Method for producing high-purity alkali metal sulfides

Abstract

One embodiment of the present invention discloses a method for producing high purity alkali metal sulfide, comprising the steps of mixing an alkali metal sulfide salt precursor and a reducing agent to form a mixture, reducing the mixture to obtain an alkali metal sulfide, and purifying the resulting alkali metal sulfide.

Description

Technical Field

[0001] The present invention relates to a method for producing high-purity alkali metal sulfides.

Background Art

[0002] As a method for producing lithium sulfide, which is an example of an alkali metal sulfide, a chemical reaction using Li2CO3 and H2S has been widely used in the Li2S synthesis step. However, there is a problem that additional equipment for storing H2S gas, which is a toxic substance, and exhaust facilities for control purposes are required. In addition, in the case of unreacted H2S, a process is necessary in which the substance is completely burned with a burner or the like and then neutralized with a sodium hydroxide solution and treated with sodium sulfide.

[0003] Further, in the above method, for the synthesis of Li2S, additional equipment installation and processes for H2S gas management are generated, resulting in a problem in productivity. Since a larger amount of H2S is used during mass production of Li2S, in addition to the basic material cost for performing the synthesis process, the cost input for equipment setup and maintenance for maintaining stability increases, so the economic efficiency may decrease as a mass production method.

[0004] In the case of LiOH (Lithium hydroxide) used as a raw material in other conventional lithium sulfide production methods, the price is higher than that of Li2SO4 (Lithium sulfate). In this technology, since the synthesis is carried out in a dry ball milling process, the process is simple compared to other methods. However, due to the problem of basic raw material prices, the economic efficiency decreases when shifting to the mass production stage. Furthermore, the purity of the powder synthesized by the milling process is relatively low, and an additional high-purity process is required.

[0005] Therefore, there is a need for a method for synthesizing economically and highly pure alkali metal sulfides.

Summary of the Invention

Problems to be Solved by the Invention

[0006] The problem that this invention aims to solve is to provide an economical and novel method for producing high-purity alkali metal sulfides. [Means for solving the problem]

[0007] One embodiment of the present invention discloses a method for producing a high-purity alkali metal sulfide, comprising the steps of: mixing an alkali metal sulfide precursor with a reducing agent to form a mixture; reducing the mixture to obtain an alkali metal sulfide; and purifying the obtained alkali metal sulfide. [Effects of the Invention]

[0008] According to the present invention, alkali metal sulfides can be synthesized economically and with high purity. [Brief explanation of the drawing]

[0009] [Figure 1] Figure 1 is a flowchart showing a method for producing high-purity alkali metal sulfides according to one embodiment of the present invention. [Figure 2] Figure 2 is a graph showing the results of Raman spectroscopy analysis of lithium sulfide after the purification process. [Figure 3] Figure 3 is a graph showing the results of Raman spectroscopy analysis of lithium sulfide before the purification process. [Figure 4] Figure 4 is a graph showing the results of X-ray diffraction analysis (XRD) of lithium sulfide produced by one embodiment compared to commercially available lithium sulfide. [Figure 5] Figure 5 is a graph showing a comparison of the full width at half maximum (FWHM) of lithium sulfide produced according to one embodiment with commercially available lithium sulfide. [Figure 6] Figure 6 is a graph showing the results of confirming the peak of lithium sulfide reduction depending on the heat treatment temperature during lithium sulfide production according to one embodiment. [Figure 7]Figure 7 is a graph showing the change in Gibbs free energy according to the heat treatment temperature and carbon input amount during lithium sulfide production according to one embodiment. [Figure 8] Figure 8 is a graph showing the results of confirming the peak of lithium sulfide reduction depending on the amount of carbon input during lithium sulfide production according to one embodiment. [Figure 9] Figure 9 is a photograph showing the lithium sulfide obtained after the purification process by centrifugation according to one embodiment, where the lithium sulfide separated from impurities and the impurities have been removed. [Best Mode for Carrying Out the Invention]

[0010] One embodiment of the present invention discloses a method for producing a high-purity alkali metal sulfide, comprising the steps of: mixing an alkali metal sulfide precursor with a reducing agent to form a mixture; reducing the mixture to obtain an alkali metal sulfide; and purifying the obtained alkali metal sulfide.

[0011] In this embodiment, the mixing step can be performed to form the mixture in a nonpolar solvent.

[0012] In this embodiment, the reduction step can be performed by reducing the mixture with a reducing gas.

[0013] In this embodiment, the reducing gas may include at least one selected from the group consisting of carbon monoxide, nitrogen, argon, helium, neon, methane, ethane, propane, and butane, and hydrogen.

[0014] In this embodiment, the reduction step may include a heat treatment step.

[0015] In this embodiment, the heat treatment step can be performed at a temperature of 800°C to 1500°C.

[0016] In this embodiment, the purification step can purify the alkali metal sulfide obtained using a polar solvent.

[0017] In this embodiment, the polarity index of the polar solvent can be 4.3 to 6.2.

[0018] In this embodiment, in the purification step, the obtained alkali metal sulfide can be dissolved in the polar solvent.

[0019] In this embodiment, after the purification step, a step of crystallizing the alkali metal sulfide can further be included.

[0020] In this embodiment, the crystallization step can be carried out by heat treatment at 300°C to 1000°C for crystallization.

Embodiments for Carrying out the Invention

[0021] The above objects, other objects, features and advantages of the present invention will be easily understood through the following preferred embodiments related to the accompanying drawings. However, the present invention is not limited to the embodiments described herein and may be embodied in other forms. Rather, the embodiments introduced herein are provided to make the disclosed content thorough and complete and to enable the idea of the present invention to be fully conveyed to those of ordinary skill in the art.

[0022] In describing each drawing, similar reference numerals were used for similar components. In the accompanying drawings, the dimensions of the structures are shown enlarged for clarity of the invention. Terms such as "first," "second," etc., may be used to describe various components, but such components should not be limited by such terms. Such terms are used solely for the purpose of distinguishing one component from another. For example, without departing from the scope of the invention, the first component may be named the second component, and similarly, the second component may be named the first component. A singular expression includes plural expressions unless the context clearly indicates otherwise.

[0023] In this specification, terms such as “includes” or “have” are intended to indicate the presence of features, figures, steps, actions, components, parts, or combinations thereof described herein, and should be understood not to preemptively exclude the possibility of the presence or addition of one or more other features, figures, steps, actions, components, parts, or combinations thereof. Furthermore, when a part such as a layer, membrane, region, or plate is said to be “on top” of another part, this includes not only when it is “directly on top” of the other part, but also when there is another part in between. Conversely, when a part such as a layer, membrane, region, or plate is said to be “below” another part, this includes not only when it is “directly below” the other part, but also when there is another part in between.

[0024] Unless otherwise specified, all numbers, values, and / or expressions used herein to represent quantities of components, reaction conditions, polymer compositions, and formulations should be understood to be modified in all cases by the term “approximately,” as these numbers are approximations that reflect the various uncertainties in measurement that arise in obtaining these values, among which the numbers are inherently different. Furthermore, where a numerical range is disclosed herein, the range is continuous and, unless otherwise indicated, includes all values ​​from the minimum to the maximum value of that range. Furthermore, where such a range refers to an integer, unless otherwise indicated, it includes all integers from the minimum to the maximum value of that range.

[0025] Figure 1 is a flowchart showing a method for producing high-purity alkali metal sulfides according to one embodiment of the present invention.

[0026] Referring to Figure 1, the method for producing high-purity alkali metal sulfide according to this embodiment may include the steps of: mixing a precursor and a reducing agent to form a mixture S100; reducing the mixture to obtain alkali metal sulfide S200; purifying the obtained alkali metal sulfide S300; drying the purified alkali metal sulfide S400; and crystallizing the dried alkali metal sulfide S500.

[0027] The precursor may be a compound containing alkali and sulfur elements. For example, the precursor may be lithium sulfate, sodium sulfate, potassium sulfate, etc. These compounds may be hydrated or anhydrous.

[0028] The reducing agent may contain carbon. The reducing agent is a carbon-based reducing agent, and its material is not particularly limited; it may be a gas, liquid, or solid phase. Furthermore, the reducing agent may contain non-carbon atoms, as long as it can serve as a source of carbon.

[0029] Such reducing agents may include at least one of the solid carbonaceous materials such as coal, coke, black smoke, carbon black, fullerenes, carbon tubes, charcoal, carbide, elemental carbon, and its allotropes. A typical example of elemental carbon and its allotropes is graphite. The reducing agent may also be a metallic reducing agent such as LiBH4.

[0030] On the other hand, if the reducing agent content is too low, the reduction efficiency will decrease, but conversely, if the reducing agent content is too high, excessive impurities may be generated, which can reduce the manufacturing efficiency. Therefore, the precursor and reducing agent may be mixed in a molar ratio of approximately 1:1 to approximately 1:5. For example, they may be mixed in molar ratios of 1:1 to 1:5, 1:1 to 1:4, 1:2 to 1:4, 1:3 to 1:6, or 1:3 to 1:5.

[0031] The precursor and the reducing agent can be uniformly mixed to form a mixture (S100).

[0032] Milling, stirring, homogenizing, and ultrasonic mixing can be used to mix the precursor and reducing agent. For example, the mixing of the precursor and reducing agent can be carried out by a milling process. The milling process may be selected from the group consisting of ball mills, attrition mills, vibration mills, disk mills, jet mills, rotor mills, pearl mills, and combinations thereof.

[0033] A solvent may be added when mixing the precursor and reducing agent. However, the solvent is not always necessary and may be omitted when mixing the precursor and reducing agent.

[0034] The solvent used when mixing the precursor and the reducing agent may be a nonpolar solvent. Nonpolar solvents may be selected from the group consisting of, for example, cyclohexane, hexane, n-heptane, and combinations thereof.

[0035] On the other hand, if a solvent is used when mixing the precursor and the reducing agent, a drying step to remove the solvent may be included. Drying may be carried out by various methods, such as vacuum drying, hot air drying, freeze-drying, or spray drying.

[0036] After forming the mixture, the mixture can be reduced to obtain an alkali metal sulfide (S200).

[0037] During the reduction of a mixture, a reducing gas can be supplied. The reducing gas may contain hydrogen. In one example, the reducing gas may be hydrogen. Alternatively, the reducing gas may be a mixture of hydrogen and at least one selected from the group consisting of carbon monoxide, nitrogen, argon, helium, neon, methane, ethane, propane, and butane. By supplying a reducing gas containing hydrogen during the reduction of a mixture in this way, the reducing effect of the carbon-containing reducing agent can be further enhanced.

[0038] On the other hand, reduction can be performed together with heat treatment. The heat treatment temperature can be approximately 700-900°C. Furthermore, the heat treatment time can be approximately 1 to 24 hours.

[0039] Step S300, which involves purifying the obtained alkali metal sulfide, is a step to remove the reducing agent and unreacted precursors mixed with the obtained alkali metal sulfide. After mixing the obtained alkali metal sulfide with an organic solvent, only the purified solvent is extracted.

[0040] The solvent used in the purification process may be a polar solvent. Specifically, a polar solvent may have a polarity index between 4.3 and 6.2. For example, such a polar solvent may be ethanol. In such a solvent, only the resulting alkali metal sulfide dissolves, and the remaining impurities settle, allowing for easy separation of the alkali metal sulfide from the impurities and obtaining high-purity alkali metal sulfide.

[0041] The purification may include a centrifugation step. The centrifugation step may be performed at approximately 1000 to 5000 rpm for approximately 5 to 60 minutes. Specifically, for example, it may be performed at 1000 to 4000, 1000 to 3000, 2000 to 5000, 2000 to 4000, 2000 to 3000, 3000 to 5000, or 3000 to 4000 rpm for 5 to 50, 5 to 40, 5 to 30, 10 to 60, 10 to 50, 10 to 40, 10 to 30, 20 to 60, 20 to 50, 20 to 40, 30 to 60, or 30 to 50 minutes.

[0042] As another example, alkali metal sulfides and impurities can be separated by various methods such as filtering, crimping dewatering, and filter pressing.

[0043] Next, the solvent in which the purified alkali metal sulfide is dissolved can be separated and dried (S400), and then heat-treated to crystallize the alkali metal sulfide (S500).

[0044] Drying may be carried out by vacuum drying, hot air drying, freeze-drying, spray drying, etc., which can remove the solvent.

[0045] Alkali metal sulfides from which the solvent has been removed can be crystallized by heat treatment. Here, the heat treatment temperature can be 300°C to 930°C. On the other hand, the purity of the alkali metal sulfide can be further improved by supplying at least one reducing gas from the group consisting of carbon monoxide, nitrogen, hydrogen, argon, helium, neon, methane, ethane, propane, and butane during the heat treatment for crystallization.

[0046] On the other hand, a preliminary crystallization step may be further included between the drying step S400 and the crystallization step S500. The preliminary crystallization step involves pre-crystallizing the alkali metal sulfide before the crystallization step S500. By performing the crystallization step S500 after the preliminary crystallization step, it is possible to produce perfectly crystalline alkali metal sulfide necessary for lithium-sulfur batteries and the like.

[0047] However, when alkali metal sulfides are used as raw materials in synthesis, such as solid electrolytes, excellent reactivity is required, so a pre-crystallization step may not be necessary. In other words, by making the alkali metal sulfides have relatively low crystallinity, chemical reactions such as decomposition may become more active.

[0048] The production of alkali metal sulfides according to the embodiment described above can be carried out in a glove box or dry room that is not exposed to moisture and oxygen.

[0049] The configuration and operation of the present invention will be described in more detail below through preferred embodiments of the present invention. However, these are presented as preferred examples of the present invention and should not be construed as limiting the present invention in any way.

[0050] <Embodiment 1: Production of alkali metal sulfide (lithium sulfide)> Li2SO4 and carbon were mixed in a 1:4 molar ratio by milling in heptane solvent. The mixed mixture was heat-treated at 800°C for 12 hours in a 3% hydrogen and argon (Ar) gas atmosphere to synthesize Li2S.

[0051] The synthesized Li2S was dissolved in ethanol solvent (Polarity index = 5.2) and then centrifuged at 3000 RPM for 30 minutes. This allowed impurities other than the Li2S dissolved in ethanol to settle, separating only the Li2S dissolved in ethanol.

[0052] The ethanol solvent in which the Li2S was dissolved was separated, extracted, and vacuum-dried, then heat-treated at 600°C for 5 hours. This caused the solvent ethanol component to evaporate, yielding high-purity Li2S.

[0053] <Experimental Example 1: Raman Spectroscopy and XRD (X-ray Diffraction Spectroscopy) Analysis> Figure 2 is a graph showing the Raman spectroscopy results for lithium sulfide after the purification process, and Figure 3 is a graph showing the Raman spectroscopy results for lithium sulfide before the purification process. Furthermore, Figure 4 is a graph showing the X-ray diffraction spectroscopy (XRD) results for lithium sulfide produced by one embodiment compared with commercially available lithium sulfide.

[0054] Raman spectroscopy analysis of the lithium sulfide produced in Embodiment 1 confirmed that the main bond in the sample after the purification process is Li2S, as shown in Figure 2. On the other hand, Raman spectroscopy analysis of the sample before the purification process confirmed that Li2S and carbon were mixed, as shown in Figure 3. In other words, according to the present invention, impurities in the sample were removed by the purification process.

[0055] Furthermore, when the main XRD peak of lithium sulfide produced in Embodiment 1 was examined, it was found to be the same as that of commercially available Li2S, as shown in Figure 4. This indicates that the purification process in Embodiment 1 is effective.

[0056] <Experimental Example 2: Size Analysis> Figure 5 is a graph showing a comparison of the full width at half maximum (FWHM) of lithium sulfide produced according to one embodiment with commercially available lithium sulfide.

[0057] Furthermore, when the XRD peak of the lithium sulfide produced in Embodiment 1 was examined, as shown in Figure 5, the average width at half maximum appeared at approximately 92% compared to the peak morphology of commercially available lithium sulfide, indicating excellent properties from the standpoint of crystallinity.

[0058] Furthermore, when measuring the particle size of the powder, as shown in Table 1 below, the lithium sulfide produced in Embodiment 1 showed an average size of approximately 22.8 nm, while the commercially available product showed an average size of approximately 19.4 nm, indicating that they are at a comparable level.

[0059] [Table 1]

[0060] <Experimental Example 3: XRD and Ellingham Diagram Analysis Based on Heat Treatment Temperature and Carbon Ratio> Figure 6 is a graph showing the results of confirming the peak of lithium sulfide reduction according to the heat treatment temperature during lithium sulfide production according to one embodiment, and Figure 7 is a graph showing the change in Gibbs free energy according to the heat treatment temperature and carbon input amount during lithium sulfide production according to one embodiment. Furthermore, Figure 8 is a graph showing the results of confirming the peak of lithium sulfide reduction according to the carbon input amount during lithium sulfide production according to one embodiment.

[0061] Similar to Embodiment 1, XRD peaks and Ellingham diagrams were observed at different temperatures during the heat treatment for lithium sulfide synthesis. As shown in Figure 6, it was confirmed that a clear peak for Li2S, which is reduced from Li2SO4, is generated when the reaction temperature is 800°C.

[0062] Furthermore, as confirmed by examining the Ellingham diagram, it was found that the decrease in Gibbs free energy increases with increasing heat treatment temperature and carbon input, as shown in Figure 7. When Li2SO4 and carbon were heat-treated in a 1:4 ratio, a large change in Gibbs free energy of approximately -250 kJ / mol was observed at approximately 800°C.

[0063] Furthermore, as shown in Figure 8, it was confirmed that the greater the amount of carbon added, the clearer the peak of lithium sulfide reduced from Li2SO4 becomes.

[0064] <Experimental Example 4: Confirmation of Lithium Sulfide after Purification and Crystallization> As a result of purifying lithium sulfide using different solvents in the purification process, as in Embodiment 1 described above, it was found that lithium sulfide dissolves in solvents within a specific polarity range, as shown in Table 2 below.

[0065] [Table 2]

[0066] Furthermore, following the purification process using a centrifuge as in Embodiment 1, it was confirmed that the lithium sulfide layer dissolved in the solvent and the layer of carbon-containing impurities were separated, as shown in the left photograph of Figure 9. Subsequently, the solvent, from which the precipitate impurities had been removed using a filter, was collected, and after vacuum drying and additional heat treatment, a high-purity final product was confirmed as shown in the right photograph of Figure 9. In summary, these results show that the lithium sulfide produced as in Embodiment 1 has improved purity compared to lithium sulfide produced by conventional synthesis methods.

[0067] Thus, the present invention has been described with reference to one embodiment shown in the drawings, but this is merely illustrative, and those with ordinary skill in the art will understand that various modifications and variations of embodiments are possible therefrom. Therefore, the true scope of technical protection of the present invention should be determined by the technical idea of ​​the appended claims.

Claims

1. The steps include mixing an alkali metal sulfide precursor with a reducing agent to form a mixture, The steps include reducing the aforementioned mixture to obtain an alkali metal sulfide, The step includes purifying the obtained alkali metal sulfide, A method for producing a high-purity alkali metal sulfide, wherein the mixture is formed in a nonpolar solvent during the mixing process.

2. The method for producing a high-purity alkali metal sulfide according to Claim 1, wherein the reduction is carried out under a reducing gas atmosphere.

3. The method for producing a high-purity alkali metal sulfide according to claim 2, wherein the reducing gas comprises at least one selected from the group consisting of carbon monoxide, nitrogen, argon, helium, neon, methane, ethane, propane, and butane, and hydrogen.

4. The method for producing a high-purity alkali metal sulfide according to claim 1, wherein the reduction step includes a heat treatment step.

5. The method for producing a high-purity alkali metal sulfide according to claim 4, wherein the heat treatment step is performed at 800°C to 1500°C.

6. The method for producing a high-purity alkali metal sulfide according to claim 1, wherein the purification step involves purifying the alkali metal sulfide obtained using a polar solvent.

7. The method for producing a high-purity alkali metal sulfide according to claim 6, wherein the polarity index of the polar solvent is greater than 4.3 and less than 6.

2.

8. The method for producing a high-purity alkali metal sulfide according to claim 6, wherein in the purification step, the obtained alkali metal sulfide is dissolved in the polar solvent.

9. A method for producing a high-purity alkali metal sulfide according to claim 1, further comprising the step of crystallizing the alkali metal sulfide after the purification step.

10. The method for producing a high-purity alkali metal sulfide according to claim 9, wherein the crystallization step is performed by heat treatment at 300°C to 1000°C to crystallize the material.

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