Lithium sulfide and method for preparing lithium sulfide

Abstract

The invention belongs to the technical field of chemical materials, and particularly discloses a method for preparing lithium sulfide, which comprises the following steps: mixing lithium salt and ammonium sulfide for reaction to obtain lithium sulfide; wherein the lithium salt comprises at least one of lithium hexafluorophosphate, lithium difluorophosphate and lithium tetrafluoroborate. According to the method for preparing the lithium sulfide, the lithium salt and the ammonium sulfide can fully react under the mild condition, the reaction time is short, the reaction yield is high, and meanwhile the purity of the obtained lithium sulfide is high.

Description

Technical Field

[0001] The present application belongs to the field of chemical materials, and specifically relates to lithium sulfide and a preparation method thereof. Background Art

[0002] Currently, the preparation conditions for lithium sulfide with low impurity content, high purity, and high yield are harsh, making it difficult to mass-produce. For example, in the related art, anhydrous sodium sulfide and lithium iodide are used as reactants. First, the reactants need to undergo a long pretreatment. The treated sodium sulfide and lithium iodide are then reacted at a molar ratio of 0.4:2 at 120°C for 12 hours. The reactants are then washed with an organic solvent, and the by-products are separated by using a tetrahydrofuran solution. Finally, the lithium sulfide product is dried, etc. This method is time-consuming and labor-intensive. For another example, in the related art, lithium hydride is mixed with elemental sulfur and mechanically ball-milled to produce lithium sulfide. This reaction is vigorous, heat generation is difficult to control, and the resulting lithium sulfide has low purity. As can be seen, the preparation methods in the related art are time-consuming and labor-intensive, the resulting lithium sulfide has low purity, and are not conducive to industrial mass production. It would be of great significance to develop a method for preparing lithium sulfide under mild conditions and suitable for mass production. Summary of the Invention

[0003] The present application aims to solve, at least to some extent, one of the technical problems in the related art. To this end, the present application provides a method for preparing lithium sulfide with mild reaction conditions, short reaction time, high reaction yield, or high product purity, and lithium sulfide prepared by the method.

[0004] In a first aspect of the present application, a method for preparing lithium sulfide is proposed, comprising: mixing a lithium salt and ammonium sulfide to react to obtain lithium sulfide; wherein the lithium salt comprises at least one of lithium hexafluorophosphate, lithium difluorophosphate and lithium tetrafluoroborate.

[0005] The preparation method of the first aspect of the present application has at least the following beneficial effects: mild reaction conditions, short reaction time, high reaction yield (such as yield ≥98%) and high product purity.

[0006] In addition, the preparation method according to the above embodiment of the present application may also have the following additional technical features: In some embodiments, the lithium salt and ammonium sulfide react in an organic solvent comprising at least one of an ester solvent and an ether solvent. The reaction is carried out in an ether or ester organic solvent. Ether or ester solvents have good solubility for both the lithium salt and ammonium sulfide, helping to fully disperse the reactants in the reaction system, increasing the reaction rate and ensuring a more complete reaction while requiring less solvent. Furthermore, ester and ether solvents are chemically stable and are less likely to react with the lithium salt and ammonium sulfide, thereby improving product purity.

[0007] In some embodiments, the ester organic solvent includes at least one of dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, ethylene carbonate, ethyl acetate, and methyl acetate; and the ether solvent includes at least one of ethylene glycol dimethyl ether, ethylene glycol diethyl ether, diethylene glycol dimethyl ether, methyl tert-butyl ether, and diethyl ether. These organic solvents have excellent solubility, stable chemical properties, are unlikely to react with raw materials, and are highly safe.

[0008] In some embodiments, the mass ratio of the lithium salt to the organic solvent is 1:3 to 1:20, specifically 1:4 to 1:8, thereby helping the lithium salt to completely dissolve in the organic solvent without wasting the organic solvent.

[0009] In some embodiments, the reaction of the lithium salt and ammonium sulfide satisfies at least one of the following conditions: The reaction temperature is 15°C to 70°C, specifically 40°C to 50°C; The reaction time is 2h~6h, specifically 3h~4h; The reaction atmosphere is an inert atmosphere.

[0010] This is conducive to the smooth progress of the reaction, improves the reaction efficiency, reduces side reactions, and does not cause time waste.

[0011] In some embodiments, the molar ratio of the lithium salt to the ammonium sulfide is 2:1 to 2.1:1. This can promote a more complete reaction and increase the reaction rate.

[0012] In some embodiments, after the mixing reaction of the lithium salt and the ammonium sulfide is completed, at least one of the following post-processing steps is further included: The lithium salt and the ammonium sulfide are mixed and reacted to obtain a mixed solution containing lithium sulfide, and the mixed solution is subjected to solid-liquid separation to obtain a crude lithium sulfide product; Drying the crude lithium sulfide product at 50° C. to 120° C. and -0.095 MPa for 1 h to 5 h to obtain a dry crude lithium sulfide product; The dried crude lithium sulfide product is calcined at 400° C. to 800° C. and −0.095 MPa for 1 h to 5 h.

[0013] The above post-processing steps are beneficial for removing the filtrate and impurities generated in the side reactions, thereby improving the purity of lithium sulfide.

[0014] In some embodiments, the yield of lithium sulfide is ≥98%, which is conducive to achieving industrial mass production.

[0015] In a second aspect, the present application provides lithium sulfide prepared by the aforementioned method for preparing lithium sulfide. This lithium sulfide is high in purity and low in impurities. When used to prepare a sulfide electrolyte, it can improve the electrolyte's ionic conductivity and interfacial stability, thereby enhancing the electrochemical performance and safety of the battery.

[0016] In some embodiments, the purity of the lithium sulfide is ≥99.95%.

[0017] This application has at least the following beneficial effects: 1. In the present application, a lithium salt (at least one of lithium hexafluorophosphate, lithium difluorophosphate, and lithium tetrafluoroborate) is reacted with ammonium sulfide in an ether or ester organic solvent to obtain lithium sulfide. The reaction is homogeneous, the reaction conditions are mild, and the lithium salt conversion rate is high.

[0018] 2. The lithium salt used in this application is one of the commonly used battery raw materials with high quality. The lithium sulfide product generated from this lithium salt has few impurities and high quality.

[0019] 3. This application uses ester or ether solvents as solvents. Lithium salts have high solubility in ester or ether solvents, require less solvent, and have high reaction efficiency. Compared with alcohol solvents, they will not cause the decomposition of lithium salts to produce a large amount of impurities, which in turn will lead to low product yield and purity.

[0020] 4. In the method of the present application, the generated lithium sulfide is insoluble in ester or ether solvents. After the reaction is completed, the lithium sulfide product can be directly filtered, which makes separation and purification simple and the product quality high.

[0021] 5. The by-products of this application are ammonium hexafluorophosphate, ammonium difluorophosphate and ammonium tetrafluoroborate, which can be recycled and used as electrolyte or raw materials for synthesizing electrolyte. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 This is the XRD spectrum of lithium sulfide prepared in Example 1 of the present application. DETAILED DESCRIPTION

[0023] The embodiments of the present application are described in detail below, which are intended to explain the present application and should not be construed as limiting the present application.

[0024] This application is based on the following discoveries and understandings of the inventors: The inventors have found that the current preparation conditions for obtaining lithium sulfide with low impurity content, high purity and high yield are harsh and not conducive to mass production. For example, in the related art, anhydrous sodium sulfide and lithium iodide are used as reactants. First, the reactants are pre-treated for a long time, and then the treated sodium sulfide and lithium iodide are reacted at a molar ratio of 0.4:2 at 120°C for 12 hours. Then, they are washed with an organic solvent, and the by-products are separated by tetrahydrofuran solution. Finally, the lithium sulfide product is dried and other operations are performed. This method is time-consuming and labor-intensive. For example, in the related art, lithium hydride and elemental sulfur are mixed and mechanically ball-milled to obtain lithium sulfide. The reaction is violent, the heat generation is difficult to control, and the purity of the prepared lithium sulfide is low. It can be seen that the preparation methods in the related art are time-consuming and labor-intensive, the purity of the obtained lithium sulfide is low, and it is not conducive to achieving industrial mass production.

[0025] To this end, the inventors of this application conducted in-depth research on the preparation of lithium sulfide. On the one hand, to achieve large-scale production, the reaction conditions of the synthesis route should preferably avoid routes that require high temperature and high pressure conditions, which are energy-intensive and difficult to control. On the other hand, the lithium salt and sulfur salt involved in the reaction need to have appropriate reactivity, neither too high a reactivity that makes the reaction violent and difficult to operate, nor too low a reactivity that requires harsh conditions. As a result, the inventors developed a method for preparing lithium sulfide that uses high-quality lithium salts and sulfur salts to react under mild conditions to produce high-purity lithium sulfide.

[0026] The first aspect of the present application provides a method for preparing lithium sulfide, comprising: The lithium salt is mixed with ammonium sulfide to react to obtain lithium sulfide; Wherein, the lithium salt includes at least one of lithium hexafluorophosphate, lithium difluorophosphate and lithium tetrafluoroborate.

[0027] The reaction equation is as follows:

[0028] Where X can be PF6 - PO2F2 - 、BF4 - At least one of .

[0029] In this application, lithium salts and ammonium sulfide are subjected to a double decomposition reaction to prepare lithium sulfide. Compared with other reaction routes, the principle is simple, the reaction conditions are mild, the reaction rate is fast, and the product yield is high. The lithium salts of the above reaction are lithium salts such as lithium hexafluorophosphate, lithium difluorophosphate and lithium tetrafluoroborate, which have good chemical stability, are not easy to decompose at room temperature and pressure, and are suitable for large-scale transportation and storage in industry. Similarly, ammonium sulfide is widely used in the chemical industry as a sulfur source and is relatively easier to obtain. Moreover, ammonium sulfide is stable at room temperature and is easy to store and transport. Moreover, the reaction activity between the two is high, and it is easy to react to generate lithium sulfide. The raw materials are easy to obtain, the reaction conditions are mild, and the product yield is high. The by-products are ammonium hexafluorophosphate, ammonium difluorophosphate and ammonium tetrafluoroborate, which can be recycled as electrolytes or synthetic electrolyte raw materials.

[0030] Furthermore, lithium salts such as lithium hexafluorophosphate, lithium difluorophosphate and lithium tetrafluoroborate are also commonly used raw materials for batteries. Battery-grade lithium hexafluorophosphate, lithium difluorophosphate and lithium tetrafluoroborate have a certain guarantee for product quality during the production process. Lithium salts are of high quality, low impurities, high purity and stable chemical properties. Therefore, battery-grade lithium salts can be used in the method for preparing lithium sulfide of the present application, which can directly reduce the introduction of impurities in the reaction, help to improve the purity of the product, and at the same time, the sources of such lithium salts are relatively wide.

[0031] In some embodiments, the lithium salt and ammonium sulfide react in an organic solvent comprising at least one of an ester solvent and an ether solvent. The lithium salt and ammonium sulfide undergo a metathesis reaction in the organic solvent system to produce lithium sulfide. The organic solvent has good solubility for both the lithium salt and ammonium sulfide, thereby facilitating full dispersion of the reactants in the reaction system, increasing the contact area between the reactants, and improving the reaction rate, thereby ensuring a more complete reaction.

[0032] Specifically, commonly used organic solvents include alcohols, esters, and ethers. The inventors discovered that when using alcoholic solvents to dissolve lithium salts, due to the presence of hydroxyl groups in alcoholic solvents, they are highly reactive and easily react with certain groups in the lithium salt, causing the lithium salt to decompose and produce a large amount of impurities, thereby reducing product yield and purity. For example, lithium hexafluorophosphate (LiPF6) and the alcoholic solvent (ROH) can react as follows: LiPF6 + ROH = LiPF5(OR) + HF. LiPF5(OR) further decomposes to produce impurities such as HF, PF5, POF3, and ROPOF2. The HF produced by the reaction itself can catalyze subsequent alcoholysis reactions, creating a vicious cycle.

[0033] Compared to alcohol solvents, ester and ether solvents are more chemically stable and less likely to react with lithium salts, ensuring a stable reaction system. They also offer better solubility and require less solvent, resulting in cost savings and a wider range of applications. Furthermore, the resulting lithium sulfide is insoluble in ester and ether solvents, making subsequent separation and purification steps relatively simple.

[0034] In some embodiments, the ester organic solvent includes at least one of dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, ethylene carbonate, ethyl acetate, and methyl acetate. In some embodiments, the ether solvent includes at least one of ethylene glycol dimethyl ether, ethylene glycol diethyl ether, diethylene glycol dimethyl ether, methyl tert-butyl ether, and diethyl ether. As a result, the chemical properties are more stable, the solubility is better, and it is less likely to react with lithium salts, thereby further ensuring the stability of the reaction system and making subsequent separation and purification steps easier.

[0035] In some embodiments, the mass ratio of the lithium salt to the organic solvent is 1:3 to 1:20, specifically 1:3, 1:4, 1:5, 1:6, 1:7, 1:8, 1:9, 1:10, 1:11, 1:12, 1:13, 1:14, 1:15, 1:16, 1:17, 1:18, 1:19, 1:20, etc. In some embodiments, the mass ratio of the lithium salt to the organic solvent is 1:4 to 1:8. This helps to completely dissolve the lithium salt in the organic solvent without wasting the organic solvent.

[0036] In some embodiments, the temperature for reacting the lithium salt and ammonium sulfide is 15°C to 70°C, specifically 15°C, 20°C, 25°C, 30°C, 35°C, 40°C, 45°C, 50°C, 55°C, 60°C, 65°C, 70°C, etc. In some embodiments, the temperature for reacting the lithium salt and ammonium sulfide can be 40°C to 50°C. Under the above temperature conditions, the reaction efficiency can be further improved, and the conditions are mild, there are few side reactions, the equipment requirements are low, and industrial production can be easily achieved. It can basically avoid problems such as low reaction rate caused by too low temperature or high equipment requirements caused by too high temperature.

[0037] In some embodiments, the lithium salt and ammonium sulfide are reacted for 2 to 6 hours, specifically 2 hours, 3 hours, 4 hours, 5 hours, 6 hours, etc. In some embodiments, the lithium salt and ammonium sulfide are reacted for 3 to 4 hours. Within this reaction time, the reaction is substantially complete, the reaction efficiency is high, time is not wasted, and the cost is low.

[0038] In some embodiments, the reaction of the lithium salt and ammonium sulfide can be carried out in an inert atmosphere. In some embodiments, the inert atmosphere comprises at least one of nitrogen and argon. This ensures a smooth reaction, reduces side reactions, and improves product purity and yield.

[0039] In some embodiments, the molar ratio of the lithium salt to the ammonium sulfide is 2:1 to 2.1:1, specifically 2:1, 2.01:1, 2.02:1, 2.03:1, 2.04:1, 2.05:1, 2.06:1, 2.07:1, 2.08:1, 2.09:1, 2.1:1, etc. This can further increase the reaction rate and promote a more complete reaction.

[0040] In some embodiments, after the mixing reaction of the lithium salt and the ammonium sulfide is completed, at least one of the following steps is further included: The lithium salt and the ammonium sulfide are mixed and reacted to obtain a mixed solution containing lithium sulfide, and the mixed solution is subjected to solid-liquid separation to obtain a crude lithium sulfide product; Drying the crude lithium sulfide product at 50° C. to 120° C. and -0.095 MPa for 1 h to 5 h to obtain a dry crude lithium sulfide product; The dried crude lithium sulfide product is calcined at 400° C. to 800° C. and −0.095 MPa for 1 h to 5 h.

[0041] Specifically, after the lithium salt and ammonium sulfide react in an organic solvent, a mixed solution containing lithium sulfide is directly obtained, which can be precipitated by solid-liquid separation. The specific filtration method can be selected according to actual needs, including but not limited to suction filtration.

[0042] It is understood that the crude lithium sulfide precipitate obtained by direct solid-liquid separation generally contains residual organic solvents, water, and impurities. These residual organic solvents, water, and volatile impurities can be removed by drying. In some embodiments, the drying temperature can be 50°C, 60°C, 70°C, 80°C, 90°C, 100°C, 110°C, 120°C, etc.; the drying pressure can be -0.095 MPa, etc.; and the drying time can be 1 hour, 2 hours, 3 hours, 4 hours, 5 hours, etc. Drying under the above conditions can achieve a higher purity of lithium sulfide.

[0043] It is understandable that some non-volatile impurities may still exist in the dried lithium sulfide after filtration and drying, so they can be further removed by calcination. At the same time, high-temperature calcination can make the crystal structure of lithium sulfide more perfect. Specifically, the calcination temperature can be 400°C, 450°C, 500°C, 550°C, 600°C, 650°C, 700°C, 750°C, 800°C, etc.; the calcination pressure can be specifically 0.095Mpa, etc.; the calcination time can be specifically 1h, 2h, 3h, 4h, 5h, etc. In this way, the structure of lithium sulfide can be further improved and the purity can be higher.

[0044] In some embodiments, the yield of lithium sulfide prepared by the method of the present application is ≥98%, specifically, 98%, 98.5%, 99.0%, 99.5%, 100%, etc. Therefore, the method can achieve industrial mass production.

[0045] In a second aspect, the present application provides lithium sulfide prepared by the aforementioned method for preparing lithium sulfide. This lithium sulfide is high in purity and low in impurities. When used to prepare a sulfide electrolyte, it can improve the electrolyte's ionic conductivity and interfacial stability, thereby enhancing the electrochemical performance and safety of the battery.

[0046] In some embodiments, the lithium sulfide has a purity of ≥99.95%, specifically 99.95%, 99.96%, 99.97%, 99.98%, 99.99%, 100%, etc. Thus, the lithium sulfide can be used to prepare a sulfide electrolyte for use in all-solid-state batteries, effectively improving the electrochemical performance and safety of the battery.

[0047] The embodiments of the present application are described in detail below.

[0048] Example 1: 1. Purge argon into a dry reactor to maintain an argon-free atmosphere, add 311.42 g of lithium hexafluorophosphate, 68.14 g of ammonium sulfide, and 1245.68 g of ethyl methyl carbonate, start stirring, and heat to 40°C for 3 h; 2. After the reaction is completed, the filter cake is filtered to obtain a filter cake, which is washed with ethyl methyl carbonate. The washed filter cake is dried at a vacuum degree of -0.095 MPa and a temperature of 80° C. for 3 h to obtain crude lithium sulfide; 3. The crude lithium sulfide was calcined at a vacuum degree of -0.095 MPa and a temperature of 600°C for 3 hours to obtain lithium sulfide.

[0049] Example 2-42 The method is the same as that in Example 1, and the specific parameters that are different are shown in Table 1.

[0050] Example 43 1. Purge argon into a dry reactor to maintain an argon-free atmosphere, add 311.42 g of lithium hexafluorophosphate, 68.14 g of ammonium sulfide, and 1245.68 g of ethanol, start stirring, and heat to 40°C for 3 h; 2. After the reaction is completed, the filter cake is filtered to obtain a filter cake, which is washed with ethyl methyl carbonate. The washed filter cake is dried at a vacuum degree of -0.095 MPa and a temperature of 80° C. for 3 h to obtain crude lithium sulfide; 3. The crude lithium sulfide was calcined at a vacuum degree of -0.095 MPa and a temperature of 600°C for 3 h to obtain lithium sulfide.

[0051] Comparative Example 1 1. Purge argon into a dry reactor to maintain an argon-free atmosphere, add 86.90 g of lithium chloride, 68.14 g of ammonium sulfide and 1245.68 g of ethyl methyl carbonate, start stirring, and heat to 40 ° C for 3 h; 2. After the reaction is completed, the filter cake is filtered to obtain a filter cake, which is washed with ethyl methyl carbonate. The washed filter cake is dried at a vacuum degree of -0.095 MPa and a temperature of 80° C. for 3 h to obtain crude lithium sulfide; 3. The crude lithium sulfide was calcined at a vacuum degree of -0.095 MPa and a temperature of 600°C for 3 h to obtain lithium sulfide.

[0052] Performance testing: 1. Lithium sulfide structure test: X-ray diffraction was used to test the structure of the product. The XRD spectrum of the lithium sulfide prepared in Example 1 is shown in FIG. Figure 1 .

[0053] 2. Yield: The yield is calculated based on the final lithium sulfide product. That is, the mass of the lithium sulfide after calcination is measured and its mass ratio to the theoretical yield of lithium sulfide (calculated based on the raw material dosage and reaction equation) is calculated, which is the yield.

[0054] 3. Purity: Metal ions (Na, K, Ca, Fe, Mg, Al, Cr, Pb, Ni, Co), PF6 - / BF4 - / PO2F2 - The purity of the product was then calculated by the difference method.

[0055]

[0056] It can be seen from the above test data that, compared with Comparative Example 1, the embodiment of the present application obtains lithium sulfide by mixing and reacting at least one of lithium hexafluorophosphate, lithium difluorophosphate and lithium tetrafluoroborate with ammonium sulfide. The reaction can be fully reacted under mild conditions, the reaction time is short and the reaction yield is high, and the purity of the obtained lithium sulfide is high.

[0057] Specifically, it can be seen from Examples 1-9 that the purity and yield of the prepared lithium sulfide do not change significantly when different lithium salts and organic solvents are selected, indicating that the types of lithium salts and organic solvents used in this application can ensure the smooth preparation of lithium sulfide and can achieve higher purity and yield.

[0058] It can be seen from Examples 1 and 10-16 that the purity and yield of lithium sulfide are higher when the mass ratio of lithium salt to organic solvent is 1:3-1:20, while the purity and yield of lithium sulfide are better when the mass ratio of lithium salt to organic solvent is 1:4-1:8.

[0059] It can be seen from Examples 1 and 17-23 that the purity and yield of lithium sulfide are higher when the reaction temperature is 15°C to 70°C, and the purity and yield of lithium sulfide are better when the reaction temperature is 40°C to 50°C.

[0060] It can be seen from Examples 1 and 24-27 that the purity and yield of lithium sulfide are higher when the reaction time is 2h~6h, and the purity and yield of lithium sulfide are better when the reaction time is 3h~4h.

[0061] It can be seen from Examples 1 and 28-30 that the molar ratio of lithium salt to ammonium sulfide is 2:1 to 2.1:1, and the purity and yield of lithium sulfide are high.

[0062] It can be seen from Examples 1 and 31-36 that the purity and yield of lithium sulfide are high when the drying temperature is 50° C. to 120° C. and the drying time is 1 h to 5 h.

[0063] It can be seen from Examples 1 and 37-43 that the purity and yield of lithium sulfide are high when the calcination temperature is 400° C. to 800° C. and the calcination time is 1 h to 5 h.

[0064] In the description of this specification, the reference terms "one embodiment," "some embodiments," "example," "specific example," or "some examples" mean that the specific features, structures, materials, or characteristics described in conjunction with the embodiment or example are included in at least one embodiment or example of the present invention. In this specification, the schematic representations of the above terms do not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials, or characteristics described can be combined in any one or more embodiments or examples in a suitable manner. In addition, those skilled in the art can combine and combine different embodiments or examples described in this specification and features of different embodiments or examples without contradiction.

[0065] Although the embodiments of the present invention have been shown and described above, it will be understood that the above embodiments are illustrative and are not to be construed as limitations on the present invention. A person skilled in the art may change, modify, replace and modify the above embodiments within the scope of the present invention.

Claims

1. A method for preparing lithium sulfide, characterized in that: include: Mixing lithium salt and ammonium sulfide to react to obtain lithium sulfide; Wherein, the lithium salt includes at least one of lithium hexafluorophosphate, lithium difluorophosphate and lithium tetrafluoroborate.

2. The method according to claim 1, characterized in that The lithium salt and the ammonium sulfide react in an organic solvent, and the organic solvent includes at least one of an ester solvent and an ether solvent.

3. The method according to claim 2, characterized in that The ester solvent includes at least one of dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, ethylene carbonate, ethyl acetate, and methyl acetate; and / or the ether solvent includes at least one of ethylene glycol dimethyl ether, ethylene glycol diethyl ether, diethylene glycol dimethyl ether, methyl tert-butyl ether, and diethyl ether.

4. The method according to claim 2, characterized in that The mass ratio of the lithium salt to the organic solvent is 1:3 to 1:

20.

5. The method according to claim 4, characterized in that The mass ratio of the lithium salt to the organic solvent is 1:4 to 1:

8.

6. The method according to claim 1, characterized in that The mixing reaction of the lithium salt and ammonium sulfide satisfies at least one of the following conditions: The reaction temperature is 15°C~70°C; Reaction time is 2h~6h; The reaction atmosphere is an inert atmosphere.

7. The method according to claim 6, characterized in that The mixing reaction of the lithium salt and ammonium sulfide satisfies at least one of the following conditions: The reaction temperature is 40°C~50°C; The reaction time is 3h~4h.

8. The method according to claim 1, characterized in that The molar ratio of the lithium salt to the ammonium sulfide is 2:1 to 2.1:

1.

9. The method according to claim 1, characterized in that After the mixing reaction of the lithium salt and the ammonium sulfide is completed, at least one of the following steps is further included: The lithium salt and the ammonium sulfide are mixed and reacted to obtain a mixed solution containing lithium sulfide, and the mixed solution is subjected to solid-liquid separation to obtain a crude lithium sulfide product; Drying the crude lithium sulfide product at 50° C. to 120° C. and -0.095 MPa for 1 h to 5 h to obtain a dry crude lithium sulfide product; The dried crude lithium sulfide product is calcined at 400° C. to 800° C. and −0.095 MPa for 1 h to 5 h.

10. The method according to any one of claims 1 to 9, characterized in that The yield of the lithium sulfide is ≥98%.

11. A lithium sulfide, characterized in that: Prepared according to the method according to any one of claims 1 to 10.

12. The lithium sulfide according to claim 11, characterized in that The purity of the lithium sulfide is ≥99.95%.

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