Purification method of lithium sulfide

Abstract

The invention provides a purification method of lithium sulfide. The purification method comprises the following steps: (1) dissolving crude lithium sulfide in a solvent to form a first solution; (2) carbon disulfide is added into the crude product solution for a reaction, a second solution is obtained, and the second solution contains lithium trithiocarbonate formed through the reaction and lithium trithiocarbonate which exists or does not exist; (3) separating lithium trithiocarbonate and existing or non-existing lithium trithiocarbonate from the second solution in a crystallization manner; and (4) heating the crystallized product obtained in the step (3) in two stages in an inert atmosphere, with the temperature of the first stage being 50-85 DEG C and the temperature of the second stage being 100-200 DEG C to obtain a pure lithium sulfide product, and then drying in the inert atmosphere to obtain a high-purity lithium sulfide finished product. The purification method is simple and high in purification efficiency.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of material synthesis / lithium-sulfur solid-state battery, and particularly relates to a lithium sulfide purification method. BACKGROUND

[0002] Lithium-sulfur batteries have become a hot candidate for the next generation of energy storage technology due to their ultra-high theoretical energy density, low cost and high theoretical specific capacity.

[0003] However, traditional lithium-sulfur batteries with liquid electrolytes face key challenges such as polysulfide shuttle effect, lithium dendrite growth and short cycle life. The introduction of solid-state electrolytes (SSE) provides a new way to solve these problems and promotes the development of lithium-sulfur solid-state batteries (All-Solid-State Li-S Battery, ASLSB). Lithium sulfide, as a key raw material for sulfide solid-state electrolytes, plays a crucial role in the large-scale application of solid-state batteries. Currently, due to the high sensitivity of lithium sulfide to water and oxygen, its preparation method mainly depends on the neutralization reaction of toxic hydrogen sulfide and lithium hydroxide, as well as the direct sulfuration reaction of sulfur and lithium metal. In the neutralization reaction, whether in aqueous solution or in organic solvent, there is inevitably incomplete conversion of lithium hydroxide, or a large amount of lithium hydroxide / hydrogen sulfide lithium, which is difficult to separate and purify. In the direct sulfuration reaction, sulfur vaporizes to form various Sx (x = 1 ~ 20) molecular sulfur forms. After reacting with lithium metal, a series of Li2S x (x = 1 ~ 20) mixtures are obtained, which are difficult to separate and purify, resulting in low yield and low purity, which limits the further application of lithium sulfide. SUMMARY

[0004] To solve the problems in the background art, the present application provides a lithium sulfide purification method, which can effectively remove impurities in the crude lithium sulfide, improve the purity of the lithium sulfide, and has a high yield. The technical solution of the present application to solve the above technical problems is as follows: The present application provides a lithium sulfide purification method, comprising the following steps: (1) dissolving the crude lithium sulfide in a solvent to form a first solution; (2) adding carbon disulfide to the crude solution to obtain a second solution, the second solution containing lithium trithiocarbonate formed by reaction, and, optionally, lithium hydrogen trithiocarbonate; (3) separating lithium trithiocarbonate and, optionally, lithium hydrogen trithiocarbonate from the second solution by crystallization; (4) The crystalline product obtained in step (3) is heated in two stages under an inert atmosphere, the first stage at a temperature of 50-85°C and the second stage at a temperature of 100-200°C, to obtain pure lithium sulfide, which is then dried under reduced pressure in an inert atmosphere to obtain high-purity lithium sulfide product.

[0005] According to the above scheme, the solvent in step (1) is one or a combination of several of the following: alcohol, amide, carbonate, carboxylate, carboxylic acid lactone, and ether.

[0006] Further, the solvent is selected from one or a combination of several of the following: alcohol such as methanol, ethanol, amide such as N,N-dimethylformamide, carbonate such as dimethyl carbonate, carboxylate such as ethyl acetate, carboxylic acid lactone such as γ-butyrolactone, and ether such as tetrahydrofuran.

[0007] According to the above scheme, the mass concentration of the crude lithium sulfide in the first solution in step (1) is 5%-40%, preferably 10%-20%.

[0008] According to the above scheme, the molar ratio of carbon disulfide to lithium sulfide in the crude lithium sulfide is 1:1-1.5:1, preferably 1:1-1.2:1.

[0009] According to the above scheme, the reaction temperature in step (2) is 5-50°C, preferably 25-50°C, and the reaction temperature is lower than the boiling point of the selected solvent.

[0010] The reaction pressure in step (2) is -0.085 MPa-0.25 MPa, preferably 0 MPa-0.1 MPa.

[0011] The reaction time in step (2) is 5-10 h.

[0012] According to the above scheme, the specific method of crystallization in step (3) is as follows: the second solution is filtered, concentrated, and crystallized; if necessary, further purification is performed by recrystallization, i.e., the second solution is filtered, concentrated, crystallized, and recrystallized.

[0013] Further, when concentrated and crystallized, 60%-90% of the solid is precipitated.

[0014] According to the above scheme, the first-stage heating reaction in step (4) is performed under negative pressure, and the gaseous byproduct H2S produced by decomposition can be absorbed by sodium hydroxide aqueous solution or lithium hydroxide aqueous solution.

[0015] According to the above scheme, the reaction time in the first stage of step (4) is 1-5 h, and the reaction time in the second stage is 1-5 h.

[0016] According to the above scheme, the drying of the lithium sulfide in step (4) is performed by reduced pressure drying, and the drying temperature is 50-200°C, preferably 150-180°C, and the pressure is -0.1 MPa to 0.1 MPa, preferably -0.1 MPa to 0.08 MPa. The residual solvent can be removed by the reduced pressure drying.

[0017] According to the above scheme, the inert atmosphere gas in step (4) can be selected from one or a combination of nitrogen, argon and helium.

[0018] The crude lithium sulfide prepared by the prior art method generally contains lithium hydroxide (LiOH), lithium carbonate (Li2CO3), Li2Sx (x=2-8), lithium hydrosulfide (LiHS) and other impurities. In the present application, carbon disulfide (CS2) is added to the crude lithium sulfide solution, and lithium sulfide (Li2S) reacts with carbon disulfide (CS2) to convert into lithium trithiocarbonate (Li2CS3). The specific reaction equation is as follows: Li2S + CS2→ Li2CS3 If the crude lithium sulfide contains lithium hydrosulfide (LiHS) impurities, lithium hydrosulfide (LiHS) reacts with carbon disulfide to convert into lithium trithiocarbonate (LiHCS3). The specific reaction equation is as follows: LiHS + CS2→ LiHCS3 Other impurities in the crude lithium sulfide, such as lithium hydroxide (LiOH), lithium carbonate (Li2CO3) and Li2Sx (x=2-8), lithium hydroxide (LiOH) reacts with carbon disulfide (CS2) to generate lithium carbonate (Li2CO3), and lithium carbonate (Li2CO3) and Li2Sx (x=2-8) do not react with carbon disulfide. Lithium trithiocarbonate (Li2CS3) or lithium trithiocarbonate (Li2CS3) and lithium trithiocarbonate (LiHCS3) are precipitated by crystallization, thereby removing other impurities (lithium carbonate (Li2CO3) and Li2Sx (x=2-8) and the like).

[0019] The mixture of lithium trithiocarbonate (Li2CS3) or lithium trithiocarbonate (Li2CS3) and lithium trithiocarbonate (LiHCS3) is heated in two stages. In the first stage, the main reaction is that lithium trithiocarbonate (LiHCS3) loses hydrogen sulfide after heating to convert into lithium trithiocarbonate (Li2CS3). In the second stage, the main reaction is that lithium trithiocarbonate (Li2CS3) decomposes after heating to convert into lithium sulfide. The specific reaction equation is as follows: 2LiHCS3→ Li2CS3+ H2S ↑ Li2CS3→ Li2S+CS2↑ By the purification method, the impurities in the crude lithium sulfide can be removed, the purification of lithium sulfide is realized, and the lithium hydrosulfide impurities can be converted into lithium sulfide.

[0020] The beneficial effects of the present application are: (1) The present application utilizes the reaction characteristics that carbon disulfide reacts with lithium sulfide and lithium hydrosulfide to obtain lithium trithiocarbonate and lithium hydrogen trithiocarbonate, while other impurities do not react with carbon disulfide, and the solubility difference between lithium trithiocarbonate and lithium hydrogen trithiocarbonate and other impurities. The lithium sulfide crude product is reacted with carbon disulfide and crystallized to convert Li2S and LiHS into lithium trithiocarbonate and lithium hydrogen trithiocarbonate, and separate them from other impurities. Further, the lithium trithiocarbonate and lithium hydrogen trithiocarbonate are heated in stages to obtain pure lithium sulfide. This method can effectively remove impurities in the lithium sulfide crude product, has high purification efficiency, and the purity of the obtained lithium sulfide is above 99.95%, meeting the requirements of battery-grade lithium sulfide purity, and has high yield.

[0021] (2) The purification method of the present application is simple, has mild reaction conditions, low energy consumption, high safety, and the raw material carbon disulfide is widely available and low in price, making it suitable for wide application. DETAILED DESCRIPTION

[0022] The principles and features of the present application are described below in conjunction with specific examples, which are used to explain the present application and are not intended to limit the scope of the present application.

[0023] The present application provides a lithium sulfide purification method. Carbon disulfide reacts with lithium sulfide and lithium hydrosulfide to obtain lithium trithiocarbonate and lithium hydrogen trithiocarbonate, while other impurities do not react with carbon disulfide. The reaction characteristics and the solubility difference between lithium trithiocarbonate and lithium hydrogen trithiocarbonate and other impurities are utilized. The lithium sulfide crude product is reacted with carbon disulfide and crystallized to convert Li2S and LiHS into lithium trithiocarbonate and lithium hydrogen trithiocarbonate, and separate them from other impurities. Further, the lithium trithiocarbonate and lithium hydrogen trithiocarbonate are heated in stages to obtain pure lithium sulfide. The following are specific examples.

[0024] The crude lithium sulfide in the examples is prepared by the existing high-temperature solid-phase reaction method. The preparation method is as follows: metal lithium and sulfur are pre-mixed by ball milling under high-purity argon protection, then reacted at 500-700°C for 5h, crushed and sieved, and then vacuum dried at 120°C for 4h to obtain lithium sulfide crude product. The purity of the above-prepared lithium sulfide crude product is 90%.

[0025] Example 1 1. 20.0g of lithium sulfide crude product with a purity of 90% is dissolved in 80.0g of tetrahydrofuran solution to obtain a tetrahydrofuran solution with a lithium sulfide mass percentage of 20%; 2. To the above tetrahydrofuran solution, 32.8 g of carbon disulfide (molar ratio of carbon disulfide to lithium sulfide is 1.1:1.0) was slowly added, and the reaction was carried out at 40°C for 8 h under 0.1 MPa to obtain a reaction solution containing lithium trithiocarbonate and lithium bis-trithiocarbonate; 3. The above reaction solution containing lithium trithiocarbonate and lithium bis-trithiocarbonate was filtered, concentrated, crystallized and recrystallized to obtain 42.4 g of lithium trithiocarbonate and lithium bis-trithiocarbonate (crystalline solid precipitation rate is 80%); 4. The lithium trithiocarbonate and lithium bis-trithiocarbonate after crystallization and purification were heated and decomposed at 60°C for 2 h under nitrogen protection, the tail gas was absorbed by 10% lithium hydroxide aqueous solution, and then the temperature was increased to 150°C for heating and decomposition for 3 h, and the temperature was maintained at -0.09 MPa for drying for 6 h to obtain high-purity lithium sulfide product with a purity of 99.95% and a yield of 83%.

[0026] Example 2 1. 20.0 g of lithium sulfide crude product with a purity of 90% was dissolved in 100.0 g of dimethyl carbonate solution to obtain a dimethyl carbonate solution with a lithium sulfide mass percentage of 18%; 2. To the above dimethyl carbonate solution, 29.8 g of carbon disulfide (molar ratio of carbon disulfide to lithium sulfide is 1.0:1.0) was slowly added, and the reaction was carried out at 30°C for 7 h under 0.2 MPa to obtain a reaction solution containing lithium trithiocarbonate and lithium bis-trithiocarbonate; 3. The above reaction solution containing lithium trithiocarbonate and lithium bis-trithiocarbonate was filtered, concentrated, crystallized and recrystallized to obtain 39.8 g of lithium trithiocarbonate and lithium bis-trithiocarbonate (crystalline solid precipitation rate is 75%); 4. The lithium trithiocarbonate and lithium bis-trithiocarbonate after crystallization and purification were heated and decomposed at 75°C for 2 h under nitrogen protection, the tail gas was absorbed by 10% lithium hydroxide aqueous solution, and then the temperature was increased to 180°C for heating and decomposition for 3 h, and the temperature was maintained at 0.02 MPa for drying for 8 h to obtain high-purity lithium sulfide product with a purity of 99.98% and a yield of 85%.

[0027] Example 3 1. 20.0 g of lithium sulfide crude product with a purity of 90% was dissolved in 75.0 g of ethyl acetate solution to obtain an ethyl acetate solution with a lithium sulfide mass percentage of 25%; 2. To the above ethyl acetate solution, 35.8 g of carbon disulfide (molar ratio of carbon disulfide to lithium sulfide crude product is 1.2:1.0) was slowly added, and the reaction was carried out at 35°C for 8 h under -0.01 MPa to obtain a reaction solution containing lithium trithiocarbonate and lithium bis-trithiocarbonate; 3. The reaction solution containing lithium trithiocarbonate and lithium hydrogen trithiocarbonate is filtered, concentrated, crystallized and recrystallized to obtain 47.7 g of lithium trithiocarbonate and lithium hydrogen trithiocarbonate in total (crystalline solid yield 90%); 4. The lithium trithiocarbonate and lithium hydrogen trithiocarbonate after crystallization and purification are heated and decomposed under nitrogen protection at 80°C for 1 h, the tail gas is absorbed by 10% sodium hydroxide aqueous solution, and then the temperature is increased to 130°C for heating and decomposition for 5 h. After the reaction is completed, the product of high-purity lithium sulfide with a purity of 99.96% is obtained by drying at 150°C and -0.1 MPa for 5 h, and the yield is 84%.

[0028] Example 4 1. 20.0 g of lithium sulfide crude product with a purity of 90% is dissolved in 120.0 g of methanol solution to obtain a methanol solution with a lithium sulfide mass percentage of 15%; 2. 32.8 g of carbon disulfide (molar ratio of carbon disulfide to lithium sulfide crude product is 1.1:1.0) is slowly added to the above methanol solution, and the reaction is carried out at 0.15 MPa and 5°C for 10 h to obtain a reaction solution containing lithium trithiocarbonate and lithium hydrogen trithiocarbonate; 3. The reaction solution containing lithium trithiocarbonate and lithium hydrogen trithiocarbonate is filtered, concentrated, crystallized and recrystallized to obtain 34.5 g of lithium trithiocarbonate and lithium hydrogen trithiocarbonate in total (crystalline solid yield 65%); 4. The lithium trithiocarbonate and lithium hydrogen trithiocarbonate after crystallization and purification are heated and decomposed under nitrogen protection at 65°C for 3 h, the tail gas is absorbed by 10% sodium hydroxide aqueous solution, and then the temperature is increased to 110°C for heating and decomposition for 3 h. After the reaction is completed, the product of high-purity lithium sulfide with a purity of 99.95% is obtained by drying at 160°C and -0.1 MPa for 5 h, and the yield is 81%.

[0029] Example 5 1. 20.0 g of lithium sulfide crude product with a purity of 90% is dissolved in 120.0 g of DMAC solution to obtain a DMAC solution with a lithium sulfide mass percentage of 15%; 2. 32.8 g of carbon disulfide (molar ratio of carbon disulfide to lithium sulfide crude product is 1.1:1.0) is slowly added to the above methanol solution, and the reaction is carried out at 0.25 MPa and 50°C for 5 h to obtain a reaction solution containing lithium trithiocarbonate and lithium hydrogen trithiocarbonate; 3. The reaction solution containing lithium trithiocarbonate and lithium hydrogen trithiocarbonate is filtered, concentrated, crystallized and recrystallized to obtain 45.1 g of lithium trithiocarbonate and lithium hydrogen trithiocarbonate in total (crystalline solid yield 85%); 4. The lithium sulfide product with purity of 99.95% was obtained by heating the lithium sulfide crude product and lithium thiosulfate at 85℃ for 2h under nitrogen protection, absorbing the tail gas with 10% lithium hydroxide aqueous solution, then heating at 200℃ for 3h, drying at 150℃ and -0.1MPa for 7h, and the yield was 82%.

[0030] Comparative Example 1 The difference between Comparative Example 1 and Example 1 is only that the molar ratio of carbon disulfide to lithium sulfide in the second step reaction is 0.5:1.0, and the purity of the final product is improved from 90% to 95.21%, and the yield is 73%.

[0031] Comparative Example 2 The difference between Comparative Example 2 and Example 2 is only that the reaction temperature of lithium sulfide and carbon disulfide in the second step reaction is 60℃, and the purity of the final product is improved from 90% to 92.16%, and the yield is 71%.

[0032] Comparative Example 3 The difference between Comparative Example 3 and Example 3 is only that the lithium thiosulfate and lithium thiosulfate are not heated in stages, but directly heated at 100℃ for 5h, and the purity of the final product is improved from 90% to 93.43%, and the yield is 76%.

[0033] From Comparative Example 1 and Example 1, it can be seen that when the amount of lithium sulfide crude product is much larger than that of carbon disulfide, the amount of sodium thiosulfate or sodium thiosulfate obtained by the reaction is small, and the purity of the lithium sulfide purified by this process is difficult to improve. From Comparative Example 2 and Example 2, it can also be verified that a too high reaction temperature will cause carbon disulfide to escape, and the amount of lithium sulfide purified by sodium thiosulfate or sodium thiosulfate is small, and part of the unreacted lithium sulfide crude product carries impurities into the final product, resulting in little improvement in product purity. Comparative Example 3 and Example 3 show that the final sodium thiosulfate and sodium thiosulfate need to be heated in stages, and part of the gaseous by-product is decomposed at a lower temperature to prevent the gaseous by-product from reacting with lithium sulfide at a high temperature to produce impurities that remain in the final product, making it difficult to improve the purity. Therefore, the above comparative examples and examples can prove that the method for purifying lithium sulfide according to the present application can obtain a lithium sulfide product with very high purity.

[0034] The above description is only a preferred embodiment of the present application and is not intended to limit the present application. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present application shall be included in the protection scope of the present application.

Claims

1. A method for purifying lithium sulfide, characterized in that, Includes the following steps: (1) Dissolve crude lithium sulfide in a solvent to form a first solution; (2) Carbon disulfide is added to the crude product solution to react and a second solution is obtained. The second solution contains lithium trisulfide formed by the reaction, and lithium trisulfide bicarbonate, which may or may not be present. (3) Lithium trisulfate carbonate and lithium trisulfate bicarbonate, present or absent, are separated from the second solution by crystallization. (4) The crystalline product obtained in step (3) is heated in two stages under an inert atmosphere. The temperature of the first stage is 50~85 ℃ and the temperature of the second stage is 100~200 ℃ to obtain pure lithium sulfide. Then, it is dried under an inert atmosphere to obtain high-purity lithium sulfide product.

2. The method for purifying lithium sulfide according to claim 1, characterized in that, In step (1), the solvent is one or a combination of several of the following: alcohol, amide, carbonate, carboxylic acid ester, carboxylic acid lactone, and ether.

3. The method for purifying lithium sulfide according to claim 1, characterized in that, In step (2), the molar ratio of carbon disulfide to lithium sulfide in the crude lithium sulfide is 1:1 to 1.5:

1.

4. The method for purifying lithium sulfide according to claim 1, characterized in that, The reaction temperature in step (2) is 5~50 ℃ and the reaction pressure is -0.085MPa~0.25MPa.

5. The method for purifying lithium sulfide according to claim 1, characterized in that, The reaction time in step (2) is 5~10h.

6. The method for purifying lithium sulfide according to claim 1, characterized in that, The specific method for crystallization in step (3) is to filter, concentrate, and crystallize the second solution; or to filter, concentrate, crystallize, and recrystallize the second solution.

7. The method for purifying lithium sulfide according to claim 1, characterized in that, In step (4), the first stage is carried out under negative pressure, and gaseous byproducts are generated by decomposition, which are absorbed by sodium hydroxide aqueous solution or lithium hydroxide aqueous solution.

8. The method for purifying lithium sulfide according to claim 1, characterized in that, In step (4), the reaction time for the first stage is 1-5 hours, and the reaction time for the second stage is 1-5 hours.

9. The method for purifying lithium sulfide according to claim 1, characterized in that, In step (4), the temperature for drying lithium sulfide is 50~200℃ and the pressure is -0.1MPa~0.1 MPa.

10. The method for purifying lithium sulfide according to claim 1, characterized in that, The inert gas is selected from one or a combination of nitrogen, argon, and helium.