Lithium sulfide and method for producing sulfide-based solid electrolyte

Abstract

To provide lithium sulfide capable of synthesizing a sulfide-based solid electrolyte excellent in ion conductivity, and to provide a method for producing a sulfide-based solid electrolyte using the lithium sulfide.SOLUTION: The lithium sulfide has a carbon content of ≤ 2.0 mass% and an oxygen content of ≤ 5.0 mass%. The impurities are preferably one or more selected from carbon, lithium sulfate, lithium carbonate, and lithium oxide. The method for producing a sulfide-based solid electrolyte of the present invention is characterized by using the above-mentioned lithium sulfide as a raw material.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to lithium sulfide suitable as a constituent material of sulfide-based solid electrolyte materials used in, for example, all-solid-state batteries, and to a method for producing a sulfide-based solid electrolyte using this lithium sulfide. [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. In Patent Document 1, the sulfur oxide content, SiO2 content, Al content, and Ca content of the produced lithium sulfide particles are specified.

[0006] Patent Document 2 discloses a method for obtaining lithium sulfide particles by reacting lithium hydroxide with hydrogen sulfide. In Patent Document 2, the lithium sulfide particles produced have layered cracks on the surface and an average particle diameter d 50 The thickness is set to 0.1 mm or more and 1.5 mm or less, and the oxygen concentration in the range from the surface to a depth of 5 nm is set to 20.0 atom % or less.

[0007] Patent Document 3 discloses a method for obtaining lithium sulfide powder by reacting lithium hydroxide with hydrogen sulfide. In Patent Document 3, the produced lithium sulfide powder has a solvent content of 0.1% by mass or less, a lithium hydroxide content of 0.3% by mass or less, and a lithium sulfide content of 98.0% by mass or more.

[0008] Patent Document 4 also discloses a method for producing lithium sulfide by mixing lithium sulfate and a carbon material and heating the mixture. This method also discloses that both the lithium sulfate and the carbon material are converted into fine particles to increase the reaction area and thereby reduce the amount of unreacted raw materials.

[0009] Furthermore, 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]

[0010] [Patent Document 1] Japanese Patent Application Laid-Open No. 2006-151725 [Patent Document 2] Japanese Patent Application Publication No. 2019-147731 [Patent Document 3] Japanese Patent Application Publication No. 2023-116632 [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]

[0011] 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.

[0012] 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.

[0013] In contrast to this, in Patent Documents 4 and 5, lithium sulfide is produced by reducing lithium sulfate using a carbon material, so there is no need to use an organic solvent or hydrogen sulfide, and handling and management are relatively easy.

[0014] 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. Here, in lithium sulfide produced by reducing lithium sulfate using a carbon material as in Patent Documents 4 and 5, it is required that specific impurities be restricted in order to improve the ionic conductivity of the sulfide-based solid electrolyte.

[0015] The present invention has been made in view of the above-mentioned circumstances, and an object of the present invention is to provide lithium sulfide that can be used to synthesize a sulfide-based solid electrolyte having excellent ion conductivity, and a method for producing a sulfide-based solid electrolyte using this lithium sulfide. [Means for solving the problem]

[0016] In order to solve the above problems, lithium sulfide according to a first aspect of the present invention is characterized in that the carbon content is 2.0% by mass or less and the oxygen content is 5.0% by mass or less.

[0017] According to the lithium sulfide of the first aspect of the present invention, the carbon content is 2.0 mass % or less and the oxygen content is 5.0 mass % or less, so that it is possible to sufficiently reduce the amount of impurities contained in the sulfide-based solid electrolyte produced using this lithium sulfide as a raw material, and therefore it is possible to improve the ionic conductivity of the sulfide-based solid electrolyte produced using this lithium sulfide as a raw material.

[0018] The lithium sulfide of Aspect 2 of the present invention is characterized in that, in the lithium sulfide of Aspect 1 of the present invention, the impurities contained therein are one or more kinds selected from carbon, lithium sulfate, lithium carbonate, and lithium oxide.

[0019] According to the lithium sulfide of the second aspect of the present invention, the impurities contained therein are one or more selected from carbon, lithium sulfate, lithium carbonate, and lithium oxide, and the contents of these impurities are limited so that the carbon content is 2.0% by mass or less and the oxygen content is 5.0% by mass or less. This makes it possible to further sufficiently reduce the amount of impurities contained in a sulfide-based solid electrolyte produced using this lithium sulfide as a raw material.

[0020] The method for producing a sulfide-based solid electrolyte according to the third aspect of the present invention is characterized by using the lithium sulfide according to the first or second aspect of the present invention.

[0021] According to the method for producing a sulfide-based solid electrolyte of Aspect 3 of the present invention, the lithium sulfide of Aspect 1 or Aspect 2 of the present invention is used, and therefore the amount of impurities is kept low, making it possible to produce a sulfide-based solid electrolyte that is high in purity and has excellent ionic conductivity. [Effects of the Invention]

[0022] According to the present invention, it is possible to provide lithium sulfide that can be used to synthesize a sulfide-based solid electrolyte having excellent ion conductivity, and a method for producing a sulfide-based solid electrolyte using this 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 lithium sulfide of this embodiment is used, for example, as a raw material for a sulfide-based solid electrolyte that constitutes a lithium ion battery. The lithium sulfide according to this embodiment has a carbon content of 2.0% by mass or less and an oxygen content of 5.0% by mass or less. Furthermore, in the lithium sulfide according to this embodiment, the impurities contained therein are preferably one or more selected from carbon, lithium sulfate, lithium carbonate, and lithium oxide.

[0026] Here, the lithium sulfide of this embodiment is produced by reducing lithium sulfate with a carbon material, as will be described later. 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 control the amount of these impurities mixed in, the carbon content and oxygen content are specified in this embodiment.

[0027] That is, in the lithium sulfide according to this embodiment, by limiting the carbon content to 2.0 mass% or less, the inclusion of unreacted carbon (C) and by-product lithium carbonate (Li2CO3) is suppressed. Furthermore, in the lithium sulfide according to this embodiment, by limiting the oxygen content to 5.0 mass% or less, contamination of the unreacted lithium sulfate (LiSO) and by-products lithium carbonate (LiCO) and lithium oxide (LiO) is suppressed.

[0028] Here, in the lithium sulfide of this embodiment, the carbon content is more preferably 1.0 mass% or less, and even more preferably 0.5 mass% or less. There is no particular lower limit for the carbon content, and it is most preferably 0 mass%. Furthermore, in the lithium sulfide of this embodiment, the oxygen content is more preferably 2.0% by mass or less, and even more preferably 0.8% by mass or less. There is no particular lower limit for the oxygen content, and it is most preferably 0% by mass.

[0029] Next, an example of a method for producing lithium sulfide according to this embodiment will be described with reference to FIG.

[0030] (Raw material preparation process S01) First, lithium sulfate and a carbon material are prepared as raw materials. Lithium sulfate may be anhydrous, free of 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 enhancing reactivity. In this embodiment, it is preferable to use lithium sulfate monohydrate whose weight loss during heating up to 120°C is in the range of 5% to 25%. The carbon material that serves as the reducing agent may be, for example, activated carbon, carbon black, etc. In this embodiment, it is preferable to use activated carbon as the carbon material.

[0031] The mixed powder of lithium sulfate and carbon material was simply mixed and stirred without granulation. Here, 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 average particle size (d50) of lithium sulfate is preferably in the range of 10 μm to 100 μm, and the average particle size (d50) of the carbon material is preferably in the range of 1 μm to 10 μm.

[0032] (Drying process S02) The prepared lithium sulfate and carbon material were placed in a vacuum furnace, and the internal pressure of the vacuum furnace was increased to 1×10 2 The vacuum is drawn until the internal pressure of the vacuum furnace is 1×10 Pa or less. After that, an inert gas is introduced to return the pressure to normal pressure, and the vacuum is drawn again to reduce the internal pressure of the vacuum furnace to 1×10 2 Pa or less. Repeat this process. Then, the mixture is heated and dried under the conditions of a heating temperature of 100° C. or higher and 250° C. or lower, and a holding time at the heating temperature of 10 hours or higher and 30 hours or lower.

[0033] (Synthesis step S03) After dehydration, the material is heated in a vacuum furnace and lithium sulfate is reduced with activated carbon to produce lithium sulfide. Here, the heating temperature in the synthesis step S03 is preferably within a range of 700° C. to 950° C. The holding time at the heating temperature is preferably within a range of 500 minutes to 3000 minutes. Furthermore, the average rate of temperature rise from room temperature (25° C.) to the heating temperature is preferably within the range of 2° C. / min to 10° C. / min.

[0034] (Cooling process S04) Next, the mixture is naturally cooled to room temperature in the vacuum furnace, and the lithium sulfide produced is recovered under an inert gas atmosphere. Here, the average cooling rate from the heating temperature to room temperature (25° C.) is preferably in the range of 1° C. / min to 20° C. / min.

[0035] Through the steps described above, lithium sulfide according to this embodiment is produced. The lithium sulfide of this embodiment is a carbon-reduced lithium sulfide obtained by reducing lithium sulfate with carbon, and the amount of impurities is sufficiently reduced. Furthermore, since evacuation is performed multiple times in the drying step S02, it is possible to sufficiently reduce the carbon content and oxygen content.

[0036] In the method for producing a sulfide-based solid electrolyte according to this embodiment, lithium sulfide according to this embodiment is used as a raw material. The lithium sulfide of this embodiment has a carbon content of 2.0 mass % or less and an oxygen content of 5.0 mass % or less, and the amount of impurities is sufficiently reduced. Therefore, the amount of impurities is also reduced in a sulfide-based solid electrolyte produced using this as a raw material, and a sulfide-based solid electrolyte with excellent properties is produced.

[0037] The lithium sulfide of this embodiment configured as described above has a carbon content of 2.0 mass % or less and an oxygen content of 5.0 mass % or less, which makes it possible to sufficiently reduce the amount of impurities contained in a sulfide-based solid electrolyte produced using this lithium sulfide as a raw material, thereby improving the ionic conductivity of the sulfide-based solid electrolyte produced using this lithium sulfide as a raw material.

[0038] In the lithium sulfide of this embodiment, when the impurities contained therein are one or more selected from carbon, lithium sulfate, lithium carbonate, and lithium oxide, the contents of these impurities contained when lithium sulfide is produced by reducing lithium sulfate with a carbon material are limited so that the carbon content is 2.0% by mass or less and the oxygen content is 5.0% by mass or less, and it becomes possible to further sufficiently reduce the amount of impurities contained in a sulfide-based solid electrolyte produced using this lithium sulfide as a raw material.

[0039] According to the method for producing a sulfide-based solid electrolyte of the present embodiment, the amount of impurities is kept low because lithium sulfide of the present embodiment is used, and therefore a sulfide-based solid electrolyte with high purity and excellent ionic conductivity can be produced.

[0040] 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]

[0041] A confirmation experiment conducted to confirm the effectiveness of the present invention will be described.

[0042] Lithium sulfate monohydrate powder (average particle size 20 μm) and activated carbon powder (average particle size 8 μm) were prepared and weighed out so that the mixing ratio (molar ratio) C / Li2SO4 was 2 or more and 4 or less. Weighed amounts of lithium sulfate monohydrate powder and activated carbon were placed in a sagger and an alumina lid was placed on top. In the drying process, the saggers are placed in a vacuum furnace, and the internal pressure of the vacuum furnace is 1×10 2 The vacuum was drawn until the internal pressure of the vacuum furnace reached 1×10 Pa or less. After that, an inert gas was introduced to return the pressure to normal pressure, and the vacuum was drawn again to reduce the internal pressure of the vacuum furnace to 1×10 2This procedure was repeated. The number of repetitions is shown in Table 1. The internal pressure of the vacuum furnace was set to 1×10 2 The sample was dried by heating under the conditions shown in Table 1 under a pressure of 0.1 Pa or less. Then, in the synthesis step, a heat treatment was carried out under the conditions shown in Table 1, followed by natural cooling to obtain lithium sulfide. From the results of XRD analysis of the obtained lithium sulfide and the raw materials used, it is believed that it contains one or more impurities selected from carbon, lithium sulfate, lithium carbonate, and lithium oxide.

[0043] The carbon content of the lithium sulfide obtained as described above was measured by a combustion infrared spectrometry. The oxygen content of the lithium sulfide was also measured by a fusion infrared spectrometry. The measurement results obtained when the sample was completely combusted and melted under appropriate measurement conditions are shown in Table 1.

[0044] Next, the obtained lithium sulfide was used to 10.35 Sn 0.27 Si 1.08 P 1.65 S 12 A sulfide-based solid electrolyte consisting of (LSSPS) was fabricated. In a glove box, lithium sulfide (LiS), tin (Sn), silicon (Si), phosphorus (P), and sulfur (S) were weighed in a non-stoichiometric ratio and mixed in a mortar. The mixture was then fired in an alumina crucible at the temperature shown in Table 1 for 6 hours to obtain a sulfide-based solid electrolyte.

[0045] The obtained sulfide-based solid electrolyte was pulverized and 0.2 g was loaded into a SUS conductivity measurement cell, and then AC impedance measurements were performed in the range of 1 Hz to 7 MHz using an SP-300 manufactured by Bio-Logic Science Instruments at room temperature (25°C) and under an applied pressure of 360 MPa. The evaluation results are shown in Table 1.

[0046] [Table 1]

[0047] In Comparative Example 1, the oxygen content of the lithium sulfide was set to 5.41 mass %, and the ionic conductivity of the sulfide-based solid electrolyte produced using this lithium sulfide was 1.45 S / cm. In Comparative Example 2, the carbon content of the lithium sulfide was set to 2.85 mass %, and the ionic conductivity of the sulfide-based solid electrolyte produced using this lithium sulfide was 1.68 S / cm. In Comparative Example 3, the carbon content of the lithium sulfide was 3.34 mass % and the oxygen content was 5.85 mass %, and the ionic conductivity of the sulfide-based solid electrolyte produced using this lithium sulfide was 1.02 S / cm. In all of the comparative examples, it is believed that the lithium sulfide obtained contains one or more impurities selected from carbon, lithium sulfate, lithium carbonate, and lithium oxide, based on the results of XRD analysis of the lithium sulfide obtained and the raw materials used.

[0048] In contrast, in Examples 1 to 6 of the present invention, the carbon content of the lithium sulfide was 2.0 mass % or less and the oxygen content was 5.0 mass % or less, and the ionic conductivity of the sulfide-based solid electrolyte produced using this lithium sulfide was 2.68 S / cm or more, which was higher than that of Comparative Examples 1 to 3.

[0049] As described above, it has been confirmed that the present invention can provide lithium sulfide that can be used to synthesize a sulfide-based solid electrolyte having excellent ion conductivity, and a method for producing a sulfide-based solid electrolyte using this lithium sulfide.

Claims

1. Lithium sulfide having a carbon content of 2.0% by mass or less and an oxygen content of 5.0% by mass or less.

2. 2. The lithium sulfide according to claim 1, wherein the impurities contained therein are one or more selected from the group consisting of carbon, lithium sulfate, lithium carbonate, and lithium oxide.

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

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