A method for preparing high-purity battery-grade lithium sulfide by reducing lithium sulfate with a reducing gas

Abstract

The application discloses a method for preparing high-purity battery-grade lithium sulfide by reducing lithium sulfate with a reducing gas. The specific steps are as follows: raw material lithium sulfate and inert wave-absorbing substances are mixed, uniformly mechanically ball-milled, tablet-pressed, sieved, put into a microwave reactor, and then a reducing gas is introduced for microwave heating reaction; after cooling to room temperature, the mixture is discharged into another reactor, ethanol is added for dissolution, mechanical stirring is carried out, and then standing and filtration are performed; the inert wave-absorbing substances are recycled and reused; an ethanol solution of dissolved lithium sulfide is obtained; crude lithium sulfide is obtained by heating and distillation; after freeze-drying, the crude lithium sulfide is heated and secondarily calcined in a rotary furnace under the protection of inert gas, and finally battery-grade high-purity lithium sulfide is obtained. By mixing the inert wave-absorbing substances with the lithium sulfate and heating by the microwave method, the contact surface of the lithium sulfate and the gas can be improved, and in addition, the internal heating method can be used to avoid the lithium sulfide formed on the outer surface from wrapping the raw material lithium sulfate, so as to affect the hydrogen reduction efficiency.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of preparation of solid-state electrolyte lithium sulfide, and particularly relates to a method for preparing high-purity battery-grade lithium sulfide by reducing lithium sulfate with a reducing gas. BACKGROUND

[0002] Lithium sulfide (Li2S) is not only a core raw material for synthesizing sulfide solid-state electrolyte (SSE) but also an optimal positive electrode material for lithium-sulfur batteries. Therefore, in recent years, the demand for Li2S materials in the international market has been increasing. At present, there are ball milling method, solvent method, high-temperature reduction method and direct carbon composite method for preparing Li2S materials. The high-temperature reduction method using low-price lithium sulfate as raw material has attracted widespread attention. High-temperature reduction includes magnesium hot reduction, aluminum hot reduction and carbon hot reduction, all of which have problems such as high impurity content, difficult subsequent processing, low yield and the like. The paper (Green Chem., 2024, 26, 7231-7245) reports a method for reducing lithium sulfate with hydrogen, which has fewer impurities introduced than other high-temperature reduction methods, and the subsequent process is simple, which may improve the yield. However, it is found in the actual experiment process that the hydrogen reduction method has the following problems: 1) due to the small gas-solid contact surface, the reaction is difficult, and the reaction time is long (> 12 h); 2) the generated lithium sulfide and lithium sulfate are easy to form a low-temperature eutectic body, which causes the product to be bonded to the container, and it is difficult to take out; 3) the generated lithium sulfide is wrapped on the surface of the raw material lithium sulfate, and the internal lithium sulfate is difficult to contact with hydrogen, which causes incomplete reaction, not only affects the yield, but also there is impurity lithium sulfate in the product, and secondary ball milling and secondary high-temperature reaction are still needed to obtain high-purity Li2S product. In summary, the above problems seriously limit the industrialization process development of the hydrogen reduction method, and it is urgent to find a solution. SUMMARY

[0003] In view of the problems of long reaction time, easy sticking, and insufficient reaction in the preparation of lithium sulfide by using lithium sulfate as raw material and gas phase reduction technology, the present application aims to provide a method for preparing high-purity battery-grade lithium sulfide by reducing lithium sulfate with reducing gas, which introduces microwave heating into the gas phase reduction process of lithium sulfate, and can improve the heating method through the heating process to solve the problem of too long heating time (traditional heating mode uses thermal radiation and thermal convection to heat the surface of the material first, and then heats the inside of the material through heat conduction, which has low heating efficiency and requires a long time, while microwave heating uses wave-absorbing materials to absorb microwaves to make molecules move at high speed to generate heat, which heats from the inside without the need for heat conduction process, and has fast and uniform heating speed), the added wave-absorbing material can dilute lithium sulfate particles on one hand, increase the gas contact area, and make lithium sulfate fully react with hydrogen to improve the reaction efficiency; on the other hand, it can also avoid the formation of eutectic, and solve the problem of difficult subsequent processing. The added wave-absorbing material can improve the wave-absorbing efficiency, which is conducive to microwave heating, and the wave-absorbing material can be reused through recycling, reducing the cost. Compared with the traditional tube furnace heating method, the process greatly improves the yield of lithium sulfide, and has operability for actual industrial production.

[0004] In order to achieve the above-mentioned purpose of the present application, the following technical solutions are adopted:

[0005] A method for preparing high-purity battery-grade lithium sulfide by reducing lithium sulfate with reducing gas, comprising the following steps: uniformly mixing lithium sulfate and inert wave-absorbing material, tabletting and sieving, placing in a microwave reaction tube, introducing reducing gas, opening the microwave reactor to heat, after microwave heating is completed, cooling to room temperature, uniformly crushing the obtained mixture, adding ethanol for dissolution, mechanical stirring, standing and filtering, recycling the inert wave-absorbing material for reuse, obtaining an ethanol solution of dissolved lithium sulfide, heating and distilling to obtain crude lithium sulfide, and then placing it into a rotary furnace, introducing inert gas, and calcining to remove organic carbon and sulfur, to obtain battery-grade high-purity lithium sulfide product.

[0006] Further, the inert wave-absorbing material is one of silicon, silicon carbide, silicon nitride, silicon boride, zirconium oxide, and aluminum oxide, and the mass ratio of lithium sulfate to inert wave-absorbing material is 1:1 to 1:9.

[0007] Further, the pressure for tabletting and sieving of the lithium sulfate and inert material mixture is 5-50 Mpa, the tabletting time is 10-60 min, and the particle size is 5-100 mesh.

[0008] Further, the reducing gas is hydrogen, carbon monoxide, a mixture of hydrogen and carbon monoxide, a mixture of hydrogen and inert gas, or a mixture of carbon monoxide and inert gas; the gas flow rate of the reducing gas is in the range of 10-50 mL·min -1 .

[0009] Furthermore, when the reducing gas is a mixture of hydrogen and an inert gas or a mixture of carbon monoxide and an inert gas, the volume concentration of hydrogen or carbon monoxide is 5-100%; the inert gas is nitrogen, argon or helium, and the purity of the inert gas is 99.99-99.9999%.

[0010] Furthermore, the microwave reactor has a power of 100–1350W, a heating temperature of 650–850℃, and a heating time of 1–5 h.

[0011] Furthermore, the mechanical stirring speed is 80~350 r·min. -1 The stirring time is 1~5 hours.

[0012] Furthermore, freeze drying is performed with a vacuum level ranging from 1 to 10 Pa, a freezing temperature ranging from -150 to -120 °C, and a freeze drying time ranging from 12 to 36 h.

[0013] Furthermore, the rotation speed of the rotary kiln is 1~4 r·min -1 .

[0014] Furthermore, the inert gas used in the calcination atmosphere is one of nitrogen, helium, or argon, with a purity of 99.99~99.9999%; the calcination temperature range is 400~600℃.

[0015] Compared with the existing hydrogen reduction lithium sulfate technology, the method of this invention has the following characteristics:

[0016] 1) To address the current challenges of difficult gas-solid phase contact, long reduction reaction time, and difficulty in achieving a complete reaction in the high-temperature reduction of solid lithium sulfate by hydrogen, and the easy formation of a eutectic during the reaction, this invention adds an inert microwave-absorbing material to the lithium sulfate mixture and heats it by microwave. This not only increases the contact area between lithium sulfate and the gas but also prevents lithium sulfide from forming on the outer surface and encapsulating the raw material lithium sulfate, thus affecting the efficiency of hydrogen reduction, through internal heating.

[0017] 2) In the process of removing organic carbon, the present invention uses freeze drying, which is conducive to the formation of loose crude lithium sulfide, and then uses a rotary kiln gas heating method, which is conducive to uniform solid heating and higher efficiency in removing organic carbon.

[0018] 3) The present invention combines microwave heating reduction, freeze drying and rotary kiln secondary roasting processes to form lithium sulfide with high yield, few by-products, and easy separation and purification of reaction products, which is conducive to large-scale industrial production. The product yield is as high as 90% and the purity is 99.9%. Attached Figure Description

[0019] Figure 1 The image shows the XRD pattern of lithium sulfide from Example 1. Detailed Implementation

[0020] The method described in this invention will be explained in detail below with specific examples.

[0021] Example 1

[0022] Weigh 6.0 g of lithium sulfate and 14.0 g of silicon carbide, with a mass ratio of lithium sulfate to silicon carbide of 3:7. Mix them in a ball mill. Compress the powder into tablets at 20 MPa for 30 min using a tablet press. Sift the resulting 30-mesh reactant and load it into a quartz boat. Place the boat in a microwave reaction tube and microwave at 40 mL / min. -1 A 20% hydrogen gas mixture (diluted with 99.99% argon) was introduced at a rate of 10 min, and then the microwave reactor was turned on. The microwave reaction power was set to 900 W, and the reaction was carried out at 700 °C for 5 h. After the reaction was completed, microwave heating was stopped, and after cooling to room temperature, a mixture of crude lithium sulfide and silicon carbide was obtained. The mixture was then pulverized under an argon atmosphere and quickly transferred to a reaction vessel. Ethanol (containing less than 1 ppm of water) was added under an argon atmosphere to dissolve the lithium sulfide, and the mixture was stirred at 200 r·min. -1 The mixture was stirred at a certain speed for 3 h and allowed to stand for 2 h to obtain an ethanol solution of lithium sulfide on top and a silicon carbide solid on the bottom. Under argon protection, the mixture was filtered to obtain silicon carbide and an ethanol solution containing dissolved lithium sulfide. The ethanol solution of lithium sulfide was transferred to an evaporator and heated for distillation to recover ethanol. Crude lithium sulfide was collected in a collector at the bottom of the reactor. The crude lithium sulfide was freeze-dried for 24 h under a vacuum of 2 Pa and a temperature of -120 °C to remove the solvent. The dried lithium sulfide was then rapidly transferred to a rotary kiln and refluxed at a rate of 40 mL / min. -1 99.99% argon gas was introduced at a rate of 10 min at 500 °C at a rate of 2 r·min. -1 The reaction was carried out at a rotational rate of 2 h to obtain a white lithium sulfide product with a yield of 92%. X-ray diffraction analysis showed the following results: Figure 1 As shown in the XRD pattern, only the characteristic diffraction peaks of lithium sulfide (corresponding to card PDF#04-004-0924) were observed in the X-ray diffraction, with no other impurity peaks; dry particle size analysis of the product showed D... 50 =1.5μm; ICP-MS analysis showed that the lithium sulfide purity reached 99.97%, the composition of metal impurities is shown in Table 1, and the ionic conductivity was 4.2 ms·cm. -1 .

[0023] Table 1. Inductively Coupled Plasma Mass Spectrometry Test Results of Lithium Sulfide Product in Example 1

[0024]

[0025] Example 2

[0026] Weigh 2.0 g of lithium sulfate and 18.0 g of silicon, with a lithium sulfate to silicon mass ratio of 1:9. Mix them in a ball mill. Compress the powder into tablets at 5 MPa for 60 min using a tablet press. Sift the resulting 10-mesh reactant material and load it into a quartz boat. Place the boat in a microwave reaction tube and microwave at 20 mL / min. -1 A mixture of CO:H2 (volume ratio 1:9) was introduced at a rate of 1:15 for 15 min. The microwave reactor was then turned on, and the microwave reaction power was set to 500 W. The reaction was carried out at 650 °C for 5 h. After the reaction was completed, microwave heating was stopped, and the mixture was allowed to cool to room temperature to obtain a crude lithium sulfide and silicon mixture. The mixture was then pulverized under a nitrogen atmosphere and quickly transferred to a reaction vessel. Ethanol (water content less than 1 ppm) was added under a nitrogen atmosphere to dissolve the lithium sulfide, and the mixture was stirred at 80 r·min⁻¹. -1 The mixture was stirred at a certain speed for 5 hours and allowed to stand for 2 hours, resulting in an ethanol solution of lithium sulfide on top and a silicon solid on the bottom. Under nitrogen protection, the mixture was filtered to obtain silicon and an ethanol solution containing dissolved lithium sulfide. The lithium sulfide ethanol solution was transferred to an evaporator and heated for distillation to recover ethanol. Crude lithium sulfide was collected in the reactor's collector. The crude lithium sulfide was freeze-dried at a vacuum of 4 Pa ​​and a temperature of -125 °C for 28 hours to remove the solvent. The dried lithium sulfide was then rapidly transferred to a rotary kiln and evaporated at a rate of 40 mL / min. -1 99.99% nitrogen gas was introduced at a rate of 10 min at 550 °C at a rate of 3 r·min. -1 The reaction was carried out at a rotational rate of 2 h to obtain a white lithium sulfide product with a yield of 90% and a purity of 99.94%. 50 =2.6 μm, ionic conductivity 3.2 ms·cm -1 .

[0027] Example 3

[0028] Weigh 2.5 g of lithium sulfate and 17.5 g of silicon nitride, with a mass ratio of lithium sulfate to silicon nitride of 1:7. Mix them in a ball mill. Compress the powder into tablets at 10 MPa for 50 min using a tablet press. Sift the resulting 20-mesh reactant material and load it into a quartz boat. Place the boat in a microwave reaction tube and microwave at 30 mL / min. -1A mixture of 5% carbon monoxide (diluted with 99.99% argon) was introduced at a rate of 20 min. The microwave reactor was then turned on, and the microwave reaction power was set to 550 W. The reaction was carried out at 850 °C for 1 h. After the reaction was completed, microwave heating was stopped, and the mixture was allowed to cool to room temperature to obtain a mixture of crude lithium sulfide and silicon nitride. The mixture was pulverized under a helium atmosphere and quickly transferred to a reaction vessel. Ethanol (containing less than 1 ppm of water) was added under a helium atmosphere to dissolve the lithium sulfide, and the mixture was stirred at 100 r·min. -1 The mixture was stirred at a certain speed for 4 h and allowed to stand for 2 h, resulting in an ethanol solution of lithium sulfide on top and a silicon nitride solid on the bottom. Under helium protection, the mixture was filtered to obtain silicon nitride and an ethanol solution containing dissolved lithium sulfide. The lithium sulfide ethanol solution was transferred to an evaporator and heated for distillation to recover ethanol. Crude lithium sulfide was collected in the reactor's collector. The crude lithium sulfide was freeze-dried at a vacuum of 6 Pa and a temperature of -130 °C for 32 h to remove the solvent. The dried lithium sulfide was then rapidly transferred to a rotary kiln and evaporated at a rate of 30 mL / min. -1 99.999% helium gas was introduced at a rate of 10 min at 600 °C at a rate of 3 r·min. -1 The reaction was carried out at a rotation rate of 1 h to obtain a white lithium sulfide product with a yield of 93% and a purity of 99.96%. 50 =2.8 μm, ionic conductivity 4.0 ms·cm -1 .

[0029] Example 4

[0030] Weigh 4.0 g of lithium sulfate and 16.0 g of zirconium oxide, with a mass ratio of lithium sulfate to zirconium oxide of 1:4. Mix them in a ball mill. Compress the powder into tablets at 15 MPa for 40 min using a tablet press. Sift the resulting 30-mesh reactant material and load it into a quartz boat. Place the boat in a microwave reaction tube and microwave at 30 mL / min. -1 Pure hydrogen gas was introduced at a rate of 15 min, the microwave reactor was turned on, the microwave reaction power was set to 600 W, and the reaction was carried out at 800 °C for 2 h. After the reaction was completed, microwave heating was stopped, and after cooling to room temperature, a mixture of crude lithium sulfide and zirconium oxide was obtained. The above mixture was pulverized under an argon atmosphere and quickly transferred to a reaction vessel. Ethanol (water content less than 1 ppm) was added under an argon atmosphere to dissolve the lithium sulfide, and the mixture was stirred at 150 r·min. -1The mixture was stirred at a certain speed for 3 hours and allowed to stand for 2 hours, resulting in an ethanol solution of lithium sulfide on top and a solid zirconium oxide on the bottom. Under argon protection, the mixture was filtered to obtain zirconium oxide and an ethanol solution containing dissolved lithium sulfide. The ethanol solution of lithium sulfide was transferred to an evaporator and heated for distillation to recover ethanol. Crude lithium sulfide was collected in the reactor's collector. The crude lithium sulfide was freeze-dried for 16 hours under a vacuum of 8 Pa and a temperature of -135 °C to remove the solvent. The dried lithium sulfide was then rapidly transferred to a rotary kiln and evaporated at a rate of 30 mL / min. -1 99.99% argon gas was introduced at a rate of 10 min at 450 °C at a rate of 4 r·min. -1 The reaction was carried out at a rotational rate of 100°C for 2 hours to obtain a white lithium sulfide product with a yield of 92% and a purity of 99.94%. 50 =3.6 μm, ionic conductivity 2.6 ms·cm -1 .

[0031] Example 5

[0032] Weigh 6 g of lithium sulfate and 14 g of silicon boride, with a mass ratio of lithium sulfate to silicon boride of 3:7. Mix them in a ball mill. Compress the powder into tablets at 25 MPa for 40 min using a tablet press. Sift the resulting 10-mesh reactant material and load it into a quartz boat. Place the boat in a microwave reaction tube and microwave at 40 mL / min. -1 A 20% CO mixture (diluted with 99.999% helium) was introduced at a rate of 10 min. The microwave reactor was then turned on, and the microwave reaction power was set to 650 W. The reaction was carried out at 750 °C for 2 h. After the reaction was completed, microwave heating was stopped, and the mixture was allowed to cool to room temperature to obtain a mixture of crude lithium sulfide and silicon boride. The mixture was pulverized under a nitrogen atmosphere and quickly transferred to a reaction vessel. Ethanol (containing less than 1 ppm of water) was added under a nitrogen atmosphere to dissolve the lithium sulfide, and the mixture was stirred at 250 r·min. -1 The mixture was stirred at a certain speed for 1 h and allowed to stand for 2 h, resulting in an ethanol solution of lithium sulfide on top and a silicon boride solid on the bottom. Under argon protection, the mixture was filtered to obtain silicon boride and an ethanol solution containing dissolved lithium sulfide. The lithium sulfide ethanol solution was transferred to an evaporator and heated for distillation to recover ethanol. Crude lithium sulfide was collected in the reactor's collector. The crude lithium sulfide was freeze-dried for 14 h under a vacuum of 8 Pa and a temperature of -140 °C to remove the solvent. The dried lithium sulfide was then rapidly transferred to a rotary kiln and evaporated at a rate of 30 mL / min. -1 99.9999% nitrogen gas was introduced at a rate of 1 r·min for 10 min at 550 °C. -1 The reaction was carried out at a rotational rate of 100°C for 3 hours to obtain a white lithium sulfide product with a yield of 89% and a purity of 99.92%. 50=2.4 μm, ionic conductivity 3.0 ms·cm -1 .

[0033] Example 6

[0034] Weigh 8.0 g of lithium sulfate and 12.0 g of α-alumina, with a mass ratio of lithium sulfate to α-alumina of 2:3. Mix them in a ball mill. Compress the powder into tablets at 40 MPa for 20 min using a tablet press. Sift the tablets to obtain 20-mesh reactants. Load the tablets into a quartz boat and place it in a microwave reaction tube. Incubate at 20 mL / min. -1 Pure carbon monoxide was introduced at a rate of 10 min, the microwave reactor was turned on, the microwave reaction power was set to 700 W, and the reaction was carried out at 750 °C for 3 h. After the reaction was completed, microwave heating was stopped, and after cooling to room temperature, a mixture of crude lithium sulfide and alumina was obtained. The above mixture was pulverized under a helium atmosphere and quickly transferred to a reaction vessel. Ethanol (water content less than 1 ppm) was added under a helium atmosphere to dissolve the lithium sulfide, and the mixture was stirred at 300 r·min. -1 The mixture was stirred at a constant speed for 1.5 h and allowed to stand for 2 h, resulting in an ethanol solution of lithium sulfide on top and alumina solid on the bottom. Under helium protection, the mixture was filtered to obtain alumina and an ethanol solution containing dissolved lithium sulfide. The ethanol solution of lithium sulfide was transferred to an evaporator and heated for distillation to recover ethanol. Crude lithium sulfide was collected in the reactor's collector. The crude lithium sulfide was freeze-dried for 12 h under a vacuum of 10 Pa and a temperature of -150 °C to remove the solvent. The dried lithium sulfide was then rapidly transferred to a rotary kiln and evaporated at a rate of 20 mL / min. -1 99.99% helium gas was introduced at a rate of 10 min at 500 °C at a rate of 2 r·min. -1 The reaction was carried out at a rotational rate of 100% for 2 hours to obtain a white lithium sulfide product with a yield of 90% and a purity of 99.93%. 50 =1.8 μm, ionic conductivity 3.8 mS·cm -1 .

[0035] Example 7

[0036] Weigh 8.0 g of lithium sulfate and 12.0 g of silicon carbide in a mass ratio of 2:3, and ball mill them together. The resulting powder is then compressed into tablets at 30 MPa for 30 min using a tablet press. The tablets are sieved to obtain a 20-mesh reactant, loaded into a quartz boat, placed in a microwave reaction tube, and microwaved at 50 mL / min. -1A mixture of CO:H2 (volume ratio 2:8) was introduced at a rate of 10 min. The microwave reactor was then turned on, and the microwave reaction power was set to 750 W. The reaction was carried out at 700 °C for 4 h. After the reaction was completed, microwave heating was stopped, and the mixture was allowed to cool to room temperature to obtain a mixture of crude lithium sulfide and silicon carbide. The mixture was then pulverized under a nitrogen atmosphere and quickly transferred to a reaction vessel. Ethanol (water content less than 1 ppm) was added under a nitrogen atmosphere to dissolve the lithium sulfide, and the mixture was stirred at 150 r·min. -1 The mixture was stirred at a certain speed for 3 hours and allowed to stand for 2 hours, resulting in an ethanol solution of lithium sulfide on top and a silicon carbide solid on the bottom. Under nitrogen protection, the mixture was filtered to obtain silicon carbide and an ethanol solution containing dissolved lithium sulfide. The ethanol solution of lithium sulfide was transferred to an evaporator and heated for distillation to recover ethanol. Crude lithium sulfide was collected in the collector of the reaction vessel. The crude lithium sulfide was freeze-dried for 24 hours under a vacuum of 1 Pa and a temperature of -120 °C to remove the solvent. The dried lithium sulfide was then rapidly transferred to a rotary kiln and evaporated at a rate of 50 mL / min. -1 99.999% nitrogen gas was introduced at a rate of 1 r·min for 10 min at 600 °C. -1 The reaction was carried out at a rotational rate of 2 h to obtain a white lithium sulfide product with a yield of 90% and a purity of 99.96%. 50 =1.4 μm, ionic conductivity 4.0 ms·cm -1 .

[0037] Example 8

[0038] Weigh 10.0 g of lithium sulfate and 10.0 g of silicon nitride in a mass ratio of 1:1, and ball mill them together. The resulting powder is then compressed into tablets at 20 MPa for 20 min using a tablet press. The tablets are sieved to obtain a 20-mesh reactant, loaded into a quartz boat, placed in a microwave reaction tube, and microwaved at 40 mL / min. -1 A hydrogen mixture with a volume concentration of 40% (diluted with 99.99% argon) was introduced at a rate of 15 min. The microwave reactor was then turned on, and the microwave reaction power was set to 800 W. The reaction was carried out at 650 °C for 5 h. After the reaction was completed, microwave heating was stopped, and the mixture was allowed to cool to room temperature to obtain a mixture of crude lithium sulfide and silicon nitride. The mixture was pulverized under an argon atmosphere and quickly transferred to a reaction vessel. Ethanol (containing less than 1 ppm of water) was added under an argon atmosphere to dissolve the lithium sulfide, and the mixture was stirred at a rate of 200 r·min. -1The mixture was stirred at a certain speed for 3 h, then allowed to stand for 2 h, resulting in an ethanol solution of lithium sulfide on top and a silicon nitride solid on the bottom. Under argon protection, the mixture was filtered to obtain silicon nitride and an ethanol solution containing dissolved lithium sulfide. The lithium sulfide ethanol solution was transferred to an evaporator and heated for distillation to recover ethanol. Crude lithium sulfide was collected in the reactor's collector. The crude lithium sulfide was freeze-dried at a vacuum of 1 Pa and a temperature of -150 °C for 36 h to remove the solvent. The dried lithium sulfide was then rapidly transferred to a rotary kiln and evaporated at a rate of 40 mL / min. -1 99.99% argon gas was introduced at a rate of 1 r·min for 15 min at 600 °C. -1 The reaction was carried out at a rotation rate of 0.5% for 3 hours to obtain a white lithium sulfide product with a yield of 92% and a purity of 99.98%. 50 =1.42 μm, ionic conductivity 3.8 ms·cm -1 .

[0039] Comparative Example 1:

[0040] Weigh 6.0 g of lithium sulfate, ball-mill it in a ball mill, load it into a quartz boat, place it in a tube furnace, and heat it at a rate of 40 mL / min. -1 After passing pure hydrogen gas at a rate of 10 min, the reaction was carried out at 750 °C for 12 h. After the reaction was completed, heating was stopped, and the mixture was allowed to cool to room temperature to obtain crude lithium sulfide (with some melt adhering to the quartz boat). Ethanol (containing less than 1 ppm of water) was added under an argon atmosphere to dissolve the lithium sulfide, and the mixture was then subjected to a reaction at 250 r·min. -1 The mixture was stirred at a certain speed for 3 h, allowed to stand for 2 h, and then the lithium sulfide ethanol solution was transferred to an evaporator for heating and distillation. The lithium sulfide was collected in the collector of the reaction vessel, dried under vacuum at 80 °C for 12 h, and then transferred again to a tube furnace at a rate of 40 mL / min. -1 Argon gas was introduced, and the reaction was carried out at 500 °C for 3 h to obtain a white lithium sulfide product with a yield of 30% and a purity of 99.0%. 50 =66.3 μm, ionic conductivity 1.0 ms·cm -1 .

[0041] Comparative Example 2:

[0042] Weigh 8.0 g of lithium sulfate, ball-mill it in a ball mill, load it into a quartz boat, place it in a tube furnace, and heat it at a rate of 20 mL / min. -1After introducing 5% hydrogen gas (diluted with 99.99% argon) at a rate of 15 min, the reaction was carried out at 800 °C for 16 h. After the reaction was completed, heating was stopped, and crude lithium sulfide (with some adhesion) was obtained after cooling to room temperature. The product was transferred to a ball mill and ball-milled a second time under argon protection. Then, it was transferred to a quartz boat and reacted again at 700 °C for 8 h under argon protection. Ethanol (with a water content of less than 1 ppm) was added under an argon atmosphere to dissolve the lithium sulfide, and the reaction was carried out at a rate of 200 r·min. -1 The mixture was stirred at a certain speed for 4 h, allowed to stand for 2 h, and then the lithium sulfide ethanol solution was transferred to an evaporator and heated for distillation. The lithium sulfide was collected in the collector of the reaction vessel, dried under vacuum at 70 °C for 14 h, and then ball-milled again under argon protection. The resulting solution was transferred to a quartz boat and placed in a tube furnace, where it was heated at a speed of 40 mL / min. -1 Argon gas was introduced, and the reaction was carried out at 500℃ for 3 hours to obtain a white lithium sulfide product with a yield of 65% and a purity of 99.5%. 50 =8.8 μm, ionic conductivity 3.0 ms·cm -1 .

[0043] Comparative Example 3:

[0044] Weigh 6.0 g of lithium sulfate and ball mill it in a ball mill. Compress the powder into tablets at 20 MPa for 30 min using a tablet press. Sieve the tablets to obtain 30-mesh reactants. Load the reactants into a quartz boat and place it in a tube furnace. Press the furnace at 40 mL / min. -1 A hydrogen mixture with a volume concentration of 20% (diluted with 99.99% argon) was introduced at a rate of 10 min, and then the heating program of the tube furnace was turned on. The reaction was carried out at 700 °C for 5 h. After the reaction was completed, heating was stopped, and crude lithium sulfide was obtained after cooling to room temperature (some melt adhered to the quartz boat). The crude lithium sulfide was then pulverized under an argon atmosphere and quickly transferred to a reaction vessel. Ethanol (with a water content of less than 1 ppm) was added under an argon atmosphere to dissolve the lithium sulfide, and the mixture was stirred at a rate of 200 r·min. -1 The mixture was stirred at a certain speed for 3 h and allowed to stand for 2 h to obtain an ethanol solution of lithium sulfide on top and a small amount of lithium sulfate solid on the bottom. Under argon protection, the mixture was filtered to obtain lithium sulfate and an ethanol solution containing dissolved lithium sulfide. The ethanol solution of lithium sulfide was transferred to an evaporator and heated for distillation to recover ethanol. Crude lithium sulfide was collected in a collector at the bottom of the reactor. The crude lithium sulfide was freeze-dried for 24 h under a vacuum of 2 Pa and a temperature of -120 °C to remove the solvent. The dried lithium sulfide was then rapidly transferred to a tube furnace and evaporated at a rate of 40 mL / min. -1 99.99% argon gas was introduced at a rate of 10 min at 500 °C at a rate of 2 r·min. -1The reaction was carried out at a rotational rate of 2 h to obtain a white lithium sulfide product with a yield of 42% and a purity of 99.6%. 50 =9.8 μm, ionic conductivity 2.7 ms·cm -1 .

Claims

1. A method for preparing high-purity battery-grade lithium sulfide by reducing lithium sulfate with a reducing gas, characterized in that, The process includes the following steps: lithium sulfate and an inert microwave absorbing material are ball-milled and mixed evenly, pressed into tablets and sieved, placed in a microwave reaction tube, a reducing gas is introduced, the microwave reactor is turned on and heated, after microwave heating is completed, the mixture is cooled to room temperature, the obtained mixture is pulverized evenly, dissolved in ethanol, mechanically stirred, allowed to stand and filtered, the inert microwave absorbing material is recovered and reused to obtain an ethanol solution of dissolved lithium sulfide, heated and distilled to obtain crude lithium sulfide, after freeze-drying, it is placed in a rotary kiln, inert gas is introduced, calcined to remove organic carbon and sulfur, and battery-grade high-purity lithium sulfide product is obtained; The inert absorbing material is one of silicon, silicon carbide, silicon nitride, silicon boride, zirconium oxide, and aluminum oxide, and the mass ratio of lithium sulfate to the inert absorbing material is 1:1 to 1:

9. The microwave reactor has a power of 100–1350W, a heating temperature of 650–850℃, and a heating time of 1–5 hours. Freeze-drying, with a vacuum degree ranging from 1 to 10 Pa; a freezing temperature range from -150 to -120 ℃; and a freeze-drying time range from 12 to 36 h.

2. The method for preparing high-purity battery-grade lithium sulfide by reducing lithium sulfate with a reducing gas as described in claim 1, characterized in that, The pressure for compressing and sieving the mixture of lithium sulfate and inert substances is 5~50 MPa, the compression time is 10~60 min, and the particle size is 5~100 mesh.

3. The method for preparing high-purity battery-grade lithium sulfide by reducing lithium sulfate with a reducing gas as described in claim 1, characterized in that, The reducing gas is hydrogen, carbon monoxide, a mixture of hydrogen and carbon monoxide, a mixture of hydrogen and an inert gas, or a mixture of carbon monoxide and an inert gas; the ventilation rate of the reducing gas ranges from 10 to 50 mL / min. -1 .

4. The method for preparing high-purity battery-grade lithium sulfide by reducing lithium sulfate with a reducing gas as described in claim 1, characterized in that, When the reducing gas is a mixture of hydrogen and an inert gas or a mixture of carbon monoxide and an inert gas, the volume concentration of hydrogen or carbon monoxide is 5-100%; the inert gas is nitrogen, argon or helium, and the purity of the inert gas is 99.99-99.9999%.

5. The method for preparing high-purity battery-grade lithium sulfide by reducing lithium sulfate with a reducing gas as described in claim 1, characterized in that, The mechanical stirring speed is 80~350 r·min. -1 The stirring time is 1~5 hours.

6. The method for preparing high-purity battery-grade lithium sulfide by reducing lithium sulfate with a reducing gas as described in claim 1, characterized in that, The rotation speed of the rotary kiln is 1~4 r·min -1 .

7. The method for preparing high-purity battery-grade lithium sulfide by reducing lithium sulfate with a reducing gas as described in claim 1, characterized in that, The inert gas used in the roasting atmosphere is one of nitrogen, helium, or argon, with a purity of 99.99~99.9999%; the roasting temperature range is 400~600℃.

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