Method for producing lithium sulfide, and method for producing sulfide-based solid electrolyte

Abstract

A method for producing lithium sulfide according to the present invention is characterized by having a raw material mixing step (S1) in which lithium sulfate and a carbon material are mixed to form a mixed raw material, and a thermal reduction step (S2) in which the mixed raw material is heat-treated in a non-oxidizing atmosphere and the lithium sulfate is thermally reduced to generate lithium sulfide, the method also being characterized in that a carbon powder having a specific surface area measured by the BET method of 55 m2 / g or greater is used as the carbon material. A method for producing a sulfide-based solid electrolyte according to the present invention is characterized in that lithium sulfide produced through the method for producing lithium sulfide according to the present invention is used as a raw material.

Description

Method for producing lithium sulfide and method for producing sulfide-based solid electrolyte

[0001] This invention relates to a method for producing lithium sulfide, which is suitable as a constituent material for sulfide-based solid electrolyte materials used in, for example, all-solid-state batteries, and a method for producing a sulfide-based solid electrolyte. This application claims priority based on Japanese Patent Application No. 2024-122304, filed on July 29, 2024, the contents of which are incorporated herein by reference.

[0002] Lithium-ion batteries are widely used as power sources in a wide range of vehicles, from electric vehicles (EVs) and hybrid electric vehicles (HEVs) to electronic devices such as mobile phones and laptops. Conventional lithium-ion batteries contain lithium hexafluorophosphate (LiPF) in an organic solvent as an electrolyte. ６ An organic electrolyte solution containing a lithium salt such as CI, CI 6000, is used.

[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 using such organic electrolytes, all-solid-state lithium ion batteries using sulfide-based solid electrolytes have been proposed. Examples of currently proposed sulfide-based solid electrolytes include Li ２ S-P ２ S ５ system, Li ２ S-P ２ S ３ system, Li ２ S-SiS ２ system, Li ２ S-Ga ２ S ２ system, Li ２ S-GeS ２ In any of these sulfide-based solid electrolytes, lithium sulfide (Li ２ S) is used.

[0005] As a method for producing lithium sulfide, for example, Patent Document 1 discloses a method in which lithium hydroxide is reacted with hydrogen sulfide in an aprotic organic solvent to produce lithium hydrosulfide, and lithium sulfide particle powder is obtained from the lithium hydrosulfide.

[0006] Furthermore, Patent Document 2 discloses a method for obtaining lithium sulfide by repeating a reaction cycle in which metallic lithium is reacted with sulfur gas or hydrogen sulfide to produce lithium sulfide on the surface of metallic lithium, unreacted metallic lithium is then melted, diffused into and permeated into the lithium sulfide that has already been produced, and the unreacted metallic lithium is then reacted again with sulfur gas or hydrogen sulfide.

[0007] Patent Document 3 proposes a method for producing lithium sulfide by reacting lithium carbonate with hydrogen sulfide. Patent Document 4 discloses a method for producing lithium sulfide by mixing lithium sulfate with a carbon material and heating the mixture. Patent Document 5 discloses a method for obtaining lithium sulfide by thermally reducing lithium sulfate with a carbon material in a vacuum atmosphere.

[0008] Japanese Unexamined Patent Publication No. 2006-151725 (A) Japanese Unexamined Patent Publication No. 09-110404 (A) Japanese Unexamined Patent Application No. 2012-221819 (A) Japanese Unexamined Patent Application No. 2013-227180 (A) Japanese Unexamined Patent Application No. 2021-147251 (A)

[0009] However, the method disclosed in Patent Document 1 requires the use of an aprotic organic solvent and the organic solvent used must be treated separately, which results in problems such as a complicated production process and high production costs. In addition, there is a risk that part of the aprotic organic solvent may remain in the produced lithium sulfide.

[0010] Furthermore, the inventions disclosed in Patent Documents 2 and 3 require the use of toxic hydrogen sulfide gas, which has the problem of high equipment costs due to the need to maintain the airtightness of the reaction apparatus, treat unreacted hydrogen sulfide gas, etc. Furthermore, if the reaction is insufficient, there is a risk that unreacted substances will remain in the produced lithium sulfide.

[0011] On the other hand, in Patent Documents 4 and 5, lithium sulfide is produced by reducing lithium sulfate with a carbon material, so there is no need to use an organic solvent or hydrogen sulfide, and handling and management are relatively easy.

[0012] Recently, there has been a demand for sulfide-based solid electrolytes that are used in high-power batteries to have even better ionic conductivity. Therefore, there is a demand for further improvement in the purity of lithium sulfide, which is a raw material for sulfide-based solid electrolytes. Patent Document 4 proposes that both lithium sulfate and a carbon material are made into fine particles to increase the reaction area and thereby reduce the generation of unreacted raw materials.

[0013] However, the production method described in Patent Document 4 requires that both lithium sulfate and a carbon material be converted into fine particles, which complicates the process, increases costs, and reduces production efficiency. Furthermore, when production is performed using equipment such as a rotary kiln, the fine particle powder tends to float inside the equipment, making it difficult to discharge it from the outlet side, which could prevent stable production of lithium sulfide.

[0014] Furthermore, in Patent Document 5, lithium sulfide is produced by reducing lithium sulfate monohydrate with carbon such as activated carbon in an argon or vacuum atmosphere, but there is a risk that by-products such as lithium carbonate and lithium oxide and unreacted raw materials may be mixed into the produced lithium sulfide.

[0015] 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 suppress the incorporation of impurities and enable stable production of high-purity lithium sulfide, and a method for producing a sulfide-based solid electrolyte using lithium sulfide produced by this method for producing lithium sulfide.

[0016] In order to solve the above problems, a method for producing lithium sulfide according to a first aspect of the present invention includes a raw material mixing step of mixing lithium sulfate and a carbon material to form a mixed raw material, and a thermal reduction step of heat-treating the mixed raw material in a non-oxidizing atmosphere to thermally reduce the lithium sulfate to produce lithium sulfide, wherein the carbon material is a carbon material having a specific surface area of ​​55 m as measured by a BET method. ２ The present invention is characterized by using carbon powder with a carbon content of 1 / g or more.

[0017] According to the method for producing lithium sulfide of Aspect 1 of the present invention, the carbon material has a specific surface area of ​​55 m ２ / g or more of carbon powder is used, and therefore in a thermal reduction step in which a mixed raw material obtained by mixing this carbon powder as a carbon material with lithium sulfate is heat-treated under a non-oxidizing atmosphere, the reduction reaction of lithium sulfate by the carbon material (carbon powder) is promoted, and the generation of unreacted substances (lithium sulfate, carbon material) and by-products (lithium carbonate and lithium oxide) can be suppressed. Therefore, it becomes possible to stably and efficiently produce high-quality lithium sulfide with few impurities.

[0018] A method for producing lithium sulfide according to a second aspect of the present invention is characterized in that, in the method for producing lithium sulfide according to the first aspect of the present invention, the thermal reduction step comprises carrying out a heat treatment under conditions in which the atmosphere has a dew point of −60° C. or less and the heating temperature is 700° C. or more and 900° C. or less.

[0019] According to the method for producing lithium sulfide of Aspect 2 of the present invention, the atmosphere in the thermal reduction step has a dew point of −60°C or lower, which further suppresses the generation of by-products such as lithium oxide and lithium carbonate. Furthermore, the heating temperature in the thermal reduction step is set within a range of 700°C or higher and 900°C or lower, which further promotes the reduction reaction and suppresses sintering and adhesion of the mixed raw materials. Therefore, it becomes possible to produce high-quality lithium sulfide with fewer impurities more stably and efficiently.

[0020] The method for producing a sulfide-based solid electrolyte according to Aspect 3 of the present invention is characterized in that lithium sulfide produced by the method for producing lithium sulfide according to Aspect 1 or Aspect 2 of the present invention is used as a raw material.

[0021] According to the method for producing a sulfide-based solid electrolyte of Aspect 3 of the present invention, high-quality lithium sulfide with few impurities produced by the method for producing lithium sulfide according to Aspect 1 or Aspect 2 of the present invention is used as a raw material, and therefore, a sulfide-based solid electrolyte with excellent ionic conductivity can be produced.

[0022] According to the present invention, it is possible to provide a method for producing lithium sulfide that can suppress the incorporation of impurities and enable stable production of high-purity lithium sulfide, and a method for producing a sulfide-based solid electrolyte that uses lithium sulfide produced by this method for producing lithium sulfide.

[0023] FIG. 1 is a flow chart showing an example of a method for producing lithium sulfide according to an embodiment of the present invention.

[0024] 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.

[0025] The method for producing lithium sulfide according to the present embodiment is, for example, to produce lithium sulfide (Li ２ The sulfide-based solid electrolyte material has high ionic conductivity, is non-flammable, and is highly safe, and therefore is suitable as a material for on-board batteries of electric vehicles (EVs) and hybrid electric vehicles (HEVs).

[0026] In the method for producing lithium sulfide according to the present embodiment, lithium sulfide is produced by reducing lithium sulfate with a carbon material, as will be described later. ２ SO ４ +2C → Li ２ S + 2CO ２ At this time, impurities mixed into lithium sulfide include unreacted carbon (C) and lithium sulfate (Li ２ SO ４ ), by-product lithium carbonate (Li ２ CO３ ), lithium oxide (Li ２ In order to obtain high-purity, high-quality lithium sulfide, it is necessary to suppress the inclusion of these impurities.

[0027] Here, the method for producing lithium sulfide according to this embodiment will be described with reference to the flow diagram of Fig. 1. As shown in Fig. 1, the method for producing lithium sulfide according to this embodiment includes a raw material mixing step S01, a thermal reduction step S02, and a cooling step S03.

[0028] (Raw material mixing step S01) First, lithium sulfate as a lithium source and a carbon material acting as a reducing agent are blended and mixed so that the blending ratio (molar ratio) is within a predetermined range to form a mixed raw material. Then, in the method for producing lithium sulfide according to the present embodiment, a carbon material having a specific surface area measured by the BET method of 55 m is used as the carbon material. ２ / g or more of carbon powder is used.

[0029] A carbon material that acts as a reducing agent and has a specific surface area of ​​55 m ２ By using carbon powder of 1 / g or more, the reduction reaction of lithium sulfate is promoted, and the unreacted carbon (C) and lithium sulfate (Li ２ SO ４ ) and the by-product lithium carbonate (Li ２ CO ３ ), lithium oxide (Li ２ O) can be suppressed, thereby suppressing the incorporation of these substances and enabling the production of high-purity lithium sulfide.

[0030] Here, the specific surface area of ​​the carbon powder used as the carbon material is 100 m ２ / g or more, and ２ It is more preferable that the specific surface area of ​​the carbon powder used as the carbon material is 3000 m / g or more. ２ / g or less, and ２ It is more preferable that the SiO2 content is 1 / g or less.

[0031] In this embodiment, the average particle size (d50) of the carbon powder used as the carbon material is preferably in the range of 10 nm or more and 100 μm or less. When the average particle size (d50) of the carbon powder used as the carbon material is 10 nm or more, the carbon powder is easy to handle and the work efficiency is further improved. On the other hand, when the average particle size (d50) of the carbon powder is 100 μm or less, it is easy to mix uniformly with lithium sulfate, and the reduction reaction can be further promoted. The average particle size (d50) of the carbon powder used as the carbon material is more preferably 100 nm or more, and even more preferably 1 μm or more. On the other hand, the average particle size (d50) of the carbon powder used as the carbon material is more preferably 50 μm or less, and even more preferably 10 μm or less.

[0032] Lithium sulfate may be anhydrous without water of crystallization, or may be monohydrate. In the case of lithium sulfate monohydrate, the volume change during heating causes fine cracks to form on the surface of lithium sulfate due to the removal of water of crystallization, increasing the surface area and enhancing reactivity. The average particle size (d50) of lithium sulfate is not particularly limited, but is preferably in the range of 1 μm to 100 μm.

[0033] The mixing ratio (molar ratio) of lithium sulfate and carbon material is C / Li ２ SO ４ is preferably in the range of 2 or more and 4 or less. There is no particular limitation on the method for mixing lithium sulfate and the carbon material, and various existing methods can be appropriately selected and used.

[0034] (Thermal Reduction Step S02) Next, the mixed raw material described above is charged into a heat treatment furnace and heat-treated in a non-oxidizing atmosphere (a vacuum atmosphere or an inert gas atmosphere), and lithium sulfate is thermally reduced with a carbon material (carbon powder) to produce lithium sulfide. Here, in this embodiment, it is preferable to use an atmosphere with a dew point of −60° C. or lower. By setting the dew point to −60° C. or lower, it is possible to suppress the oxidation reaction of lithium sulfate and the carbon material, and further suppress the production of by-products. It is more preferable that the dew point of the atmosphere is −70° C. or lower, and even more preferably −80° C. or lower.

[0035] In this embodiment, the heating temperature in the thermal reduction step S02 is preferably set within a range of 700°C or higher and 950°C or lower. By setting the heating temperature in the thermal reduction step S02 to 700°C or higher, the reduction reaction of lithium sulfate can be further promoted. On the other hand, by setting the heating temperature in the thermal reduction step S02 to 950°C or lower, it is possible to prevent the mixed raw material from being sintered and adhering to the inside of the furnace, and the reduction reaction of lithium sulfate can be stably progressed. The heating temperature in the thermal reduction step S02 is more preferably 750°C or higher, and even more preferably 780°C or higher. The heating temperature in the thermal reduction step S02 is more preferably 900°C or lower, and even more preferably 880°C or lower.

[0036] Furthermore, in this embodiment, the holding time at the heating temperature is preferably within a range of 1 minute to 600 minutes, and the rate of temperature rise to the heating temperature is preferably within a range of 1° C. / min to 100° C. / min.

[0037] (Cooling step S03) Next, the mixture is naturally cooled to room temperature in the heat treatment furnace, and the resulting lithium sulfide is recovered. The cooling rate to room temperature is preferably in the range of 1° C. / min to 100° C. / min.

[0038] Through the above steps, unreacted carbon (C) and lithium sulfate (Li ２ SO ４ ), by-product lithium carbonate (Li ２ CO ３), lithium oxide (Li ２ O) is suppressed, and high-purity lithium sulfide is produced.

[0039] In the method for producing a sulfide-based 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 in a sulfide-based solid electrolyte produced using this as a raw material is also reduced, making it possible to produce a sulfide-based solid electrolyte with excellent properties such as ionic conductivity.

[0040] According to the method for producing lithium sulfide of this embodiment configured as described above, a carbon material having a specific surface area of ​​55 m as measured by the BET method is used. ２ Since carbon powder of 1 / g or more is used, in the thermal reduction step S02 in which the mixed raw material in which this carbon powder is mixed with lithium sulfate as a carbon material is heat-treated under a non-oxidizing atmosphere, the reduction reaction of lithium sulfate by the carbon material (carbon powder) is promoted, and the unreacted carbon (C), lithium sulfate (Li ２ SO ４ ) and the by-product lithium carbonate (Li ２ CO ３ ), lithium oxide (Li ２ Therefore, it is possible to stably and efficiently produce high-quality lithium sulfide with few impurities.

[0041] In the method for producing lithium sulfide according to the present embodiment, when the atmosphere in the thermal reduction step S02 is set to a dew point of −60° C. or lower, the by-product lithium carbonate (Li ２ CO ３ ), lithium oxide (Li ２Furthermore, when the heating temperature in the thermal reduction step S02 is set within a range of 700°C or higher and 900°C or lower, the reduction reaction of lithium sulfate can be further promoted and sintering and adhesion of the mixed raw material can be suppressed. Therefore, it is possible to more stably and efficiently produce high-quality lithium sulfide with fewer impurities.

[0042] According to the method for producing a sulfide-based solid electrolyte of the present embodiment, high-quality lithium sulfide with few impurities produced by the method for producing lithium sulfide of the present embodiment is used as a raw material, so that a sulfide-based solid electrolyte with excellent ionic conductivity can be produced.

[0043] Although one embodiment of the present invention has been described above, the present invention is not limited thereto and can be appropriately modified within the scope of the technical concept of the invention. In this embodiment, lithium sulfate monohydrate is used as the lithium sulfate, but the present invention is not limited thereto and an anhydrous lithium sulfate having no water of crystallization may also be used.

[0044] A confirmation experiment conducted to confirm the effectiveness of the present invention will be described.

[0045] Lithium sulfate monohydrate powder (average particle size 20 μm) and the carbon material (carbon powder) shown in Table 1 were prepared and mixed in a molar ratio of C / Li ２ SO ４ The specific surface area of ​​the carbon material (carbon powder) shown in Table 1 was measured by the BET method.

[0046] The weighed lithium sulfate monohydrate powder and a carbon material were mixed, and the resulting mixed raw material was transferred to an alumina board. The mixed raw material was then heated in a tubular furnace in a vacuum atmosphere (5 Pa, dew point shown in Table 1) up to the heating temperature shown in Table 1, whereby lithium sulfate was thermally reduced to produce lithium sulfide. The mixture was then naturally cooled in the furnace, and the lithium sulfide was removed.

[0047] The lithium sulfide obtained as described above was pulverized in an agate mortar and then subjected to powder X-ray diffraction measurement to confirm the presence or absence of impurities. The evaluation results are shown in Table 1.

[0048]

[0049] In the comparative example, the average particle size of the carbon material (carbon powder) is 0.048 μm, but the specific surface area is 36.3 m ２ / g, and the unreacted lithium sulfate (Li ２ SO ４ ), by-product lithium oxide (Li ２ In contrast, in Examples 1 to 9 of the present invention, the specific surface area of ​​the carbon material (carbon powder) was 55.0 m ２ / g or more, and no unreacted materials or by-products were found to be present in the produced lithium sulfide.

[0050] As described above, it has been confirmed that the present invention can provide a method for producing lithium sulfide that can suppress the incorporation of impurities and stably produce high-purity lithium sulfide.

[0051] It is possible to provide a method for producing lithium sulfide that can suppress the incorporation of impurities and enable stable production of high-purity lithium sulfide, and a method for producing a sulfide-based solid electrolyte that uses lithium sulfide produced by this method for producing lithium sulfide.

Claims

1. A method for producing a lithium sulfide-containing carbonaceous material, comprising: a raw material mixing step of mixing lithium sulfate with a carbonaceous material to form a mixed raw material; and a thermal reduction step of heat-treating the mixed raw material in a non-oxidizing atmosphere to thermally reduce the lithium sulfate to produce lithium sulfide, wherein the carbonaceous material has a specific surface area of ​​55 m as measured by the BET method. ２ / g or more of carbon powder.

2. The method for producing lithium sulfide according to claim 1, wherein the thermal reduction step is performed in an atmosphere having a dew point of -60°C or lower and a heating temperature of 700°C or higher but 900°C or lower.

3. A method for producing a sulfide-based solid electrolyte, characterized in that lithium sulfide produced by the method for producing lithium sulfide according to claim 1 or 2 is used as a raw material.

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