Method for producing lithium sulfide

Abstract

The present invention relates to a method for producing lithium sulfide. More specifically, provided is a method for preparing high-performance lithium sulfide, the lithium sulfide prepared by using a mixed solvent of polar and non-polar solvents allowing the particle diameter to be controlled and impurities in the lithium sulfide to be reduced.

Description

Method for producing lithium sulfide

[0001] Cross-citation with related application(s)

[0002] This application claims the benefit of priority to Korean Patent Application No. 10-2024-0092020, filed July 11, 2024, the entire disclosure of which is incorporated herein by reference.

[0003]

[0004] The present invention relates to a method for producing lithium sulfide.

[0005]

[0006] With the recent development of the secondary battery industry, the potential for using solid electrolyte materials is increasing. Among these, sulfide-based solid electrolytes, synthesized using lithium sulfide (Li2S), lithium chloride (LiCl), and phosphorus pentasulfide (P2S5), are considered to possess higher ionic conductivity and stability over a wide voltage range compared to oxide-based solid electrolytes.

[0007]

[0008] Among the above raw materials, lithium sulfide (Li2S) has the highest melting point, and its properties significantly influence the performance of solid electrolytes. Therefore, the synthesis of lithium sulfide with superior performance as a solid electrolyte raw material is crucial.

[0009]

[0010] Lithium sulfide, the main raw material for sulfide-based solid electrolytes, is typically manufactured by reacting a solid lithium precursor with a gaseous sulfur precursor. Previous research on lithium sulfide production has primarily focused on methods for synthesizing lithium sulfide, particularly in relation to the selection of precursors. Since then, research has focused on improving the performance of manufactured lithium sulfide, in addition to lithium sulfide synthesis. These improvements in lithium sulfide performance primarily involve techniques for controlling the manufactured lithium sulfide particles. For example, techniques for controlling the particle size of lithium sulfide or the specific surface area of ​​lithium sulfide have been proposed (JP2019-156691A).

[0011]

[0012] Among them, the particle size of the raw material lithium sulfide (Li2S) is a very important factor in the manufacture of solid electrolytes. If the particles are too large, the reactivity may be low during electrolyte synthesis, and if the particles are too small, they may not be well dispersed during the reaction due to the agglomeration phenomenon between particles. As a result, unreacted lithium sulfide (Li2S) may remain even after the solid electrolyte synthesis. Therefore, a technology for controlling the particle size of lithium sulfide (Li2S) at an appropriate level is necessary. However, most of the conventional technologies for synthesizing lithium sulfide using lithium hydroxide (LiOH) and hydrogen sulfide (H2S) as raw materials are limited to the synthesis of lithium sulfide (Li2S), and a separate additional process is required to control the particle size of the synthesized lithium sulfide (Li2S).

[0013]

[0014] Meanwhile, impurities contained within lithium sulfide can negatively impact the performance of solid electrolytes. Therefore, in addition to technologies for controlling the lithium sulfide particles themselves, some research has been conducted to reduce impurities within lithium sulfide. Among these, lithium hydroxide rapidly reacts with carbon dioxide in the air to form carbonate impurities. To remove these impurities, a technique has been proposed that removes them at extremely high temperatures, exceeding 1,000°C. However, this has hindered commercial application.

[0015]

[0016] Accordingly, research is needed on a method for producing high-purity lithium sulfide while solving the above-mentioned problems and controlling the particle size of the produced lithium sulfide.

[0017]

[0018] The present invention relates to a method for producing lithium sulfide, which can obtain high-purity lithium sulfide with particle size control and effective removal of impurities.

[0019]

[0020] The present invention provides a method for producing lithium sulfide, comprising the steps of: preparing a mixture comprising lithium hydroxide, a polar solvent, and a non-polar solvent (step 1); reacting the mixture with hydrogen sulfide gas (step 2); removing the solvent from the reaction product under vacuum conditions after completion of the reaction in step 2 (step 3); and heat-treating the reaction product by secondarily introducing hydrogen sulfide gas (step 4).

[0021]

[0022] In addition, the present invention provides lithium sulfide manufactured according to the manufacturing method of the present invention.

[0023]

[0024] The method for producing lithium sulfide according to the present invention can produce high-purity lithium sulfide by minimizing impurities in lithium sulfide. More specifically, the particle size of lithium sulfide produced using a mixed solvent of polar solvent and non-polar solvent can be controlled, and high-performance lithium sulfide with reduced impurities in lithium sulfide can be produced through post-reaction post-treatment.

[0025]

[0026] In the present invention, terms such as first, second, etc. are used to describe various components, and the terms are used only for the purpose of distinguishing one component from another.

[0027] Additionally, the terms used herein are for the purpose of describing exemplary embodiments only and are not intended to limit the present invention.

[0028] Singular expressions include plural expressions unless the context clearly indicates otherwise.

[0029] In this specification, the terms “comprise,” “include,” or “have” are intended to describe a feature, number, step, component, or combination thereof implemented, but do not exclude the possibility of one or more other features, numbers, steps, components, combinations, or additions thereof.

[0030] Additionally, in this specification, when each layer or element is referred to as being formed “on” or “over” each layer or element, it means that each layer or element is formed directly on each layer or element, or that other layers or elements may be additionally formed between each layer, on the object, or on the substrate.

[0031] The present invention is susceptible to various modifications and takes various forms. Specific embodiments are illustrated and described in detail below. However, this does not limit the invention to a specific disclosed form, but rather encompasses all modifications, equivalents, and alternatives falling within the spirit and technical scope of the present invention.

[0032]

[0033] Hereinafter, the present invention will be described in detail.

[0034]

[0035] The present invention provides a method for producing lithium sulfide, comprising the steps of: preparing a mixture comprising lithium hydroxide, a polar solvent, and a non-polar solvent (step 1); reacting the mixture with hydrogen sulfide gas (step 2); removing the solvent from the reaction product under vacuum conditions after completion of the reaction in step 2 (step 3); and heat-treating the reaction product by secondarily introducing hydrogen sulfide gas (step 4).

[0036]

[0037] Lithium sulfide can be produced by reacting lithium hydroxide with hydrogen sulfide gas. This reaction can be carried out with or without a solvent. A solvent-free process has the advantage of eliminating the need for solvent removal or recovery steps. However, the removal of the reaction water generated by the acid-base reaction is difficult and the reaction temperature is high, making the use of a solvent advantageous from a process economic perspective.

[0038]

[0039] Meanwhile, when synthesizing lithium sulfide (Li2S) using an organic solvent, the reaction proceeds in two stages.

[0040] Reaction 1: LiOH + H2S → LiSH + H2O

[0041] Reaction 2: 2LiSH → Li2S + H2S

[0042]

[0043] In the processes using the above solvents, it has been common to use a single type of solvent. However, when a single polar solvent is used, the thermal decomposition rate of the intermediate product, LiSH, is slow, and a high thermal decomposition temperature is required. In addition, when a non-polar solvent, such as an aromatic or aliphatic solvent, is used, the reaction rate is relatively slow, so that the water and lithium hydroxide produced by the acid-base neutralization reaction of the starting materials, lithium hydroxide and hydrogen sulfide, and the final product, lithium sulfide, coexist, and there is a problem that the reverse reaction of lithium sulfide produced by moisture proceeds.

[0044]

[0045] In order to manufacture high-purity lithium sulfide (Li2S), research is needed on an effective manufacturing method for removing impurities and residual solvents generated during the reaction process.

[0046]

[0047] Accordingly, the inventors of the present invention have completed the present invention by confirming that high-purity lithium sulfide can be produced while controlling the particle size to a certain level through a series of production steps including the steps of producing a mixture including lithium hydroxide, a polar solvent, and a non-polar solvent, reacting the mixture with hydrogen sulfide gas to produce lithium sulfide, and then heat-treating the mixture by secondary injection of hydrogen sulfide gas.

[0048]

[0049] Step 1 of the present invention is a step of preparing a mixture comprising lithium hydroxide, a polar solvent, and a non-polar solvent.

[0050]

[0051] Examples of the polar solvent include N-methylpyrrolidone (NMP), methanol, ethanol, n-propanol, isopropanol, n-butanol, isobutanol, n-pentanol, diethyl ether, diisopropyl ether, t-butylmethyl ether, phenyl methyl ether, diethoxyethane, tetrahydrofuran, etc., and in the present invention, these may be used alone or in a mixture of two or more thereof.

[0052]

[0053] In addition, the non-polar solvent may include, for example, aromatic hydrocarbon solvents such as toluene, xylene, ethylbenzene, decalin, and 1,2,3,4-tetrahydronaphthalene; aliphatic hydrocarbon solvents such as hexane, pentane, 2-ethylhexane, heptane, octane, decane, cyclohexane, methyl cyclohexane, hexene, heptene, and cyclohexene; and in the present invention, these may be used alone or in combination of two or more.

[0054]

[0055] The polar solvent usable in the present invention is a solvent having excellent solubility in raw material LiOH, intermediate product LiSH, and water, and the non-polar solvent is a solvent having non-reactivity with raw materials such as LiOH and H2S and thermal stability, and when a polar solvent and a non-polar solvent having the above characteristics are mixed, the reactivity can be improved by controlling the particle size of the lithium sulfide produced by adjusting the solubility of the raw material and intermediate product according to the solvent ratio. Preferably, a solvent mixture including one of the polar solvents and one of the non-polar solvents can be used.

[0056]

[0057] In addition, in the present invention, the solvent may contain a non-polar solvent in an amount greater than that of the polar solvent. For example, the polar solvent may be contained in an amount of 1 to 50 parts by weight based on 100 parts by weight of the polar solvent and the non-polar solvent in total. That is, the polar solvent may be contained in an amount of 1 to 50 parts by weight and the non-polar solvent may be contained in an amount of 50 to 99 parts by weight. When the polar solvent and the non-polar solvent are contained in the above ranges in the mixed solvent, it is easy to remove the water generated by the subsequent solvent removal process and the neutralization reaction, and it induces high dispersion of the reactant lithium hydroxide in the solvent, making it easy to control the particle size. More preferably, the polar solvent may be contained in an amount of 1.5 parts by weight or more, 2 parts by weight or more, 3 parts by weight or more, 4 parts by weight or more, or 5 parts by weight or more, but 50 parts by weight or less, 45 parts by weight or less, or 40 parts by weight or less based on 100 parts by weight of the polar solvent and the non-polar solvent in total.

[0058]

[0059] Additionally, in the present invention, lithium hydroxide may be used in a ratio of 1:5 to 1:10 based on the total mass of the polar solvent and the non-polar solvent. When lithium hydroxide and the solvent are used in the above range, high dispersion of lithium hydroxide is induced, which is advantageous for controlling the particle size of the lithium sulfide produced. Preferably, lithium hydroxide and the solvent may be used in a ratio of 1:5.5 to 1:9.5, 1:6 to 1:9, or 1:7 to 1:8 based on mass.

[0060]

[0061] Next, step 2 of the present invention is a step (step 2) of reacting the mixture prepared in step 1 with hydrogen sulfide gas.

[0062]

[0063] The mixture containing lithium hydroxide and hydrogen sulfide manufactured in the above step 1 is manufactured into lithium sulfide through a gas-liquid phase reaction. Preferably, the step 2 may be carried out at a temperature condition of 70°C to 200°C. More specifically, the step 1 may be carried out at a temperature condition of 80°C or higher, 90°C or higher, 100°C or higher, or 110°C or higher, and step 2 may be carried out at a temperature condition of 180°C or lower, 160°C or lower, 150°C or lower, or 140°C or lower.

[0064]

[0065] According to one embodiment of the present invention, in step 2, hydrogen sulfide gas may be introduced at 0.1 L / min to 5 L / min. When hydrogen sulfide gas is used in the above range, an appropriate reaction rate can be maintained, and the amount of unreacted lithium hydroxide can be reduced, while the amount of water produced by the neutralization reaction with the produced lithium sulfide can be appropriately controlled to suppress reverse reactions. Preferably, in step 2, hydrogen sulfide gas may be introduced at 0.15 L / min or more, or 0.2 L / min or more, and 4 L / min or less, 3 L / min or less, 2 L / min or less, or 1 L / min or less.

[0066]

[0067] Next, when step 2 of the present invention is completed, step 3 of the present invention is performed. Step 3 of the present invention is a step of removing the solvent under vacuum conditions from the reaction product after the completion of the reaction of step 2. Whether the reaction of step 2 is completed can be confirmed by observing whether water is generated in the reaction of the lithium hydroxide raw material and hydrogen sulfide gas. That is, step 3 of the present invention is a step of preparing a heat treatment step through secondary injection of hydrogen sulfide gas, which will be described later, by drying and removing the solvent under a vacuum atmosphere after confirming that no additional water is generated.

[0068]

[0069] The solvent removal step may be carried out at a temperature that is the same as or different from the temperature of the reaction step of step 2, and there is no particular limitation as long as it is a temperature suitable for removing the solvent while minimizing physical and chemical modification of the reaction product. For example, the step 3 may be carried out at a temperature of 60°C to 200°C, or 70°C to 190°C, or 80°C to 180°C. In addition, the step 3 may be carried out at a pressure of 1 mbar to 50 mbar, or 5 mbar to 45 mbar, or 10 mbar to 40 mbar. In addition, the step 3 may be carried out for 1 hour to 10 hours, or 2 hours to 7 hours, or 3 hours to 5 hours.

[0070]

[0071] Step 4 of the present invention is a step of secondarily introducing hydrogen sulfide gas and performing heat treatment. As described above, the lithium sulfide produced through steps 1 to 3 may contain unreacted lithium hydroxide raw material and lithium carbonate. Accordingly, as a process for converting unreacted lithium hydroxide and lithium carbonate into lithium sulfide, by secondarily introducing hydrogen sulfide gas and performing heat treatment separately from the first step of introducing hydrogen sulfide gas in step 2 described above, the unreacted lithium hydroxide raw material and lithium carbonate in the lithium sulfide can be removed, and high-purity lithium sulfide can be produced.

[0072]

[0073] Preferably, step 4 is performed by increasing the temperature in an inert gas atmosphere, and then switching to hydrogen sulfide gas when the target temperature is reached to perform heat treatment. At this time, step 4 may be performed under a temperature condition of 250°C to 400°C. If performed below 250°C, it may not be easy to remove lithium carbonate, and if performed above 400°C, there may be a problem of lowering process efficiency. More preferably, it may be performed at 260°C or higher, 270°C or higher, 280°C or higher, 290°C or higher, or 300°C or higher, but 390°C or lower, 380°C or lower, 370°C or lower, 360°C or lower, or 350°C or lower.

[0074]

[0075] Meanwhile, there is no particular limitation on examples of inert gases used in the temperature raising process in step 4, but for example, nitrogen, argon, helium, and a mixture thereof may be used.

[0076]

[0077] Preferably, in the step 4, hydrogen sulfide may be secondarily introduced so that the molar ratio of the lithium hydroxide raw material of step 1 and hydrogen sulfide gas becomes 1:5 to 1:50. Injecting hydrogen sulfide gas in an amount within the above range facilitates contact between lithium carbonate contained in lithium sulfide and hydrogen sulfide, thereby advantageously converting lithium carbonate into lithium sulfide, and if it is outside the above range, a problem may arise in which lithium carbonate is not effectively converted into lithium sulfide. More preferably, the molar ratio of lithium hydroxide and 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.

[0078]

[0079] 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, there may be a problem in which impurities are not effectively removed, and if the heat treatment step is performed for more than 10 hours, there may be a problem in which process efficiency is reduced. More preferably, the heat treatment step of Step 4 may be performed for 4 hours or more, or 5 hours or more, but 8 hours or less, or 7 hours or less.

[0080]

[0081] According to one embodiment of the present invention, the average particle diameter (D) of lithium hydroxide 50 ) is 600 um to 700 um, and the average particle size (D) of the manufactured lithium sulfide 50 ) can be from 10um to 650um. In this specification, the average particle diameter (D 50 ) means the particle diameter at the 50% point of the cumulative distribution of particle counts according to particle size (particle diameter). The average particle diameter (D 50 ) can be measured using a laser diffraction scattering particle size distribution measuring device, and the particle size at the point where 50% of the cumulative particle number distribution according to particle size is calculated, and this is the average particle diameter (D 50 ) is defined as. When the average particle diameter of lithium sulfide is controlled within the above range, it is easy to synthesize a solid electrolyte manufactured using lithium sulfide as a raw material, and purity and ionic conductivity can be increased. Preferably, the average particle diameter (50) of the lithium hydroxide may be 600 um or more or 650 um or more and 700 um or less, and the average particle diameter (D) of the lithium sulfide 50 ) may be 10 um or more, 650 um or less, 500 um or less, 200 um or less, or 100 um or less.

[0082]

[0083] According to the method for producing lithium sulfide of the present invention described above, the produced lithium sulfide can contain lithium carbonate in an amount of 5,000 ppm or less based on the mass of the produced lithium sulfide as a result of IC (Ion chromatography) analysis. Preferably, the produced lithium sulfide can contain lithium carbonate in an amount of 4,500 ppm or less, 4,200 ppm or less, 4,000 ppm or less, 3,500 ppm or less, or 3,000 ppm or less based on the mass of the lithium sulfide.

[0084]

[0085] Meanwhile, the present invention provides lithium sulfide manufactured according to the above-described manufacturing method. As described above, the lithium sulfide of the present invention is a high-purity lithium sulfide having a particle size controlled to a certain level and an amount of carbonate-based impurities that are difficult to remove of 5,000 ppm or less, thereby enabling excellent performance in the manufacture of solid electrolyte materials.

[0086]

[0087] Hereinafter, the functions and effects of the invention will be described in more detail through specific examples. However, these examples are provided merely as examples of the invention and do not define the scope of the invention.

[0088]

[0089] [Example]

[0090] Example 1

[0091] The raw material LiOH (60 g) with an average particle size (D50) of 650 μm and a solvent containing a non-polar solvent Xylene and a polar solvent N-methylpyrrolidone (NMP) were used. The starting material LiOH (60 g) was added, and the mixed solvent (480 g, 2 wt% N-methylpyrrolidone (NMP), 98 wt% Xylene) was mixed, and then stirred at 200 to 400 rpm. The synthesis temperature was carried out at 120 ℃, and when the temperature was raised to that temperature, hydrogen sulfide (H2S) was injected at a rate of 0.2 L / min to 1 L / min. The reaction was terminated when water generation due to acid-base neutralization reaction was completed, thereby producing lithium sulfide. The produced lithium sulfide was dried in a vacuum dryer at 100 ℃ and 20 mbar for 3 hours to remove the solvent, and then transferred to a heat treatment device.

[0092] The temperature of the heat treatment device was raised to 350°C in a nitrogen environment. Thereafter, lithium sulfide was produced by heat treatment at 350°C for 6 hours while hydrogen sulfide (H2S) was added for a second time so that the molar ratio of lithium hydroxide (LiOH) and hydrogen sulfide (H2S) was 1:10.

[0093]

[0094] Example 2

[0095] Lithium sulfide was prepared in the same manner as in Example 1, except that a mixed solvent of 10% NMP and 90% Xylene was used.

[0096]

[0097] Example 3

[0098] Lithium sulfide was prepared in the same manner as in Example 1, except that a mixed solvent of 15% NMP and 85% Xylene was used.

[0099]

[0100] Comparative Example 1

[0101] Lithium sulfide was prepared in the same manner as in Example 1, except for the heat treatment process.

[0102]

[0103] Comparative Example 2

[0104] Lithium sulfide was prepared in the same manner as in Example 1, except that a solvent of 100% xylene was used.

[0105]

[0106] Experimental example

[0107] (1) Measurement of lithium sulfide particle size

[0108] The D50 of LiOH and manufactured lithium sulfide used in the examples and comparative examples was measured using a laser diffraction particle size measuring device, and the results are shown in Table 1 below.

[0109]

[0110] (2) Purity analysis

[0111] The unreacted lithium hydroxide and lithium carbonate (Li2CO3) contents based on the mass of lithium sulfide prepared in the above examples and comparative examples were measured using ion chromatography (IC). The IC analyzer used was Thermofisher's ICS-6000, and the results are shown in Table 1 below.

[0112]

[0113] Lithium hydroxide average particle size (um) Whether solvent heat treatment was performed Average particle size of Li2S (um) LiOH content in Li2S (wt%) Carbonate content (ppm) Lithium hydroxide 650----9,480 Example 1650 NMP 2%, Xylene 98% O 150.03 2,656 Example 2650 NMP 10%, Xylene 90% O 250.02 2,306 Example 3650 NMP 20%, Xylene 80% O 330.03 2,303 Comparative example 1650 NMP 2%, Xylene 98% X 181.2 29,390 Comparative example 2650 NMP 0%, Xylene 100% O 650 0.03 4,680

[0114] As can be confirmed in Table 1 above, according to the method for producing lithium sulfide of the present invention, it was confirmed that it is possible to produce high-performance lithium sulfide having a particle size of 100 ㎛ or less and a very low impurity content.

[0115]

Claims

1. A step of preparing a mixture containing lithium hydroxide, a polar solvent and a non-polar solvent (step 1); A step of reacting the above mixture with hydrogen sulfide gas (step 2); After the completion of the reaction in step 2 above, a step (step 3) of removing the solvent from the reaction product under vacuum conditions; and Including a step (step 4) of heat treatment by secondary injection of hydrogen sulfide gas, Method for producing lithium sulfide.

2. In paragraph 1, The polar solvent comprises at least one selected from the group consisting of N-methylpyrrolidone (NMP), methanol, ethanol, n-propanol, isopropanol, n-butanol, isobutanol, n-pentanol, diethyl ether, diisopropyl ether, t-butylmethyl ether, phenyl methyl ether, diethoxyethane and tetrahydrofuran. Method for producing lithium sulfide.

3. In paragraph 1, The nonpolar solvent comprises at least one selected from the group consisting of toluene, xylene, ethylbenzene, decalin, 1,2,3,4-tetrahydronaphthalene, hexane, pentane, 2-ethyl hexane, heptane, octane, decane, cyclohexane, methyl cyclohexane, hexene, heptene and cyclohexene. Method for producing lithium sulfide.

4. In paragraph 1, In step 1, with respect to the total of 100 parts by weight of the polar solvent and the non-polar solvent, the polar solvent is included in an amount of 1 to 50 parts by weight. Method for producing lithium sulfide.

5. In paragraph 1, Lithium hydroxide is used in a ratio of 1:5 to 1:10 based on the total mass of polar solvent and non-polar solvent. Method for producing lithium sulfide.

6. In paragraph 1, The above step 2 is carried out under temperature conditions of 70 ℃ to 200 ℃. Method for producing lithium sulfide.

7. In paragraph 1, In step 2, hydrogen sulfide gas is introduced at 0.1 L / min to 5 L / min. Method for producing lithium sulfide.

8. In paragraph 1, The above step 3 is carried out under temperature conditions of 60 ℃ to 200 ℃. Method for producing lithium sulfide.

9. In paragraph 1, The above step 3 is carried out under pressure conditions of 1 mbar to 50 mbar. Method for producing lithium sulfide.

10. In paragraph 1, The above step 4 is carried out under temperature conditions of 250 ℃ to 400 ℃. Method for producing lithium sulfide.

11. In paragraph 1, In the above step 4, hydrogen sulfide is secondarily added so that the molar ratio of the lithium hydroxide raw material of step 1 and hydrogen sulfide gas is 1:5 to 1:

50. Method for producing lithium sulfide.

12. In paragraph 1, The average particle size (D) of lithium hydroxide in step 1 50 ) is 600 um to 700 um, The average particle size of manufactured lithium sulfide (D 50 ) is 10 um to 650 um, Method for producing lithium sulfide.

13. In paragraph 1, According to the IC (Ion chromatography) analysis results, the manufactured lithium sulfide contains lithium carbonate at 5,000 ppm or less by mass. Method for producing lithium sulfide.

14. Lithium sulfide manufactured by a manufacturing method according to any one of claims 1 to 13.