1 / 2It may be less than 0.5 kPa. As in Comparative Example 2, if the solubility parameter is high, the crystal structure will collapse. (2) It is important that the second solvent has a low vapor pressure (low boiling point). Furthermore, the lower the vapor pressure (boiling point) of the second solvent, the higher the crystallinity and the fewer impurities a solid electrolyte can be obtained. For example, the vapor pressure of the second solvent may be 0.5 kPa or less, and the boiling point of the second solvent may be 150°C or more.

[0091] The results are summarized in Table 1. Here, the peak intensity ratio of the main peak (near 30°) to the impurity peak (near 26.5°) in Table 1 is calculated based on Comparative Example 1.

[0092] [TG-MS measurement] The obtained solid electrolyte powder was subjected to TG-MS measurement. The measurement conditions were as follows: the sample weight was 10 mg, the temperature rise rate was 10°C / min, helium was used as the carrier gas, and the gas flow rate was 80 ml / min.

[0093] The results of DTG (Differential Thermogravimetry) are shown in Figure 8. The results of TICC (Total Ion Current Chromatogram) measurements are shown in Figure 9.

[0094] 8 and 9, it was found that there are three temperature regions in which mass loss and gas generation occur with increasing temperature in solid electrolytes: (1) Region 1, from room temperature to less than 170°C; (2) Region 2, from 170°C to less than 250°C; and (3) Region 3, from 250°C to less than 300°C.

[0095] Next, the gas generated in each region was analyzed by MS. Figure 10 shows the MS spectrum of each region in Comparative Example 1. Figure 11 shows the MS spectrum of each region in Example 3.

[0096] Comparing Figures 10 and 11, it was confirmed that although common gases were present, different gases were generated in each region. Of these, we focused on the common peaks at m / z = 192 and m / z = 256. Considering the components of the solid electrolyte, we speculated that these peaks were cyclic sulfur (S6 and S8). Therefore, we attempted to identify these peaks. Figure 12 shows the MS spectrum of cyclic sulfur S8. Figure 13 shows the MS spectrum of cyclic sulfur S6.

[0097] 10 to 13, the peaks at m / z=192 and m / z=256 appearing in the MS spectra of Fig. 10 and Fig. 11 are believed to be derived from cyclic sulfurs S6 and S8. Furthermore, based on the magnitude of the peak intensities, the main component of the gas generated from the solid electrolyte is believed to be cyclic sulfur S8.

[0098] FIG. 14 shows the results of TICC focusing on m / z=256. Here, the peak appearing in region 2 of 170°C or higher but lower than 250°C is referred to as the first peak, and the peak appearing in region 3 of 250°C or higher but lower than 300°C is referred to as the second peak. As can be observed from FIG. 14, in Comparative Examples 1 and 2, the peak intensity P2 of the second peak is higher than the peak intensity P1 of the first peak. On the other hand, in Examples 1 to 5, the peak intensity P1 of the first peak is higher than the peak intensity P2 of the second peak. In this respect, a significant difference was observed between the results of Examples 1 to 5 and Comparative Example 1.

[0099] The peak intensities of the first and second peaks were further investigated. Table 1 shows the peak intensity ratios P1 / P2 of the examples and comparative examples. As can be seen from Table 1, the peak intensity ratios P1 / P2 of comparative examples 1 and 2 were less than 1.00. On the other hand, the peak intensity ratios P1 / P2 of examples 1 to 5 were greater than 1.00, specifically in the range of 1.19 to 2.10.

[0100] Furthermore, the TICC results focusing on m / z=192 are shown in Figure 15. As can be seen from Figure 15, the peak intensities of the first and second peaks showed a similar tendency to the results for m / z=256.

[0101] [Ionic conductivity measurement] The ionic conductivity of the obtained solid electrolyte powder was measured by the AC impedance method. Specifically, the procedure is as follows: 80 mg of solid electrolyte powder was placed inside a metal cylinder with a diameter of 1 cm and pressed under 360 MPa to form a pellet. Stainless steel electrodes were attached to both ends of this pellet to form a cell, and the ionic conductivity of the cell was measured by the AC impedance method. The results are shown in Table 1.

[0102] As can be seen from Table 1, the ionic conductivity of Examples 1 to 5, in which the second solvent was added, was improved compared to Comparative Example 1, in which no second solvent was added. This is thought to be due to the increase in the main peak of Li6PS5Cl, which has an argyrodite structure, in XRD measurement. Furthermore, Examples 1 to 3 showed a tendency for the ionic conductivity to increase as the vapor pressure of the second solvent decreased. Furthermore, Examples 3 to 5 showed that the ionic conductivity was highest when the content of the second solvent was 30 wt%. This is thought to be because the amount of impurities was the lowest among Examples 3 to 5. Comparative Example 2 had an extremely low ionic conductivity. This is thought to be due to the addition of NMP disrupting the crystalline structure of the solid electrolyte.

[0103] [Table 1]

Claims

1. A solid electrolyte solution in which a solid electrolyte material is dissolved in a solvent, the solid electrolyte material comprises lithium, phosphorus, sulfur, and a halogen; the solvent includes a first solvent capable of dissolving the solid electrolyte material and a second solvent incapable of dissolving the solid electrolyte material, The solubility parameter of the second solvent is 10.5 (cal / cm) １／２ or less, and the vapor pressure is 0.5 kPa or less; Solid electrolyte solution.

2. 2. The solid electrolyte solution according to claim 1, wherein the content of the second solvent in the solvent is 5 wt % or more and 50 wt % or less.

3. The solid electrolyte solution according to claim 1 or 2, wherein the second solvent has a carbon number of 7 or more and 10 or less.

4. A preparation step of preparing the solid electrolyte solution according to any one of claims 1 to 3; and removing the solvent from the solid electrolyte solution. Method for producing solid electrolytes.

5. the removing step includes a first heating step of heating the solid electrolyte solution at a temperature of 50°C or higher and 120°C or lower, and a second heating step of heating the solid electrolyte solution at a temperature of 140°C or higher and 200°C or lower after the first heating step. The method for producing the solid electrolyte according to claim 4 .

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