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

Abstract

To provide a method for producing lithium sulfide capable of suppressing mixing of impurities and stably producing high-purity lithium sulfide, and a method for producing a sulfide-based solid electrolyte using lithium sulfide produced by the method for producing lithium sulfide.SOLUTION: A method for producing lithium sulfide according to the present invention includes a material mixing step of mixing lithium sulfate and a carbonaceous material to form a mixed material, and a thermal reducing step of heat-treating the mixed material in a non-oxidizing atmosphere to thermally reduce the lithium sulfate, thereby producing lithium sulfide, wherein a carbonaceous powder having a specific surface area of 55m2 / g or more as measured by a BET method is used as the carbonaceous material. The method for producing a sulfide-based solid electrolyte of the present invention is characterized by using lithium sulfide produced by the method for producing lithium sulfide of the present invention as a raw material.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a method for producing lithium sulfide suitable as a constituent material of sulfide-based solid electrolyte materials used in, for example, all-solid-state batteries, and a method for producing sulfide-based solid electrolytes. [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] To improve the safety and durability of conventional lithium-ion batteries that use organic electrolytes, all-solid-state lithium-ion batteries using sulfide-based solid electrolytes have been proposed. Examples of sulfide-based solid electrolytes currently proposed include Li2S-P2S5, Li2S-P2S3, Li2S-SiS2, Li2S-Ga2S2, and Li2S-GeS2. In all of these sulfide-based solid electrolytes, lithium sulfide (Li2S) is used as a constituent material.

[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 this 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, and then unreacted metallic lithium is melted and diffused and penetrated into the lithium sulfide that has already been produced, and then the unreacted metallic lithium is 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 and 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. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] Japanese Patent Application Laid-Open No. 2006-151725 [Patent Document 2] Japanese Patent Application Publication No. 09-110404 [Patent Document 3] Japanese Patent Application Laid-Open No. 2012-221819 [Patent Document 4] Japanese Patent Application Laid-Open No. 2013-227180 [Patent Document 5] Patent Publication No. 2021-147251 Summary of the Invention [Problem to be solved by the invention]

[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 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 leads to problems such as 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, which does not require the use of an organic solvent or hydrogen sulfide, and is relatively easy to handle and manage.

[0012] Recently, sulfide-based solid electrolytes for high-power batteries are required to have even better ionic conductivity, and therefore, lithium sulfide, which is a raw material for sulfide-based solid electrolytes, is required to have even higher purity. In Patent Document 4, both the lithium sulfate and the carbon material are made into fine particles to increase the reaction area, thereby reducing the amount 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 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. [Means for solving the problem]

[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 carbon powder used is characterized by a carbon content of 1000 ppm or more.

[0017] According to the method for producing lithium sulfide of the first aspect of the present invention, the carbon material has a specific surface area of ​​55 m 2 / g or more of carbon powder is used. In the thermal reduction step, in which the carbon powder is used as a carbon material and mixed with lithium sulfate to form a mixed raw material, 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 lower and the heating temperature is 700° C. or higher and 900° C. or lower.

[0019] According to the method for producing lithium sulfide of the second aspect 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. In addition, the heating temperature in the thermal reduction step is set within the range of 700°C to 900°C, 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 the third aspect of the present invention is characterized in that lithium sulfide produced by the method for producing lithium sulfide according to the first or second aspect of the present invention is used as a raw material.

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

[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. [Brief explanation of the drawings]

[0023] [Figure 1] FIG. 1 is a flow chart showing an example of a method for producing lithium sulfide according to an embodiment of the present invention. DETAILED DESCRIPTION OF THE 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 this embodiment is for producing lithium sulfide (LiS), which is a constituent material of a sulfide-based solid electrolyte used as a solid electrolyte in an all-solid-state battery. Sulfide-based 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).

[0026] In the method for producing lithium sulfide according to the present embodiment, as will be described later, lithium sulfide is produced by reducing lithium sulfate with a carbon material. Li2SO4+2C → Li2S+2CO2 At this time, impurities that may be mixed into the lithium sulfide include unreacted carbon (C), lithium sulfate (Li2SO4), and by-products such as lithium carbonate (Li2CO3) and lithium oxide (Li2O). 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 chart of FIG. 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 process S01) First, lithium sulfate, which serves as a lithium source, and a carbon material, which acts as a reducing agent, are blended so that the blend ratio (molar ratio) falls within a predetermined range, and mixed to form a mixed raw material. In the method for producing lithium sulfide according to the present embodiment, a carbon material having a specific surface area of ​​55 m as measured by the BET method is used. 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 ​​55m 2 By using carbon powder with a concentration of 0.1g or more, the reduction reaction of lithium sulfate is promoted, and it is possible to suppress the remaining unreacted carbon (C) and lithium sulfate (Li2SO4) and the production of by-products such as lithium carbonate (Li2CO3) and lithium oxide (Li2O). This suppresses the incorporation of these substances and makes it possible to produce 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 1000m 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 2000m 2 It is more preferable that the saturation rate 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 within the range of 10 nm to 100 μm. When the average particle size (d50) of the carbon powder used as the carbon material is 10 nm or more, the carbon powder becomes 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, the carbon powder is easily mixed uniformly with lithium sulfate, and the reduction reaction can be further accelerated. 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, while 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, having no water of crystallization, or may be monohydrate. In the case of lithium sulfate monohydrate, a 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 making it possible to enhance reactivity. Although there is no particular limitation on the average particle size (d50) of lithium sulfate, it is preferable that it be within the range of 1 μm or more and 100 μm or less.

[0033] Furthermore, the mixing ratio (molar ratio) of lithium sulfate and the carbon material, C / Li2SO4, is preferably in the range of 2 or more and 4 or less. The method for mixing lithium sulfate and the carbon material is not particularly limited, and various existing methods can be appropriately selected and used.

[0034] (Thermal reduction step S02) Next, the mixed raw material is charged into a heat treatment furnace and heat-treated in a non-oxidizing atmosphere (vacuum atmosphere or inert gas atmosphere), and lithium sulfate is thermally reduced with a carbon material (carbon powder) to produce lithium sulfide. In this embodiment, the atmosphere preferably has 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 generation of by-products. The dew point of the atmosphere is more preferably −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 accelerated. On the other hand, by setting the heating temperature in the thermal reduction step S02 to 950°C or lower, the mixed raw material can be prevented from sintering 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 the range of 1 minute to 600 minutes. The rate of temperature rise up to the heating temperature is preferably within the range of 1° C. / min to 100° C. / min.

[0037] (Cooling process S03) Next, the mixture is naturally cooled to room temperature in the heat treatment furnace, and the produced 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-described process, the incorporation of unreacted carbon (C), lithium sulfate (LiSO), and by-products such as lithium carbonate (LiCO) and lithium oxide (LiO) is suppressed, resulting in the production of high-purity lithium sulfide.

[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. The lithium sulfide produced by the method for producing lithium sulfide according to the present embodiment has high purity as described above, and therefore 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 / g or more of carbon powder is used. In the thermal reduction step S02, in which the carbon powder is mixed with lithium sulfate as a carbon material and the resulting mixed raw material is heat-treated under a non-oxidizing atmosphere, the reduction reaction of lithium sulfate by the carbon material (carbon powder) is promoted, and it is possible to suppress the remaining unreacted carbon (C) and lithium sulfate (Li2SO4) and the generation of by-products such as lithium carbonate (Li2CO3) and lithium oxide (Li2O). Therefore, it becomes 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 has a dew point of −60° C. or lower, the generation of by-products such as lithium carbonate (LiCO) and lithium oxide (LiO) can be further suppressed. Furthermore, 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, thereby enabling more stable and efficient production of 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 to this and can be modified as appropriate within the scope of the technical idea of ​​the invention. In this embodiment, lithium sulfate monohydrate is used as lithium sulfate, but the present invention is not limited to this, and anhydrous lithium sulfate containing no water of crystallization may also be used. [Example]

[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 weighed so that the mixing ratio (molar ratio) C / Li2SO4 was 2.07. The specific surface area of ​​the carbon material (carbon powder) shown in Table 1 was measured by the BET method.

[0046] The weighed powder of lithium sulfate monohydrate and the carbon material were mixed, and the resulting mixed material was transferred to an alumina board. The mixture 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, and lithium sulfate was thermally reduced to produce lithium sulfide. Thereafter, the mixture was naturally cooled in the furnace, and the lithium sulfide was taken out.

[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] [Table 1]

[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, it was confirmed that the lithium sulfide produced contained unreacted lithium sulfate (Li2SO4) and by-product lithium oxide (Li2O). 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.

Claims

1. 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, The carbon material has a specific surface area of ​​55 m as measured by the BET method. ２ / g or more of carbon powder.

2. 2. The method for producing lithium sulfide according to claim 1, wherein the thermal reduction step performs heat treatment under conditions of an atmosphere having a dew point of −60° C. or lower and a heating temperature of 700° C. or higher and 900° C. or lower.

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

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