Lithium sulfide manufacturing method

Abstract

To provide a method for producing lithium sulfide by which high purity lithium sulfide can be obtained by effectively removing carbonate-based impurities in the production of lithium sulfide.SOLUTION: A method for producing lithium sulfide comprises: a step (step 1) of pretreating a lithium hydroxide raw material under an inert gas atmosphere; a step (step 2) of performing a dry reaction between the pretreated lithium hydroxide raw material and hydrogen sulfide by primary injection of hydrogen sulfide gas; a step (step 3) of raising the temperature of a reaction product under an inert gas atmosphere after completion of the reaction in the step (2); and a step (step 4) of performing a heat treatment by secondary injection of hydrogen sulfide gas.SELECTED DRAWING: None

Description

[Technical Field]

[0001] [CROSS-REFERENCE TO RELATED APPLICATIONS] This application claims the benefit of priority based on Korean Patent Application No. 10-2023-0049611, filed April 14, 2023, and all contents disclosed in the documents of said Korean patent application are incorporated herein by reference. The present invention relates to a method for producing lithium sulfide, and more particularly to a method for producing high-purity lithium sulfide by using hydrogen sulfide in a treatment step after the production of lithium sulfide. [Background technology]

[0002] Recently, with the development of the secondary battery industry, the use of solid electrolyte materials has become more feasible. Among them, sulfide-based solid electrolytes, synthesized using lithium sulfide (Li2S), lithium chloride (LiCl), and phosphorus pentasulfide (P2S5) as raw materials, are considered to have higher ionic conductivity and stability over a wide voltage range compared to oxide-based solid electrolytes.

[0003] Lithium sulfide, the main raw material for sulfide-based solid electrolytes, is generally produced by reacting a solid lithium precursor with a gaseous sulfur precursor. Previous research into the production of lithium sulfide has focused on the synthesis of lithium sulfide, which involves selecting a lithium sulfide precursor. Since then, in addition to lithium sulfide synthesis, research into improving the performance of the produced lithium sulfide has been attracting attention. The improvement of lithium sulfide performance has primarily focused on techniques for controlling the produced lithium sulfide particles. For example, a technique for controlling the particle size or specific surface area of ​​lithium sulfide (JP 2019-156691A) has been proposed.

[0004] Meanwhile, impurities contained in lithium sulfide may adversely affect the performance of solid electrolytes. Therefore, in addition to technologies for controlling lithium sulfide particles themselves, some research has been conducted to reduce impurities in lithium sulfide. Methods for producing lithium sulfide include a wet process using a solvent and a dry process not using a solvent. In the wet process, the impurity content can be reduced only by effectively removing the solvent after producing lithium sulfide. However, there are limitations to how effectively the solvent can be removed, and additional costs are incurred for solvent removal.

[0005] In addition, lithium hydroxide (LiOH), one of the lithium precursors used in the production of lithium sulfide, generally contains some carbonate impurities. In order to convert the carbonate impurities present in lithium hydroxide into lithium sulfide, a method has been proposed in which lithium hydroxide is heated at temperatures above 1,000°C to convert it into lithium oxide (Li2O), and then lithium sulfide is synthesized. However, this requires extremely high temperatures, making it difficult to apply to commercial processes.

[0006] Research into a technology for solving the above problems in the method for producing lithium sulfide and producing high-purity lithium sulfide is currently required. Summary of the Invention [Problem to be solved by the invention]

[0007] The present invention relates to a method for producing lithium sulfide, which can effectively remove impurities during the production of lithium sulfide to obtain high-purity lithium sulfide. [Means for solving the problem]

[0008] The present invention provides a method for producing lithium sulfide, comprising: a step (Step 1) of pretreating a lithium hydroxide raw material in an inert gas atmosphere; a step (Step 2) of first introducing hydrogen sulfide gas and dry-reacting the pretreated lithium hydroxide raw material with hydrogen sulfide; a step (Step 3) of heating the reaction product in an inert gas atmosphere after completion of the reaction in Step 2; and a step (Step 4) of secondly introducing hydrogen sulfide gas and heat-treating the reaction product. The present invention also provides lithium sulfide produced by the production method of the present invention. [Effects of the Invention]

[0009] The method for producing lithium sulfide according to the present invention can produce high-purity lithium sulfide by minimizing impurities in the lithium sulfide. More specifically, it can produce lithium sulfide with excellent performance as a raw material for a solid electrolyte by minimizing carbonate-based impurities in the produced lithium sulfide. [Brief explanation of the drawings]

[0010] [Figure 1] 1 is a graph showing the results of X-ray diffraction (XRD) analysis of lithium sulfide prepared according to an embodiment and commercial lithium sulfide. DETAILED DESCRIPTION OF THE INVENTION

[0011] In the present invention, terms such as "first" and "second" are used to describe various components, and the terms are used only to distinguish one component from another. Furthermore, the terms used in this specification are merely used to describe exemplary embodiments and are not intended to limit the present invention. The singular expression includes the plural expression unless the context clearly indicates otherwise. In this specification, the terms "comprises," "comprises," or "having" are intended to describe one or more features, numbers, steps, components, or combinations thereof that may be implemented, but do not exclude the possibility of one or more other features, numbers, steps, components, combinations, or additions thereof. Furthermore, in this specification, when a layer or element is referred to as being formed "on" or "on" another layer or element, it means that the layer or element is formed directly on the other layer or element, or that other layers or elements can be additionally formed between the layers, on the object, or on the substrate. The present invention can be modified in various ways and can have various forms, so that specific embodiments are exemplified and described in detail below, but it should be understood that this is not intended to limit the present invention to the specific disclosed embodiments, and that the present invention includes all modifications, equivalents, and alternatives falling within the spirit and technical scope of the present invention.

[0012] The present invention will be described in detail below.

[0013] The present invention provides a method for producing lithium sulfide, comprising: a step (Step 1) of pretreating a lithium hydroxide raw material in an inert gas atmosphere; a step (Step 2) of first introducing hydrogen sulfide gas and dry-reacting the pretreated lithium hydroxide raw material with hydrogen sulfide; a step (Step 3) of heating the reaction product in an inert gas atmosphere after completion of the reaction in Step 2; and a step (Step 4) of secondly introducing hydrogen sulfide gas and heat-treating the reaction product.

[0014] The "lithium hydroxide raw material" as used herein is not particularly limited as long as it is in the form of a lithium hydroxide salt, and may be, for example, lithium hydroxide monohydrate. Furthermore, the "lithium hydroxide raw material" as used herein may be in a form containing trace amounts of impurities. Furthermore, the particle size of the "lithium hydroxide raw material" is not particularly limited in the present invention.

[0015] Lithium hydroxide or lithium hydroxide monohydrate has a lower decomposition temperature than other lithium salts, allowing lithium sulfide to be synthesized under relatively mild conditions, making it suitable for use as a lithium precursor in the production of lithium sulfide. However, lithium hydroxide and lithium hydroxide monohydrate are hygroscopic and are rapidly converted into lithium carbonate (Li2CO3) by carbon dioxide (CO2) when exposed to the atmosphere. However, the converted lithium carbonate decomposes only at high temperatures, making it difficult to remove using conventional methods. It also remains as an impurity even after the production of lithium sulfide, potentially adversely affecting the performance of the solid electrolyte, i.e., reducing ionic conductivity.

[0016] Therefore, the inventors of the present invention confirmed that high-purity lithium sulfide can be produced by first introducing hydrogen sulfide gas, which is used as a sulfur precursor in the production of lithium sulfide, to react with a lithium hydroxide raw material, and then performing a second step of introducing hydrogen sulfide gas and heat treatment, thereby effectively reducing unreacted raw materials and lithium carbonate.Furthermore, the present invention does not use a solvent in the reaction of the lithium hydroxide raw material with hydrogen sulfide gas, thereby preventing the generation of additional impurities due to the use of a solvent.

[0017] Step 1 of the present invention is a step of pretreating a lithium hydroxide raw material under an inert gas atmosphere, and is a step of raising the temperature of the lithium hydroxide raw material through Step 1 to prepare for the reaction so that the reaction with hydrogen sulfide can be facilitated in the reaction step of Step 2. Furthermore, when lithium hydroxide monohydrate is used as the lithium hydroxide raw material, step 1 of the present invention can convert the lithium hydroxide monohydrate to anhydrous lithium hydroxide monohydrate, thereby enabling the reaction with hydrogen sulfide gas in the step described below.

[0018] The inert gas is not particularly limited, but examples thereof include nitrogen, argon, helium, and mixtures thereof.

[0019] Furthermore, Step 1 may be performed at a temperature between 170°C and 450°C. If Step 1 is performed at a temperature below 170°C, moisture in the lithium hydroxide raw material may not be easily removed, which may cause adverse effects such as a reverse reaction during lithium sulfide synthesis, and if Step 1 is performed at a temperature above 450°C, the lithium hydroxide raw material may melt. Preferably, Step 1 may be performed at a temperature of 175°C or higher, 180°C or higher, 185°C or higher, 190°C or higher, or 195°C or higher, and 225°C or lower, 220°C or lower, 215°C or lower, 210°C or lower, or 205°C or lower.

[0020] Step 2 of the present invention is a step of first introducing hydrogen sulfide gas and dry-reacting the pretreated lithium hydroxide raw material with hydrogen sulfide. As described above, the present invention relates to a production method in which a solid lithium hydroxide raw material is used as a lithium precursor and gaseous hydrogen sulfide is used as a sulfur precursor, and they are dry-reacted in the absence of a solvent.

[0021] Step 2 of the present invention may be carried out at a temperature that is the same as or different from the temperature of the pretreatment step of Step 1. More specifically, the reaction step of Step 2 may be carried out at the same temperature as the pretreatment step of the raw material, or may be carried out by increasing the temperature to 50°C, 60°C, 70°C, 80°C, 90°C, 100°C, 110°C, 120°C, 130°C, 140°C, or 150°C, or decreasing the temperature to 50°C, 60°C, 70°C, 80°C, 90°C, 100°C, 110°C, 120°C, 130°C, 140°C, or 150°C, based on the temperature of Step 1.

[0022] Preferably, in step 2, hydrogen sulfide gas may be initially introduced so that the molar ratio of lithium hydroxide raw material to hydrogen sulfide gas in step 1 is 1:0.5 to 1:50. If the molar ratio of hydrogen sulfide gas to lithium hydroxide raw material is less than 1:0.5, the reactivity of hydrogen sulfide and lithium hydroxide raw material may be reduced, resulting in a longer reaction time and reduced process efficiency. More preferably, hydrogen sulfide gas may be initially introduced so that the molar ratio of lithium hydroxide to hydrogen sulfide gas is 1:1 to 1:40, 1:1 to 1:35, or 1:1 to 1:30.

[0023] Additionally, step 2 of the present invention may be performed while removing water. By removing water generated during the reaction between the lithium hydroxide raw material and hydrogen sulfide gas from the reaction system, it is possible to prevent aggregation due to moisture absorption by lithium hydroxide and promote the reaction. Techniques widely known in the art can be used to remove water during the reaction step. For example, water can be removed by condensing the vapor water generated along with the hydrogen sulfide gas into a liquid using a condenser, and then reintroducing the gaseous hydrogen sulfide into the reactor. If even a small amount of water cannot be properly removed during this process, lithium sulfide may be converted back to lithium hydroxide through a reverse reaction, and lithium hydroxide may remain in the reaction product containing lithium sulfide.

[0024] Step 3 of the present invention is a step of heating the reaction product under an inert gas atmosphere after the completion of the reaction of Step 2. The completion of the reaction can be confirmed by observing whether or not water is generated in the reaction of the lithium hydroxide raw material with hydrogen sulfide gas. That is, Step 3 of the present invention is a step of heating the reaction product under an inert gas atmosphere after confirming that no additional water is generated, in preparation for a heat treatment step involving the secondary introduction of hydrogen sulfide gas.

[0025] Meanwhile, the inert gas in step 3 may be the same as or different from that in step 1, and the above description of the inert gas is to be referred to.

[0026] Preferably, Step 3 may be performed at 230°C to 400°C. If Step 3 is performed at a temperature below 230°C, lithium carbonate, an impurity, may not be effectively removed in Step 4, which will be described later. If Step 3 is performed at a temperature above 400°C, process efficiency may be reduced. More preferably, Step 3 may be performed at a temperature of 235°C or higher, 240°C or higher, 245°C or higher, or 250°C or higher, and 390°C or lower, 380°C or lower, 370°C or lower, 360°C or lower, or 350°C or lower. Furthermore, Step 3 of the present invention may be performed at a temperature higher than Step 1.

[0027] Step 4 of the present invention is a step of secondly introducing hydrogen sulfide gas and performing heat treatment. As described above, unreacted lithium hydroxide raw material and lithium carbonate may exist in the lithium sulfide produced through steps 1 to 3. Therefore, this is a process for converting the unreacted lithium hydroxide and lithium carbonate into lithium sulfide. By secondly introducing hydrogen sulfide gas and performing heat treatment separately from the first introduction of hydrogen sulfide gas in step 2 described above, it is possible to remove the unreacted lithium hydroxide raw material and lithium carbonate from the lithium sulfide, thereby producing high-purity lithium sulfide.

[0028] On the other hand, step 4 of the present invention may be performed under temperature conditions that are the same as or different from the temperature raised in step 3, and may be performed at the same temperature as the temperature in step 3, for example.

[0029] Preferably, in step 4, hydrogen sulfide may be secondarily introduced so that the molar ratio of the lithium hydroxide raw material to hydrogen sulfide gas in step 1 is 1:5 to 1:50. Introducing hydrogen sulfide gas in this range facilitates contact of the lithium carbonate contained in the lithium sulfide with hydrogen sulfide, thereby advantageously converting the lithium carbonate to lithium sulfide; if the amount is outside this range, there is a risk that the lithium carbonate may not be effectively converted to lithium sulfide. More preferably, the molar ratio of lithium hydroxide to hydrogen sulfide gas in step 1 may be 1:6 to 1:45, or 1:7 to 1:40, or 1:8 to 1:35, or 1:9 to 1:30.

[0030] Preferably, the molar ratio of the hydrogen sulfide gas initially charged in step 2 to the hydrogen sulfide gas subsequently charged in step 4 may be 1:0.5 to 1:50. Adding hydrogen sulfide gas within this range is advantageous in that it facilitates contact between hydrogen sulfide and lithium hydroxide and improves reactivity. Adding a small amount outside this range may reduce reactivity and leave unreacted lithium hydroxide, while adding an excess amount may reduce process efficiency. More preferably, the molar ratio of the hydrogen sulfide gas initially charged to the hydrogen sulfide gas subsequently charged may be 1:8 to 1:45, 1:10 to 1:40, or 1:10 to 1:30.

[0031] Meanwhile, the heat treatment step of step 4 may be performed for 3 to 10 hours. If the heat treatment step is performed for less than 3 hours, impurities may not be effectively removed, and if it is performed for more than 10 hours, process efficiency may be reduced. More preferably, the heat treatment step of step 4 may be performed for 4 hours or more, or 5 hours or more, and 8 hours or less, or 7 hours or less.

[0032] Meanwhile, according to one embodiment of the present invention, steps 1 to 4 may be carried out in a one-pot process. Here, the term "one-pot process" means that the series of steps of the present invention are carried out in one reaction vessel without isolation of any intermediates before recovery of the final product.

[0033] If the drying step of the lithium hydroxide raw material, the synthesis step performed by adding lithium sulfide, and the heat-treatment step were performed in separate reactors, additional costs could be incurred during the process of transporting the solid raw material, and as mentioned above, the lithium hydroxide raw material could be rapidly converted to lithium carbonate when exposed to the atmosphere, which could result in contamination of the dried raw material. According to one embodiment, in a method for producing lithium sulfide, the lithium hydroxide raw material is introduced into a reactor and dried, and then hydrogen sulfide is first added without adding a solvent to carry out a reaction to synthesize lithium sulfide. After heating in a nitrogen atmosphere, hydrogen sulfide is secondly added and heat-treated. By performing these series of processes in the same reactor, it is possible to prevent the contamination issues caused by the transportation and exposure to the atmosphere.

[0034] In addition, when the above-mentioned one-pot reaction is performed, the type of reactor is not particularly limited, and examples thereof include a batch reactor, a semi-batch reactor, a flow reactor, and a fluidized reactor.

[0035] According to the method for producing lithium sulfide of the present invention, the lithium sulfide may contain 5,000 ppm or less of lithium carbonate based on the mass of the produced lithium sulfide as a result of IC (ion chromatography) analysis, and preferably, 4,500 ppm or less, 4,200 ppm or less, 4,000 ppm or less, 3,500 ppm or less, 3,000 ppm or less, or 2,000 ppm or less of lithium carbonate based on the mass of lithium sulfide.

[0036] Meanwhile, the present invention provides lithium sulfide prepared by the above-described method. As described above, the lithium sulfide of the present invention is high-purity lithium sulfide having an amount of carbonate-based impurities that are difficult to remove of 5,000 ppm or less, and can achieve excellent performance when preparing a solid electrolyte material.

[0037] The functions and effects of the present invention will be described in more detail below through specific examples of the present invention. However, these examples are presented only as examples of the present invention and do not define the scope of the invention.

[0038] [Example]

[0039] Example 1 17 g of lithium hydroxide monohydrate (LiOH·H2O) was pretreated by heating to 200°C in a nitrogen (N2) atmosphere and maintaining the temperature for 2 hours. Subsequently, lithium hydroxide (LiOH) was transferred to a reactor, and hydrogen sulfide (HS) was added in a single infusion to achieve a 1:1 molar ratio. The reaction was continued at 200°C, removing the by-product water, until no more water was produced. Once no more water was produced, the addition of HS was discontinued, and the reaction product, including lithium sulfide (Li2S), was recovered from the reactor and transferred to a heat treatment device. The temperature of the heat treatment device was then raised to 250°C in a nitrogen atmosphere. Subsequently, hydrogen sulfide (HS) was added in a second infusion to achieve a 1:10 molar ratio of the lithium hydroxide (LiOH) and hydrogen sulfide (HS). Heat treatment was continued at 250°C for 6 hours to produce lithium sulfide.

[0040] Example 2 Lithium sulfide was produced by the same production method as in Example 1, except that the heat treatment temperature was changed to 300°C.

[0041] Example 3 Lithium sulfide was produced by the same production method as in Example 1, except that the heat treatment temperature was changed to 350°C.

[0042] Example 4 72 g of lithium hydroxide monohydrate (LiOH·HO) was placed in a reactor and heated to 200°C in a nitrogen (N) atmosphere. Nitrogen was injected so that the molar ratio of lithium hydroxide monohydrate to nitrogen was 1:5. After the reactor temperature reached 200°C, the temperature was maintained for 5 hours for pretreatment. Nitrogen injection was stopped, and hydrogen sulfide was added in a single injection to achieve a 1:1 molar ratio of lithium hydroxide monohydrate to hydrogen sulfide. The reaction was carried out at 200°C for 5 hours while removing the by-product water. After confirming that no further water was being generated, hydrogen sulfide injection was stopped, and the temperature was raised to 350°C in a nitrogen atmosphere. Hydrogen sulfide was then added in a second injection to achieve a 1:10 molar ratio of lithium hydroxide monohydrate to hydrogen sulfide, and the reactor was heat-treated at 350°C for 6 hours. The pretreatment, reaction, and heat treatment steps of the above process were carried out in one pot without opening the reactor to produce lithium sulfide.

[0043] Comparative Example 1 17g of lithium hydroxide (LiOH) was added and heated to 200°C in a nitrogen (N2) environment. After reaching 200°C, hydrogen sulfide (HS) was added in the first batch so that the molar ratio of lithium hydroxide (LiOH) to hydrogen sulfide (HS) was 1:1, and the reaction was continued while removing the by-product water until no more water was produced.

[0044] Experimental Example 1: X-ray diffraction analysis X-ray diffraction (XRD) analysis was performed on the lithium sulfide prepared in Example 1. The XRD equipment used was Rigaku's Ultima IV, and measurements were performed at room temperature. The results are shown in FIG. As can be seen from FIG. 1, the XRD analysis results showed that the lithium sulfide prepared according to the present invention had peaks at the same positions as those of the commercial lithium sulfide powder used as a reference material.

[0045] Experimental Example 2: Analysis of Lithium Carbonate Content The lithium carbonate (Li2CO3) content of the lithium sulfide prepared in the examples and comparative examples was measured based on the mass of the raw material or lithium sulfide using ion chromatography (IC). The IC analyzer used was a Thermofisher ICS-6000, and the results are shown in Table 1 below.

[0046] [Table 1]

[0047] As can be seen from Table 1 above, it was confirmed that the method for producing lithium sulfide of the present invention can effectively remove carbonate-based impurities contained in raw materials to produce high-purity lithium sulfide.

Claims

1. A step (step 1) of pretreating a lithium hydroxide raw material under an inert gas atmosphere; a step (step 2) of first introducing hydrogen sulfide gas to dry-react the pretreated lithium hydroxide raw material with hydrogen sulfide; After the reaction of step 2 is completed, a step (step 3) of heating the reaction product to 250°C to 400°C in an inert gas atmosphere; Step 4: Secondly introducing hydrogen sulfide gas at the same temperature as the temperature raised in step 3 to perform heat treatment; In step 4, hydrogen sulfide is secondarily introduced so that the molar ratio of lithium hydroxide raw material to hydrogen sulfide gas in step 1 is 1:5 to 1:50; The method for producing lithium sulfide, wherein the molar ratio of the hydrogen sulfide gas primarily introduced in step 2 to the hydrogen sulfide gas secondarily introduced in step 4 is 1:10 to 1:

30.

2. Step 1 is carried out at 170 to 450°C. The method for producing lithium sulfide according to claim 1.

3. In step 2, hydrogen sulfide gas is initially introduced so that the molar ratio of lithium hydroxide raw material to hydrogen sulfide gas in step 1 is 1:0.5 to 1:

50. The method for producing lithium sulfide according to claim 1.

4. Step 2 is carried out while removing water. The method for producing lithium sulfide according to claim 1.

5. Step 4 is carried out for 3 to 10 hours. The method for producing lithium sulfide according to claim 1.

6. Steps 1 to 4 are carried out in a one-pot process. The method for producing lithium sulfide according to claim 1.

7. According to the results of IC (ion chromatography) analysis, the lithium carbonate content is 5,000 ppm or less based on the mass of the produced lithium sulfide. The method for producing lithium sulfide according to claim 1.

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