How lithium sulfide is produced

Abstract

The present invention provides a method for producing lithium sulfide, comprising a lithium sulfide synthesis step of synthesizing lithium sulfide powder by reacting lithium sulfate powder with a reaction gas containing carbon monoxide, and the method may further comprise a lithium sulfate pulverization step of pulverizing the lithium sulfate powder to a predetermined size prior to the lithium sulfide synthesis step.

Description

[Technical field]

[0001] The present invention relates to a method for producing lithium sulfide (Li2S). [Background technology]

[0002] Recently, there has been an increasing need for lithium sulfide as a raw material for solid electrolytes in all-solid-state lithium secondary batteries. All-solid-state lithium secondary batteries are currently commercially available lithium secondary batteries in which the organic liquid electrolyte and separator are replaced with a solid electrolyte. Solid electrolytes are non-flammable or flame-retardant, making them safer than liquid electrolytes. Solid electrolytes are divided into oxide-based and sulfide-based. Sulfide-based solid electrolytes have higher lithium ion conductivity and are stable over a wide voltage range compared to oxide-based solid electrolytes, and are therefore primarily used as solid electrolytes.

[0003] Lithium sulfide does not exist as a natural mineral, so it can be obtained synthetically from other lithium compounds. In general, lithium sulfide can be produced by a synthetic process that uses lithium hydroxide or lithium carbonate as a lithium source. For example, the production method for lithium sulfide involves the combination of a solid lithium source (e.g., lithium hydroxide or lithium carbonate) and a gaseous sulfur source (S or CS 2(g) ) to produce lithium sulfide. However, lithium hydroxide or lithium carbonate is difficult to pulverize into fine powder, and the reaction efficiency with the gas, which is the sulfur source, decreases, so it may be difficult to produce high-purity lithium sulfide. In addition, the size of lithium sulfide increases during the synthesis process, so the reactivity may decrease during the synthesis process of the solid electrolyte. Furthermore, lithium hydroxide and lithium carbonate used as lithium sources are expensive, so the production cost of lithium sulfide may increase. Summary of the Invention [Problem to be solved by the invention]

[0004] An object of the present invention is to provide a method for producing lithium sulfide, which can increase the purity of lithium sulfide powder while reducing the production cost of lithium sulfide. Another object of the present invention is to provide a method for producing lithium sulfide, which can reduce the size of the produced lithium sulfide powder and increase the reactivity of the powder. [Means for solving the problem]

[0005] The method for producing lithium sulfide of the present invention is characterized by including a lithium sulfide synthesis step of reacting lithium sulfate powder with a reaction gas containing carbon monoxide to synthesize lithium sulfide powder.

[0006] The method for producing lithium sulfide of the present invention may further include a lithium sulfate pulverizing step of pulverizing the lithium sulfate powder to a predetermined size before the lithium sulfide synthesis step. Furthermore, the lithium sulfate pulverizing step may pulverize the lithium sulfate powder to a powder having a size of 0.01 to 10.0 μm.

[0007] The lithium sulfide producing step of the present invention may further include a lithium sulfate preheating step of heating the lithium sulfate powder to remove moisture remaining in the lithium sulfate powder after the lithium sulfate pulverizing step. The lithium sulfate preheating step can be performed by heating lithium sulfate hydrate to 100 to 400°C.

[0008] The method for producing lithium sulfide according to the present invention may further include a step of heat-treating the lithium sulfide powder after the lithium sulfide synthesis step, in which the lithium sulfide powder is heat-treated to increase the crystallinity of the lithium sulfide powder. The lithium sulfide powder heat-treating step may be performed at a heat treatment temperature higher than the synthesis temperature of the lithium sulfide synthesis step. The synthesis temperature in the lithium sulfide synthesis step may be 650 to 840°C, and the heat treatment temperature in the lithium sulfide powder heat treatment step may be 850 to 930°C.

[0009] The reaction gas may further contain hydrogen and a carrier gas. Also, the reaction gas may contain 10 to 50 vol% of the carbon monoxide based on the total volume of the reaction gas, 50 to 90 vol% of hydrogen and the carrier gas, and the carrier gas and hydrogen may be contained in a volume ratio of 90:10 to 99:1. Furthermore, the reaction gas may contain 10 to 50 vol% of the carbon monoxide, 0.5 to 10 vol% of the hydrogen, and 45 to 90 vol% of the carrier gas based on the total volume of the reaction gas.

[0010] The reaction gas may further contain hydrogen, and may contain 90 to 99.5 vol % of the carbon monoxide and 0.5 to 10 vol % of the hydrogen, based on the total volume of the reaction gas. The lithium sulfide powder can be used as a solid electrolyte in an all-solid-state battery. Effect of the Invention

[0011] The method for producing lithium sulfide of the present invention has the effect of reducing production costs by using lithium sulfate or lithium sulfate hydrate as a lithium source. The method for producing lithium sulfide of the present invention does not use carbon, so that no carbon remains in the produced lithium sulfide powder, and the purity of the lithium sulfide powder is increased. The method for producing lithium sulfide of the present invention uses lithium sulfate which can be pulverized to a relatively small size compared to other lithium sources, and therefore has the effect of producing lithium sulfide powder with a smaller size. The method for producing lithium sulfide of the present invention uses lithium sulfate pulverized to small sizes, which has an increased surface area and increased reactivity with gas, thereby increasing the crystallinity of the lithium sulfide powder. The method for producing lithium sulfide of the present invention produces lithium sulfide powder in a fine powder form, which has the effect of increasing the reactivity of the lithium sulfate powder with other powders during the synthesis of the solid electrolyte. [Brief description of the drawings]

[0012] [Figure 1]FIG. 1 is a process diagram of a method for producing lithium sulfide according to an embodiment of the present invention. [Diagram 2] 1 is a SEM photograph of lithium sulfate powder used in a specific example of the present invention before being pulverized. [Diagram 3] 1 is a SEM photograph of the lithium sulfate powder used in a specific embodiment of the present invention after pulverization. [Figure 4] 1 is an XRD graph of lithium sulfide powder synthesized according to a specific embodiment of the present invention; [Diagram 5] 1 is an XRD graph of commercial lithium sulfide powder. [Figure 6] 5 is a graph showing the charge / discharge performance of half cells made using the synthesized lithium sulfide powder of FIG. 4 and a commercial lithium sulfide powder. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0013] Hereinafter, a method for producing lithium sulfide according to an embodiment of the present invention will be described in detail. First, a method for producing lithium sulfide according to an embodiment of the present invention will be described. FIG. 1 is a flow chart of a method for producing lithium sulfide according to one embodiment of the present invention.

[0014] 1, a method for producing lithium sulfide according to an embodiment of the present invention includes a step S10 of grinding lithium sulfate and a step S30 of synthesizing lithium sulfide. The method for producing lithium sulfide may further include a step S20 of preheating lithium sulfate and a step S40 of heat treating lithium sulfide powder.

[0015] The method for producing lithium sulfide may use lithium sulfate or lithium sulfate hydrate as a lithium source. The method for producing lithium sulfide may use a gas containing carbon monoxide (CO) as a reactant gas. The reactant gas may further contain hydrogen. The reactant gas may further contain hydrogen and a carrier gas. The carrier gas may contain at least one selected from nitrogen (N2) and argon (Ar).

[0016] The method for producing lithium sulfide is a reaction according to the following reaction formula 1) or 2), in which lithium sulfide can be produced by reacting lithium sulfate with carbon monoxide. Li2SO4+4CO → Li2S+4CO2------- 1) Li2SO4+2CO+2H2 → Li2S+2CO2+2H2O ------- 2)

[0017] The method for producing lithium sulfide uses lithium sulfate or lithium sulfate hydrate as a lithium source, thereby reducing production costs. The method for producing lithium sulfide does not use carbon, so that no carbon remains in the lithium sulfide powder produced, thereby increasing the purity of the lithium sulfide powder produced. The method for producing lithium sulfide uses lithium sulfate that can be pulverized to a relatively small size compared to other lithium sources, thereby reducing the size of the lithium sulfide powder produced. The method for producing lithium sulfide uses lithium sulfate pulverized to a small size, thereby increasing the surface area and reactivity with gas, thereby increasing the crystallinity of the lithium sulfide powder. The method for producing lithium sulfide produces lithium sulfide in a fine powder, so that the reactivity of the lithium sulfate powder with other powders can be increased during the synthesis of a solid electrolyte.

[0018] The lithium sulfate pulverizing step S10 is a step of pulverizing the lithium sulfate powder to a predetermined size. The lithium sulfate powder may be a relatively coarse powder of various sizes as a raw material. For example, the lithium sulfate powder may be a lithium sulfate powder having an average particle size of about 200 μm. In order to have certain characteristics, it is necessary to use a powder having a predetermined range of sizes for the lithium sulfate powder. Therefore, the lithium sulfate pulverizing step S10 can pulverize the lithium sulfate powder to a powder having a predetermined size. The lithium sulfate powder can be pulverized to a powder having a diameter of preferably 0.01 to 10.0 μm. Also, the lithium sulfate powder can be pulverized to a powder having an average particle size of 0.1 to 1.0 μm. The lithium sulfate pulverizing step S10 can be performed using a pulverizing device such as a ball mill device.

[0019] The lithium sulfate pre-heating step S20 is a step of pre-heating lithium sulfate powder to remove moisture remaining in the lithium sulfate powder. The lithium sulfate pre-heating step S20 can be selectively performed when lithium sulfate hydrate is used as the lithium source. The lithium sulfate hydrate powder can be Li2SO4·nH2O. The lithium sulfate pre-heating step S20 can be performed to remove moisture remaining in the lithium sulfate powder even when lithium sulfate powder is used as the lithium source.

[0020] The lithium sulfate pre-heating step S20 can be performed by heating lithium sulfate hydrate powder or lithium sulfate powder at a pre-heating temperature of 100 to 400° C. The lithium sulfate pre-heating step S20 can be performed according to the reaction represented by the following formula 3). Li2SO4·H2O → Li2SO4+H2O------- 3)

[0021] The lithium sulfide synthesis step S30 is a step of synthesizing lithium sulfide powder by reacting lithium sulfate powder with a reaction gas. The lithium sulfide powder can be synthesized by reacting lithium sulfate powder with a reaction gas according to the reaction of the above formula 1) or formula 2).

[0022] The lithium sulfide synthesis step S30 can be performed at a synthesis temperature of 650 to 840°C for a synthesis time of 0.5 to 5 hours. The synthesis temperature in the lithium sulfide synthesis step S30 can be set to a temperature lower than the melting point of lithium sulfate. The lithium sulfide synthesis step S30 can be performed at a synthesis temperature lower than 845°C, which is the melting point of lithium sulfate. In addition, the lithium sulfide synthesis step S30 can be performed by heating the lithium sulfate powder while supplying a reaction gas. The lithium sulfide synthesis step S30 can synthesize lithium sulfide while separating oxygen from the lithium sulfate powder.

[0023] The reaction gas may include carbon monoxide (CO). The reaction gas may include carbon monoxide (CO) and a carrier gas. The reaction gas may include carbon monoxide (CO) and hydrogen (H2). The reaction gas may include carbon monoxide (CO), hydrogen (H2) and a carrier gas. The carrier gas may include at least one selected from nitrogen (N2) and argon (Ar). When the reaction gas is a mixed gas of carbon monoxide and a carrier gas, the carbon monoxide can be mixed at 10 to 50 vol% based on the total volume of the reaction gas, and the carrier gas can be mixed at 50 to 90 vol%. The carbon monoxide can be mixed at 25 to 40 vol% based on the total volume of the reaction gas, and the carrier gas can be mixed at 50 to 90 vol%. When the reaction gas is a mixed gas of carbon monoxide and hydrogen, the carbon monoxide can be mixed at 90 to 99.5 vol% based on the total volume of the reaction gas, and the hydrogen can be mixed at 0.5 to 10 vol%. When the reaction gas is a mixed gas of carbon monoxide, hydrogen, and a carrier gas, carbon monoxide can be mixed at 10 to 50 vol% based on the total volume of the reaction gas, and hydrogen and the carrier gas can be mixed at 50 to 90 vol%. In this case, the hydrogen can be mixed with the carbon monoxide in a state where it is mixed with the carrier gas. Furthermore, the carrier gas and hydrogen can be mixed at a volume ratio of 90:10 to 99:1. The hydrogen can be mixed with the carbon monoxide separately from the carrier gas. In this case, the reaction gas can be mixed with 10 to 50 vol% carbon monoxide, 0.5 to 10 vol% hydrogen, and 45 to 90 vol% carrier gas.

[0024] The carbon monoxide can react with oxygen in lithium sulfate to generate carbon dioxide, thereby allowing lithium sulfide to be synthesized. The hydrogen can react with oxygen in lithium sulfate to generate water vapor (H2O), thereby allowing lithium sulfide to be synthesized. In addition, the carrier gas can separate carbon dioxide and water vapor (H2O), which are generated during the synthesis of lithium sulfide, from lithium sulfide and transport them to the outside. Therefore, the carrier gas can allow lithium sulfide powder to be efficiently synthesized with high purity.

[0025] The heat treatment step S40 of the lithium sulfide powder is a step of increasing the crystallinity of the lithium sulfide powder by heat treating the synthesized lithium sulfide powder. The heat treatment step of the lithium sulfide powder can be performed while supplying the reaction gas used in the synthesis step of lithium sulfide. The heat treatment step of the lithium sulfide powder can be selectively performed, and can be performed when it is necessary to increase the crystallinity of the lithium sulfide powder synthesized in the synthesis step of lithium sulfide.

[0026] The heat treatment step of the lithium sulfide powder can be performed at a heat treatment temperature of 850 to 930°C for a heat treatment time of 0.5 to 5 hours. The heat treatment temperature in the heat treatment step of the lithium sulfide powder can be set to a temperature lower than the melting point of lithium sulfide. Also, the heat treatment temperature can be performed at a temperature higher than the synthesis temperature. For example, the heat treatment temperature can be set in a temperature range higher than 845°C, which is the melting point of lithium sulfate, and lower than 938°C, which is the melting point of lithium sulfide. If the heat treatment temperature is too high, the lithium sulfide powder may partially melt, resulting in a decrease in crystallinity. Also, if the heat treatment temperature is too low, the degree of increase in crystallinity of the lithium sulfide powder may be small.

[0027] The method for producing lithium sulfide of the present invention will be described below with reference to more specific examples. FIG. 2 is a SEM photograph of the lithium sulfate powder used in the specific embodiment of the present invention before pulverization. FIG. 3 is a SEM photograph of the lithium sulfate powder used in the specific embodiment of the present invention after pulverization. FIG. 4 is an XRD graph of the lithium sulfide powder synthesized in the specific embodiment of the present invention. FIG. 5 is an XRD graph of a commercial lithium sulfide powder. FIG. 6 is a graph showing the charge / discharge performance of a half cell made using the synthesized lithium sulfide powder of FIG. 4 and the commercial lithium sulfide powder.

[0028] In this example, lithium sulfate monohydrate (Li2SO4·H2O) was used as a lithium source. As shown in FIG. 2, it was confirmed that the lithium sulfate monohydrate had a size of about 200 μm. The lithium sulfate monohydrate was pulverized using a ball mill. As shown in FIG. 3, the lithium sulfate monohydrate was pulverized into a powder having a diameter of about 0.1 μm. The pulverized lithium sulfate powder was placed in an alumina boat and heated in a tube furnace. The lithium sulfate powder was synthesized at a synthesis temperature of 780° C. for a synthesis time of 5 hours. In this example, the lithium sulfate pre-heating step was not performed. In addition, in this example, the lithium sulfide powder heat treatment step was not performed.

[0029] In this example, the crystallinity of the synthesized lithium sulfide powder was evaluated, and a solid electrolyte was produced and evaluated for various properties. In addition, in this example, a commercially available lithium sulfide powder was purchased as a reference example for comparative evaluation and was also evaluated.

[0030] [Evaluation of Crystal Phase] As shown in the XRD graph of Figure 4, the powder synthesized in the example only has Li2S peaks, and no peaks of unreacted phases or secondary phases are observed. Therefore, it can be confirmed that the synthesized powder is a lithium sulfide powder having crystallinity.

[0031] As shown in the XRD graph of the commercial lithium sulfide powder of the reference example in FIG. 5, it shows peaks at the same positions as in FIG. 4. Therefore, it can be confirmed that the synthesized powder has the same phase as the commercial lithium sulfide powder.

[0032] [Production of solid electrolyte powder] 4.28g of the lithium sulfide powder synthesized above, 4.141g of commercial P2S5 powder, and 1.58g of commercial LiCl powder were mixed, and then placed in a 250cc zirconia container together with 300g of 3mm diameter zirconia balls and 40g of Solvent, sealed, and milled at 600rpm for 20 hours with a planetary mill to produce a mixed powder. Since the synthesized lithium sulfide powder is sensitive to moisture, the powder mixing and sealing process was carried out in a glove box filled with sufficiently dried Ar gas.

[0033] The milled mixed powder was dried in a vacuum oven at 110°C for 15 hours, and then heat-treated in a quartz tube sealed with Ar gas at 360°C for 5 hours to produce a solid electrolyte powder. The heat-treated solid electrolyte powder was analyzed by X-ray diffraction to confirm the Li6PS5Cl peak.

[0034] [Measurement of ionic conductivity] The produced solid electrolyte powder was uniaxially pressed at a pressure of 560 MPa in a glove box to produce pellets, and the ionic conductivity was measured by an AC impedance method. According to the evaluation results, the solid electrolyte powder mixed with lithium sulfide powder of the example was measured to have an ionic conductivity of 2.09 mS / cm. On the other hand, the solid electrolyte powder mixed with lithium sulfide powder of the reference example was measured to have an ionic conductivity of 2.10 mS / cm. Therefore, it can be confirmed that the solid electrolyte powder of the example has good ionic conductivity.

[0035] [Preparation of half-cell and evaluation of charge / discharge performance] 150 mg of the solid electrolyte powder of the embodiment was uniaxially pressed at a pressure of 450 MPa in a mold with a diameter of 12 mm to prepare a solid electrolyte pellet. This molding process was carried out in a glove box filled with argon gas. A lithium composite oxide (LiNi 0.8 Co 0.1 Mn 0.1A positive electrode material made by mixing 57.6 parts by mass of O2 powder, 38.4 parts by mass of solid electrolyte powder whose particle size was adjusted to an average particle size of 1 μm, and 4 parts by mass of carbon black conductive material was placed on the plate and uniaxially pressed at a pressure of 550 MPa. Furthermore, indium (In) powder was pressed at a pressure of 217 MPa as a negative electrode material to form a half cell. The manufactured half cell was charged and discharged at 30°C, 0.05C, and in the charge / discharge range of 0.6 to 3.6 V.

[0036] Similarly, a half-cell was fabricated using a solid electrolyte powder mixed with commercial lithium sulfide powder. As shown in Fig. 6, it can be confirmed that the half-cells according to Example (a) and Reference Example (b) show equivalent charge / discharge performance. Therefore, it can be confirmed that the lithium sulfide powder according to the example has the same characteristics as the commercial lithium sulfide powder when used as a solid electrolyte powder.

[0037] The evaluation results for the lithium sulfide powders of the Examples and Reference Examples are summarized in Table 1 below. [Table 1]

[0038] As seen from the above specific examples, it has been confirmed that the method for producing lithium sulfide powder according to the embodiments of the present invention can be applied to the production of lithium sulfide powder for solid electrolyte powder. The method for producing lithium sulfide powder according to the embodiments of the present invention uses relatively inexpensive lithium sulfate as a lithium source, so that lithium sulfide powder can be easily produced at low cost.

[0039] What has been described above is merely one embodiment for carrying out the method for producing lithium sulfide according to the present invention, and the present invention is not limited to the above embodiment. As claimed in the following claims, it can be said that the technical spirit of the present invention is within the scope of the invention to the extent that anyone having ordinary knowledge in the field to which the invention belongs can make various modifications without departing from the gist of the present invention.

Claims

1. 1. A method for producing lithium sulfide, comprising: a step of synthesizing lithium sulfide powder by reacting lithium sulfate powder with a reaction gas containing carbon monoxide.

2. Prior to the synthesis of lithium sulfide, 2. The method of claim 1, further comprising the step of pulverizing the lithium sulfate powder to a predetermined size.

3. 3. The method of claim 2, wherein the lithium sulfate pulverizing step comprises pulverizing the lithium sulfate powder to a powder having a diameter of 0.01 to 10.0 μm.

4. After the lithium sulfate grinding step, 3. The method for producing lithium sulfide according to claim 2, further comprising a pre-heating step of heating the lithium sulfate powder to remove moisture remaining in the lithium sulfate powder.

5. The method for producing lithium sulfide according to claim 4, wherein the lithium sulfate pre-heating step is performed by heating lithium sulfate hydrate to 100 to 400°C.

6. After the lithium sulfide synthesis step, 2. The method of claim 1, further comprising a heat treatment step of increasing the crystallinity of the lithium sulfide powder by heat treating the synthesized lithium sulfide powder.

7. 7. The method of claim 6, wherein the heat treatment of the lithium sulfide powder is performed at a heat treatment temperature higher than a synthesis temperature of the lithium sulfide synthesis step.

8. The synthesis temperature in the lithium sulfide synthesis step is 650 to 840° C., The method for producing lithium sulfide according to claim 7, wherein the heat treatment temperature in the heat treatment of the lithium sulfide powder is 850 to 930°C.

9. The method for producing lithium sulfide according to claim 1, wherein the reaction gas further comprises hydrogen and a carrier gas.

10. The reaction gas is The carbon monoxide is contained in an amount of 10 to 50 vol % based on the total volume of the reaction gas, and hydrogen and a carrier gas are contained in an amount of 50 to 90 vol %, The method for producing lithium sulfide according to claim 9, wherein the carrier gas and hydrogen are contained in a volume ratio of 90:10 to 99:

1.

11. The method for producing lithium sulfide according to claim 9, wherein the reaction gas contains 10 to 50 vol% of the carbon monoxide, 0.5 to 10 vol% of the hydrogen, and 45 to 90 vol% of the carrier gas, based on a total volume of the reaction gas.

12. The method for producing lithium sulfide according to claim 1, wherein the reaction gas further contains hydrogen, and the carbon monoxide is contained in an amount of 90 to 99.5 vol% and the hydrogen is contained in an amount of 0.5 to 10 vol% based on a total volume of the reaction gas.

13. The method for producing lithium sulfide according to claim 1, wherein the lithium sulfide powder is used as a solid electrolyte of an all-solid-state battery.

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