Lithium sulfide, preparation method therefor, and method for preparing solid electrolyte for secondary battery by using same

Abstract

A method for preparing lithium sulfide, according to the present invention, may comprise the steps of: preparing a base solvent; dissolving a lithium source in the base solvent so as to prepare a first base source; dissolving a sulfur source in the base solvent so as to prepare a second base source; mixing the first base source and the second base source and adding a catalyst so as to prepare a lithium sulfide source; reacting the lithium sulfide source so as to prepare preliminary lithium sulfide; and washing and drying the preliminary lithium sulfide so as to prepare lithium sulfide.

Description

Lithium sulfide, its production method, and its production method for a solid electrolyte for a secondary battery using the same

[0001] The present invention relates to lithium sulfide, a method for producing the same, and a method for producing a solid electrolyte for a secondary battery using the same, and more specifically, to lithium sulfide produced by mixing a first base source and a second base source, each of which is a lithium source and a sulfur source dissolved in a base solvent in which a surfactant is dissolved, adding a catalyst, and reacting the mixture, a method for producing the same, and a method for producing a solid electrolyte for a secondary battery using the same.

[0002] Lithium-ion batteries are widely used, particularly as power supplies for home appliances. These secondary batteries use an organic liquid electrolyte containing an organic solvent, and lithium ions move from one electrode to the other during charging and discharging. However, because organic solvents are flammable, interest in all-solid-state lithium-ion batteries that do not use them is growing.

[0003] All-solid-state lithium-ion batteries are manufactured by configuring the entire battery in a solid form using a solid electrolyte, such as lithium (Li), phosphorus (P), sulfur (S), and a halogen. One of the starting materials for manufacturing these solid electrolytes is lithium sulfide (Li2S). The technical characteristics of this starting material, such as purity, particle size, and porosity, are crucial for obtaining a high-purity solid electrolyte. Accordingly, active research and development is underway to develop efficient methods for manufacturing lithium sulfide (Li2S).

[0004] For example, US 2020 / 165129 A1 (Albemarle) relates to Li2S powder and its preparation, wherein such powder has an average particle size of 250 μm to 1,500 μm and a BET surface area of ​​1 to 100 m 2 / g. The manufacturing method comprises the steps of a) heating lithium hydroxide monohydrate having an average particle size in the range of 150 μm to 2,000 μm in the absence of air in a temperature-controlled unit to a reaction temperature of 150°C to 450°C while flowing an inert gas upward or through it until the residual water content of the crystallized lithium hydroxide formed becomes less than 5 wt%, and b) overflowing or crossing a sulfur source into the anhydrous lithium hydroxide formed in the first step. However, such Li2S powders are problematic due to their large average particle size exceeding 100 μm, which means that further processing is required before use, especially when such Li2S powders are used in the manufacture of battery components.

[0005] For another example, US 2015 / 0246811 (Arkema France) discloses a process for preparing alkali metal sulfides, which comprises at least one step a) of reacting at least one oxygen-containing compound of said alkali metal with at least one sulfur-containing compound of the formula I: RS(=O)n-Sx-R'. Two embodiments for carrying out said step a) are disclosed. A first embodiment is carried out at a temperature of 150°C to 500°C, preferably 150°C to 400°C, preferably 200°C to 350°C, in the presence of at least one catalyst, which has the purpose of increasing the kinetics of the reaction. A second embodiment is carried out at a temperature of preferably 300°C to 800°C, preferably 300°C to 600°C, optionally in the absence of a catalyst. In step a), preferably water is added, or alternatively, hydrogen can be used. In the embodiment, the preliminary step is carried out under nitrogen flow at temperatures of 550°C and 250°C. This process has the problem of requiring a complex raw material (RS(=O)n-Sx-R') and the use of a catalyst or high reaction temperature.

[0006] As another example, Ohsaki et al. (Powder Technology, 387, July 2021, 415-420) describe the synthesis of solid electrolyte particles of Li3PS4 with size control in the submicron scale using a liquid-phase agitation method starting from fine Li2S particles, which must undergo a wet milling or dissolution-precipitation process before being used to prepare solid electrolyte particles of Li3PS4.

[0007] As another example, US 2016 / 0104916 (Idemitsu Kosan Co., Ltd.) discloses a method for producing a solid electrolyte, which comprises contacting an alkali metal sulfide, one or more sulfur compounds, and a halogen compound in a solvent. The production of the alkali metal sulfide, specifically Li2S, is disclosed with reference to methods known in the art. For example, the reaction of lithium hydroxide and hydrogen sulfide in a hydrocarbon solvent is initiated at 70°C to 300°C. The lithium sulfide can be modified using a solvent including a polar solvent to obtain a large specific surface area. In Production Example 1, toluene is used. The particle size of the alkali metal sulfide used as a raw material is not limited, and in practice, the particle size may exceed 100 μm, since the step of reducing the particle size is not always advantageous in terms of cost.

[0008] As another example, an anode material and a method for producing the same are also disclosed in US 2013 / 0295464 (Idemitsu Kosan Co., Ltd.). Hydrogen sulfide and alkali metal hydroxides can be used as raw materials. Production Example 1 initiates a reaction between lithium hydroxide and hydrogen sulfide in N-methyl-2-pyrrolidone (NMP) at 130°C.

[0009] The technical problem to be solved by the present invention is to provide a method for producing lithium sulfide with improved crystallinity.

[0010] Another technical problem to be solved by the present invention is to provide a method for producing lithium sulfide having high purity and improved yield.

[0011] Another technical problem that the present invention seeks to solve is to provide a method for producing lithium sulfide with a shortened production time.

[0012] Another technical problem that the present invention seeks to solve is to provide a method for producing lithium sulfide with a simplified manufacturing process.

[0013] Another technical problem that the present invention seeks to solve is to provide a method for producing lithium sulfide with reduced manufacturing process costs.

[0014] Another technical problem that the present invention seeks to solve is to provide a method for producing lithium sulfide that is easy to mass-produce.

[0015] The technical problems to be solved by the present invention are not limited to those described above.

[0016] To solve the above technical problem, the present invention provides a method for producing lithium sulfide.

[0017] According to one embodiment, the method for producing lithium sulfide may include the steps of preparing a base solvent, dissolving a lithium source in the base solvent to prepare a first base source, dissolving a sulfur source in the base solvent to prepare a second base source, mixing the first base source and the second base source and adding a catalyst to prepare a lithium sulfide source, reacting the lithium sulfide source to produce a preliminary lithium sulfide, and washing and drying the preliminary lithium sulfide to produce the lithium sulfide.

[0018] According to one embodiment, the base solvent may include one prepared by dissolving a surfactant in an organic solvent.

[0019] According to one embodiment, the organic solvent may include any one of acetonitrile, butanol, isopropanol, N-methyl-2-pyrrolidone (NMP), acetone, tetrahydrofuran, methanol, dimethylacetamide, or ethanol.

[0020] In one embodiment, the surfactant may include any one of hydrazine hydrate, acetone, ethylene glycol, or Triton X-165.

[0021] In one embodiment, the lithium source may include any one of lithium bromide (Li bromide), lithium hydroxide (Li hydroxide), lithium fluoride (LiF), lithium nitrate (Li nitrate), lithium carbonate (Li carbonate), lithium acetate (Li acetate), lithium iodide (Li iodide), lithium chloride (LiCl), or lithium bis(trifluoromethanesulfonyl)imide (LiTFSI).

[0022] In one embodiment, the sulfur source may comprise sodium sulfide (Na2S).

[0023] According to one embodiment, the catalyst may include any one of thioacetamide, gallic acid, L-ascorbic acid, thiourea, cyanuric acid, cetyltrimethylammonium bromide (CTAB), or graphite.

[0024] According to one embodiment, the crystallinity of the lithium sulfide may be controlled depending on the time for which the lithium sulfide source is reacted.

[0025] According to one embodiment, the time for reacting the lithium sulfide source may be controlled to be more than 1 hour and less than 4 hours, thereby improving the crystallinity of the lithium sulfide.

[0026] According to one embodiment, the surfactant may include acetone, the organic solvent may include ethanol, the lithium source may include lithium chloride (LiCl), the sulfur source may include sodium sulfide (Na2S), and the catalyst may include graphite.

[0027] According to one embodiment, the surfactant, the organic solvent, the lithium source, and the catalyst may include materials selected to improve the crystallinity of the lithium sulfide.

[0028] In one embodiment, the surfactant may include Triton X-165, the organic solvent may include any one of acetone, tetrahydrofuran, methanol, or ethanol, the lithium source may include any one of lithium acetate, lithium iodide, lithium chloride (LiCl), or lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), and the catalyst may include any one of cyanuric acid, cetyltrimethylammonium bromide (CTAB), or graphite.

[0029] According to one embodiment, the shape of the particles of the lithium sulfide may be controlled depending on the type of the lithium source.

[0030] According to one embodiment, the lithium source may be selected as the lithium bis(trifluoromethanesulfonyl)imide or the lithium iodide, and the shape of the lithium sulfide particle may be controlled to be in the form of a flake, and the lithium source may be selected as the lithium acetate or the lithium chloride, and the shape of the lithium sulfide particle may be controlled to be in the form of a spherical.

[0031] According to one embodiment, the catalyst may be graphite, and the weight ratio of the graphite to the weight of the lithium source in the lithium sulfide source may be controlled to be 0.1 wt% or more and 2 wt% or less.

[0032] According to one embodiment, the catalyst may include cyanuric acid, and a weight ratio of the cyanuric acid to the weight of the lithium source in the lithium sulfide source is controlled to be greater than 1 wt% and less than 10 wt%.

[0033] According to one embodiment, the catalyst may include cetyltrimethylammonium bromide, and a weight ratio of cetyltrimethylammonium bromide relative to the weight of the lithium source in the lithium sulfide source is controlled to be greater than 1 wt% and less than 5 wt%.

[0034] In order to solve the above technical problem, the present invention provides a method for manufacturing a solid electrolyte for a secondary battery using lithium sulfide manufactured by the above-described manufacturing method.

[0035] According to one embodiment, the method for manufacturing the solid electrolyte may include the steps of preparing lithium sulfide, germanium sulfide, and phosphorus pentasulfide manufactured by the above-described manufacturing method, ball milling the lithium sulfide, the germanium sulfide, and the phosphorus pentasulfide to manufacture a precursor, and processing the precursor into a pellet form and heat-treating the precursor to manufacture the solid electrolyte.

[0036] According to one embodiment, in the step of manufacturing the precursor, the lithium sulfide, the germanium sulfide, and the phosphorus pentasulfide may be ball milled in an argon atmosphere for 10 hours, and in the step of manufacturing the solid electrolyte, the precursor processed into a pellet form may be heat-treated at 550° C. for 8 hours in an argon atmosphere.

[0037] A method for producing lithium sulfide according to the present invention may include a step of preparing a base solvent, a step of dissolving a lithium source in the base solvent to prepare a first base source, a step of dissolving a sulfur source (e.g., sodium sulfide) in the base solvent to prepare a second base source, a step of mixing the first base source and the second base source and adding a catalyst to prepare a lithium sulfide source, a step of reacting the lithium sulfide source to prepare a preliminary lithium sulfide, and a step of washing and drying the preliminary lithium sulfide to prepare the lithium sulfide.

[0038] In the step of preparing the base solvent, the base solvent can be prepared by dissolving a surfactant in an organic solvent. Then, due to the organic solvent (for example, any one of acetone, tetrahydrofuran, methanol, or ethanol) and the surfactant (Triton X-165), lithium sulfide having improved crystallinity and prevented particle aggregation can be provided.

[0039] And, in the step of preparing the first base source, lithium sulfide having improved crystallinity and reduced particle size can be provided due to the lithium source (any one of lithium acetate, lithium iodide, lithium chloride, or lithium bis(trifluoromethanesulfonyl)imide).

[0040] In addition, in the step of manufacturing the lithium sulfide source, lithium sulfide having improved crystallinity and reduced particle size can be provided due to the catalyst (any one of cyanuric acid, cetyltrimethylammonium bromide, or graphite) in the lithium sulfide source.

[0041] Moreover, unlike conventional techniques, the additional heat treatment process and impurity purification process after the step of manufacturing the lithium sulfide can be omitted. Accordingly, the manufacturing process of the lithium sulfide can be simplified, and since high-temperature heat energy is not required, the manufacturing process cost of the lithium sulfide can be reduced. Accordingly, a method for manufacturing the lithium sulfide that is easy to mass-produce can be provided.

[0042] Therefore, the lithium sulfide manufactured by the above-described manufacturing method is of high purity and has sulfur ions (S) of the lithium sulfide. 2- ) forms a face-centered cubic lattice, and lithium ions (Li + ) may have an antifluorite structure provided therein. Accordingly, a solid electrolyte (e.g., Li) using the lithium sulfide 10 GeP2S 12 ) and applying the solid electrolyte to a lithium secondary battery can improve the coulombic efficiency and capacity retention rate of the lithium secondary battery.

[0043] Figure 1 is a flowchart illustrating a method for producing lithium sulfide according to an embodiment of the present invention.

[0044] FIG. 2 is a drawing for explaining a method for manufacturing a base solvent according to an embodiment of the present invention.

[0045] FIG. 3 is a drawing for explaining a method for manufacturing a first source according to an embodiment of the present invention.

[0046] FIG. 4 is a drawing for explaining a method for manufacturing a second source according to an embodiment of the present invention.

[0047] FIG. 5 is a drawing for explaining a method for manufacturing a lithium sulfide source according to an embodiment of the present invention.

[0048] FIG. 6 is a drawing for explaining a preliminary lithium sulfide according to an embodiment of the present invention.

[0049] FIG. 7 is a drawing for explaining a method for producing lithium sulfide according to an embodiment of the present invention.

[0050] FIG. 8 is a drawing for explaining a method for manufacturing a precursor according to an embodiment of the present invention.

[0051] FIG. 9 is a drawing for explaining a method for manufacturing a solid electrolyte according to an embodiment of the present invention.

[0052] Figure 10 is a graph for comparing the crystallinity of lithium sulfide according to Experimental Examples 1-1 to 1-5 of the present invention.

[0053] Figure 11 is a photograph for comparing the particle sizes of lithium sulfide according to Experimental Examples 1-1 to 1-5 of the present invention.

[0054] Figure 12 is a graph for comparing the crystallinity of lithium sulfide according to Experimental Examples 1-2 and 2-1 to 2-3 of the present invention.

[0055] Figure 13 is a photograph for comparing the particle sizes of lithium sulfide according to Experimental Examples 1-2 and 2-1 to 2-3 of the present invention.

[0056] Figure 14 is a graph for comparing the crystallinity of lithium sulfide according to Experimental Examples 2-3 and 3-1 to 3-8 of the present invention.

[0057] Figure 15 is a photograph for comparing the size and shape of lithium sulfide particles according to Experimental Examples 2-3 and 3-1 to 3-8 of the present invention.

[0058] Figure 16 is a graph for comparing the crystallinity of lithium sulfide according to Experimental Examples 2-3 and 4-1 to 4-8 of the present invention.

[0059] Figure 17 is a photograph for comparing the size and shape of lithium sulfide particles according to Experimental Examples 2-3, 4-1, and 4-5 to 4-8 of the present invention.

[0060] Figure 18 is a graph for comparing the crystallinity of lithium sulfide according to Experimental Examples 2-3 and 5-1 to 5-6 of the present invention.

[0061] Figure 19 is a photograph for comparing the particle sizes of lithium sulfide according to Experimental Examples 2-3 and 6-1 to 6-5 of the present invention.

[0062] FIG. 20 is a photograph for comparing the particle sizes of lithium sulfide according to Experimental Example 5-2 and Experimental Examples 7-1 to 7-5 according to the present invention.

[0063] FIG. 21 is a graph for comparing the crystallinity of lithium sulfide according to Experimental Examples 5-6, 8-2, 8-3, and 8-5 of the present invention.

[0064] Figure 22 is a photograph for comparing the size and shape of lithium sulfide particles according to Experimental Examples 5-6 and Experimental Examples 8-1 to 8-5 of the present invention.

[0065] Figure 23 is a photograph for comparing the particle sizes of lithium sulfide according to Experimental Example 5-4 and Experimental Examples 9-1 to 9-5 of the present invention.

[0066] Figure 24 is a photograph for comparing the particle sizes of lithium sulfide according to Experimental Example 5-3 and Experimental Examples 10-1 to 10-5 of the present invention.

[0067] Figure 25 is a graph for comparing the particle sizes of lithium sulfide according to Experimental Example 5-5 and Experimental Examples 11-1 to 11-3 of the present invention.

[0068] Figure 26 is a graph for comparing the purity and yield of lithium sulfide according to Experimental Examples 2-3 and 5-1 to 5-6 of the present invention.

[0069] Figure 27 is a graph showing the performance of a full cell according to Experimental Examples 1 to 4 of the present invention.

[0070] Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the attached drawings. However, the technical concept of 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 ensure that the disclosed content is thorough and complete and to sufficiently convey the spirit of the present invention to those skilled in the art.

[0071] In this specification, when a component is referred to as being on another component, it means that it can be formed directly on the other component, or a third component may be interposed between them. In addition, in the drawings, the thicknesses of films and regions are exaggerated for the purpose of effectively explaining the technical contents.

[0072] Also, although terms such as first, second, and third have been used to describe various components in various embodiments of this specification, these components should not be limited by these terms. These terms are only used to distinguish one component from another. Thus, what is referred to as a first component in one embodiment may be referred to as a second component in another embodiment. Each embodiment described and illustrated herein also includes its complementary embodiments. Also, the term "and / or" has been used herein to mean including at least one of the components listed before and after.

[0073] In the specification, singular expressions include plural expressions unless the context clearly dictates otherwise. In addition, terms such as "comprise" or "have" are intended to specify the presence of a feature, number, step, component, or combination thereof described in the specification, and should not be construed as excluding the presence or addition of one or more other features, numbers, steps, components, or combinations thereof. In addition, the term "connection" is used in the present specification to mean both indirectly connecting multiple components and directly connecting them.

[0074] In addition, when describing the present invention below, if it is determined that a detailed description of a related known function or configuration may unnecessarily obscure the gist of the present invention, the detailed description will be omitted.

[0075]

[0076] FIG. 1 is a flowchart for explaining a method for producing lithium sulfide according to an embodiment of the present invention, FIG. 2 is a diagram for explaining a method for producing a base solvent according to an embodiment of the present invention, FIG. 3 is a diagram for explaining a method for producing a first source according to an embodiment of the present invention, FIG. 4 is a diagram for explaining a method for producing a second source according to an embodiment of the present invention, FIG. 5 is a diagram for explaining a method for producing a lithium sulfide source according to an embodiment of the present invention, FIG. 6 is a diagram for explaining a preliminary lithium sulfide according to an embodiment of the present invention, and FIG. 7 is a diagram for explaining a method for producing lithium sulfide according to an embodiment of the present invention.

[0077] Referring to FIGS. 1 and 2, a base solvent (100) is prepared (S100).

[0078] The base solvent (100) can be prepared by dissolving a surfactant (120) in an organic solvent (110). For example, the organic solvent (110) can be any one of acetonitrile, butanol, isopropanol, N-methyl-2-pyrrolidone (NMP), acetone, tetrahydrofuran, methanol, dimethylacetamide, or ethanol. For example, the surfactant (120) can be any one of hydrazine hydrate, acetone, ethylene glycol, or Triton X-165.

[0079] And, depending on the type of the organic solvent (110) and the surfactant (120), the crystallinity of the lithium sulfide (300) described below can be controlled.

[0080] According to one embodiment, the organic solvent (110) may be selected from any one of acetone, tetrahydrofuran, methanol, or ethanol. In addition, the surfactant (120) may be selected from Triton X-165. Accordingly, the crystallinity of the lithium sulfide (300) described below may be improved. In addition, the aggregation of the particles of the lithium sulfide (300) described below may be prevented.

[0081] Referring to FIGS. 1 and 3, a first base source (220) is prepared by dissolving a lithium source (210) in the base solvent (100) (S200).

[0082] The lithium source (210) may be, for example, any one of lithium bromide (Li bromide), lithium hydroxide (Li hydroxide), lithium fluoride (LiF), lithium nitrate (Li nitrate), lithium carbonate (Li carbonate), lithium acetate (Li acetate), lithium iodide (Li iodide), lithium chloride (LiCl), or lithium bis(trifluoromethanesulfonyl)imide (LiTFSI).

[0083] And, depending on the type of the lithium source (210) provided to the base solvent (100), the crystallinity of lithium sulfide (300) described below can be controlled.

[0084] According to one embodiment, the lithium source (210) may be selected from any one of lithium acetate (Li acetate), lithium iodide (Li iodide), lithium chloride (LiCl), or lithium bis(trifluoromethanesulfonyl)imide (LiTFSI). Accordingly, the crystallinity of lithium sulfide (300) described below may be improved.

[0085] In addition, depending on the type of the lithium source (210) provided to the base solvent (100), the shape of the particles of lithium sulfide (300) described below and the average size of the particles can be controlled.

[0086] According to one embodiment, when the lithium source (210) is selected as lithium chloride (LiCl), the shape of the particles of lithium sulfide (300) described below can be controlled to be spherical, and the average size of the particles can be controlled to be 500 nm or more and 1 μm or less.

[0087] And, when the lithium source (210) is selected as lithium acetate, the shape of the particles of lithium sulfide (300) described later can be controlled to be spherical, and the average size of the particles can be controlled to be 50 nm or more and 200 nm or less.

[0088] And, when the lithium source (210) is selected as lithium iodide, the shape of the particles of lithium sulfide (300) described later can be controlled to be in the form of flakes, and the average size of the particles can be controlled to be 500 nm or more and 1 μm or less.

[0089] And, when the lithium source (210) is selected as lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), the particle shape of the lithium sulfide (300) described below can be controlled to be in the form of flakes, and the average size of the particles can be controlled to be 50 nm or more and 200 nm or less.

[0090] Referring to FIG. 1 and FIG. 4, a second base source (240) is prepared by dissolving a sulfur source (230) in the base solvent (100) (S300).

[0091] The sulfur source (230) may be, for example, sodium sulfide (Na2S). Accordingly, the sulfur source (230) can be easily dissolved in the base solvent (100), so that the second base source (240) can be easily manufactured.

[0092] Referring to FIG. 1 and FIG. 5, the first base source (220) and the second base source (240) are mixed and a catalyst (250) is added to produce a lithium sulfide source (260) (S400).

[0093] The catalyst (250) may be, for example, any one of thioacetamide, gallic acid, L-ascorbic acid, thiourea, cyanuric acid, cetyltrimethylammonium bromide (CTAB), or graphite.

[0094] And, depending on the type of the catalyst (250), the crystallinity of lithium sulfide (300) described below can be controlled.

[0095] According to one embodiment, the catalyst (250) may be selected from any one of cyanuric acid, cetyltrimethylammonium bromide (CTAB), or graphite. Accordingly, the crystallinity of lithium sulfide (300), which will be described later, may be improved.

[0096] In addition, the average size of the particles of lithium sulfide (300) described below can be controlled according to the weight ratio of the catalyst (250) to the weight of the lithium source (210) of the first base source (220) in the lithium sulfide source (260).

[0097] According to one embodiment, the catalyst (250) is graphite, and the weight ratio of the catalyst (250) to the weight of the lithium source (210) in the lithium sulfide source (260) can be controlled to be 0.1 wt% or more and 2 wt% or less. Accordingly, the average particle size of the lithium sulfide (300) described below can be reduced.

[0098] In contrast, when the weight ratio of the catalyst (250) to the weight of the lithium source (210) in the lithium sulfide source (260) exceeds 2 wt%, the average size of the particles of the lithium sulfide (300) described below may increase.

[0099] Therefore, according to an embodiment of the present application, the catalyst (250) is graphite, and the weight ratio of the catalyst (250) to the weight of the lithium source (210) in the lithium sulfide source (260) can be controlled to be 0.1 wt% or more and 2 wt% or less. Accordingly, lithium sulfide (300) described below with a reduced average particle size can be provided.

[0100] In addition, the average size and crystallinity of the particles of lithium sulfide (300), which will be described later, can be controlled according to the weight ratio of the catalyst (250) to the weight of the lithium source (210) of the first base source (220) in the lithium sulfide source (260).

[0101] According to one embodiment, the catalyst (250) is cyanuric acid, and the weight ratio of the catalyst (250) to the weight of the lithium source (210) in the lithium sulfide source (260) can be controlled to be greater than 1 wt% and less than 10 wt%. Accordingly, the average particle size of the lithium sulfide (300) described below can be reduced and crystallinity can be improved.

[0102] In contrast, when the weight ratio of the catalyst (250) to the weight of the lithium source (210) in the lithium sulfide source (260) is controlled to 1 wt% or less or 10 wt% or more, the average size of the particles of the lithium sulfide (300) described below may increase and the crystallinity may decrease.

[0103] Therefore, according to an embodiment of the present application, the catalyst (250) is cyanuric acid, and the weight ratio of the catalyst (250) to the weight of the lithium source (210) in the lithium sulfide source (260) can be controlled to be greater than 1 wt% and less than 10 wt%. Accordingly, lithium sulfide (300) described below with a reduced average particle size and improved crystallinity can be provided.

[0104] According to one embodiment, the catalyst (250) is cetyltrimethylammonium bromide (CTAB), and the weight ratio of the catalyst (250) to the weight of the lithium source (210) in the lithium sulfide source (260) can be controlled to be greater than 1 wt% and less than 5 wt%. Accordingly, the average particle size of the lithium sulfide (300) described below can be reduced and crystallinity can be improved.

[0105] In contrast, when the weight ratio of the catalyst (250) to the weight of the lithium source (210) in the lithium sulfide source (260) is controlled to 1 wt% or less or 5 wt% or more, the average size of the particles of the lithium sulfide (300) described below may increase and the crystallinity may decrease.

[0106] Therefore, according to an embodiment of the present application, the catalyst (250) is cetyltrimethylammonium bromide (CTAB), and the weight ratio of the catalyst (250) to the weight of the lithium source (210) in the lithium sulfide source (260) can be controlled to be greater than 1 wt% and less than 5 wt%. Accordingly, lithium sulfide (300) described below with a reduced average particle size and improved crystallinity can be provided.

[0107] Referring to FIG. 1 and FIG. 6, the lithium sulfide source (260) is reacted to produce a preliminary lithium sulfide (310) (S500).

[0108] In the above step S500, the lithium sulfide source (260) may be metathesized. Accordingly, a solution containing the reserve lithium sulfide (310) and a residual salt may be prepared. For example, the residual salt may be sodium chloride (NaCl).

[0109] And, depending on the time for reacting the lithium sulfide source (260), the crystallinity of the lithium sulfide (300) described below can be controlled. For example, the reaction of the lithium sulfide source (260) can be a metathesis reaction.

[0110] According to one embodiment, the time for reacting the lithium sulfide source (260) can be controlled to be more than 1 hour and less than 4 hours. Accordingly, the crystallinity of the lithium sulfide (300) described below can be improved. For example, the lithium sulfide source (260) can be manufactured using ethanol as the organic solvent (110), acetone as the surfactant (120), lithium chloride (LiCl) as the lithium source (220), sodium sulfide (Na2S) as the sulfur source (230), and graphite as the catalyst.

[0111] In contrast, if the time for reacting the lithium sulfide source (260) is controlled to 1 hour or less or 4 hours or more, the crystallinity of the lithium sulfide (300) described below may be reduced.

[0112] Accordingly, according to the embodiment of the present application, the reaction time of the lithium sulfide source (260) can be controlled to be more than 1 hour and less than 4 hours. Accordingly, the lithium sulfide (300) described below with improved crystallinity can be provided. As a result, the manufacturing time of the lithium sulfide (300) described below can be shortened.

[0113] Referring to FIG. 1 and FIG. 7, the lithium sulfide (300) described above is manufactured by washing and drying the preliminary lithium sulfide (310) (S600).

[0114] According to one embodiment, in step S500, the lithium sulfide (300) can be easily manufactured by washing the solution containing the preliminary lithium sulfide (310) and the residual salt produced by metathesis of the lithium sulfide source (260) multiple times with a washing solvent and vacuum drying. For example, the washing solvent can be ethanol and deionized water. For example, the vacuum drying can be performed overnight.

[0115] According to an embodiment of the present application, unlike conventional techniques, an additional heat treatment process and an impurity purification process can be omitted after step S600. Accordingly, the manufacturing process of lithium sulfide (300) is simplified, and since high-temperature heat energy is not required, the manufacturing process cost of lithium sulfide (300) can be reduced. Accordingly, mass production of lithium sulfide (300) can be facilitated.

[0116] In conclusion, the method for producing lithium sulfide (300) according to an embodiment of the present application may include a step of preparing the base solvent (100), a step of dissolving the lithium source (210) in the base solvent (100) to prepare the first base source (220), a step of dissolving the sulfur source (230) in the base solvent (100) to prepare the second base source (240), a step of mixing the first base source (220) and the second base source (240) and adding the catalyst (250) to produce the lithium sulfide source (260), a step of reacting the lithium sulfide source (260) to produce the preliminary lithium sulfide (310), and a step of washing and drying the preliminary lithium sulfide (310) to produce the lithium sulfide (300).

[0117] In the step of preparing the base solvent (100), the base solvent (100) can be prepared by dissolving the surfactant (120) in the organic solvent (110). In addition, due to the organic solvent (110, for example, any one of acetone, tetrahydrofuran, methanol, or ethanol) and the surfactant (120, Triton X-165), the lithium sulfide (300) having improved crystallinity and prevented aggregation of particles can be provided.

[0118] And, in the step of preparing the first base source (220), the lithium sulfide (300) having improved crystallinity and reduced particle size can be provided due to the lithium source (210, any one of lithium acetate, lithium iodide, lithium chloride, or lithium bis(trifluoromethanesulfonyl)imide).

[0119] In addition, in the step of manufacturing the lithium sulfide source (260), the lithium sulfide (300) having improved crystallinity and reduced particle size can be provided due to the catalyst (250, any one of cyanuric acid, cetyltrimethylammonium bromide, or graphite) in the lithium sulfide source (260).

[0120] In addition, unlike conventional technologies, the additional heat treatment process and the impurity purification process after the step of manufacturing the lithium sulfide can be omitted. Accordingly, the manufacturing process of the lithium sulfide (300) is simplified, and since high-temperature heat energy is not required, the manufacturing process cost of the lithium sulfide (300) can be reduced. Accordingly, a method for manufacturing the lithium sulfide (300) that is easy to mass-produce can be provided.

[0121]

[0122] Hereinafter, with reference to FIGS. 8 and 9, a method for manufacturing a solid electrolyte for a secondary battery using lithium sulfide manufactured by the above-described manufacturing method is described.

[0123] FIG. 8 is a drawing for explaining a method for manufacturing a precursor according to an embodiment of the present invention, and FIG. 9 is a drawing for explaining a method for manufacturing a solid electrolyte according to an embodiment of the present invention.

[0124] Lithium sulfide (300) manufactured by the method described above with reference to FIGS. 1 to 7 is provided. The lithium sulfide (300) is of high purity, and the sulfur ion (S) of the lithium sulfide 2- ) forms a face-centered cubic lattice, and lithium ions (Li + ) may have an antifluorite structure provided therein. That is, the lithium sulfide (300) may be an ionic compound belonging to the Fm3m space group with high symmetry of the cubic system.

[0125] Referring to FIG. 8, lithium sulfide (300), germanium sulfide (400), and phosphorus pentasulfide (500) are prepared, and the lithium sulfide (300), the germanium sulfide (400), and the phosphorus pentasulfide (500) are ball milled to manufacture a precursor (600).

[0126] In the step of manufacturing the precursor (600), the lithium sulfide (300), the germanium sulfide (400), and the phosphorus pentasulfide (500) may be mixed in a stoichiometric molar ratio and then ball milled in an inert atmosphere. For example, the molar ratio of the lithium sulfide (300), the germanium sulfide (400), and the phosphorus pentasulfide (500) may be controlled to 5:1:1. For example, the inert atmosphere may be an argon atmosphere. For example, the ball milling time may be controlled to 10 hours.

[0127] Referring to FIG. 9, the precursor (600) is processed into a pellet shape and heat-treated to manufacture a solid electrolyte (700).

[0128] In the step of manufacturing the solid electrolyte (700), the precursor (600) processed into a pellet form may be heat-treated in an inert atmosphere. For example, the inert atmosphere may be an argon atmosphere. For example, the heat treatment may be performed at 550°C for 8 hours. For example, the solid electrolyte (700) may be Li 10 GeP2S 12 Accordingly, when the manufactured solid electrolyte (700) is applied as an electrolyte of a lithium secondary battery, the coulombic efficiency and capacity retention rate of the lithium secondary battery can be improved.

[0129]

[0130] Hereinafter, experimental examples and characteristic evaluation results of lithium sulfide according to embodiments of the present invention are described.

[0131]

[0132] Lithium sulfide according to experimental examples

[0133] A base solvent was prepared by dissolving a surfactant in an organic solvent.

[0134] Then, a lithium source and a sulfur source (Na2S, 0.8 g, 1.03 M) were dissolved in the base solvent, respectively, to prepare a first base source and a second base source.

[0135] Then, after mixing the first base source and the second base source, a catalyst was added to prepare a lithium sulfide source.

[0136] Then, the lithium sulfide source was reacted for 1 to 10 hours to produce preliminary lithium sulfide.

[0137] Then, the above-mentioned preliminary lithium sulfide was washed several times with ethanol and deionized water and vacuum-dried (overnight) to produce lithium sulfide (Li2S).

[0138]

[0139] ClassificationOrganic solventSurfactant (weight ratio to weight of organic solvent)Lithium sourceCatalyst (weight ratio to weight of lithium source)Reaction timeExperimental example 1-1Ethanol(25mL)Acetone(2wt%)LiCl(2.0M, 0.86g)Graphite(1wt%)1hExperimental example 1-2Ethanol(25mL)Acetone(2wt%)LiCl(2.0M, 0.86g)Graphite(1wt%)2hExperimental example 1-3Ethanol(25mL)Acetone(2wt%)LiCl(2.0M, 0.86g)Graphite(1wt%)4hExperimental example 1-4Ethanol(25mL)Acetone(2wt%)LiCl(2.0M, 0.86g)Graphite(1wt%)5hExperimental example 1-5Ethanol(25mL)Acetone(2wt%)LiCl(2.0M, 0.86g)Graphite(1wt%)10hExperimental Example 2-1Ethanol(25mL)Hydrazine hydrate(2wt%)LiCl(2.0M, 0.86g)Graphite(1wt%)2hExperimental Example 2-2Ethanol(25mL)EthyleneGlycol(2wt%)LiCl(2.0M, 0.86g)Graphite(1wt%)2hExperimental Example 2-3Ethanol(25mL)Triton X-165(2wt%)LiCl(2.0M, 0.86g)Graphite(1wt%)2hExperimental Example 3-1Ethanol(25mL)Triton X-165(2wt%)Li bromide(2.0M, 0.86g)Graphite(1wt%)2hExperimental Example 3-2Ethanol(25mL)Triton X-165(2wt%)Li hydroxide(2.0M, 0.86g)Graphite(1wt%)2hExperimental Example 3-3Ethanol(25mL)Triton X-165(2wt%)LiF(2.0M, 0.86g)Graphite(1wt%)2hExperimental Example 3-4Ethanol(25mL)Triton X-165(2wt%)Li nitrate(2.0M, 0.86g)Graphite(1wt%)2hExperimental Example 3-5Ethanol(25mL)Triton X-165(2wt%)Li carbonate(2.0M, 0.86g)Graphite(1wt%)2hExperimental Example 3-6Ethanol(25mL)Triton X-165(2wt%)Li iodide(2.0M, 0.86g)Graphite(1wt%)2hExperimental Example 3-7Ethanol(25mL)Triton X-165(2wt%)LiTFSI(2.0M, 0.86g)Graphite(1wt%)2hExperimental Example 3-8Ethanol(25mL)Triton X-165(2wt%)Li acetate(2.0M, 0.86g)Graphite(1wt%)2hExperimental Example 4-1Acetonitrile(25mL)Triton X-165(2wt%)LiCl(2.0M, 0.86g)Graphite(1wt%)2hExperimental Example 4-2 Butanol (25 mL) Triton X-165 (2 wt%) LiCl (2.0 M, 0.86 g) Graphite (1 wt%) 2 h Experimental example 4-3 Isopropanol (25 mL) Triton X-165 (2 wt%) LiCl (2.0 M, 0.86 g) Graphite (1 wt%) 2 h Experimental example 4-4 NMP (25 mL) Triton X-165 (2 wt%) LiCl (2.0 M, 0.86 g) Graphite (1 wt%) 2 h Experimental example 4-5 Acetone (25 mL) Triton X-165 (2 wt%) LiCl (2.0 M, 0.86 g) Graphite (1 wt%) 2 h Experimental example 4-6 Tetrahydrofuran (25 mL) Triton X-165(2wt%)LiCl(2.0M, 0.86g)Graphite(1wt%)2hExperimental Example 4-7Methanol(25mL)Triton X-165(2wt%)LiCl(2.0M, 0.86g)Graphite(1wt%)2hExperimental Example 4-8Dimethylacetamide(25mL)Triton X-165(2wt%)LiCl(2.0M, 0.86g)Graphite(1wt%)2hExperimental Example 5-1Ethanol(25mL)Triton X-165(2wt%)LiCl(2.0M, 0.86g)Thioacetamide(1wt%)2hExperimental Example 5-2Ethanol(25mL)Triton X-165(2wt%)LiCl(2.0M, 0.86g)Gallic acid(1wt%)2hExperimental example 5-3Ethanol(25mL)Triton X-165(2wt%)LiCl(2.0M, 0.86g)L-Ascorbic acid(1wt%)2hExperimental example 5-4Ethanol(25mL)Triton X-165(2wt%)LiCl(2.0M, 0.86g)Thiourea(1wt%)2hExperimental example 5-5Ethanol(25mL)Triton X-165(2wt%)LiCl(2.0M, 0.86g)CTAB(1wt%)2hExperimental example 5-6Ethanol(25mL)Triton X-165(2wt%)LiCl(2.0M, 0.86g)Cyanuric acid(1wt%)2hExperimental example 6-1 Ethanol (25 mL) Triton X-165 (2 wt%) LiCl (2.0 M, 0.86 g) Graphite (0.1 wt%) 2 h Experimental example 6-2 Ethanol (25 mL) Triton X-165 (2 wt%) LiCl (2.0 M, 0.86 g) Graphite (0.5 wt%) 2 h Experimental example 6-3 Ethanol (25 mL) Triton X-165 (2 wt%) LiCl (2.0 M, 0.86 g) Graphite (2 wt%) 2 h Experimental example 6-4 Ethanol (25 mL) Triton X-165 (2 wt%) LiCl (2.0 M, 0.86 g) Graphite (5 wt%) 2 h Experimental example 6-5 Ethanol (25 mL) Triton X-165(2wt%)LiCl(2.0M, 0.86g)Graphite(10wt%)2hExperimental Example 7-1Ethanol(25mL)Triton X-165(2wt%)LiCl(2.0M, 0.86g)Gallic acid(0.5wt%)2hExperimental Example 7-2Ethanol(25mL)Triton X-165(2wt%)LiCl(2.0M, 0.86g)Gallic acid(2wt%)2hExperimental Example 7-3Ethanol(25mL)Triton X-165(2wt%)LiCl(2.0M, 0.86g)Gallic acid(3wt%)2hExperimental Example 7-4Ethanol(25mL)Triton X-165(2wt%)LiCl(2.0M, 0.86g)Gallic acid(5wt%)2hExperimental Example 7-5Ethanol(25mL)Triton X-165(2wt%)LiCl(2.0M, 0.86g)Gallic acid(10wt%)2hExperimental Example 8-1Ethanol(25mL)Triton X-165(2wt%)LiCl(2.0M, 0.86g)Cyanuric acid(0.1wt%)2hExperimental Example 8-2Ethanol(25mL)Triton X-165(2wt%)LiCl(2.0M, 0.86g)Cyanuric acid(0.5wt%)2hExperimental Example 8-3Ethanol(25mL)Triton X-165(2wt%)LiCl(2.0M, 0.86g)Cyanuric acid(2wt%)2hExperimental Example 8-4 Ethanol (25 mL) Triton X-165 (2 wt%) LiCl (2.0 M, 0.86 g) Cyanuric acid (5 wt%) 2 h Experimental example 8-5 Ethanol (25 mL) Triton X-165 (2 wt%) LiCl (2.0 M, 0.86 g) Cyanuric acid (10 wt%) 2 h Experimental example 9-1 Ethanol (25 mL) Triton X-165 (2 wt%) LiCl (2.0 M, 0.86 g) Thiourea (2 wt%) 2 h Experimental example 9-2 Ethanol (25 mL) Triton X-165 (2 wt%) LiCl (2.0 M, 0.86 g) Thiourea (3 wt%) 2 h Experimental example 9-3 Ethanol (25 mL) Triton X-165(2wt%)LiCl(2.0M, 0.86g)Thiourea(4wt%)2hExperimental Example 9-4Ethanol(25mL)Triton X-165(2wt%)LiCl(2.0M, 0.86g)Thiourea(5wt%)2hExperimental Example 9-5Ethanol(25mL)Triton X-165(2wt%)LiCl(2.0M, 0.86g)Thiourea(10wt%)2hExperimental Example 10-1Ethanol(25mL)Triton X-165(2wt%)LiCl(2.0M, 0.86g)L-Ascorbic acid(0.5wt%)2hExperimental Example 10-2Ethanol(25mL)Triton X-165(2wt%)LiCl(2.0M, 0.86g)L-Ascorbic acid(2wt%)2hExperimental Example 10-3Ethanol(25mL)Triton X-165(2wt%)LiCl(2.0M, 0.86g)L-Ascorbic acid(3wt%)2hExperimental Example 10-4Ethanol(25mL)Triton X-165(2wt%)LiCl(2.0M, 0.86g)L-Ascorbic acid(5wt%)2hExperimental Example 10-5Ethanol(25mL)Triton X-165(2wt%)LiCl(2.0M, 0.86g)L-Ascorbic acid(10wt%)2hExperimental Example 11-1Ethanol(25mL)Triton X-165(2wt%)LiCl(2.0M, 0.86g)CTAB(3wt%)2hExperimental Example 11-2Ethanol (25mL)Triton

[0140]

[0141] FIG. 10 is a graph for comparing the crystallinity of lithium sulfide according to Experimental Examples 1-1 to 1-5 of the present invention, and FIG. 11 is a photograph for comparing the particle sizes of lithium sulfide according to Experimental Examples 1-1 to 1-5 of the present invention.

[0142] Referring to (a) of Fig. 10, the lithium sulfides according to Experimental Examples 1-1 (1h), 1-2 (2h), 1-3 (4h), 1-4 (5h), and 1-5 (10h) were analyzed by XRD, and referring to (b) of Fig. 10, the crystallinity of the lithium sulfides according to Experimental Examples 1-1 (1h), 1-2 (2h), 1-3 (4h), 1-4 (5h), and 1-5 (10h) was graphically represented. Referring to Fig. 11, the lithium sulfides according to Experimental Examples 1-2 (2h), 1-3 (4h), 1-4 (5h), and 1-5 (10h) were photographed by SEM.

[0143] As can be seen from (a) to (b) of Fig. 10, among the lithium sulfides according to Experimental Examples 1-1 to 1-5, it can be seen that the crystallinity of the lithium sulfide according to Experimental Example 1-2 is the best.

[0144] Therefore, in the method for producing lithium sulfide according to the embodiment of the present application, it can be seen that the method of controlling the time for reacting the lithium sulfide source to be more than 1 hour and less than 4 hours is a method for improving the crystallinity of lithium sulfide.

[0145] As can be seen in Fig. 11, the average particle size of the lithium sulfide according to Experimental Example 1-2 is the smallest. In addition, it can be seen that as the reaction time of the lithium sulfide source increases, the average particle size of the lithium sulfide increases.

[0146]

[0147] FIG. 12 is a graph for comparing the crystallinity of lithium sulfide according to Experimental Examples 1-2 and 2-1 to 2-3 of the present invention, and FIG. 13 is a photograph for comparing the particle sizes of lithium sulfide according to Experimental Examples 1-2 and 2-1 to 2-3 of the present invention.

[0148] Referring to (a) of Fig. 12, the lithium sulfides according to Experimental Examples 1-2 (Acetone), 2-1 (Hydrazine hydrate), 2-2 (Ethylene glycol), and 2-3 (Triton X-165) were analyzed by XRD, and referring to (b) of Fig. 12, the crystallinity of the lithium sulfides according to Experimental Examples 1-2 (Acetone), 2-1 (Hydrazine hydrate), 2-2 (Ethylene glycol), and 2-3 (Triton X-165) is shown in a graph. Referring to Fig. 13, the lithium sulfides according to Experimental Examples 1-2 (Acetone), 2-1 (Hydrazine hydrate), 2-2 (Ethylene glycol), and 2-3 (Triton X-165) were photographed by SEM.

[0149] As can be seen in (a) and (b) of Fig. 12, among the lithium sulfides according to Experimental Examples 1-2 and 2-1 to 2-3, it can be seen that the crystallinity of the lithium sulfide according to Experimental Example 2-3 is the best.

[0150] Therefore, in the method for producing lithium sulfide according to the embodiment of the present application, it can be seen that the method of using Triton X-165 as a surfactant is a method for improving the crystallinity of lithium sulfide.

[0151] As can be seen in Fig. 13, the particle size of the lithium sulfide according to Experimental Example 1-2 is 35 nm to 140 nm, the particle size of the lithium sulfide according to Experimental Example 2-1 is 50 nm to 180 nm, the particle size of the lithium sulfide according to Experimental Example 2-3 is 150 nm to 280 nm, and the particle size of the lithium sulfide according to Experimental Example 2-2 is 150 nm to 350 nm.

[0152] And, among the lithium sulfides according to Experimental Examples 1-2 and 2-1 to 2-3, it can be seen that the particle size of the lithium sulfide according to Experimental Example 2-3 is the most uniform and dense. In addition, considering that the crystallinity and yield of the lithium sulfide according to Experimental Example 2-3 are the best, Triton X-165 can be used as a surfactant in the method for producing lithium sulfide according to the embodiment of the present application.

[0153]

[0154] FIG. 14 is a graph for comparing the crystallinity of lithium sulfide according to Experimental Examples 2-3 and 3-1 to 3-8 of the present invention, and FIG. 15 is a photograph for comparing the size and shape of particles of lithium sulfide according to Experimental Examples 2-3 and 3-1 to 3-8 of the present invention.

[0155] Referring to (a) of FIG. 14, lithium sulfides according to Experimental Example 2-3 (LiCl), Experimental Example 3-1 (Li bromide), Experimental Example 3-2 (Li hydroxide), Experimental Example 3-3 (LiF), Experimental Example 3-4 (Li nitrate), Experimental Example 3-5 (Li carbonate), Experimental Example 3-6 (Li iodide), Experimental Example 3-7 (LiTFSI), and Experimental Example 3-8 (Li acetate) were analyzed by XRD, and referring to (b) of FIG. 14, lithium sulfides according to Experimental Example 2-3 (LiCl), Experimental Example 3-1 (Li bromide), Experimental Example 3-2 (Li hydroxide), Experimental Example 3-3 (LiF), Experimental Example 3-4 (Li nitrate), Experimental Example 3-5 (Li carbonate), Experimental Example 3-6 (Li iodide), Experimental Example 3-7 (LiTFSI), and Experimental Example 3-8 (Li acetate) were analyzed by XRD. The crystallinity of lithium is shown graphically. Referring to Fig. 15, the lithium sulfides according to Experimental Example 2-3 (LiCl), Experimental Example 3-1 (Li bromide), Experimental Example 3-2 (Li hydroxide), Experimental Example 3-3 (LiF), Experimental Example 3-4 (Li nitrate), Experimental Example 3-5 (Li carbonate), Experimental Example 3-6 (Li iodide), Experimental Example 3-7 (LiTFSI), and Experimental Example 3-8 (Li acetate) were photographed using SEM.

[0156] As can be seen from (a) and (b) of Fig. 14, among the lithium sulfides according to Experimental Examples 2-3 and 3-1 to 3-8, it can be seen that the lithium sulfide according to Experimental Example 2-3 has the best crystallinity.

[0157] And, it can be seen that the lithium sulfide according to Experimental Examples 2-3, 3-6, 3-7, and 3-8 has relatively superior crystallinity than the lithium sulfide according to Experimental Examples 3-1 to 3-5.

[0158] Therefore, in the method for producing lithium sulfide according to the embodiment of the present application, it can be seen that the method of using any one of lithium acetate (Li acetate), lithium iodide (Li iodide), lithium chloride (LiCl), or lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) as a lithium source is a method for improving the crystallinity of lithium sulfide.

[0159] As can be seen in Fig. 15, the lithium sulfide according to Experimental Example 2-3 has a spherical particle shape and an average particle size of 500 nm to 1 μm, the lithium sulfide according to Experimental Example 3-6 has a flake particle shape and an average particle size of 500 nm to 1 μm, the lithium sulfide according to Experimental Example 3-7 has a flake particle shape and an average particle size of 50 nm to 200 nm, and the lithium sulfide according to Experimental Example 3-8 has a spherical particle shape and an average particle size of 50 nm to 200 nm.

[0160] Therefore, in the method for producing lithium sulfide according to the embodiment of the present application, it can be seen that the shape and average size of the particles of lithium sulfide are controlled depending on the type of lithium source.

[0161]

[0162] FIG. 16 is a graph for comparing the crystallinity of lithium sulfide according to Experimental Examples 2-3 and 4-1 to 4-8 of the present invention, and FIG. 17 is a photograph for comparing the size and shape of particles of lithium sulfide according to Experimental Examples 2-3, 4-1, and 4-5 to 4-8 of the present invention.

[0163] Referring to (a) of Fig. 16, lithium sulfide according to Experimental Example 2-3 (Ethanol), Experimental Example 4-1 (Acetonitrile), Experimental Example 4-2 (Butanol), Experimental Example 4-3 (Isopropanol), Experimental Example 4-4 (NMP), Experimental Example 4-5 (Acetone), Experimental Example 4-6 (Tetrahydrofuran), Experimental Example 4-7 (Methanol), and Experimental Example 4-8 (Dimethylacetamide) was analyzed by XRD, and referring to (b) of Fig. 16, lithium sulfide according to Experimental Example 2-3 (Ethanol), Experimental Example 4-1 (Acetonitrile), Experimental Example 4-2 (Butanol), Experimental Example 4-3 (Isopropanol), Experimental Example 4-4 (NMP), Experimental Example 4-5 (Acetone), Experimental Example 4-6 (Tetrahydrofuran), Experimental Example 4-7 (Methanol), and Experimental Example The crystallinity of the lithium sulfide according to Experimental Example 4-8 (Dimethylacetamide) is shown graphically. Referring to Fig. 17, the lithium sulfide according to Experimental Example 2-3 (Ethanol), Experimental Example 4-1 (Acetonitrile), Experimental Example 4-5 (Acetone), Experimental Example 4-6 (Tetrahydrofuran), Experimental Example 4-7 (Methanol), and Experimental Example 4-8 (Dimethylacetamide) were photographed using SEM.

[0164] As can be seen in (a), (b) and (c) of FIG. 16, among the lithium sulfides according to Experimental Examples 2-3 and Experimental Examples 4-1 to 4-8, it can be seen that the crystallinity of the lithium sulfide according to Experimental Example 2-3 is the best.

[0165] And, it can be seen that the lithium sulfide according to Experimental Examples 2-3, 4-5, 4-6, and 4-7 has excellent crystallinity and suppressed aggregation of lithium sulfide particles compared to the lithium sulfide according to Experimental Examples 4-1 to 4-4.

[0166] Therefore, in the method for producing lithium sulfide according to the embodiment of the present application, it can be seen that the method of using any one of acetone, tetrahydrofuran, methanol, or ethanol as an organic solvent not only improves the crystallinity of lithium sulfide but also suppresses agglomeration of lithium sulfide particles.

[0167]

[0168] Figure 18 is a graph for comparing the crystallinity of lithium sulfide according to Experimental Examples 2-3 and 5-1 to 5-6 of the present invention.

[0169] Referring to (a) of FIG. 18, lithium sulfide according to Experimental Example 2-3 (Graphite), Experimental Example 5-1 (Thioacetamide), Experimental Example 5-2 (Gallic acid), Experimental Example 5-3 (L-Ascorbic acid), Experimental Example 5-4 (CTAB), and Experimental Example 5-5 (Cyanuric acid) was analyzed by XRD, and referring to (b) of FIG. 18, the crystallinity of lithium sulfide according to Experimental Example 2-3 (Graphite), Experimental Example 5-1 (Thioacetamide), Experimental Example 5-2 (Gallic acid), Experimental Example 5-3 (L-Ascorbic acid), Experimental Example 5-4 (CTAB), and Experimental Example 5-5 (Cyanuric acid) was graphically represented.

[0170] As can be seen from (a) and (b) of Fig. 18, among the lithium sulfides according to Experimental Examples 2-3 and 5-1 to 5-5, it can be seen that the crystallinity of the lithium sulfide according to Experimental Example 2-3 is the best.

[0171]

[0172] Figure 19 is a photograph for comparing the particle sizes of lithium sulfide according to Experimental Examples 2-3 and 6-1 to 6-5 of the present invention.

[0173] Referring to Fig. 19, lithium sulfide according to Experimental Example 2-3 (1 wt%), Experimental Example 6-1 (0.1 wt%), Experimental Example 6-2 (0.5 wt%), Experimental Example 6-3 (2 wt%), Experimental Example 6-4 (5 wt%), and Experimental Example 6-5 (10 wt%) was photographed using SEM.

[0174] As can be seen in Fig. 19, it can be seen that the particle size of the lithium sulfide decreases in the order of the lithium sulfide according to Experimental Examples 6-1, 6-2, 2-3, and 6-3. In addition, it can be seen that the particle size of the lithium sulfide increases in the order of the lithium sulfide according to Experimental Examples 6-4 and 6-5, starting from the lithium sulfide according to Experimental Examples 6-3. That is, it can be seen that the particle size of the lithium sulfide decreases as the weight ratio of the catalyst increases from 0.1 wt% to 0.2 wt%, but the particle size of the lithium sulfide increases when the weight ratio of the catalyst exceeds 2 wt%.

[0175] Therefore, in the method for producing lithium sulfide according to an embodiment of the present application, it can be seen that the method of controlling the weight ratio of the catalyst (graphite) to the weight of the lithium source (LiCl) in the lithium sulfide source to 0.1 wt% or more and 2 wt% or less is a method for reducing the particle size of lithium sulfide.

[0176]

[0177] FIG. 20 is a photograph for comparing the particle sizes of lithium sulfide according to Experimental Example 5-2 and Experimental Examples 7-1 to 7-5 according to the present invention.

[0178] Referring to Fig. 20, lithium sulfide according to Experimental Example 5-2 (1 wt%), Experimental Example 7-1 (0.5 wt%), Experimental Example 7-2 (2 wt%), Experimental Example 7-2 (3 wt%), Experimental Example 7-4 (5 wt%), and Experimental Example 7-5 (10 wt%) was photographed using SEM.

[0179] As can be seen in Fig. 20, it can be seen that the particle size of the lithium sulfide decreases in the order of the lithium sulfide according to Experimental Examples 7-1, 5-2, 7-2, and 7-3. In addition, it can be seen that the particle size of the lithium sulfide increases in the order of the lithium sulfide according to Experimental Examples 7-4 and 7-5, starting from the lithium sulfide according to Experimental Example 7-3. Specifically, it can be seen that the average particle size of the lithium sulfide according to Experimental Example 7-3 is the smallest at 500 nm.

[0180] Therefore, in the method for producing lithium sulfide according to an embodiment of the present application, it can be seen that the method of controlling the weight ratio of the catalyst (Gallic acid) to the weight of the lithium source (LiCl) in the lithium sulfide source to be more than 2 wt% and less than 5 wt% is a method for reducing the size of the lithium sulfide particles.

[0181]

[0182] FIG. 21 is a graph for comparing the crystallinity of lithium sulfide according to Experimental Examples 5-6, 8-2, 8-3, and 8-5 of the present invention, and FIG. 22 is a photograph for comparing the size and shape of particles of lithium sulfide according to Experimental Examples 5-6, and 8-1 to 8-5 of the present invention.

[0183] Referring to (a) of Fig. 21, lithium sulfide according to Experimental Examples 5-6 (1 wt%), 8-2 (0.5 wt%), 8-3 (2 wt%), and 8-5 (10 wt%) was analyzed by XRD, and referring to (b) of Fig. 21, the crystallinity of lithium sulfide according to Experimental Examples 5-6 (1 wt%), 8-2 (0.5 wt%), 8-3 (2 wt%), and 8-5 (10 wt%) was graphically represented. Referring to Fig. 22, the lithium sulfide according to Experimental Example 5-6 (1 wt%), Experimental Example 8-1 (0.1 wt%), Experimental Example 8-2 (0.5 wt%), Experimental Example 8-3 (2 wt%), Experimental Example 8-4 (5 wt%), and Experimental Example 8-5 (10 wt%) was photographed using SEM.

[0184] Referring to (a) and (b) of FIG. 21, among the lithium sulfides according to Experimental Examples 5-6, 8-2, 8-3, and 8-5, it can be seen that the crystallinity of the lithium sulfide according to Experimental Example 8-3 is remarkably excellent.

[0185] Therefore, in the method for producing lithium sulfide according to an embodiment of the present application, it can be seen that the method of controlling the weight ratio of the catalyst (Cyanuric acid) to the weight of the lithium source (LiCl) in the lithium sulfide source to be more than 1 wt% and less than 10 wt% is a method for improving the crystallinity of lithium sulfide.

[0186] As can be seen in Fig. 22, the proportion of the lithium sulfide particles having a nano rod shape increases in the order of lithium sulfide according to Experimental Example 8-1, Experimental Example 8-2, Experimental Example 5-6, Experimental Example 8-3, Experimental Example 8-4, and Experimental Example 8-5.

[0187]

[0188] Figure 23 is a photograph for comparing the particle sizes of lithium sulfide according to Experimental Example 5-4 and Experimental Examples 9-1 to 9-5 of the present invention.

[0189] Referring to Fig. 23, lithium sulfide according to Experimental Example 5-1 (1 wt%), Experimental Example 9-1 (2 wt%), Experimental Example 9-2 (3 wt%), Experimental Example 9-3 (4 wt%), Experimental Example 9-4 (5 wt%), and Experimental Example 9-5 (10 wt%) was photographed using SEM.

[0190] As can be seen in Fig. 23, among the lithium sulfides according to Experimental Example 5-1 and Experimental Examples 9-1 to 9-5, the lithium sulfide according to Experimental Example 9-2 has the smallest average particle size and has a spherical shape.

[0191] Therefore, in the method for producing lithium sulfide according to an embodiment of the present application, it can be seen that the method of controlling the weight ratio of the catalyst (Thiourea) to the weight of the lithium source (LiCl) in the lithium sulfide source to be more than 2 wt% and less than 4 wt% is a method for reducing the particle size of lithium sulfide.

[0192]

[0193] Figure 24 is a photograph for comparing the particle sizes of lithium sulfide according to Experimental Example 5-3 and Experimental Examples 10-1 to 10-5 of the present invention.

[0194] Referring to Fig. 24, lithium sulfide according to Experimental Example 5-3 (1 wt%), Experimental Example 10-1 (0.5 wt%), Experimental Example 10-2 (2 wt%), Experimental Example 10-3 (3 wt%), Experimental Example 10-4 (5 wt%), and Experimental Example 10-5 (10 wt%) was photographed using SEM.

[0195] As can be seen in Fig. 24, it can be seen that the particle size of the lithium sulfide decreases in the order of the lithium sulfide according to Experimental Examples 10-1, 5-3, 10-2, and 10-3. In addition, it can be seen that the particle size of the lithium sulfide increases in the order of the lithium sulfide according to Experimental Examples 10-4 and 10-5, starting from the lithium sulfide according to Experimental Example 10-3. That is, it can be seen that the particle size of the lithium sulfide decreases as the weight ratio of the catalyst increases from 0.5 wt% to 3 wt%, but the particle size of the lithium sulfide increases when the weight ratio of the catalyst exceeds 3 wt%.

[0196] Therefore, in the method for producing lithium sulfide according to an embodiment of the present application, it can be seen that the method of controlling the weight ratio of the catalyst (L-Ascorbic acid) to the weight of the lithium source (LiCl) in the lithium sulfide source to 0.5 wt% or more and 3 wt% or less is a method for reducing the particle size of lithium sulfide.

[0197]

[0198] Figure 25 is a graph for comparing the particle sizes of lithium sulfide according to Experimental Example 5-5 and Experimental Examples 11-1 to 11-3 of the present invention.

[0199] Referring to Fig. 25, lithium sulfide according to Experimental Example 5-5 (1 wt%), Experimental Example 11-1 (3 wt%), Experimental Example 11-2 (5 wt%), and Experimental Example 11-3 (10 wt%) was photographed using SEM.

[0200] As can be seen in Fig. 25, among the lithium sulfides according to Experimental Examples 5-5, 11-1, 11-2, and 11-3, the lithium sulfide according to Experimental Example 11-1 has the smallest average particle size and the best crystallinity.

[0201] Therefore, in the method for producing lithium sulfide according to an embodiment of the present application, it can be seen that the method of controlling the weight ratio of the catalyst (CTAB) to the weight of the lithium source (LiCl) in the lithium sulfide source to be more than 1 wt% and less than 5 wt% is a method for reducing the size of lithium sulfide particles and improving crystallinity.

[0202]

[0203] Figure 26 is a graph for comparing the purity and yield of lithium sulfide according to Experimental Examples 2-3 and 5-1 to 5-6 of the present invention.

[0204] Referring to Fig. 26, the purity and yield of lithium sulfide according to Experimental Example 2-3 (Graphite), Experimental Example 5-1 (Thioacetamide), Experimental Example 5-2 (Gallic acid), Experimental Example 5-3 (L-Ascorbic acid), Experimental Example 5-4 (CTAB), and Experimental Example 5-5 (Cyanuric acid) are shown graphically.

[0205] As can be seen in Fig. 26, the purity of the lithium sulfide according to Experimental Examples 2-3, 5-1 to 5-5 is substantially close to 100%.

[0206] And, among the lithium sulfides according to Experimental Examples 2-3, 5-1 to 5-5, it can be seen that the yield of the lithium sulfide according to Experimental Example 5-5 is the best, and the yield of the lithium sulfide according to Experimental Example 5-5 is the best, followed by the yield of the lithium sulfide according to Experimental Example 5-4 and Experimental Example 2-3.

[0207] In addition, considering the crystallinity of the lithium sulfide according to Experimental Examples 2-3, 5-1 to 5-5 described in FIG. 18, it can be seen that in the method for producing lithium sulfide according to the embodiment of the present application, the method of using any one of graphite, cyanuric acid, or cetyltrimethylammonium bromide (CTAB) as a catalyst is a method for improving the crystallinity, purity, and yield of lithium sulfide.

[0208]

[0209] Solid electrolyte according to Experimental Example 1

[0210] Lithium sulfide (Li2S), germanium sulfide (GeS2), and phosphorus pentasulfide (P2S5) were provided in a glove box and mixed so that the molar ratio of lithium sulfide (Li2S), germanium sulfide (GeS2), and phosphorus pentasulfide (P2S5) according to Experimental Example 6-3 was 5:1:1, and then ball milled (10 hours) in an argon (Ar) atmosphere to manufacture a precursor.

[0211] Then, after processing the precursor into a pellet form, it was heat-treated at 550°C for 8 hours in an argon (Ar) atmosphere and naturally cooled to room temperature to manufacture a solid electrolyte.

[0212]

[0213] Full cell (LFP) according to Experimental Example 1

[0214] A lithium iron phosphate (LFP)-based anode was prepared as the positive electrode, a graphite-based anode was prepared as the negative electrode, a solid electrolyte according to Experimental Example 1 was prepared as the electrolyte, and a porous polypropylene separator was prepared as the separator.

[0215] Then, a full cell was assembled in the order of the positive electrode, the separator, the solid electrolyte, and the negative electrode.

[0216]

[0217] Full cell (NMC622) according to Experimental Example 2

[0218] NMC622 (LiNi) as the anode 0.6 Mn 0.2 Co 0.2 A full cell according to Experimental Example 2 was manufactured in the same manner as the full cell according to Experimental Example 1, except that an O2-based anode was prepared.

[0219]

[0220] Full cell (NCA) according to Experimental Example 3

[0221] A full cell according to Experimental Example 3 was manufactured in the same manner as the full cell according to Experimental Example 1, except that an NCA (LiNiCoAlO2)-based positive electrode was prepared as the positive electrode.

[0222]

[0223] Full cell (LCO) according to Experimental Example 4

[0224] A full cell according to Experimental Example 4 was manufactured in the same manner as the full cell according to Experimental Example 1, except that an NCO(LiCoO2)-based positive electrode was prepared as the positive electrode.

[0225]

[0226] Figure 27 is a graph showing the performance of a full cell according to Experimental Examples 1 to 4 of the present invention.

[0227] Referring to (a) of Fig. 27, the capacity of the full cell (LFP) according to Experimental Example 1 was measured while performing 175 charge / discharge cycles, and the Coulombic efficiency was calculated and presented together in a graph. Referring to (b) of Fig. 27, the capacity of the full cell (NMC622) according to Experimental Example 2 was measured while performing 300 charge / discharge cycles, and the Coulombic efficiency was calculated and presented together in a graph. Referring to (c) of Fig. 27, the capacity of the full cell (NCA) according to Experimental Example 3 was measured while performing 350 charge / discharge cycles, and the Coulombic efficiency was calculated and presented together in a graph. Referring to (d) of Fig. 27, the capacity of the full cell (LCO) according to Experimental Example 4 was measured while performing 250 charge / discharge cycles, and the Coulombic efficiency was calculated and presented together in a graph.

[0228] As can be seen from (a) to (d) of FIG. 27, the initial capacity of the full cell according to Experimental Example 1 is approximately 150 mAh / g, which is stably maintained even after 175 charge / discharge cycles. In addition, the coulombic efficiency of the full cell according to Experimental Example 1 is substantially close to 100%. Therefore, it can be seen that the capacity retention rate and coulombic efficiency for the charge / discharge cycles of the full cell according to Experimental Example 1 are excellent.

[0229] And, it can be seen that the initial capacity of the full cell according to Experimental Example 2 is about 150 mAh / g, but the capacity decreases to about 75 mAh / g after 300 charge / discharge cycles. And, it can be seen that the coulombic efficiency of the full cell according to Experimental Example 2 is practically close to 100%. Therefore, it can be seen that the capacity retention rate of the full cell according to Experimental Example 2 with respect to the charge / discharge cycle is significantly lower than that of the full cell according to Experimental Example 1.

[0230] And, it can be seen that the initial capacity of the full cell according to Experimental Example 3 is about 200 mAh / g, but the capacity decreases to about 175 mAh / g after 350 charge / discharge cycles. And, it can be seen that the coulombic efficiency of the full cell according to Experimental Example 3 is practically close to 100%. Therefore, it can be seen that the capacity retention rate of the full cell according to Experimental Example 3 with respect to the charge / discharge cycle is lower than that of the full cell according to Experimental Example 1.

[0231] And, it can be seen that the initial capacity of the full cell according to Experimental Example 4 is about 160 mAh / g, but the capacity decreases to about 110 mAh / g after 250 charge / discharge cycles. And, it can be seen that the coulombic efficiency of the full cell according to Experimental Example 3 is practically close to 100%. Therefore, it can be seen that the capacity retention rate of the full cell according to Experimental Example 4 with respect to the charge / discharge cycle is lower than that of the full cell according to Experimental Example 1.

[0232]

[0233] While the present invention has been described in detail using preferred embodiments, the scope of the present invention is not limited to the specific embodiments described above, and should be interpreted in accordance with the appended claims. Furthermore, those skilled in the art will appreciate that numerous modifications and variations are possible without departing from the scope of the present invention.

[0234] Lithium sulfide according to an embodiment of the present invention can be applied to a solid electrolyte of a lithium secondary battery and used in various devices such as electric vehicles, electric airplanes, mobile devices, humanoid robots, and ESS.

Claims

1. Step of preparing base solvent; A step of preparing a first base source by dissolving a lithium source in the above base solvent; A step of preparing a second base source by dissolving a sulfur source in the above base solvent; A step of preparing a lithium sulfide source by mixing the first base source and the second base source and adding a catalyst; A step of producing preliminary lithium sulfide by reacting the above lithium sulfide source; and A method for producing lithium sulfide, comprising the step of washing and drying the above-mentioned preliminary lithium sulfide to produce lithium sulfide.

2. In paragraph 1, A method for producing lithium sulfide, comprising dissolving a surfactant in an organic solvent, wherein the base solvent is prepared.

3. In paragraph 2, A method for producing lithium sulfide, wherein the organic solvent comprises any one of acetonitrile, butanol, isopropanol, N-methyl-2-pyrrolidone (NMP), acetone, tetrahydrofuran, methanol, dimethylacetamide, or ethanol.

4. In paragraph 2, A method for producing lithium sulfide, wherein the surfactant comprises any one of hydrazine hydrate, acetone, ethylene glycol, or Triton X-165.

5. In paragraph 1, A method for producing lithium sulfide, wherein the lithium source comprises any one of lithium bromide (Li bromide), lithium hydroxide (Li hydroxide), lithium fluoride (LiF), lithium nitrate (Li nitrate), lithium carbonate (Li carbonate), lithium acetate (Li acetate), lithium iodide (Li iodide), lithium chloride (LiCl), or lithium bis(trifluoromethanesulfonyl)imide (LiTFSI).

6. In paragraph 1, A method for producing lithium sulfide, wherein the sulfur source is sodium sulfide (Na2S).

7. In paragraph 1, A method for producing lithium sulfide, wherein the catalyst comprises any one of thioacetamide, gallic acid, L-ascorbic acid, thiourea, cyanuric acid, cetyltrimethylammonium bromide (CTAB), or graphite.

8. A method for producing lithium sulfide by reacting a first base source in which a lithium source is dissolved in a base solvent, a second base source in which a sulfur source is dissolved in the base solvent, and a lithium sulfide source including a catalyst, The above base solvent comprises a surfactant dissolved in an organic solvent, A method for producing lithium sulfide, comprising controlling the crystallinity of the lithium sulfide according to the time for reacting the lithium sulfide source.

9. In paragraph 8, A method for producing lithium sulfide, wherein the time for reacting the lithium sulfide source is controlled to be more than 1 hour and less than 4 hours, thereby improving the crystallinity of the lithium sulfide.

10. In paragraph 9, The above surfactant contains acetone, The organic solvent includes ethanol, The above lithium source includes lithium chloride (LiCl), The above sulfur source includes sodium sulfide (Na2S), A method for producing lithium sulfide comprising the above catalyst and graphite.

11. A method for producing lithium sulfide by reacting a first base source in which a lithium source is dissolved in a base solvent, a second base source in which a sulfur source is dissolved in the base solvent, and a lithium sulfide source including a catalyst, The above base solvent comprises a surfactant dissolved in an organic solvent, A method for producing lithium sulfide, wherein the surfactant, the organic solvent, the lithium source, and the catalyst are selected as materials that improve the crystallinity of the lithium sulfide.

12. In paragraph 11, The above surfactant comprises Triton X-165, The organic solvent comprises any one of acetone, tetrahydrofuran, methanol, or ethanol, The lithium source comprises any one of lithium acetate (Li acetate), lithium iodide (Li iodide), lithium chloride (LiCl), or lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), A method for producing lithium sulfide, wherein the catalyst comprises any one of cyanuric acid, cetyltrimethylammonium bromide (CTAB), or graphite.

13. In paragraph 12, A method for producing lithium sulfide, comprising controlling the shape of particles of the lithium sulfide according to the type of the lithium source.

14. In paragraph 13, The lithium source is selected from the lithium bis(trifluoromethanesulfonyl)imide or the lithium iodide, and the shape of the lithium sulfide particles is controlled to be in the form of flakes. A method for producing lithium sulfide, wherein the lithium source is selected as lithium acetate or lithium chloride, and the shape of the lithium sulfide particles is controlled to be spherical.

15. In paragraph 12, A method for producing lithium sulfide, wherein the catalyst is graphite, and the weight ratio of the graphite to the weight of the lithium source in the lithium sulfide source is controlled to be 0.1 wt% or more and 2 wt% or less.

16. In paragraph 12, A method for producing lithium sulfide, wherein the catalyst is cyanuric acid, and the weight ratio of the cyanuric acid to the weight of the lithium source in the lithium sulfide source is controlled to be greater than 1 wt% and less than 10 wt%.

17. In paragraph 12, A method for producing lithium sulfide, wherein the catalyst is cetyltrimethylammonium bromide, and the weight ratio of cetyltrimethylammonium bromide to the weight of the lithium source in the lithium sulfide source is controlled to be greater than 1 wt% and less than 5 wt%.

18. A step of preparing lithium sulfide, germanium sulfide, and phosphorus pentasulfide manufactured by the method for manufacturing lithium sulfide according to any one of paragraphs 1, 8, or 11; A step of manufacturing a precursor by ball milling the lithium sulfide, the germanium sulfide, and the phosphorus pentasulfide; A method for manufacturing a solid electrolyte for a secondary battery, comprising the step of processing the precursor into a pellet form and heat-treating it to manufacture a solid electrolyte.

19. In paragraph 18, In the step of manufacturing the precursor, the lithium sulfide, the germanium sulfide, and the phosphorus pentasulfide are ball milled in an argon atmosphere for 10 hours, A method for manufacturing a solid electrolyte for a secondary battery, comprising, in the step of manufacturing the solid electrolyte, heat-treating the precursor processed into a pellet form at 550°C for 8 hours in an argon atmosphere.

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