Lithium sulfide and method for preparing same

Abstract

The present invention relates to lithium sulfide and a method for preparing same, and provides lithium sulfide comprising peak A at 2.2 ppm to 2.5 ppm, peak B at 1.2 ppm to 2.1 ppm, peak C at -0.7 ppm to -0.5 ppm , peak D at 2.9 ppm to 3.1 ppm, and peak E at -0.2 ppm to 1.15 ppm, wherein, with respect to the integral ratio of the peaks, 5 ≤ A / B and 1 ≤ B / C.

Description

Lithium sulfide and method of manufacturing the same

[0001] This relates to lithium sulfide and a method for manufacturing the same.

[0002] This application claims priority to Korean Patent Application No. 10-2024-0191894, filed on December 19, 2024, the entire contents of which are incorporated herein by reference.

[0003] Secondary batteries are widely used in everything from small electronic devices such as mobile phones or laptops to large devices such as electric vehicles (EVs) or energy storage systems (ESS). As the application fields of secondary batteries expand to all areas of daily life, there is a growing demand for not only performance such as high energy density and long lifespan, but also stability.

[0004] Conventionally, most electrolytes used in lithium-ion batteries were liquid electrolytes utilizing organic solvents. However, due to risks such as leakage or fire associated with these liquid electrolytes, strict packaging was required; consequently, there were limitations in increasing energy density beyond a certain level due to this strict packaging. Consequently, the need for all-solid-state batteries utilizing inorganic solid electrolytes instead of organic liquid electrolytes has emerged.

[0005] The above-mentioned all-solid-state battery allows for the safe fabrication of battery cells by excluding organic solvents such as liquid electrolytes. Furthermore, since inorganic solid electrolytes maintain stability without decomposing over a wide voltage range, they offer the advantage of enabling the use of high-voltage electrode materials.

[0006] The above solid electrolytes are classified into oxide-based and sulfide-based types, and the sulfide-based solid electrolytes have the characteristic of high ionic conductivity compared to the oxide-based solid electrolytes. The main raw material for the sulfide-based solid electrolytes is lithium sulfide (Li2S). Various methods are utilized for the synthesis of lithium sulfide, such as synthesis methods using a high-energy ball mill, synthesis methods using a wet plasma process, and wet / dry methods using lithium metal.

[0007] Specifically, solvent extraction is used to increase the purity of even low-purity lithium sulfide. However, there is a problem in that the purification effect is insufficient due to the loss of sulfur during the drying process after solvent extraction.

[0008] One embodiment of the present invention comprises the step of mixing a carbon raw material and a lithium compound and heat-treating them to obtain a thermal reduction product;

[0009] A step of purifying the above thermal reduction product by a solvent extraction method; and

[0010] The method includes the step of heat-treating the above-mentioned purified product; and

[0011] In the step of purifying the above thermal reduction product by a solvent extraction method,

[0012] A method for manufacturing lithium sulfide is provided, wherein, prior to solvent extraction, an additional sulfur raw material is added to the thermal reduction product, and then purified by solvent extraction.

[0013] The above additional sulfur raw material may be solid sulfur (S).

[0014] The amount of the additional sulfur raw material added above may be controlled so that the S / Li molar ratio of the thermal reduction product is in the range of 0.45 to 0.55.

[0015] In the step of purifying the above thermal reduction product by a solvent extraction method,

[0016] The above solvent may be ethanol, a solvent having a carbon chain length of 2 or more, or a mixture thereof.

[0017] The above lithium compound may include lithium sulfate.

[0018] A lithium sulfide purification having an optimal range of peaks can be obtained. This improves the efficiency of subsequent steps, thereby improving the purity and yield of the final product.

[0019] Figure 1 is NMR data of lithium sulfide after a heat treatment step.

[0020] Figure 2 is an enlarged view of the NMR data of lithium sulfide that has undergone a heat treatment step.

[0021] Terms such as first, second, and third are used to describe various parts, components, regions, layers, and / or sections, but are not limited thereto. These terms are used solely to distinguish one part, component, region, layer, or section from another part, component, region, layer, or section. Accordingly, the first part, component, region, layer, or section described below may be referred to as the second part, component, region, layer, or section without departing from the scope of the present invention.

[0022] The technical terms used herein are for the reference of specific embodiments only and are not intended to limit the invention. The singular forms used herein include plural forms unless phrases clearly indicate otherwise. As used in the specification, the meaning of “comprising” specifies certain characteristics, areas, integers, steps, actions, elements, and / or components, and does not exclude the presence or addition of other characteristics, areas, integers, steps, actions, elements, and / or components.

[0023] When it is stated that one part is "above" or "on" another part, it may be directly above or on the other part, or other parts may be involved in between. In contrast, when it is stated that one part is "directly above" another part, no other parts are interposed in between.

[0024] Unless otherwise defined, all terms used herein, including technical and scientific terms, have the same meaning as generally understood by those skilled in the art to which the present invention pertains. Terms defined in commonly used dictionaries are further interpreted to have meanings consistent with relevant technical literature and the present disclosure, and are not interpreted in an ideal or highly formal sense unless otherwise defined. Additionally, unless specifically noted, % means weight %, and 1 ppm is 0.0001 weight %.

[0025] Hereinafter, embodiments of the present invention are described in detail so that those skilled in the art can easily implement the invention. However, the present invention may be embodied in various different forms and is not limited to the embodiments described herein.

[0026] Lithium sulfide can be manufactured by various methods, but in one embodiment of the present invention, it can be manufactured by a method of heat-treating a lithium raw material together with carbon. This technology is called thermal reduction carbothermal technology.

[0027] In this case, the lithium raw material may be a raw material containing the lithium and sulfur components required for lithium sulfide.

[0028] When lithium sulfide is manufactured through this reaction, a purification process is required to remove residual carbon components and impurities.

[0029] Generally, a solvent extraction method is used, but in one embodiment of the present invention, the effect of removing impurities can be improved by applying an optimal solvent.

[0030] In addition, we propose a new method to replenish sulfur components lost due to the reaction during solvent extraction and to undergo a purification process.

[0031] Lithium sulfide obtained by this method may have the following characteristics.

[0032] Lithium sulfide can be obtained, which includes, for the 7Li NMR peaks, peak A at 2.2 - 2.5 ppm, peak B at 1.2 - 2.1 ppm, peak C at -0.7 - -0.5 ppm, peak D at 2.9 - 3.1 ppm, and peak E at -0.2 - 1.15 ppm, and for the integration ratio of the peaks, 5 ≤ A / B and 1 ≤ B / C.

[0033] At this time, the measurement and interpretation methods of 7Li NMR will be described in detail in the experimental examples described later.

[0034] While it is necessary to verify through various analyses whether the interpretation of such NMRs chemically corresponds accurately to each component, the inventors intend to quickly confirm the characteristics of purified lithium sulfide through simple NMR analysis.

[0035] Through this review, we aim to secure lithium sulfide with an optimal range of NMR peaks to resolve potential issues that may arise when applied as a battery material in advance.

[0036] In the above NMR peaks, the inventors' interpretation suggests that the peak in the range of 2.2 - 2.5 ppm appears to be a Li2S peak. The peak in the range of 1.2 - 2.1 ppm appears to be a lithium polysulfide peak. The peak in the range of -0.7 - -0.5 ppm appears to be a Li2SO4 peak. The peak in the range of 2.9 - 3.1 ppm appears to be a Li2O peak. The peak in the range of -0.2 - 1.15 ppm appears to be a Li2CO3 peak.

[0037] Each component can cause the following problems when lithium sulfide is used as a battery material (especially, sulfide-based solid electrolytes).

[0038] The above lithium polysulfide has the advantage of being able to partially compensate for the loss of sulfur that occurs during the synthesis of azirodite. However, if present in excess, unreacted Li2S and sulfur remain in the synthesized azirodite, which may cause a problem of reduced ion conductivity.

[0039] If lithium sulfate (Li2SO4) and lithium carbonate (Li2CO3) do not meet the appropriate range, impurities such as lithium phosphate (Li3PO4) may be generated during the synthesis of azirodite, which can lead to a problem of reduced ion conductivity.

[0040] In the case of lithium oxide (Li2O), if the appropriate range is not satisfied, impurities such as lithium chloride (LiCl) and lithium phosphate (Li3PO4) are generated during the synthesis of azirodite, which may lead to a problem of reduced ion conductivity.

[0041] By appropriately controlling the NMR peaks mentioned above, the composition of the final impurities can be controlled within an optimal range through the heat treatment step following the purification stage. It is crucial to ensure that the mechanisms of thermal decomposition or reduction of impurities through heat treatment function effectively, while simultaneously securing a lithium sulfide yield.

[0042] To achieve these objectives simultaneously, the lithium sulfide obtained through the purification process needs to be controlled to have an NMR peak within the aforementioned range.

[0043] More specifically, 8 ≤ A / B ≤ 50.

[0044] If the ratio of lithium sulfide to lithium polysulfide, inferred from the peak above, satisfies the above range, it can partially compensate for the sulfur loss occurring during the synthesis of azirodite.

[0045] More specifically, 2 ≤ B / C ≤ 3.

[0046] If the ratio of lithium polysulfide and lithium sulfate, as estimated from the peak above, satisfies the above range, the ion conductivity problem can be improved by controlling the appropriate range of impurities such as lithium phosphate.

[0047] More specifically, C / D ≤ 20.

[0048] If the ratio of lithium sulfate and lithium oxide, as estimated from the peak above, satisfies the above range, the decrease in ion conductivity can be prevented by appropriately controlling impurities such as lithium chloride and lithium phosphate. More specifically, 5 ≤ C / D ≤ 9.

[0049] More specifically, D / E ≤ 3.

[0050] If the ratio of lithium oxide and lithium carbonate, as inferred from the peak above, satisfies the above range, the problem of reduced ion conductivity can be solved by controlling the optimal content of lithium chloride and lithium phosphate during the manufacture of azirodite. More specifically, 0 < D / E ≤ 0.4.

[0051] As described above, after solvent extraction purification, an additional heat treatment step may be performed, and accordingly, C / D can be controlled to ≤ 2. In addition, the lithium sulfide is heat-treated after being purified by solvent extraction, and D / E ≤ 1.

[0052] This range is the range in which lithium sulfide can have the optimal effect as a battery material when applied to battery materials.

[0053]

[0054] In another embodiment of the present invention, a method for producing lithium sulfide is provided, comprising the steps of: mixing a carbon raw material and a lithium compound and heat-treating them to obtain a pyrolysis product; purifying the thermal reduction product by a solvent extraction method; and heat-treating the purified product; wherein, in the step of purifying the thermal reduction product by a solvent extraction method, an additional sulfur raw material is added to the thermal reduction product before solvent extraction, and then purified by solvent extraction.

[0055] As mentioned above, this is a method to replenish sulfur that is or may be lost during the purification step and the preceding pyrolysis reaction. If sulfur is insufficient, the yield of the final target material, lithium sulfide, may be reduced, and lithium may be converted into other forms, potentially causing issues with purity.

[0056] The above additional sulfur raw material may be solid sulfur (S).

[0057] The amount of the additional sulfur raw material added can be controlled according to the S / Li molar ratio in the total thermal reduction product. Specifically, the S / Li molar ratio of the thermal reduction product can be determined through ICP-OES analysis, and solid sulfur can be added to match the range of S / Li = 0.45 to 0.55. More specifically, it can be S / Li = 0.48 to 0.53.

[0058] By doing this, the amount of sulfur already lost in the thermal reduction reaction and the amount that may be additionally lost in the subsequent drying / heat treatment process can be replenished. Adding a larger amount of sulfur may instead result in problems such as reduced product quality due to the presence of Li2SO4 or residual carbon in the final product.

[0059] In one embodiment, the carbon raw material may include at least one of soft carbon, hard carbon, petroleum coke, coal needle coke, coal pitch coke, natural graphite, carbon black, sugars, and artificial graphite, as non-limiting examples. In one embodiment, the lithium-carbon compound may include at least one of lithium sulfate, lithium hydroxide, lithium oxide, and lithium carbonate.

[0060] In one embodiment, in the step of mixing a carbon raw material and a lithium compound and heat-treating them to obtain a thermal reduction product, the weight ratio of the lithium compound to the carbon raw material may be 0.25 to 0.5. Specifically, the ratio may be 0.30 to 0.35.

[0061] If the above ratio exceeds the upper limit, there is a problem with unreacted lithium compound raw materials, specifically lithium sulfate, remaining after heat treatment. If the above ratio exceeds the lower limit, the amount of carbon raw material used is large, resulting in a disadvantage in terms of process cost.

[0062] In one embodiment, the step of mixing a carbon raw material and a lithium compound and heat-treating them to obtain a thermal reduction product can be performed in a temperature range of 850 to 950 ℃.

[0063] In one embodiment, the step of performing the mixture of the carbon raw material and the lithium compound at a temperature range of 850 to 950 ℃ can be performed at a heating rate of 3 to 7 ℃ / min. Specifically, the heating rate can be performed at a heating rate of 4 to 6 ℃ / min.

[0064] The method may include a step of controlling the content of lithium oxide in lithium sulfide through a solvent extraction method of the above thermal reduction product. Specifically, after thermally reducing lithium sulfate in the gas phase using a reducing agent such as carbon, solvent extraction may be performed to separate the obtained lithium sulfide from the residual reducing agent.

[0065] In one embodiment, the solvent may include at least one solvent having a carbon chain length of 2 or more. Specifically, the solvent may satisfy a carbon chain length of 2 to 5, more specifically, 3 to 4.

[0066] By having a long carbon chain within the aforementioned range, the above solvent can suppress sulfur loss during the drying process and subsequently minimize sulfur loss during the drying process. If the above solvent has a carbon chain length shorter than the aforementioned range, there is a problem of causing sulfur loss during the drying process. If the above solvent has a carbon chain length longer than the aforementioned range, there is a problem of difficulty in removing the solvent through drying.

[0067] The above solvent may be, for example, an alcoholic solvent. Specifically, the alcoholic solvent having two or more carbon chains may include, as a non-limiting example, at least one of ethylene glycol, n-propanol, isopropanol, and n-butanol.

[0068] In one embodiment, the solvent in the filtration step may include two or more types of solvents. Specifically, the filtration step may include a composite solvent that essentially includes a solvent having a carbon chain length of 2 or more.

[0069] In one embodiment, the solvent having a carbon chain length of 2 or more among the composite solvents may be 30 weight% or more based on 100 weight% of the total solvent. Specifically, the solvent having a carbon chain length of 2 or more may be 30 weight% or more, more specifically 30 to 80 weight%, and even more specifically 40 to 60 weight% based on 100 weight% of the total solvent. By satisfying the above range, the S / Li content in the filtrate can be controlled as intended, and accordingly, the characteristics of the lithium sulfide produced in the subsequent process can be improved.

[0070] In the above filtration step, when the solvent comprises a plurality of solvents, the solvent having a carbon chain length of 2 or more that is necessarily included among the plurality of solvents satisfies the aforementioned range based on the total weight percentage of the solvent, thereby minimizing the loss of sulfur in the extract, which is the filtrate, and thus enabling the production of high-purity lithium sulfide with low impurities in subsequent processes.

[0071] In one embodiment, the solvent extraction step may be performed in a range of 0 to 60 ℃. Specifically, the filtration step may be performed in a range of 10 to 50 ℃, more specifically, in a range of 20 to 45 ℃.

[0072] As the above filtration step is performed within the aforementioned temperature range, the proportion of sulfur in the extract, which is the filtrate, increases, so the S / Li ratio can be controlled to a target range, and the lithium content in the extract is controlled to an appropriate ratio, resulting in less loss of sulfur and allowing for the subsequent drying process to obtain lithium sulfide with a low impurity content.

[0073] In one embodiment, the ratio of sulfur to lithium (S / Li) in the filtered filtrate obtained through the filtration step may be in the range of 0.45 to 0.55. The ratio can be adjusted by including the content of sulfur added prior to the purification step.

[0074] Specifically, the S / Li ratio may be 0.460 or higher, more specifically 0.470 or higher, and even more specifically 0.475 to 0.520 or lower. By satisfying the aforementioned range, the lithium content in the extract is controlled to an appropriate ratio, thereby allowing for the production of lithium sulfide with minimal sulfur loss. If the ratio deviates from the upper or lower limit, there is a problem in that the ratio of sulfur to lithium deviates from the appropriate range, making it impossible to obtain high-purity lithium sulfide.

[0075] In one embodiment, the sulfur content in the filtrate filtered through the filtration step may be 3.5 to 15.0 g / L. The sulfur content may be 4.0 to 11.0 g / L. By satisfying the aforementioned range of sulfur content, an extract with low sulfur loss can be prepared.

[0076] In one embodiment, the lithium content in the filtered filtrate obtained through the filtration step may be greater than 0 to 7.0 g / L. Specifically, the lithium content may be 1 to 5.0 g / L or 1.65 to 4.80 g / L. By satisfying the aforementioned ranges for the lithium content, lithium sulfide with low sulfur loss and low impurity content in the subsequent drying process can be obtained.

[0077] In one embodiment, the filtration step may be performed in an inert atmosphere. Specifically, the inert atmosphere may be performed in an inert atmosphere such as Ar, H2, He, or N2.

[0078] In one embodiment, the filtration step may extract Li2S by dissolving the thermal reduction product in a solvent and stirring at room temperature for 1 to 30 hours, specifically 20 to 27 hours. As the solvent extraction step is performed for the aforementioned time, lithium sulfide with low sulfur loss can be produced.

[0079] Specifically, the stirring can be performed at 50 to 300 rpm, specifically 50 to 200 rpm, and more specifically 100 to 200 rpm. Subsequently, unreacted carbon is filtered using a pore size filter of 0.5 to 2 μm in an inert atmosphere. Afterward, a step of drying the filtrate using a concentrator such as a rotary evaporator was performed.

[0080] Specifically, the extractable reactant may partially dissociate in solution or simultaneously achieve dynamic equilibrium with an ion pair according to the following reaction scheme 1_2.

[0081] [Reaction Equation 1_1] - Isopropane alcohol 100%

[0082] Li2S(s) + CH3CHCH3OH(l) → LiSH + CH3CHCH3O-Li ↔ 2Li + + HS - + (CH3CHCH3O) -

[0083] [Reaction Equation 1_2] - Isopropane alcohol + ethanol complex solvent

[0084] 2Li2S(s) + CH3CHCH3OH(l) + CH3CH2OH(l) → 2LiSH + CH3CHCH3O-Li + CH3CH2O-Li ↔ 4Li + + 2HS - + (CH3CH2O) - + (CH3CHCH3O) -

[0085]

[0086] The step of drying the above-mentioned filtrate can be performed in a temperature range of 20 to 160 ℃. Specifically, the temperature range can be performed in a temperature range of 30 to 150 ℃, more specifically, in a temperature range of 40 to 150 ℃.

[0087] The step of drying the above filtrate may be a step of slowly drying the solution containing lithium sulfide extracted through filtration by evaporating it. Through the drying step, the solution containing lithium sulfide may be evaporated to obtain a white powder, which can be expressed by the following reaction schemes 2_1 and 2_2.

[0088] [Reaction Equation 2_1] - Isopropane alcohol 100%

[0089] 2Li + + HS - + (CH3CHCH3O) - + n CH3CHCH3OH(l) → Li2S(s) + (n+1) CH3CHCH3OH(l)↑

[0090] [Reaction Equation 2_2] - Isopropane alcohol + ethanol complex solvent

[0091] 4Li + + 2HS - + (CH3CHCH3O) - + (CH3CH2O) - + n CH3CHCH3OH(l) + m CH3CH2OH(l)→ Li2S(s) + (n+1) CH3CHCH3OH(l)↑ + (m+1) CH3CH2OH(l)↑

[0092] The white powder obtained according to the above reaction schemes 2_1 and 2_2 may be a crude powder in which an ethanol solvent is adsorbed or absorbed around a mixture of lithium sulfide or extractable reactants.

[0093] In addition, water sulfide ions (HS) caused by heat applied during the drying process of the solution - Differences in acid-base changes occur between molecules with long carbon chains and / or complex solvents containing them, which can cause some of the hydrosulfide ions to exit the system in the form of hydrogen sulfide gas through side reactions such as reaction schemes 3_1 and 3_2 below.

[0094] [Reaction Equation 3_1] - Isopropane alcohol 100%

[0095] 2Li + + HS - + (CH3CHCH3O)- + CH3CHCH3OH(l) → 2Li(CH3CHCH3O) + H2S(g)↑

[0096] [Reaction Equation 3_2] - Isopropane alcohol + ethanol complex solvent

[0097] 2Li + + HS - + (RO)- + ROH(l) → 2Li(RO) + H2S(g)↑ (R= CH3CH 2, CH3CHCH3)

[0098] In one embodiment, the solvent having a carbon chain length of 2 or more may have an acid dissociation constant greater than 16 pKa. Specifically, the acid dissociation constant may be 16.2 pKa or more, more specifically, 16.2 pKa to 20 pKa. In one embodiment, the solvent having a carbon chain length of 2 or more may have a boiling point of 200 °C or less. Specifically, the boiling point may be 100 °C or less, more specifically, 75 to 100 °C, and more specifically, 80 to 100 °C. By satisfying the aforementioned ranges for the acid dissociation constant and the boiling point, the generation of hydrogen sulfide gas during the reaction can be suppressed.

[0099] Specifically, while ethanol has an acid dissociation constant of 16 pKa and a boiling point of approximately 78.37 °C, isopropanol has an acid dissociation constant of 16.5 pKa and a boiling point of approximately 82.5 °C. As such, since the acid dissociation constant of isopropanol is higher than that of ethanol and thus has lower acidity, the generation of hydrogen sulfide gas described in [Reaction Equation 3_1] can be suppressed when extracted with isopropanol compared to when extracted with ethanol. Accordingly, the loss of sulfur can be suppressed by using isopropanol.

[0100] By controlling the drying temperature during the step of drying the above-mentioned filtrate, the dielectric constant value of the solvent may differ, thereby changing the degree of ion dissociation. Specifically, if the drying temperature increases, the dielectric constant of the solvent decreases, making it difficult for ions to dissociate in the solvent phase. This weakens the basicity of the hydrosulfide ions, which can delay or hinder the generation of hydrogen sulfide, thereby inducing the formation of lithium sulfide in terms of the overall reaction. This is expressed as shown in Reaction Scheme 4 below.

[0101] [Reaction Equation 4_1] - Isopropane alcohol 100%

[0102] LiSH + Li(CH3CHCH3O) + CH3CHCH3OH(l) → Li2S(s) + 2CH3CHCH3OH(l)

[0103] [Reaction Equation 4_2] - Isopropane alcohol + ethanol complex solvent

[0104] LiSH + Li(RO) + ROH(l) → Li2S(s) + 2ROH(l) (R= CH3CH 2, CH3CHCH3)

[0105] In one embodiment, the method for manufacturing lithium sulfide may include a step of heat-treating the dried material after a drying step. The step of heat-treating the dried material involves applying heat to the dried material to obtain a final lithium sulfide (Li2S) powder.

[0106] In one embodiment, the dried product may be subsequently heat-treated in a temperature range of 650 to 950 ℃, specifically in a temperature range of 700 to 900 ℃. In one embodiment, the heating rate of the heat-treating step may be performed at 5 to 20 ℃ per minute. Specifically, the heating rate may be performed at 5 to 15 ℃ / min, more specifically at 8 to 12 ℃ / min.

[0107] In one embodiment, the maximum temperature holding time in the step of heat-treating the dried product may be performed for 1 to 6 hours. In one embodiment, the step of heat-treating the dried product may include a step of natural cooling after performing heat treatment for the aforementioned time.

[0108] If the upper limits of the aforementioned temperature, heating rate, and holding time are exceeded, there is a problem in which lithium sulfide is converted into lithium oxide. If the lower limits of the aforementioned temperature, heating rate, and holding time are exceeded, there is a problem in which the purity of lithium sulfide is reduced because impurities such as lithium hydroxide or lithium carbonate cannot be easily removed.

[0109] In one embodiment, the step of heat-treating the dried product may be performed in an inert gas atmosphere. The inert gas may include, for example, at least one of helium, neon, krypton, xenon, nitrogen, and argon.

[0110] In one embodiment, the step of heat-treating the dried product may be performed at a flow rate of the inert gas of 2.0 to 3.0 L / min. Specifically, the flow rate may be performed at 2.3 to 3.8 L / min.

[0111] If the above flow rate exceeds the upper limit of the aforementioned range, there is a problem that the process cost increases due to the use of an excessive amount of inert gas in the manufacturing process. If the above flow rate exceeds the lower limit of the aforementioned range, there is a problem that organic solvents or decomposition products of organic solvents evaporating from the drying material are not discharged smoothly and remain in the form of carbides, thereby lowering the purity of lithium sulfide.

[0112] Hereinafter, embodiments of the present invention will be described in detail. However, these are presented as examples and are not intended to limit the present invention, and the present invention is defined only by the scope of the claims set forth below.

[0113]

[0114] Experimental Example 1: Lithium sulfide purified by solvent extraction method

[0115] 720 g of graphite and 240 g of lithium sulfate monohydrate were mixed evenly, and then heated to 900 ℃ at a rate of 5 ℃ per minute in an inert gas (Ar) atmosphere. Subsequently, when heat-treated at 900 ℃ for 2 hours, the lithium sulfate was thermally reduced to lithium sulfide and obtained in a mixed state with graphite.

[0116] Afterward, the obtained mixture was naturally cooled in an inert gas atmosphere and stored in a glove box.

[0117] Afterwards, optionally, additional sulfur was added to the obtained mixture. The type of sulfur added was solid sulfur, and each was added according to the S / Li molar ratio in Table 1.

[0118] At this time, Sample 5 below is the result of adding solid sulfur after solvent extraction, and is data intended to analyze the effect according to the order of sulfur addition. As can be seen later, it is evident that the purity is lower compared to Sample 4, in which sulfur was added before sample extraction.

[0119] In an inert gas atmosphere, 15 g of the thermal reduction product is added to 75 mL of ethanol or a mixed solvent of isopropanol and ethanol, and stirred sufficiently at room temperature for 24 hours. After stopping the stirring, the mixed slurry is filtered, and the graphite that does not dissolve in the solvent remains in a wet state in the filtration funnel, while the remainder is obtained as a solution in the filtration flask.

[0120] Subsequently, the prepared filtrate was transferred to a 1 L round-bottom flask, mounted on a rotary evaporator, and the solution was evaporated and concentrated using a vacuum pump to proceed with the drying process. During this process, the experiment was conducted under vacuum conditions at a drying temperature of 50 ℃ for a total of 30 minutes.

[0121] Specifically, the vacuum pressure was controlled within the range of 15-200 mbar, and an external constant temperature bath was used for drying. When drying at a temperature of 60 ℃ or lower, a heated water bath was used, and when a temperature of 80 ℃ or higher was required, a constant temperature bath containing silicone oil was used.

[0122] Lithium sulfide powder was obtained by heat-treating the dried powder at 800 ℃ for 2 hours. The heating rate during heat treatment was 10 ℃ / min, and the flow rate of Ar gas was 2.5 L / min.

[0123] NMR, component analysis, etc. were performed on the samples obtained in this way, and the results are shown in Table 2 below.

[0124] Sample Name S / Li (Molar Ratio) Ethanol (v / v%) Isopropanol (v / v%) Sample 10.480 1000 Sample 20.555 1000 Sample 30.560 1000 Sample 40.550 6040 Sample 50.550 6040 Sample 60.510 1000 Sample 7 (High Purity Sample) 0.500 1000

[0125] Percentage of molecules estimated to be included (wt%) Peak ratio of each component Sample Name Li2S Polysulfide Li2SO4 Li2OL Li2CO3 Li2S+ Polysulfide Li2S / Polysulfide (A / B) Polysulfide / Li2SO4 (B / C) Li2SO4 / Li2O (C / D) Li2O / Li2CO3(D / E) Sample 1 70.720.708.6091.43.42 -0.00 - Sample 2 83.13.39.730.986.425.180.343.233.33 Sample 3 80.92.914.60.5183.827.900.2029.200.50 Sample 4 84.29.64.30.51.493.88.772.238.600.36 Sample 5 85.21.513.30086.756.800.11 - Sample 6 73.921.604.4095.53.42 -0.00 - Sample 7 (High Purity Sample)77.821.700.5099.53.58-0.00-

[0126] Figure 1 shows the NMR data of sample 4. The interpretation of each peak is as described above. The peak ranges differ for each sample and change again after undergoing a subsequent heat treatment step. These changes in peaks can ultimately be linked to the purity and yield of lithium sulfide.

[0127]

[0128] 7Li NMR Evaluation: An OXFORD 600MHz solid-state NMR spectrometer was used. The sample is loaded into the solid-state NMR rotor inside the glove box. After closing the rotor, the sample is removed from the glove box. The sample is then placed inside the NMR spectrometer and measured.

[0129] Sfo1=233.17MHz, spinning frequency=22kHz, d1=900s, 30° pulse.

[0130] The measured 7Li MAS NMR data was analyzed using the TopSpin-sola program.

[0131]

[0132] The peak analysis results obtained through 7Li NMR analysis are as follows.

[0133] Compound 7Li peak (ppm)Li2S 2.2~2.5Li2O 2.9~3.1Li2CO3 -0.2~1.15Li2SO4 -0.7 ~ -0.5Li2(S)n 1.2-2.1

[0134] These are the values ​​obtained by correcting the 7Li NMR peaks for each compound standard sample with an aqueous LiCl solution of 0 ppm. In the case of Sample 1, it can be seen that lithium sulfate and lithium carbonate, which can act as impurities, are hardly detected. However, in the case of lithium carbonate, it may not have been measured because it is included in the lithium polysulfide peak.

[0135] In the case of Sample 1, there may be problems with the target yield of lithium sulfide due to the excessive amount of lithium polysulfide. Unlike other impurities, lithium polysulfide is a substance that can affect the movement of lithium to some extent.

[0136] In the case of samples 2 and 3, the amount of lithium polysulfide decreased relatively. However, it can be seen that the amount of lithium sulfate increased significantly. This may affect the purity and yield of the lithium sulfide obtained finally.

[0137] In the case of Sample 5, only lithium sulfate is present as an impurity, but the amount exceeds the standard limit, so the same problem mentioned above may occur.

[0138] In the case of Sample 6, the amount of lithium sulfate decreased, but the amount of lithium polysulfide increased significantly, and the lithium oxide impurity increased. This can also cause problems with the purity and yield of lithium sulfide. After this purification process, additional heat treatment can be performed to finally secure the purity and yield of the obtained lithium sulfide.

[0139]

[0140] An azirodite solid electrolyte was prepared using the above Sample 4 and high-purity Sample 7. The results of confirming the ionic conductivity are shown in Table 4 below.

[0141] Sample 42.46 mS / cm Sample 72.56 mS / cm

[0142] In the case of sample 4, it can be seen that it has secured very excellent ionic conductivity similar to that of high-purity sample 7.

[0143] The present invention is not limited to the above embodiments and can be manufactured in various different forms, and those skilled in the art will understand that the invention can be implemented in other specific forms without changing the technical concept or essential features of the invention. Therefore, the embodiments described above should be understood as illustrative in all respects and not restrictive.

Claims

1. A step of mixing a carbon raw material and a lithium compound and heat-treating them to obtain a thermal reduction product; A step of purifying the above thermal reduction product by a solvent extraction method; and The method includes the step of heat-treating the above-mentioned purified product; and In the step of purifying the above thermal reduction product by a solvent extraction method, A method for manufacturing lithium sulfide, wherein, prior to solvent extraction, an additional sulfur raw material is added to the thermal reduction product, and then purified by solvent extraction.

2. In Paragraph 1, A method for manufacturing lithium sulfide, wherein the additional sulfur raw material is solid sulfur (S).

3. In Paragraph 1, A method for producing lithium sulfide, wherein the amount of the additional sulfur raw material input is controlled so that the S / Li molar ratio of the thermal reduction product is in the range of 0.45 to 0.

55.

4. In Paragraph 1, In the step of purifying the above thermal reduction product by a solvent extraction method, A method for producing lithium sulfide, wherein the solvent is ethanol, a solvent having a carbon chain length of 2 or more, or a mixture thereof.

5. In Paragraph 1, A method for manufacturing lithium sulfide, wherein the above lithium compound includes lithium sulfate.

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