Method for producing lithium sulfide, method for producing sulfide solid electrolyte, and method for evaluating lithium sulfate monohydrate

Abstract

To provide a method for producing lithium sulfide capable of reliably producing lithium sulfide with high purity, a method for producing a sulfide solid electrolyte using lithium sulfide produced by this method for producing lithium sulfide, and a method for evaluating lithium sulfate monohydrate.SOLUTION: Provided is a method for producing lithium sulfide using lithium sulfate monohydrate as a raw material, wherein the lithium sulfate monohydrate has a turbidity of 1.0% or less when suspended in water at a concentration of 0.2 g / mL. Provided is a method for producing a sulfide solid electrolyte characterized by using, as a raw material, lithium sulfide produced by the method for producing lithium sulfide according to claim 1. Also provided is a method for evaluating lithium sulfate monohydrate, for evaluating insoluble components contained in the lithium sulfate monohydrate, characterized by suspending the lithium sulfate monohydrate in water at a concentration of 0.2 g / mL, measuring turbidity of the suspension, and evaluating the presence of insoluble components contained in the lithium sulfate monohydrate.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a method for producing lithium sulfide suitable for use in, for example, a sulfide solid electrolyte of an all-solid-state battery, a method for producing a sulfide solid electrolyte using the lithium sulfide produced by the lithium sulfide production method, and a method for evaluating lithium sulfate monohydrate. [Background technology]

[0002] Lithium-ion batteries are widely used as power sources in vehicles such as EVs (electric vehicles) and HEVs (hybrid electric vehicles), as well as electronic devices such as mobile phones and laptops. Conventional lithium-ion batteries use an organic electrolyte solution, in which a lithium salt such as lithium hexafluorophosphate (LiPF6) is dissolved in an organic solvent.

[0003] These organic electrolytes are flammable and can be damaged by excessive heating or impact. In addition, in lithium-ion batteries that use metallic lithium in the negative electrode, dendrites of metallic lithium grow on the surface of the negative electrode during charging, which can cause internal short circuits between the electrodes and lead to malfunctions.

[0004] In order to improve the safety and durability of conventional lithium ion batteries that use such organic electrolytes, all-solid-state lithium ion batteries that use sulfide solid electrolytes have been proposed. Currently proposed sulfide solid electrolytes include, for example, Li2S-P2S5-based, Li2S-P2S3-based, Li2S-SiS2-based, Li2S-Ga2S2-based, and Li2S-GeS2-based.

[0005] In all of these sulfide solid electrolytes, lithium sulfide (Li2S) is used as a constituent material. As shown in Patent Documents 1 to 3, for example, lithium sulfate (Li2SO4) is used as one of the raw materials when producing lithium sulfide (Li2S). [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Laid-Open No. 2016-216312 [Patent Document 2] Japanese Patent Application Laid-Open No. 2016-216349 [Patent Document 3] Patent Publication No. 2021-147251 Summary of the Invention [Problem to be solved by the invention]

[0007] Meanwhile, sulfide solid electrolytes used as electrolytes in lithium ion batteries are required to have high ionic conductivity. In order to produce a sulfide solid electrolyte with high ionic conductivity, it is necessary to highly purify the lithium sulfide that constitutes the sulfide solid electrolyte.

[0008] In order to produce high-purity lithium sulfide, it is necessary to reduce the amount of impurities in lithium sulfate monohydrate, which is used as a raw material. Here, the amount of impurities in lithium sulfate monohydrate is usually evaluated by ICP analysis. In ICP analysis, the content of elements contained in a liquid sample is measured, and it is necessary to dissolve the measurement target in a solvent, so insoluble components cannot be evaluated. Therefore, there is a risk that high-purity lithium sulfide cannot be produced by evaluation only by ICP analysis.

[0009] The present invention has been made in view of the above-mentioned circumstances, and an object of the present invention is to provide a method for producing lithium sulfide that can reliably produce high-purity lithium sulfide, a method for producing a sulfide solid electrolyte that uses lithium sulfide produced by this method for producing lithium sulfide, and a method for evaluating lithium sulfate monohydrate. [Means for solving the problem]

[0010] In order to solve the above problems, a method for producing lithium sulfide according to a first aspect of the present invention is a method for producing lithium sulfide using lithium sulfate monohydrate as a raw material, characterized in that the lithium sulfate monohydrate has a turbidity of 1.0% or less when suspended in water at a concentration of 0.2 g / mL.

[0011] According to the method for producing lithium sulfide of the first aspect of the present invention, lithium sulfate monohydrate that has a turbidity of 1.0% or less when suspended in water at a concentration of 0.2 g / mL is used as a raw material. This makes it possible to fully evaluate insoluble components and reliably produce lithium sulfide with high purity.

[0012] The method for producing a sulfide solid electrolyte according to the second aspect of the present invention is characterized in that lithium sulfide produced by the method for producing lithium sulfide according to the first aspect of the present invention is used as a raw material.

[0013] According to the method for producing a sulfide solid electrolyte of the second aspect of the present invention, lithium sulfide produced by the method for producing lithium sulfide according to the first aspect of the present invention is used as a raw material, and therefore a sulfide solid electrolyte having excellent ionic conductivity can be produced.

[0014] The method for evaluating lithium sulfate monohydrate according to a third aspect of the present invention is a method for evaluating an insoluble component contained in lithium sulfate monohydrate, characterized by suspending the lithium sulfate monohydrate in water to a concentration of 0.2 g / mL, measuring the turbidity of the suspension, and evaluating the content of the insoluble component contained in the lithium sulfate monohydrate.

[0015] According to the method for evaluating lithium sulfate monohydrate of the third aspect of the present invention, the lithium sulfate monohydrate is suspended in water at a concentration of 0.2 g / mL, and the turbidity of the suspension is measured, thereby making it possible to evaluate the content of insoluble components in the lithium sulfate monohydrate. [Effects of the Invention]

[0016] According to the present invention, it is possible to provide a method for producing lithium sulfide that can reliably produce high-purity lithium sulfide, a method for producing a sulfide solid electrolyte that uses lithium sulfide produced by this method for producing lithium sulfide, and a method for evaluating lithium sulfate monohydrate. [Brief explanation of the drawings]

[0017] [Figure 1] FIG. 1 is a flow chart showing a method for producing lithium sulfide according to an embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0018] Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings. Note that the following embodiments are specifically described to provide a better understanding of the gist of the invention, and do not limit the present invention unless otherwise specified.

[0019] The method for producing lithium sulfide according to this embodiment is for producing lithium sulfide (LiS), which is a constituent material of a sulfide solid electrolyte used as a solid electrolyte in an all-solid-state battery. Sulfide solid electrolyte materials have high ionic conductivity, are non-flammable, and are highly safe, making them suitable as materials for on-board batteries of electric vehicles (EVs) and hybrid electric vehicles (HEVs).

[0020] Here, the method for producing lithium sulfide according to this embodiment will be described with reference to the flow chart of FIG. As shown in FIG. 1 , the method for producing lithium sulfide according to the present embodiment includes a raw material mixing step S01 of mixing lithium sulfate monohydrate and a reducing agent to obtain a mixed raw material, a heating step S02 of heat-treating the mixed raw material, and a cooling step S03 of cooling the mixed raw material to room temperature after the heating step S02.

[0021] (Raw material mixing process S01) First, lithium sulfate monohydrate and a reducing agent are blended so that the blending ratio (molar ratio) is within a predetermined range, and then these are mixed to obtain a mixed raw material. Note that there are no particular limitations on the blending method, and various existing methods can be appropriately selected and used.

[0022] In this embodiment, it is preferable to use a carbon material such as activated carbon or carbon black as the reducing agent. The average particle size (d50) of the reducing agent (carbon material) is preferably in the range of 5 μm to 15 μm.

[0023] In this embodiment, the lithium sulfate monohydrate used as a raw material has a turbidity of 1.0% or less when suspended in water at a concentration of 0.2 g / mL. This lithium sulfate monohydrate has a sufficiently low content of insoluble components and is considered to have high purity. When lithium sulfate monohydrate is suspended in water at a concentration of 0.2 g / mL, the turbidity is preferably 1.0% or less, and more preferably 0.8% or less. Here, the turbidity of a suspension of lithium sulfate monohydrate in water can be measured at room temperature using a commercially available turbidity meter.

[0024] In addition, when lithium sulfate monohydrate is heated, a volume change occurs on the surface of the lithium sulfate due to the removal of water of crystallization, which increases the surface area and enhances the reactivity. In this embodiment, it is preferable to use lithium sulfate monohydrate whose weight loss when heated to 120°C is in the range of 5% to 25%. The average particle size (d50) of the lithium sulfate monohydrate is preferably in the range of 10 μm or more and 100 μm or less.

[0025] (Heating process S02) Next, the mixed raw materials were charged into a vacuum furnace, and the internal pressure of the vacuum furnace was increased to 1×10 2 After evacuation to a pressure of 100 Pa or less, a heat treatment is carried out and lithium sulfate monohydrate is reduced with a carbon material (activated carbon) to produce lithium sulfide. The heating temperature in the heating step S02 is preferably in the range of 700° C. to 800° C. The holding time at the heating temperature is preferably in the range of 600 minutes to 1200 minutes. Furthermore, the rate of temperature rise up to the heating temperature is preferably within the range of 1° C. / min to 5° C. / min.

[0026] (Cooling process S03) Next, the mixture is naturally cooled to room temperature in the vacuum furnace, and the lithium sulfide produced is recovered. Furthermore, the cooling rate to room temperature is preferably in the range of 1° C. / min to 3° C. / min.

[0027] Through the above steps, high-purity lithium sulfide is produced.

[0028] In the method for producing a sulfide solid electrolyte according to this embodiment, lithium sulfide produced by the method for producing lithium sulfide according to this embodiment is used as a raw material. Since the lithium sulfide produced by the method for producing lithium sulfide according to this embodiment has high purity as described above, the amount of impurities is reduced in a sulfide solid electrolyte produced using this as a raw material, and a sulfide solid electrolyte with excellent properties such as ionic conductivity is produced.

[0029] Next, a method for evaluating lithium sulfate monohydrate used as a raw material in this embodiment will be described. First, lithium sulfate monohydrate is suspended in water at a concentration of 0.2 g / mL. By adjusting the concentration of lithium sulfate monohydrate in the suspension to 0.2 g / mL, it becomes possible to measure the turbidity of the suspension with high accuracy. The suspension containing the lithium sulfate monohydrate is then measured using a turbidity meter. The turbidity is used to evaluate the content of insoluble components in the lithium sulfate monohydrate. That is, when the turbidity of the suspension is high, it is evaluated that the lithium sulfate monohydrate contains a large amount of insoluble components, and when the turbidity of the suspension is low, it is evaluated that the lithium sulfate monohydrate contains a small amount of insoluble components.

[0030] According to the method for producing lithium sulfide of the present embodiment configured as described above, lithium sulfate monohydrate that has a turbidity of 1.0% or less when suspended in water at a concentration of 0.2 g / mL is used as a raw material. This makes it possible to sufficiently evaluate insoluble components and reliably produce lithium sulfide with high purity. Furthermore, when lithium sulfate monohydrate is heated, a change in volume causes fine cracks to form on the surface of the lithium sulfate due to the elimination of water of crystallization, increasing the surface area and enhancing the reactivity, thereby enabling efficient production of high-purity lithium sulfide.

[0031] According to the method for producing a sulfide solid electrolyte of the present embodiment, lithium sulfide produced using high-purity lithium sulfate monohydrate with few insoluble components is used as a raw material, and therefore a sulfide solid electrolyte with a sufficiently low amount of impurities and excellent ionic conductivity can be produced.

[0032] According to the method for evaluating lithium sulfate monohydrate of the present embodiment, lithium sulfate monohydrate is suspended in water at a concentration of 0.2 g / mL and the turbidity of the suspension is measured, so that it is possible to evaluate the content of insoluble components in lithium sulfate monohydrate.

[0033] Although one embodiment of the present invention has been described above, the present invention is not limited to this and can be modified as appropriate within the scope of the technical idea of ​​the invention. [Example]

[0034] A confirmation experiment conducted to confirm the effectiveness of the present invention will be described.

[0035] Lithium sulfate monohydrate was dissolved in water using a stirrer to a concentration of 0.2 g / mL, and the turbidity of the resulting suspension was measured using a commercially available turbidity meter (HZ-2, manufactured by Suga Test Instruments Co., Ltd.) by placing the sample in a quartz cell after standardization using a haze standard plate. The measured turbidity is shown in Table 1. Note that the amount of insoluble components in the lithium sulfate monohydrate used in Invention Example 2 was greater than that in the lithium sulfate monohydrate used in Invention Example 1. This lithium sulfate monohydrate and a reducing agent (activated carbon powder) were mixed together, and subjected to a heat treatment (heating temperature of 700°C to 800°C, holding time of 600 minutes to 1200 minutes, heating rate of 1°C / min to 5°C / min) and a cooling treatment (cooling rate of 1.5°C / min) to produce lithium sulfide.

[0036] Then, this lithium sulfide was used as a raw material, and Si, Sn, P, and S were mixed, followed by firing in an inert gas atmosphere to produce a sulfide solid electrolyte.

[0037] The ionic conductivity of the obtained sulfide solid electrolyte was measured as follows. The measurement results are shown in Table 1.

[0038] <Ionic conductivity> The obtained solid electrolyte was taken out from a glove box in an argon atmosphere, crushed in an agate mortar, and 0.3 g was weighed and filled into a stainless steel ionic conductivity measurement cell (cylindrical, inner diameter 17 mm). The ionic conductivity (mS / cm) was then measured by the AC impedance method using a Biologic Corp. measuring device "Potentio / Galvanostat SP-300" under the conditions of a measurement temperature of 25°C, a measurement frequency of 1 Hz to 1 MHz, and an applied pressure of 360 MPa to the measurement cell.

[0039] [Table 1]

[0040] In Comparative Example 1, lithium sulfate monohydrate, which had a larger amount of insoluble components than Inventive Example 2, was suspended in water at a concentration of 0.2 g / mL, and the turbidity of the suspension was high at 4.16, and the sulfide solid electrolyte produced using this as a starting material had a low ionic conductivity of 1.2 mS / cm. In Comparative Example 2, lithium sulfate monohydrate, which had a higher amount of insoluble components than Inventive Example 2 but lower than Comparative Example 1, was suspended in water at a concentration of 0.2 g / mL, resulting in a suspension with a high turbidity of 2.21, and the sulfide solid electrolyte produced using this as a starting material had a low ionic conductivity of 1.8 mS / cm.

[0041] In Inventive Example 1, the turbidity of a suspension prepared by suspending lithium sulfate monohydrate in water at a concentration of 0.2 g / mL was as low as 0.00, and the sulfide solid electrolyte produced using this suspension as a raw material had a high ionic conductivity of 3.5 mS / cm. In Inventive Example 2, the turbidity of a suspension prepared by suspending lithium sulfate monohydrate in water at a concentration of 0.2 g / mL was as low as 0.73, and the sulfide solid electrolyte produced using this suspension as a raw material had a high ionic conductivity of 3.0 mS / cm.

[0042] As described above, according to the present invention, lithium sulfide is produced using lithium sulfate monohydrate as a raw material, which has a turbidity of 1.0% or less when suspended in water at a concentration of 0.2 g / mL, and a sulfide solid electrolyte is produced using this lithium sulfide as a raw material. Therefore, the amount of impurities, including insoluble components, is sufficiently reduced, and it is possible to produce a sulfide solid electrolyte with excellent properties such as ionic conductivity.

Claims

1. A method for producing lithium sulfide using lithium sulfate monohydrate as a raw material, comprising the steps of: The method for producing lithium sulfide, wherein the lithium sulfate monohydrate has a turbidity of 1.0% or less when suspended in water at a concentration of 0.2 g / mL.

2. A method for producing a sulfide solid electrolyte, comprising using lithium sulfide produced by the method for producing lithium sulfide according to claim 1 as a raw material.

3. A method for evaluating lithium sulfate monohydrate, comprising: evaluating an insoluble component contained in lithium sulfate monohydrate; a method for evaluating lithium sulfate monohydrate, comprising suspending the lithium sulfate monohydrate in water to a concentration of 0.2 g / mL, measuring the turbidity of the suspension, and evaluating the content of insoluble components contained in the lithium sulfate monohydrate.

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