How lithium sulfide is produced

Abstract

To provide a method for more suitably producing lithium sulfide having high ion conductivity without producing by-products.SOLUTION: A method for producing lithium sulfide involves a temperature raising step (step S14) in which: lithium sulfate put into a furnace is reduced, in an atmosphere with a reduced pressure of 0.05 MPa or less, in the state where it is heated to a temperature higher than 700°C.SELECTED DRAWING: Figure 1

Description

[Technical field]

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

[0002] Lithium sulfide is known as a solid electrolyte for lithium batteries. Patent Document 1 discloses a method in which lithium hydroxide is reacted with hydrogen sulfide in an aprotic organic solvent to produce lithium hydrosulfide, and the reaction is further carried out to produce lithium sulfide.

[0003] Patent Document 2 discloses a method for producing lithium sulfide without using an organic solvent. In Patent Document 2, metallic lithium is reacted with sulfur vapor or hydrogen sulfide to generate lithium sulfide on metallic lithium. Then, the unreacted metallic lithium is heated to a high temperature to melt it, and diffuses and permeates the lithium sulfide that has already been generated, and then cooled. Then, metallic lithium is reacted again with sulfur vapor or hydrogen sulfide to generate lithium sulfide. This cycle is repeated to react 100% of metallic lithium.

[0004] Patent Document 3 discloses a method for producing lithium sulfide by reacting lithium carbonate with hydrogen sulfide.

[0005] Patent Document 4 discloses a method for producing lithium sulfide without using hydrogen sulfide. In Patent Document 4, lithium sulfate and carbon powder are each adjusted to fine particles and mixed together to increase the reaction area and reduce the amount of unreacted raw materials. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] JP 2006-151725 A [Patent Document 2] Japanese Patent Application Publication No. 9-110404 [Patent Document 3] JP 2012-221819 A [Patent Document 4] JP 2013-227180 A Summary of the Invention [Problem to be solved by the invention]

[0007] The technology described in Patent Document 1 can create a sulfide solid electrolyte with high ionic conductivity, but the cost increases because an organic solvent is used. The technology described in Patent Document 2 requires repeated reaction cycles, which increases the cost. The technology described in Patent Document 3 uses toxic hydrogen sulfide gas, which is difficult to manage and requires great care to ensure safety. The technology described in Patent Document 4 requires adjustment and mixing into fine particles, which increases the number of steps. In addition, when manufacturing using equipment such as a rotary kiln, the fine particles may scatter inside the equipment, reducing the amount of recovery.

[0008] In the conventional manufacturing method in which lithium sulfate is reduced with a carbon material without using hydrogen sulfide, the ionic conductivity of the solid electrolyte produced using the lithium sulfide produced tends to be lower than that of other manufacturing methods. It was found that the reason for this is that under many conditions, lithium carbonate or lithium oxide is produced as a by-product, lowering the purity of the lithium sulfide.

[0009] Thus, there is room for improvement in the production of lithium sulfide for use in lithium batteries that is free of by-products and has high ionic conductivity.

[0010] The present invention has been made in view of the above, and has an object to provide a more appropriate method for producing lithium sulfide which is free of by-products and has high ionic conductivity. [Means for solving the problem]

[0011] In order to solve the above-mentioned problems and achieve the object, a method for producing lithium sulfide of the present invention includes a temperature increasing step of reducing lithium sulfate charged into a furnace in an atmosphere reduced to 0.05 MPa or less while heating the lithium sulfate to a temperature higher than 700°C. Effect of the Invention

[0012] The method for producing lithium sulfide according to the present invention has an effect of more appropriately producing lithium sulfide having no by-products and high ionic conductivity. [Brief description of the drawings]

[0013] [Figure 1] FIG. 1 is a flow chart showing steps of a method for producing lithium sulfide. [Diagram 2] FIG. 2 is a schematic diagram showing an example of a production apparatus including a rotary kiln. [Diagram 3] FIG. 3 is a graph showing the test results of Example 1, and is a graph showing the measurement results of X-ray diffraction measurement. [Figure 4] FIG. 4 is a graph showing the test results of Example 2, and is a graph showing the measurement results of X-ray diffraction measurement. [Diagram 5] FIG. 5 is a graph showing the test results of Example 3, and is a graph showing the measurement results of X-ray diffraction measurement. [Figure 6] FIG. 6 is a graph showing the test results of Example 4, and is a graph showing the measurement results of X-ray diffraction measurement. [Figure 7] FIG. 7 is a graph showing the test results of Example 5, and is a graph showing the measurement results of X-ray diffraction measurement. [Figure 8] FIG. 8 is a graph showing the test results of Comparative Example 1, and is a graph showing the measurement results of X-ray diffraction measurement. [Figure 9] FIG. 9 is a graph showing the test results of Comparative Example 2, and is a graph showing the measurement results of X-ray diffraction measurement. [Figure 10] FIG. 10 is a graph showing the test results of Comparative Example 3, and is a graph showing the measurement results of X-ray diffraction measurement. [Figure 11] FIG. 11 is a graph showing the test results of each of the examples and comparative examples, and is a graph showing AC impedance measurements. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0014] The present invention will be described in detail below with reference to the drawings. Note that the present invention is not limited to the following modes for carrying out the invention (hereinafter, referred to as embodiments). Furthermore, the components in the following embodiments include those that a person skilled in the art can easily imagine, those that are substantially the same, and those that are within the so-called equivalent range. Furthermore, the components disclosed in the following embodiments can be appropriately combined.

[0015] The method for producing lithium sulfide will be described with reference to Figures 1 and 2. Figure 1 is a flow chart showing steps of the method for producing lithium sulfide. Figure 2 is a schematic diagram showing an example of a production apparatus including a rotary kiln.

[0016] In the method for producing lithium sulfide according to the present embodiment, lithium sulfate and a reducing agent as raw materials are placed in a furnace and heated to produce lithium sulfide. The reaction between lithium sulfate and the reducing agent is carried out under a reduced pressure atmosphere.

[0017] The lithium sulfate has an average particle size of preferably 10 μm or more and 100 μm or less, more preferably 15 μm or more and 80 μm or less, and further preferably 30 μm or more and 50 μm or less. The lithium sulfate, which is the raw material of the lithium sulfide production method of the present embodiment, does not need to be in the form of fine particles. Therefore, even when producing lithium sulfide using equipment such as the rotary kiln 11, for example, the possibility that powdered lithium sulfate will scatter inside the kiln body (furnace) 12 of the rotary kiln 11 and become difficult to recover is reduced.

[0018] The reducing agent is not limited as long as it is a material mainly composed of carbon, such as activated carbon. In this embodiment, the reducing agent is activated carbon. The activated carbon preferably has an average particle size of 1 μm or more and 20 μm or less.

[0019] As shown in FIG. 1, the method for producing lithium sulfide includes a preparation step (step S12) of charging lithium sulfate and a reducing agent into a furnace, a heating step (step S14) of heating the furnace, and a cooling step (step S16) of cooling the furnace.

[0020] [Preparation process] In the preparation step, lithium sulfate and a reducing agent are charged into a furnace in a predetermined molar ratio. In addition, in the preparation step, lithium sulfate and a reducing agent are mixed in a predetermined molar ratio. The molar ratio of lithium sulfate to activated carbon, which is a reducing agent, is preferably, for example, 2 to 4 in terms of the C / Li2SO4 ratio.

[0021] [Heating process] In the temperature-raising step, lithium sulfate and a reducing agent are heated in a furnace to raise the temperature. The temperature-raising step is carried out in a reduced pressure atmosphere. For example, the pressure in the furnace is reduced to 0.05 MPa or less. The pressure may be measured at a location in the furnace that is not affected by the temperature, for example, using a Proudon tube pressure gauge or a Pirani vacuum gauge. In order to carry out the temperature-raising step in a reduced pressure atmosphere, the reduction is carried out under reduced pressure, for example, by using a pump continuously during the temperature-raising step. For example, the reduction is carried out in a state where the temperature is heated to a temperature higher than 700°C.

[0022] In the temperature increasing step, lithium sulfate is reduced by a reducing agent. In the temperature increasing step, the temperature is increased to, for example, 750°C or more and 1000°C or less, preferably 850°C or more and 950°C or less. In the temperature range of 700°C or less, the reaction rate between lithium sulfate and the reducing agent is slow, and the productivity is low. In the temperature range higher than 1000°C, the temperature is increased more than necessary, and the productivity is low. In the temperature range of 950°C or more, the temperature is above the melting point of the lithium sulfate as a raw material, so that the lithium sulfate remaining unreacted dissolves. In the temperature range of 750°C or more and 1000°C or less, the reaction rate is high and the productivity is high. In the temperature range of 850°C or more and 950°C or less, lithium sulfate does not melt, and lithium sulfide that maintains a particle shape is obtained.

[0023] In the temperature-raising step, the temperature is preferably raised at a rate of 5° C. / min or more after the start of heating to reach the desired temperature range. In the temperature-raising step, the heating time in the desired temperature range is preferably 5 minutes or more and 90 minutes or less. The reason why the heating time is preferably within the above range is that if the material is held at a high temperature for a long time, grain growth gradually occurs, resulting in a decrease in reactivity.

[0024] In order to suppress the by-products of lithium carbonate and lithium oxide in the method of reducing lithium sulfate with carbon, it is important to quickly remove the gas generated during reduction. Therefore, in this embodiment, a pump with a displacement large enough for the generated gas is continuously used to maintain a reduced pressure during reduction. This allows high-purity lithium sulfide to be obtained at low cost.

[0025] It was found that the composition of the gas generated during reduction varies depending on the temperature. At temperatures between 750°C and 850°C, where the reaction begins, CO2 is the main component, and the CO ratio increases as the temperature is raised. The oxygen potential of the gas generated during reduction, in other words, the CO / CO2 ratio of the reducing atmosphere, is also an important factor in suppressing by-products. In reactions at low temperatures, the CO / CO2 ratio is outside the appropriate range, making it difficult to obtain lithium sulfide alone even if the pumping speed is increased. When the pumping speed is very fast, a high-performance pump must be prepared, which increases costs. Therefore, by performing reduction at temperatures above 850°C while reducing the pressure to below 0.05 MPa, high-purity lithium sulfide without by-products can be obtained.

[0026] [Cooling process] In the cooling step, after the heating step, the furnace is cooled to 200° C. or lower. In the cooling step, the heated furnace is naturally cooled.

[0027] After the cooling step, the lithium sulfide is removed from the furnace.

[0028] The preparation step, the heating step, and the cooling step may be carried out using, for example, a manufacturing apparatus 10 having a rotary kiln 11.

[0029] [Manufacturing equipment] The manufacturing apparatus 10 is an apparatus for manufacturing lithium sulfide by placing lithium sulfate as a raw material and activated carbon as a reducing agent in a furnace and heating them under a reduced pressure atmosphere. The manufacturing apparatus 10 includes a rotary kiln 11, a material input device 18, a material discharge pipe 24, and a pump 41.

[0030] The rotary kiln 11 includes a kiln body 12, a heater 14, and a drive unit 16. The kiln body 12 is a cylindrical member. It is preferable that the central axis of the cylindrical shape of the kiln body 12 is inclined with respect to the horizontal direction so that the end on the material charging device 18 side is vertically higher than the other end. The heater 14 heats the kiln body 12.

[0031] The driving unit 16 rotates the kiln body 12 around the central axis of the cylindrical shape as the rotation axis. The driving unit 16 includes a driving source 30 and a transmission mechanism 32. The driving source 30 generates a rotational force such as a motor. The transmission mechanism 32 transmits the rotational force of the driving source 30 to the kiln body 12.

[0032] The material charging device 18 charges lithium sulfate and activated carbon into the rotary kiln 11. The material charging device 18 includes a material storage section 21 and a material supply pipe 22. The material storage section 21 stores lithium sulfate and activated carbon. The material supply pipe 22 connects the material storage section 21 and the kiln body 12. The material supply pipe 22 is connected to the upstream side of the transport path for lithium sulfate and activated carbon in the kiln body 12. The material supply pipe 22 supplies lithium sulfate and activated carbon from the material storage section 21 to the kiln body 12.

[0033] The material discharge pipe 24 is connected to an end of the kiln body 12 opposite to the end to which the material supply pipe 22 is connected. The material discharge pipe 24 is connected to the downstream side of the transport path for lithium sulfate and activated carbon in the kiln body 12. The material discharge pipe 24 discharges lithium sulfide produced by passing through the kiln body 12.

[0034] The pump 41 is connected to the kiln body 12 via an exhaust pipe 42. The pump 41 reduces the pressure inside the kiln body 12. The pump 41 has an exhaust volume larger than the amount of gas generated in the kiln body 12 during the reaction. There are no limitations on the pump used, and a combination of a rotary pump and a mechanical booster pump or an oil diffusion pump may be used. Depending on the size of the furnace and the amount of reduction treatment performed at one time, the above conditions can be fully achieved with only a generally available rotary pump with an exhaust volume of about 150 L / min.

[0035] In the manufacturing apparatus 10 configured as above, first, a predetermined molar ratio of lithium sulfate and activated carbon is charged from the material charging device 18 into the kiln body 12 of the rotary kiln 11. The lithium sulfate and the reducing agent charged into the kiln body 12 are mixed. Then, in the temperature raising process, the pump 41 is operated to suck gas from the kiln body 12 through the exhaust pipe 42 to reduce the pressure. The pressure inside the kiln body 12 is controlled to 0.05 MPa or less. The kiln body 12 is rotated around the rotation axis by the drive unit 16 while being heated by the heater 14. The lithium sulfate and activated carbon supplied to the kiln body 12 move from the material supply pipe 22 to the material discharge pipe 24 along the conveying path by the rotation of the kiln body 12. The lithium sulfate and activated carbon moving along the conveying path are heated to a desired temperature by the heater 14. Since the lithium sulfate and the activated carbon are not in the form of fine particles, scattering in the conveying path is suppressed. Then, in the cooling step, the heater 14 is stopped to cool the kiln body 12 and the lithium sulfide. The produced lithium sulfide is then discharged from the material discharge pipe 24 and recovered.

[0036] [Effects of this embodiment] According to this embodiment, lithium sulfate and a reducing agent are placed in one furnace and heated under a reduced pressure atmosphere to produce lithium sulfide. According to this embodiment, by-products can be suppressed and gas generated during reduction can be quickly removed. In this embodiment, high-purity lithium sulfide without by-products can be obtained by performing reduction at a temperature of 850°C or higher while reducing the pressure to 0.05 MPa or less. In this embodiment, high-purity lithium sulfide without by-products can be obtained at low cost by performing reduction under reduced pressure. In this way, this embodiment can produce lithium sulfide without by-products and with high ionic conductivity.

[0037] In contrast, in order to control the atmosphere by Ar flow, the amount of gas required is about 10 to 100 times the amount of gas generated, which is not economical.

[0038] In this embodiment, the temperature is set to 700° C. or more and 1000° C. or less, preferably 850° C. or more and 950° C. or less. In this embodiment, the temperature range of 700° C. or more and 1000° C. or less can increase the reaction rate and productivity. In this embodiment, the temperature range of 850° C. or more and 950° C. or less can be used to obtain lithium sulfide that does not melt and maintains its particle shape.

[0039] In this embodiment, since it is not necessary to mix lithium sulfate used as a raw material into fine particles, an increase in the number of steps can be suppressed. When lithium sulfide is produced using a production apparatus 10 having a rotary kiln 11, scattering in the transport path can be suppressed, and a reduction in the amount of recovery can be suppressed. In this way, this embodiment can provide a method suitable for producing lithium sulfide using a production apparatus 10 having a rotary kiln 11.

[0040] In this embodiment, no toxic hydrogen sulfide gas is used in the manufacturing process, which makes it easier to manage the manufacturing process and ensures safety.

[0041] In the present embodiment, the manufacturing process uses lithium sulfate and a reducing agent, but does not use an organic solvent, thereby reducing costs.

[0042] In this embodiment, since it is not necessary to repeat the manufacturing process, the time required for manufacturing can be shortened and costs can be suppressed.

[0043] In this way, according to this embodiment, lithium sulfide can be more appropriately produced.

[0044] [Example 1] Next, a method for producing lithium sulfide will be described using a specific example 1. In a preparation step, a raw material, lithium sulfate, and a reducing agent, activated carbon, were weighed in a predetermined molar ratio in an inert atmosphere in a glove box and mixed in a mortar. The predetermined molar ratio was 1:2.4. The mixture of lithium sulfate and activated carbon was placed in a crucible made of aluminum oxide.

[0045] After the preparation process, in the temperature-raising process, the crucible containing the mixture of lithium sulfate and activated carbon was placed in a small tubular furnace and heated to 750°C in 75 minutes. During the temperature-raising process, the pressure was reduced to a predetermined level by suction using the pump 41. The state of 750°C was maintained for 60 minutes. During the temperature-raising process, the kiln body 12 was depressurized to 0.001 MPa or less by the pump 41.

[0046] After the temperature increasing step was completed, the suction by the pump 41 was stopped, and in the cooling step, the inside of the tubular furnace was naturally cooled, and the sample was recovered.

[0047] The produced sample was ground in an agate mortar, and then powder X-ray diffraction measurement was performed using a Burker D8ADVANCE device to evaluate the unreacted residue. The ground sample was weighed out at a molar ratio of 3:1 with phosphorus sulfide, and then made amorphous in a planetary ball mill using a container that was not exposed to the atmosphere. After that, 0.3 g was loaded into a SUS conductivity measurement cell in a glove box, and AC impedance measurement was performed in the measurement range of 1 Hz to 7 MHz at room temperature of 25°C with a pressure of 360 MPa applied. The measurement results of the powder X-ray diffraction measurement of the produced sample are shown in Figure 3. The measurement results of the AC impedance measurement of the produced sample are shown in Figure 11.

[0048] In Examples 2 and 3, the temperature was changed. In Examples 4 and 5, the pressure was changed. In Comparative Examples 1 and 2, the flow rate of Ar flow was changed. In Comparative Example 3, no Ar flow was performed. In Comparative Example 3, the temperature was changed.

[0049] [Example 2] In Example 2, the temperature was 850° C. and the pressure was 0.001 MPa. The results of powder X-ray diffraction measurement of the sample produced in Example 2 are shown in Figure 4. The results of AC impedance measurement of the sample produced in Example 2 are shown in Figure 11.

[0050] [Example 3] In Example 3, the temperature was 1000° C. and the pressure was 0.001 MPa. The results of powder X-ray diffraction measurement of the sample produced in Example 3 are shown in FIG 5. The results of AC impedance measurement of the sample produced in Example 3 are shown in FIG 11.

[0051] [Example 4] In Example 4, the temperature was 850° C. and the pressure was 0.05 MPa. The results of powder X-ray diffraction measurement of the sample produced in Example 4 are shown in FIG 6. The results of AC impedance measurement of the sample produced in Example 4 are shown in FIG 11.

[0052] [Example 5] In Example 5, the temperature was 850° C. and the pressure was 0.0001 MPa. The results of powder X-ray diffraction measurement of the sample produced in Example 5 are shown in FIG 7. The results of AC impedance measurement of the sample produced in Example 5 are shown in FIG 11.

[0053] [Comparative Example 1] In Comparative Example 1, the temperature was 850° C., Ar was flowed at a flow rate of 1 l / min, and the pressure was 0.1 MPa. The results of powder X-ray diffraction measurement of the sample produced in Comparative Example 1 are shown in FIG. 8. The results of AC impedance measurement of the sample produced in Comparative Example 1 are shown in FIG.

[0054] [Comparative Example 2] In Comparative Example 2, the temperature was 850° C., Ar was flowed at a flow rate of 2 l / min, and the pressure was 0.1 MPa. The results of powder X-ray diffraction measurement of the sample produced in Comparative Example 2 are shown in FIG 9. The results of AC impedance measurement of the sample produced in Comparative Example 2 are shown in FIG 11.

[0055] [Comparative Example 3] In Comparative Example 3, the temperature was 680° C., no Ar flow was performed, and the pressure was 0.001 MPa. The results of powder X-ray diffraction measurement of the sample produced in Comparative Example 3 are shown in Fig. 10. The results of AC impedance measurement of the sample produced in Comparative Example 3 were very large compared to the other Examples and Comparative Examples, and did not fall within the same display range, so they are not shown here.

[0056] The measurement results of the above-mentioned Examples 1, 2, 3, 4, 5, Comparative Example 1, Comparative Example 2, and Comparative Example 3 are shown in Table 1 below.

[0057] [Table 1]

[0058] 3 to 7 are graphs showing the test results of each Example (Example 1 to Example 5), and are graphs showing the measurement results of X-ray diffraction measurement. From Table 1, in each Example, no XRD peaks of impurities are observed. The impurities are by-products lithium carbonate and lithium oxide, and unreduced lithium sulfate. In Example 3, no by-products were observed, but lithium sulfide was melted and not obtained as powder. In Comparative Example 1, lithium carbonate was observed. In Comparative Example 2, lithium oxide was observed. In Comparative Example 2, powder was scattered. In Comparative Example 3, the generation of unreduced lithium sulfate and lithium carbonate was observed. In Comparative Example 3, the reaction rate was slow. From these, each Example can reduce the generation of lithium carbonate and unreduced lithium sulfate compared to each Comparative Example (Comparative Example 1 to Comparative Example 3). From this test, it was confirmed that by performing the temperature increasing step under a reduced pressure atmosphere, the generation of by-products and unreduced lithium sulfate can be reduced, and lithium sulfide can be more appropriately produced.

[0059] FIG. 11 is a graph showing the test results of each Example and each Comparative Example, and is a graph showing AC impedance measurement. From FIG. 11 and Table 1, in Examples 1 to 5 in which no by-products are produced, the resistance value in AC impedance is low. On the other hand, in Comparative Examples 1 and 2, the resistance value is 1.5 times or more larger than that of each Example, and Comparative Example 3 is not listed because it is a very large value. The ionic conductivity is calculated by dividing the measurement area by the resistance value and the measurement sample thickness. From these, it was confirmed that each Example can have a higher ionic conductivity than each Comparative Example. From this test, it was confirmed that lithium sulfide with no by-products and high ionic conductivity can be more appropriately produced when heating to a temperature higher than 700 ° C. in an atmosphere reduced to 0.05 MPa or less in the temperature-raising process.

[0060] From the above, in the method for producing lithium sulfide, by performing the temperature increasing step under a reduced pressure atmosphere, the generation of by-products and unreduced lithium sulfate are reduced, and lithium sulfate with high ionic conductivity is obtained. From the above, it is understood that by performing the production under the above-mentioned conditions, lithium sulfate with a desired ionic conductivity can be appropriately produced. [Explanation of symbols]

[0061] 10 Manufacturing equipment 11 Rotary Kiln 12 Kiln body (furnace) 14 Heater 16 Drive unit 18 Material feeding device 21 Material storage section 22 Material supply pipe 24 Material discharge pipe

Claims

1. A heating step in which lithium sulfate in particulate form is charged into a furnace and reduced at a temperature higher than 700° C. in an atmosphere reduced to 0.05 MPa or less while maintaining the particulate form; A method for producing lithium sulfide comprising the steps of:

2. The temperature increasing step includes heating the lithium sulfate and the reducing agent in a mixed state to 750° C. or more and 950° C. or less. The method for producing lithium sulfide according to claim 1.

3. The temperature increasing step includes heating the lithium sulfate and the reducing agent in a mixed state to a temperature of 850° C. or higher and 950° C. or lower. The method for producing lithium sulfide according to claim 2.

4. A preparation step of charging lithium sulfate in particulate form and a reducing agent into a furnace; The method for producing lithium sulfide according to any one of claims 1 to 3, comprising:

5. In the temperature increasing step, the heating time in the temperature range is 5 minutes or more and 90 minutes or less. The method for producing lithium sulfide according to claim 2 or 3.

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