Method for directly preparing high-purity Li2S by carbon thermal reduction

Abstract

The invention discloses a method for directly preparing high-purity Li2S by utilizing carbothermal reduction, which comprises the following steps: ball-milling and mixing Li2SO4.H2O and a carbon source, and carrying out first-step calcination in an air atmosphere at the calcination temperature of 450-550 DEG C to obtain a first-step calcination product; carrying out ball-milling mixing on the first-step calcined product and sublimed sulfur powder, tabletting, and carrying out second-step calcination at the calcination temperature of 700-800 DEG C under the protection of inert gas to obtain a second-step calcined product; and continuously performing third-step calcination on the second-step calcination product under the protection of inert gas at the calcination temperature of 850-950 DEG C to obtain the product Li2S. Lithium sulfate and a high-purity carbon source are adopted as main raw materials, sublimed sulfur powder is introduced in the second-step calcination process to inhibit generation of Li2O, after the carbon source is consumed in the second-step calcination process, a mixture of lithium sulfide and residual lithium sulfate is obtained, the temperature continues to rise, third-step calcination is conducted, the residual lithium sulfate is molten, volatilized and removed, and the lithium sulfide is obtained. And discharging the mixture out of the tubular furnace along with the gas flow of the inert gas to obtain high-purity lithium sulfide.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of sulfide solid-state electrolyte, and particularly relates to a method for directly preparing high-purity Li2S by carbon thermal reduction. BACKGROUND

[0002] Power batteries are main components of electric vehicles. Traditional liquid power batteries have problems such as low safety and low energy density. A full solid-state battery uses a solid-state electrolyte to replace the electrolyte in a liquid battery, which can greatly improve the safety and energy density of the battery. Among the reported solid-state electrolytes, sulfide solid-state electrolytes are of great concern due to their excellent ionic conductivity. However, the high price of sulfide solid-state electrolytes hinders the practical application process. One of the reasons for the high price of sulfide solid-state electrolytes is the high price of the synthetic raw material lithium sulfide. In addition, lithium sulfide has poor air stability and will deteriorate rapidly when exposed to air by reacting with water. The entire synthesis process of lithium sulfide is carried out in an argon glove box, which also increases the cost of synthesizing lithium sulfide.

[0003] Currently, there are related reports on the synthesis method of lithium sulfide, mainly including lithium / sulfur dry method, lithium / sulfur wet method, metathesis reaction method, metal thermal reduction reaction method, hydrogen thermal reduction reaction method and carbon thermal reduction reaction method. Carbon thermal reduction reaction is a process of reducing dry lithium sulfate and solid reducing agent powder at a certain stoichiometric ratio under inert gas protection to generate lithium sulfide at high temperature. However, the product obtained will have unreacted carbon and lithium sulfate, and subsequent impurity removal depends on liquid solvent, which has high solvent recovery cost and significant secondary pollution risk. In addition, the product morphology and crystal type need to be controlled in subsequent applications, which depends on more stages of calcination, limiting the low-cost manufacturing of lithium sulfide. For example, the Chinese patent with the publication number CN119461260A discloses a method for preparing high-purity, fine-particle-size lithium sulfide based on carbon thermal reduction. The crude product lithium sulfide obtained by carbon thermal reaction calcination is washed with alcohol solution for multiple times, and then high-purity lithium sulfide is obtained by subsequent recrystallization calcination. SUMMARY

[0004] The present application provides a method for directly preparing high-purity Li2S by carbon thermal reduction, to solve the problems of high cost and long cycle caused by organic solvent-based dissolution and impurity removal in the existing carbon thermal reduction reaction method.

[0005] To solve the above technical problems, according to one aspect of the present application, a method for directly preparing high-purity Li2S by carbon thermal reduction is provided, comprising: Step one, Li2SO4·H2O and carbon source are mixed by ball milling, and the first calcination is carried out in an air atmosphere, the calcination temperature is 450-550℃, and the first calcination product is obtained; Step two, the first step calcination product and sublimation sulfur powder are mixed by ball milling, and after tabletting, the second step calcination is carried out under the protection of inert gas, the calcination temperature is 700-800℃, and the second step calcination product is obtained; Step three, the second step calcination product is continuously calcined under the protection of inert gas, the calcination temperature is 850-950℃, and the product Li2S is obtained.

[0006] As a preferred embodiment, in step one, the molar ratio of Li2SO4·H2O and carbon source is 1:(3.4-3.9).

[0007] As a preferred embodiment, in step one, the calcination time is 2-4h.

[0008] As a preferred embodiment, in step one, the carbon source is a combination of one or more of battery-grade artificial graphite, battery-grade soft carbon and battery-grade hard carbon.

[0009] As a preferred embodiment, in step two, the mass ratio of the first step calcination product and sublimation sulfur powder is (4-3):1.

[0010] As a preferred embodiment, in step two, the calcination temperature is 780℃.

[0011] As a preferred embodiment, in step two, the inert gas is nitrogen.

[0012] As a preferred embodiment, in step two, the calcination time is 3-5h.

[0013] As a preferred embodiment, in step three, the calcination time is 1-2h.

[0014] According to another aspect of the present application, there is provided Li2S obtained according to any one of the above methods.

[0015] The present application uses lithium sulfate and high-purity carbon source as main raw materials, and introduces sublimation sulfur powder in the second step calcination process to inhibit the generation of Li2O. After the carbon source is consumed in the second step calcination, a mixture of lithium sulfide and residual lithium sulfate is obtained. The residual lithium sulfate is removed by melting and volatilization in the third step calcination, and is discharged from the tube furnace with the inert gas flow, thereby obtaining high-purity lithium sulfide.

[0016] In order to eliminate the defects of low cost, the present application uses low-cost lithium sulfate monohydrate (Li2SO4·H2O), carbon source and sulfur powder as raw materials, does not use any solvent, and obtains lithium sulfide with a purity of ≥99% through three-step calcination, without the need for further liquid phase impurity removal, thereby shortening the cycle length of large-scale preparation of lithium sulfide, and simplifying the steps and facilitating large-scale preparation. BRIEF DESCRIPTION OF DRAWINGS

[0017] Figure 1 Process flow chart for the method of the present application; Figure 2 XRD pattern of the product of the second step of Example 1; Figure 3 XRD pattern of the product of the third step of Example 1; Figure 4 XRD pattern of the product of the third step of Example 2; Figure 5 XRD pattern of the product of the third step of Example 3; Figure 6 XRD pattern of the product of the third step of Comparative Example 1; Figure 7 XRD pattern of the product of the third step of Example 4; Figure 8 XRD pattern of the product of the third step of Example 5. DETAILED DESCRIPTION

[0018] A typical embodiment of the present application provides a method for directly preparing high-purity Li2S by carbothermal reduction, the overall concept of which is to obtain lithium sulfide by three-step calcination without using any solvent, and the introduction of sublimed sulfur powder can inhibit the generation of Li2O in the carbon thermal side reaction in the second step of calcination, and is also conducive to improving the conversion rate of lithium sulfide. The third calcination utilizes a higher temperature to make the residual lithium sulfate melt and volatilize, leaving high-purity lithium sulfide. Specifically, the method provided by the present application comprises the following steps one to three.

[0019] Step one, first step calcination Lithium sulfate monohydrate (Li2SO4·H2O) and a carbon source are ball-mixed, and the first step of calcination is carried out in an air atmosphere, the calcination temperature is 450-550°C, and the calcination time is 2-4h, so as to remove the water in lithium sulfate monohydrate (Li2SO4·H2O) and the carbon source, and obtain the product of the first step of calcination.

[0020] The carbon source is a combination of one or more of battery-grade artificial graphite, battery-grade soft carbon and battery-grade hard carbon. The molar ratio of Li2SO4·H2O to the carbon source is 1:(3.4-3.9), for example: 1:3.4, 1:3.5, 1:3.6, 1:3.7, 1:3.8, 1:3.9.

[0021] In this step, ball-mixing makes the lithium sulfate monohydrate (Li2SO4·H2O) and the carbon source mixture fully contact, which is conducive to the progress of the solid-phase reaction. Exemplarily, the ball-milling speed is 500rpm, the ball-to-material ratio is 15:1, and the ball-milling time is 2h.

[0022] Step two, second step calcination The first step calcination product and sublimed sulfur powder are mixed by ball milling, and after tabletting, the second step calcination is carried out under inert gas protection to obtain the second step calcination product.

[0023] The addition of sublimed sulfur powder can inhibit the generation of lithium oxide in the reaction process of lithium sulfate and carbon. Sulfur will volatilize above 444.6℃, so the reaction will not introduce impurities.

[0024] Li2SO4+3C=S(g)+3CO(g)+Li2O Li2SO4+3CO(g)=S+3CO2(g)+Li2O Li2SO4+2C=Li2S+2CO2(g) Li2SO4+4C=Li2S+4CO(g) In this step, the calcination temperature is between 700-800℃, and the preferred calcination temperature is 780℃. The temperature should not be higher than 800℃, mainly because the side reaction will be more intense above 800℃, and the effect of adding sublimed sulfur powder will decrease.

[0025] In this step, the calcination time is 3-5h. The calcination time is also an important factor affecting the purity of lithium sulfide product. If the calcination time is less than 3h, more carbon will be left after the second step calcination, and the residual carbon will participate in the side reaction in the next step of higher third step calcination. If the calcination time is too long, the inhibitory effect of sublimed sulfur powder on the side reaction will be reduced after the sulfur powder is volatilized, and the time for the side reaction to occur will be extended.

[0026] In this step, the mass ratio of the first step calcination product and sublimed sulfur powder is (4-3):1, for example, 4:1, 3.8:1, 3.6:1, 3.5:1, 3.2:1, 3:1.

[0027] Step three, third step calcination The second step calcination product is further calcined in the third step under inert gas protection, the calcination temperature is 850-950℃, and the calcination time is 1-2h to obtain the product Li2S.

[0028] In this step, the purpose of increasing the calcination temperature is to be higher than the melting point of lithium sulfate 845℃, to remove the residual lithium sulfate in the second step calcination, and to obtain high-purity lithium sulfide as the inert atmosphere gas flow is discharged. The lithium sulfide is calcined at a higher temperature to make the crystal grains finer and the crystallinity higher, which is beneficial to the subsequent preparation of sulfide electrolyte with excellent performance.

[0029] The technical solutions claimed by the present application are further illustrated below through some examples. However, the examples and comparative examples are used to explain the embodiments of the present application and do not exceed the scope of the subject matter of the present application, and the protection scope of the present application is not limited by the examples. Unless otherwise specified, the materials and reagents used in the present application can be obtained from commercial products in the art. Example 1

[0030] Lithium sulfate monohydrate (AR) and soft carbon (battery grade) were ball-milled at a molar ratio of 1:3.7, with a ball milling speed of 500 rpm, a ball-to-material ratio of 15:1, and a ball milling time of 2 h. The obtained material was subjected to the first step of calcination under an air atmosphere, at a calcination temperature of 500 ℃ for 3 h, to obtain the first step calcination product.

[0031] The first step calcination product and sublimed sulfur powder were ball-milled at a mass ratio of 3.5:1, and then tabletted, with a ball milling speed of 200 rpm, a ball milling time of 1 h, and a ball-to-material ratio of 15:1. The obtained material was calcined under a nitrogen atmosphere, heated to 800 ℃ for 3 h, to obtain the second step calcination product.

[0032] The second step calcination product was further heated to 900 ℃ for 1 h, to obtain high-purity lithium sulfide.

[0033] The third step calcination product was analyzed by XRD Figure 2 and Figure 3 It can be obtained that, after the second step of calcination, no characteristic peaks of lithium oxide were observed in the XRD pattern of the product, which is due to the introduction of sublimed sulfur powder. After the third step of calcination, the residual lithium sulfate in the second step calcination product was removed, and only diffraction peaks of lithium sulfide were observed in the XRD pattern of the product. Example 2

[0034] Lithium sulfate monohydrate (AR) and soft carbon (battery grade) were ball-milled at a molar ratio of 1:3.7, with a ball milling speed of 500 rpm, a ball-to-material ratio of 15:1, and a ball milling time of 2 h. The obtained material was subjected to the first step of calcination under an air atmosphere, at a calcination temperature of 500 ℃ for 3 h, to obtain the first step calcination product.

[0035] The first step calcination product and sublimed sulfur powder were ball-milled at a mass ratio of 3.5:1, and then tabletted, with a ball milling speed of 200 rpm, a ball milling time of 1 h, and a ball-to-material ratio of 15:1. The obtained material was calcined under a nitrogen atmosphere, heated to 800 ℃ for 3 h, to obtain the second step calcination product.

[0036] The second step calcination product was further heated to 900 ℃ for 1 h, to obtain high-purity lithium sulfide.

[0037] The third step calcination product was analyzed by XRD Figure 4It can be obtained that some parameters in the reaction process are fine-tuned according to the range in the summary, and finally the third step calcination product also only has the diffraction peak of lithium sulfide. Example 3

[0038] Lithium sulfate monohydrate (AR) and artificial graphite (battery grade) were ball-milled at a molar ratio of 1:3.9, the ball-milling speed was 500 rpm, the ball-to-material ratio was 15:1, and the ball-milling time was 2 h. The obtained material was subjected to the first step calcination under air atmosphere, the calcination temperature was 550℃, and the calcination time was 2 h, to obtain the first step calcination product.

[0039] The first step calcination product and sublimed sulfur powder were ball-milled at a mass ratio of 3:1, and then tabletted, the ball-milling speed was 200 rpm, the ball-milling time was 1 h, and the ball-to-material ratio was 15:1. The obtained material was calcined under nitrogen protection, the temperature was raised to 720℃ and kept for 5 h, to obtain the second step calcination product.

[0040] The second step calcination product was continuously heated to 950℃ and kept for 1 h, to obtain high-purity lithium sulfide.

[0041] The third step calcination product was analyzed by XRD Figure 5 It can be obtained that some parameters in the reaction process are fine-tuned according to the range in the summary, and finally the third step calcination product also only has the diffraction peak of lithium sulfide. Example 4

[0042] Lithium sulfate monohydrate (AR) and artificial graphite (battery grade) were ball-milled at a molar ratio of 1:3.5, the ball-milling speed was 500 rpm, the ball-to-material ratio was 15:1, and the ball-milling time was 2 h. The obtained material was subjected to the first step calcination under air atmosphere, the calcination temperature was 480℃, and the calcination time was 3 h, to obtain the first step calcination product.

[0043] The first step calcination product and sublimed sulfur powder were ball-milled at a mass ratio of 3.2:1, and then tabletted, the ball-milling speed was 200 rpm, the ball-milling time was 1 h, and the ball-to-material ratio was 15:1. The obtained material was calcined under nitrogen protection, the temperature was raised to 700℃ and kept for 4 h, to obtain the second step calcination product.

[0044] The second step calcination product was continuously heated to 900℃ and kept for 1 h, to obtain high-purity lithium sulfide.

[0045] The third step calcination product was analyzed by XRD Figure 7 It can be obtained that some parameters in the reaction process are fine-tuned according to the range in the summary, and finally the third step calcination product also only has the diffraction peak of lithium sulfide. Example 5

[0046] Lithium sulfate monohydrate (AR) and artificial graphite (battery grade) were mixed by ball milling at a molar ratio of 1:3.6, a ball milling speed of 500 rpm, a ball-to-material ratio of 15:1, and a ball milling time of 2 h. The obtained material was subjected to a first-step calcination under an air atmosphere at a calcination temperature of 520 °C for a calcination time of 3 h to obtain a first-step calcination product.

[0047] The first-step calcination product and sublimed sulfur powder were mixed by ball milling at a mass ratio of 3.6:1, a ball milling speed of 200 rpm, a ball milling time of 1 h, and a ball-to-material ratio of 15:1, and then tableted. The obtained material was calcined under a nitrogen atmosphere, heated to 780 °C, and held at 780 °C for 4.5 h to obtain a second-step calcination product.

[0048] The second-step calcination product was further heated to 900 °C and held at 900 °C for 1 h to obtain a high-purity lithium sulfide.

[0049] The third-step calcination product was analyzed by XRD. Figure 8 It can be seen that, by fine-tuning some parameters in the reaction process according to the range in the disclosure, the third-step calcination product also only has diffraction peaks of lithium sulfide.

[0050] Comparative Example 1 The difference between Comparative Example 1 and Example 1 is that sublimed sulfur powder is not introduced into the first-step calcination product.

[0051] Lithium sulfate monohydrate (AR) and hard carbon (battery grade) were mixed by ball milling at a molar ratio of 1:3.4, a ball milling speed of 500 rpm, a ball-to-material ratio of 15:1, and a ball milling time of 2 h. The obtained material was subjected to a first-step calcination under an air atmosphere at a calcination temperature of 500 °C for a calcination time of 3 h to obtain a first-step calcination product.

[0052] The first-step calcination product was tableted and then subjected to a second-step calcination, heated to 750 °C, and held at 750 °C for 5 h to obtain a second-step calcination product.

[0053] The second-step calcination product was further heated to 850 °C and held at 850 °C for 2 h to obtain a lithium sulfide product. The product was analyzed by XRD. Figure 6 Comparative Examples 1, 2, and 3 obviously have diffraction peaks that do not belong to Li2S.

[0054] Table 1 shows the purity and yield of lithium sulfide obtained in Examples 1-3 and Comparative Example 1. It can be seen from the two data results that the introduction of sublimed sulfur powder in the second-step calcination process can inhibit the generation of lithium oxide in the reaction process.

[0055] Table 1

[0056] The scope of the present application is not limited to the above specific embodiments, and the present application can have various modifications and alterations, and any modification, improvement and equivalent replacement within the concept and principle of the present application should be included in the scope of protection of the present application.

Claims

1. A method for directly producing high-purity Li2S by carbothermal reduction, characterized by, The method comprises the following steps: Step 1: Li2SO4·H2O and a carbon source are mixed by ball milling, and a first-step calcination is carried out in an air atmosphere, wherein the calcination temperature is 450-550℃, and a first-step calcination product is obtained; Step 2: the first-step calcination product and sublimed sulfur powder are mixed by ball milling, and a second-step calcination is carried out under the protection of an inert gas after tabletting, wherein the calcination temperature is 700-800℃, and a second-step calcination product is obtained; Step 3: the second-step calcination product is continuously subjected to a third-step calcination under the protection of an inert gas, wherein the calcination temperature is 850-950℃, and a product Li2S is obtained.

2. The method of claim 1, wherein: In step 1, the molar ratio of Li2SO4·H2O to the carbon source is 1:(3.4-3.9).

3. The method according to claim 1 or 2, characterized in that: In step 1, the calcination time is 2-4h.

4. The method of claim 3, wherein: In step 1, the carbon source is a combination of one or more of battery-grade artificial graphite, battery-grade soft carbon and battery-grade hard carbon.

5. The method according to claim 1 or 4, characterized in that: In step 2, the mass ratio of the first-step calcination product to sublimed sulfur powder is (4-3):

1.

6. The method of claim 5, wherein: In step 2, the calcination temperature is 780℃.

7. The method of claim 6, wherein: In step 2, the inert gas is nitrogen.

8. The method of claim 7, wherein: In step 2, the calcination time is 3-5h.

9. The method of claim 1 or 8, wherein: In step 3, the calcination time is 1-2h.

10. Li2S obtained by the method according to any one of claims 1-9.

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