Sulfide, lithium sulfide-carbon composite, production method thereof

Abstract

To provide a production method for high-capacity lithium sulfide with few impurities by a simple method.SOLUTION: The production method for a sulfide with reduced oxygen atom-containing component in a crude sulfide product includes a step of mixing the crude sulfide product with sulfur, and a step of heating the sulfide after the mixing step in a hydrogen gas stream.SELECTED DRAWING: None

Description

[Technical field]

[0001] The present invention relates to a sulfide, a lithium sulfide-carbon composite, and methods for producing the same. [Background technology]

[0002] Lithium-ion secondary batteries (LIBs) are high-energy density batteries that have become indispensable in daily life as a power source for mobile devices. Large batteries for automobiles and stationary use are also being produced, and the demand is expanding year by year. However, rare metals such as nickel and cobalt are used as the positive electrode material for LIBs, and due to the uneven regional production, the price is rising and there is uncertainty about the supply, so there has been a demand for the development of batteries using raw materials that are abundant resources.

[0003] Sulfur (1673mAh / g), which is abundant and has a large capacity as a positive electrode material, is being considered as a positive electrode material for LIBs, and batteries with high energy density have been prototyped. However, when using sulfur as a battery, a lithium metal anode is used as the counter electrode, or a lithium metal anode is used and lithium is reduced and inserted into the sulfur positive electrode in advance (S+2Li + +2e=Li 2 However, batteries using metallic lithium are avoided due to serious accidents in the past, and this is an obstacle to the practical use of batteries using sulfur-based positive electrode materials.

[0004] On the other hand, lithium sulfide, which is produced by the reduction and lithium insertion of sulfur, has several promising features, such as a high capacity of 1166 mAh / g, the fact that it is a sulfur-based positive electrode material that contains lithium, allowing the use of conventional graphite anodes, and the fact that it does not undergo large volume expansion during charging and discharging like sulfur.

[0005] In addition, sulfide-based all-solid-state batteries, which have higher energy density, faster charge / discharge rate, and power density than conventional LIBs, are being developed for electric vehicles. 3 P.S. 4 , Li 6 P.S.5 Cl, Li 10 GeP 2 S 12 Lithium sulfide is one of the main raw materials for lithium ion batteries (such as lithium ion batteries), and lithium sulfide itself is being considered as a high-capacity positive electrode for all-solid-state batteries. For these reasons, lithium sulfide is becoming an extremely important substance in LIBs, and there is a demand for a more mass-producible and economical manufacturing method, as well as lithium sulfide materials that exhibit higher performance as electrodes.

[0006] There are many problems to be solved in producing lithium sulfide and applying it to electrodes or electrolyte materials. To obtain pure lithium sulfide, a method is known in which sulfur is reduced by metallic lithium in a solution of biphenyl dissolved in 1,2-dimethoxyethane (DME) (Li + Biphenyl = Li + -Biphenyl - , S+2Li + -Biphenyl - =Li 2 In addition to this method, there is a method of reacting lithium hydroxide with hydrogen sulfide (Patent Document 1, Idemitsu, 2LiOH+H 2 S=Li 2 S+2H 2 O), and large-scale facilities are required to produce and safely handle the lethal gas. A simpler, more productive and economical production method is desired. A method for producing lithium sulfide by reducing lithium sulfate with carbon black has also been proposed (see, for example, Patent Documents 2 and 3). Methods that do not use organic solvents are advantageous in terms of cost, but when carbon black or the like is used as a reducing agent, it is difficult to complete the reaction itself, and a long grinding and mixing step tends to be required.

[0007] It has also been reported that carbon and lithium sulfate are mixed at high temperatures and sintered, the carbon reduces the lithium sulfate to produce lithium sulfide, and the lithium sulfide-carbon composite functions as a positive electrode material (Patent Document 4, Non-Patent Documents 1 and 2). However, this method produces a mixture of lithium sulfide and carbon, and the capacity is only 850 mAh / gLi, which is lower than the theoretical capacity of 1166 mAh / g. 2 S(gLi 2 (S is the capacity per weight of lithium sulfide contained in the composite). In addition, even if oxygen-containing components such as lithium oxide are mixed into the lithium sulfide, this leads to a decrease in capacity. It has been reported that lithium sulfide and a carbon material are filled in a conductive container, and a direct current pulse current is passed under pressure to cause a heating reaction, resulting in a composite in which lithium sulfide and a carbon material are bonded together, which functions as the positive electrode of an all-solid-state battery (Patent Document 5). However, the charge / discharge capacity of the all-solid-state secondary battery produced using this method is only about 1 / 6 of the theoretical capacity. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] WO2016098351A1 Brochure [Patent Document 2] Patent No. 5770675 [Patent Document 3] Patent No. 6097125 [Patent Document 4] Patent No. 6059449 [Patent Document 5] Patent No. 5419020 [Non-patent literature]

[0009] [Non-Patent Document 1] Z. Li et al., Nanoscale, 7, 14385-14392 (2015). [Non-Patent Document 2] T. Seita et al., ACS Energy Lett., 5, 1-7 (2020). Summary of the Invention [Problem to be solved by the invention]

[0010] An object of the present invention is to provide a method for producing lithium sulfide having a small amount of impurities and a high capacity by a simple method. [Means for solving the problem]

[0011] Under such circumstances, the present inventors have conducted intensive research and found that the amount of oxygen atom-containing components contained in the sulfide crude product can be significantly reduced by grinding and mixing the sulfide crude product with sulfur and heating the sulfide that has been subjected to the grinding and mixing process in a hydrogen stream. The present inventors have further conducted extensive research and found that the sulfide obtained by calcining sulfate and saccharides as a carbon source in a non-oxidizing atmosphere in order to obtain a sulfide can be significantly prevented from being contaminated with carbon. The present invention is based on these new findings. The present inventors have further conducted extensive trial and error to reduce various impurities and improve the capacity as a battery material, and as a result have completed the present invention. Therefore, in a typical embodiment, the present invention provides the following: Item 1. A process for mixing a crude sulfide product with sulfur; A method for producing a sulfide having a reduced content of oxygen atom-containing components contained in a crude sulfide product, comprising the step of heating the sulfide that has been subjected to the mixing step in a hydrogen gas flow.

[0012] Item 2. The sulfide is lithium sulfide, and the oxygen atom-containing component is lithium oxide. The method according to item 1.

[0013] Item 3. The method according to Item 2, wherein the lithium sulfide is obtained by a step of firing lithium sulfate and a carbon source in a non-oxidizing atmosphere.

[0014] Item 4. The method according to Item 3, wherein the carbon source is a sugar.

[0015] Item 5. The method according to any one of items 1 to 4, wherein the heating step is carried out at 250 to 700 ° C.

[0016] Item 6. The method according to any one of Items 3 to 5, wherein the firing step is carried out at 700 to 1000° C.

[0017] Item 7. A method according to any one of claims 2 to 6, Lithium sulfide having a half-width (2θ) of a diffraction peak at a diffraction angle (2θ) of 27±0.5° as measured by X-ray diffraction using CuKα radiation of 0.15° or more and 0.50° or less.

[0018] Item 8. A step of mixing a sulfide, an organic substance, and sulfur; The mixture obtained in the mixing step is heated in a hydrogen stream. A method for producing a sulfide-carbon composite.

[0019] Item 9. The sulfide is lithium sulfide. Item 9. The method according to item 8 for producing a lithium sulfide-carbon composite.

[0020] Item 10. The organic substance is a nitrogen-containing organic substance. Item 10. The method according to item 9 for producing a lithium sulfide-carbon composite.

[0021] Item 11. The method according to any one of items 1 to 10 for producing a positive electrode material for a lithium ion battery. Effect of the Invention

[0022] According to the present invention, it is possible to provide a method for producing lithium sulfide having a small amount of impurities and a high capacity by a simple method. [Brief description of the drawings]

[0023] [Figure 1] XRD of the product obtained by removing lithium oxide from the process of mixing the crude lithium sulfide product with sulfur and heating in a hydrogen stream in Example 1 [Diagram 2] In Comparative Example 1, a crude lithium sulfide product was prepared in the same manner as in Example 1 (the step of obtaining a crude lithium sulfide product from lithium sulfate and sucrose), but the step of mixing the crude lithium sulfide product with sulfur and heating in a hydrogen gas stream according to the present invention was not performed. XRD of the resultant product [Diagram 3] Example 4: SEM observation of the product after mixing lithium sulfide and sulfur as reagents and heating in a hydrogen stream [Figure 4] SEM observation results of lithium sulfide in Comparative Example 4 [Diagram 5] The initial charge / discharge curves for evaluating the charge / discharge characteristics using the lithium sulfide of Example 4 and Comparative Example 4 as the positive electrode active material. [Figure 6] In Comparative Example 5, XRD of the crude lithium sulfide product obtained by calcining lithium sulfate monohydrate and sucrose [Figure 7] In Example 5, the lithium sulfide crude product obtained by calcining lithium sulfate monohydrate and sucrose was mixed with sulfur and acrylonitrile, and heated in a hydrogen stream to remove the lithium carbonate. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0024] Sulfide and its manufacturing method In one embodiment, the present invention provides a process for producing a process for producing a sulfide raw product, the process comprising the steps of: The present invention provides a method for producing a sulfide having a reduced content of oxygen atom-containing components in a crude sulfide product, the method comprising the step of heating the sulfide that has been subjected to the mixing step in a hydrogen gas flow.

[0025] In this embodiment, first, a step of mixing the crude sulfide product with sulfur is carried out.

[0026] Examples of sulfides that can be used as raw materials in the present invention include metal sulfides such as lithium sulfide, tin sulfide, titanium sulfide, sodium sulfide, potassium sulfide, vanadium sulfide, and iron sulfide, and lithium sulfide is preferred. As the lithium sulfide, lithium sulfide (Li2 S), lithium polysulfide (Li 2 S 8 , Li 2 S 6 , Li 2 S 4 , Li 2 S 3 , Li 2 S 2 Lithium sulfide (Li 2 These sulfides may be used alone or in combination of two or more.

[0027] In addition, metal sulfides include M x S y (wherein M represents at least one selected from the group consisting of Li, Fe, V, Mo, W, Cu, Co, Ti, Ni, and combinations thereof, and preferably contains at least Li; x and y satisfy 1≦x≦3 and 0.5≦y≦4; when M contains multiple types of metals, x and y satisfy 1≦x≦3 and 0.5≦y≦4 for each metal.)

[0028] The method for preparing the crude product of the sulfide is not particularly limited, and the product can be produced according to a method known per se. When the sulfide is lithium sulfide, the crude product of lithium sulfide can be preferably obtained by a method of calcining lithium sulfate and a carbon source in a non-oxidizing atmosphere, as described below.

[0029] In the present embodiment, examples of the oxygen atom-containing component contained in the crude sulfide product include lithium oxide, lithium carbonate, lithium hydroxide, sodium oxide, sodium carbonate, and sodium hydroxide.

[0030] The blending ratio of the crude sulfide product and sulfur is not limited, but for example, the amount of sulfur can be 0.05 to 1.50 parts by mass, preferably 0.10 to 1.20 parts by mass, per 1 part by mass of the crude sulfide product.

[0031] The method of mixing the crude product of sulfide with sulfur is not particularly limited, and examples thereof include pulverization mixing, solution mixing, and the like. In the present invention, pulverization mixing is preferred from the viewpoint of utilizing the mecha-chemical effect of applying mechanical stress such as pulverization to a substance and causing a change in physicochemical properties due to a change in crystal structure. Examples of the pulverization mixing process include a method using a ball mill, a vibration mill, a turbo mill, mechanofusion, a disk mill, and the like, and a ball mill is preferred from the viewpoint of causing mechanochemical reactions. The ball milling method may be either a wet ball milling method or a dry ball milling method.

[0032] In the ball mill method, the rotation speed, rotation time, and the size and number of balls are not particularly limited.

[0033] The rotation speed may be, for example, 100 rpm to 2000 rpm. The rotation speed may be 200 rpm or more, 300 rpm or more, or 500 rpm or more, and may be 2000 rpm or less, 1500 rpm or less, 1000 rpm or less, or 500 rpm or less.

[0034] The rotation time may be 5 minutes to 10 hours. The rotation time may be 5 minutes or more, 10 minutes or more, 15 minutes or more, 30 minutes or more, or 1 hour or more, and may be 20 hours or less, 10 hours or less, 5 hours or less, 2 hours or less, 1 hour or less, or 30 minutes or less.

[0035] The size of the balls that can be used is, for example, 0.1 mm to 50 mm in diameter. The size of the balls may be 0.1 mm or more, 1 mm or more, or 5 mm or more, and may be 50 mm or less, 25 mm or less, or 10 mm or less.

[0036] Balls of different sizes may be used in combination, for example balls with diameters of 10 mm, 5 mm, and 1 mm may be used in combination.

[0037] The grinding and mixing process may be a simple method using a mortar and pestle. The grinding and mixing process may be performed without temperature control or with temperature control such as heating and cooling. The grinding and mixing process finely grinds the crude sulfide product, the oxygen atom-containing component, and sulfur, and the reaction between sulfur and the oxygen atom-containing component proceeds in a solid phase, which makes it easier for the reaction to proceed in the subsequent heating process.

[0038] In the sulfide production method of the present invention, the sulfide that has been subjected to the mixing step is then heated in a hydrogen gas flow. The heating temperature and heating time in the heating step are not particularly limited. The heating temperature is, for example, preferably 250 to 700°C, more preferably 250 to 600°C, and even more preferably 300 to 450°C. The heating time may be 30 minutes to 20 hours. The heating time may be 30 minutes or more, 1 hour or more, 2 hours or more, 5 hours or more, or 10 hours or more, and may be 20 hours or less, 10 hours or less, 5 hours or less, 2 hours or less, 1 hour or less, or 30 minutes or less. In addition, treatment may be performed sequentially at a plurality of heating temperatures and heating times. For example, treatment may be performed under a combination of a plurality of heating conditions, such as a first heating treatment at 300°C for 30 minutes, a second heating treatment at 400°C for 1 hour, and a third heating treatment at 600°C for 2 hours. The flow rate of the hydrogen stream is not limited, but can be set, for example, in the range of 30 to 200 mL / min, preferably 50 to 150 mL / min. The above flow rate is preferable from the viewpoint of converting the remaining sulfur into hydrogen sulfide and removing it by the hydrogen stream. The heating step under the hydrogen stream generates hydrogen sulfide, and the oxygen atom-containing component contained in the sulfide crude product can be converted into a component that does not contain oxygen atoms by the reaction between the oxygen atom-containing component and the hydrogen sulfide. In an embodiment in which the sulfide crude product is lithium sulfide, the oxygen atom-containing component can be lithium oxide. In this embodiment, the content of lithium oxide contained in the lithium sulfide crude product can be reduced by the heating step under the hydrogen stream. The method is useful because the function of the sulfide, for example, electrode properties such as discharge capacity, can be improved by reducing the oxygen atom-containing component contained in the sulfide crude product. In a preferred embodiment, the ratio of the oxygen atom-containing component in the sulfide can be reduced to 3 mass% or less, preferably 1 mass% or less.

[0039] As described above, according to the present invention, the oxygen atom-containing components contained in the sulfide crude product can be reduced, thereby improving the function of the sulfide, for example, the capacity of a sulfur-based positive electrode material. Therefore, in one embodiment, the present invention provides a method for reducing the oxygen atom-containing components contained in the sulfide crude product; a method for converting the oxygen atom-containing components contained in the sulfide crude product into a sulfide; a method for improving the function of a sulfide, and the like. Specifically, the present invention provides, for example, the following methods (1) to (3): (1) mixing the crude sulfide product with sulfur; A method for reducing the amount of oxygen atom-containing components contained in a sulfide crude product, comprising the step of heating the sulfide that has been subjected to the mixing step in a hydrogen gas flow. (2) mixing the sulfide crude product with sulfur; A method for converting an oxygen atom-containing component contained in the sulfide crude product into a sulfide, comprising the step of heating the sulfide that has been subjected to the mixing step in a hydrogen gas stream. (3) mixing the sulfide crude product with sulfur; A method for improving the functionality of a sulfide, comprising the step of heating the sulfide that has been subjected to the mixing step in a hydrogen gas flow. In these embodiments, the conditions for the mixing step, the heating step, etc. can be the same as those described above for the method for producing a sulfide with reduced oxygen atom-containing components. The type of the crude sulfide product in these embodiments is also the same as that described above for the method for producing a sulfide with reduced oxygen atom-containing components, and preferably lithium sulfide.

[0040] As described above, the sulfides obtained by these methods are characterized by a low content of oxygen-atom-containing components. In a preferred embodiment, the sulfides obtained by these methods, particularly lithium sulfide, are characterized by small crystallites. Thus, in a preferred embodiment, the present invention provides lithium sulfide having a half-width (2θ) of a diffraction peak at a diffraction angle (2θ) of 27±0.5° measured by X-ray diffraction measurement using CuKα rays of 0.15° or more and 0.50° or less.

[0041] Conventionally manufactured lithium sulfide particles are often large when observed by SEM. In order to cause an electrochemical reaction in which lithium is released during charging and discharging and insulating sulfur is produced, the particles must be made fine and intimately contacted with conductive materials (such as carbon). However, Li 2 It is difficult to make S fine, and in order to bring out its properties, it is necessary to mix it with a conductive material for a long time, from several hours to several tens of hours, in a ball mill that applies high energy. The challenge is to produce fine-grained lithium sulfide suitable for use as an electrode material without using such a processing method that is not suitable for mass production. On the other hand, the solid electrolyte used in today's all-solid-state batteries has an argyrodite structure called Li 6 P.S. 5 The lithium sulfide used in this production is mixed with diphosphorus pentasulfide and lithium chloride in a high-energy ball mill, then sintered at 600 °C. When commercially available lithium sulfide is used as the raw material, lithium sulfide often remains unreacted in the product, and even if the above series of processes are repeated twice, lithium sulfide may remain as an impurity phase. Lithium sulfide is known to deteriorate easily and impair the properties of solid electrolytes, so the electrolyte is obtained by reacting lithium sulfide with a ball mill for 64 hours. In addition, when lithium sulfide comes into contact with moisture during storage, lithium hydroxide is produced, which absorbs oxygen. Oxygen combines with phosphorus, one of the main components of solid electrolytes, to produce impurities such as lithium phosphate in the solid electrolyte. The low reactivity of lithium sulfide with solid electrolytes is thought to be due to the oxygen on the surface and the large and hard particles of commercially available lithium sulfide. It is believed that if lithium sulfide that is fine and has a surface that is not contaminated with moisture could be obtained, it would be possible to produce a solid electrolyte that contains less lithium sulfide residue and does not contain oxygen, and therefore such lithium sulfide has been eagerly sought.

[0042] In the above embodiment, the lithium sulfide of the present invention has a half-width (2θ) of a diffraction peak at a diffraction angle (2θ) of 27±0.5° measured by X-ray diffraction measurement using CuKα radiation of 0.15° or more and 0.50° or less, and the half-width of such a diffraction peak reflects that the lithium sulfide of the present invention has a small crystallite size. The lithium sulfide of the present invention having a small crystallite size is preferable because it can be easily made into fine particles and can solve the above problems.

[0043] Next, a method for producing a crude product of sulfide, which is a raw material in each of the above-mentioned embodiments, will be described. Although the crude product of sulfide is not limited, a method of calcining a sulfate and a carbon source in a non-oxidizing atmosphere is preferable. In particular, by using lithium sulfate as the sulfate, lithium sulfide can be obtained as the sulfide. Therefore, in one embodiment, the present invention provides a method for producing lithium sulfide, which includes a step of calcining lithium sulfate and a carbon source in a non-oxidizing atmosphere.

[0044] In this embodiment, lithium sulfate may be anhydrous or monohydrate. The particle size of lithium sulfate is not particularly limited, but is preferably 50 μm or less, more preferably 10 μm or less, since the smaller the particle size, the more advantageous it is for mixing with the carbon source. The particle size of lithium sulfate can be measured by a laser diffraction scattering particle size distribution measurement method. The carbon source is not particularly limited, but sugars are preferred because they are easily carbonized during firing and are unlikely to remain in the lithium sulfide crude product after firing. Examples of sugars include sucrose, glucose, fructose, lactose, starch, cellulose, etc., and sucrose is preferred. These carbon sources may be used alone or in combination of two or more.

[0045] The compounding ratio of lithium sulfate and sugar is not limited, but theoretically, 2 moles of carbon are used for the reaction of 1 mole of lithium sulfate. When carbon generated by thermal decomposition of sugar is used, it is desirable to add more carbon than this, so the compounding ratio of lithium sulfate and sugar is preferably 2 moles or more of sugar in terms of carbon constituting the sugar relative to lithium sulfate. For example, when sucrose is used as the sugar, the compounding ratio of lithium sulfate to sugar is expressed by the composition formula C 12 H 22 O 11 Water is released from C 12 The number of moles of carbon is estimated assuming that 1 mole of sucrose will remain as carbon. Therefore, 1 mole of sucrose will have 12 moles of carbon, so when 6 moles of lithium sulfate are mixed with 1 mole of sucrose, the lithium sulfate:carbon ratio is 1:2. The amount of sugar used per mole of lithium sulfate is preferably 2.1 to 3.5 moles, more preferably 2.5 to 3.2 moles, calculated as the carbon contained in the sugar. If necessary, a carboxylic acid such as acetic acid may be added as an acidic substance.

[0046] The method for mixing lithium sulfate and the carbon source is not particularly limited. The mixing can be performed by dry grinding mixing or wet mixing using water. The conditions for the mixing step can be the same as those described above for the method for producing a sulfide with reduced oxygen atom-containing components.

[0047] In a preferred embodiment, in order to prevent the release of water in the calcination furnace during the calcination step, a step of heating the mixture of lithium sulfate and saccharides is preferably carried out between the step of mixing lithium sulfate and saccharides and the step of calcination. The temperature of such a heating step is preferably 100 to 250° C., more preferably 190 to 230° C. The heating step can remove hydroxyl groups contained in the saccharides by dehydration.

[0048] The temperature of the calcination step is preferably 700 to 1000°C, more preferably 900 to 1000°C, and more preferably 950 to 970°C. The calcination time is preferably maintained at the target temperature for 2 hours, more preferably 15 to 45 minutes, since the furnace is stopped when the target temperature is reached (holding time at the target temperature is 0 hours). The atmosphere used is a non-oxidizing atmosphere, and a nitrogen, argon, hydrogen atmosphere, etc. can be used. It is preferable to use hydrogen from the viewpoint of preventing a side reaction that generates impurities by converting carbon dioxide generated by the reaction to carbon monoxide. The impurities are generated by a side reaction in which lithium sulfide reacts with carbon dioxide. Therefore, it is possible to produce lithium sulfide with fewer impurities by converting carbon dioxide to carbon monoxide.

[0049] Reaction during the firing process (Li 2 SO 4 +2C=Li 2 S+2CO 2 It is preferable to use a gas stream (such as a hydrogen stream) to remove carbon dioxide generated during the reaction from the system. In a preferred embodiment, for example, hydrogen is passed through 10 g of sample at a rate of 80 to 150 ml / min, preferably 90 to 120 ml / min per minute. Hydrogen can be passed through, for example, by using an alumina tube (4 mmφ aperture, 200 mmL) nozzle to introduce hydrogen into a Tammann tube (40 mmφ aperture, 180 mmL) containing the sample. The Tammann tube is preferably not made of quartz since it comes into contact with lithium sulfide at high temperatures, and specifically, is preferably made of alumina equivalent to SSA-H or SSA-S. As mentioned above, in the reaction of the calcination process, the carbon dioxide produced reacts with lithium sulfide to a certain extent as a side reaction. This can produce impurities such as lithium oxide and lithium carbonate (Li 2 S+2CO 2 =Li 2 O+CO+S, Li 2 O+CO 2 =Li 2 CO 3 In the present invention, the sugars added in excess are converted into CO 2 Because it is oxidized by (C+CO 2=2CO) can be removed from the product. Therefore, the method of the present invention is useful because it can synthesize lithium sulfide with a reduced carbon content (typically, no carbon) as an impurity.

[0050] Specifically, it has been reported that a mixture of graphene nanoplatelet aggregates, Ketjen black, or other carbon material and lithium sulfate is sintered, the carbon reduces the lithium sulfate to generate lithium sulfide, and the lithium sulfide-carbon composite functions as a positive electrode material (Patent Document 4, Non-Patent Documents 1 and 2). However, this method produces a mixture of lithium sulfide and carbon, and the capacity is lower than the theoretical capacity of 1166 mAh / g (850 mAh / g Li 2 S(gLi 2 S is the capacity per weight of lithium sulfide contained in the composite). Therefore, a method for producing lithium sulfide that does not contain carbon and a method for synthesizing lithium sulfide material with higher capacity have been desired. In addition, a method for synthesizing lithium sulfide by reducing lithium sulfate with carbon using a carbon material such as carbon black or graphite is also known (Patent Documents 2 and 3). However, even in these methods, it is difficult to remove carbon residues and impurities. In contrast, according to the method of the present invention, lithium sulfide with a reduced carbon content as an impurity (typically, not containing carbon) can be synthesized, and the above problem can be solved. In addition, the lithium sulfide in this embodiment may contain lithium oxide, lithium carbonate, etc., but in such a case, lithium sulfide with higher purity can be obtained by subjecting it to the above-mentioned method for reducing oxygen atom-containing components contained in the crude product of the sulfide and the method for reducing metal carbonates contained in the crude product of the sulfide. The sulfide obtained in each embodiment of the present invention, especially lithium sulfide, can be used to manufacture a positive electrode material for lithium ion batteries.

[0051] Manufacturing method of sulfide-carbon composite In another embodiment, the present invention relates to a process for producing a sintered product comprising the steps of: mixing a sulfide, an organic substance, and sulfur; The mixture obtained in the mixing step is heated in a hydrogen stream. A method for producing a sulfide-carbon composite is provided.

[0052] As the sulfide in this embodiment, the above-described conditions described for the method for producing a sulfide having a reduced oxygen atom-containing component can be adopted, and among them, lithium sulfide is preferable.

[0053] Examples of organic substances include sugars such as glucose, fructose, sucrose, starch, dextrin, and cellulose, polymers such as polystyrene, polyvinyl alcohol, and polyparaphenylene vinylene, and acrylic acid monomers such as acrylic acid esters, methacrylic acid esters, and acrylic acid. Among them, nitrogen-containing organic substances are more preferable from the viewpoint of being able to include nitrogen in the carbon produced. Nitrogen, which has a different electronegativity from carbon, can impart polarity to the carbon skeleton and induce a bonding interaction between the sulfide and the carbon skeleton. Examples of nitrogen-containing organic substances include amino sugars such as glucosamine hydrochloride and N-acetylglucosamine, polymers such as chitosan, polyvinylpyrrolidone, polyacrylonitrile, and polyaniline, monomers such as methacrylamide and acrylonitrile, melamine, phenylenediamine, and phthalonitrile. These organic substances can be used alone or in combination of two or more. The blending ratio of the sulfide and the organic material is not limited, but for example, the organic material can be 0.05 to 2.0 parts by mass, preferably 0.1 to 1.0 parts by mass, per 1 part by mass of the sulfide. The blending ratio of the sulfide and the sulfur is not limited, but for example, the sulfur can be 0.05 to 0.5 parts by mass, preferably 0.1 to 0.4 parts by mass, per 1 part by mass of the sulfide. The mixing is not limited, but can be performed in the same manner as above, except that the sulfide, the organic material, and the sulfur are used as raw materials. In this embodiment, the conditions of the mixing step and the heating step can be the same as those described above for the method for producing a sulfide with reduced oxygen atom-containing components. A sulfide-carbon composite is formed by this step. In a preferred embodiment, the sulfide (preferably lithium sulfide) is composited with carbon, which is useful for showing good electrode properties.

[0054] In another preferred embodiment of the present invention, a method of heating a crude product of a sulfide with sulfur and acrylonitrile is mentioned. Thus, the present invention includes a step of mixing a crude product of a sulfide with sulfur and acrylonitrile; The present invention provides a method for producing a sulfide-carbon composite, the method comprising the step of heating the sulfide that has been subjected to the mixing step in a hydrogen gas flow.

[0055] In this embodiment, the conditions for the mixing step and the heating step may be the same as those described above for the method for producing a sulfide with reduced oxygen atom-containing components. The type of the crude sulfide product in this embodiment is also the same as that described above for the method for producing a sulfide with reduced oxygen atom-containing components, and preferably includes lithium sulfide. Therefore, in a preferred embodiment, the present invention includes a step of mixing (preferably pulverizing and mixing) the crude lithium sulfide product with sulfur and acrylonitrile, The present invention provides a method for producing a lithium sulfide-carbon composite, the method comprising the step of heating the lithium sulfide that has been subjected to the mixing step in a hydrogen gas flow.

[0056] In these embodiments, the proportion of acrylonitrile used is not limited, but for example, the amount of acrylonitrile may be 0.05 to 2.0 parts by mass, preferably 0.1 to 1.0 part by mass, per 1 part by mass of the crude sulfide product.

[0057] Even in the embodiment using acrylonitrile, the amount of oxygen atom-containing components contained in the crude sulfide product can be significantly reduced by heating a mixture of the crude sulfide product, sulfur, and acrylonitrile in a hydrogen gas flow. In addition, the crude sulfide product may contain metal carbonate, which adversely affects the function of the sulfide (electrode characteristics, etc.). Therefore, this embodiment is useful because it can improve the function of the sulfide by reducing the amount of metal carbonate. Examples of metal carbonate include lithium carbonate, sodium carbonate, potassium carbonate, and ammonium carbonate. For example, when the sulfide is lithium sulfide, the crude product of lithium sulfide may contain lithium carbonate depending on the synthesis method. It is very difficult to remove lithium carbonate. In the embodiment of the present invention using acrylonitrile, the content of not only lithium oxide but also lithium carbonate can be reduced, which is very effective.

[0058] As described above, in this embodiment, the oxygen atom-containing component contained in the sulfide crude product can be reduced, and if a metal carbonate is contained, the metal carbonate can also be reduced. As a result, the function of the sulfide-carbon composite, for example, the capacity of a sulfur-based positive electrode material, can be improved. Therefore, in one embodiment, the present invention provides a method for reducing the oxygen atom-containing component and / or metal carbonate contained in the sulfide crude product; a method for converting the oxygen atom-containing component and / or metal carbonate contained in the sulfide crude product into a sulfide; a method for improving the function of a sulfide, and the like. Specifically, the present invention provides, for example, the following methods (4) to (6): (4) mixing the crude sulfide product with sulfur and acrylonitrile; A method for reducing the amount of oxygen atom-containing components and / or metal carbonates contained in a crude sulfide product, comprising a step of heating the sulfide that has been subjected to the mixing step in a hydrogen gas stream. (5) mixing the crude sulfide product with sulfur and acrylonitrile; A method for converting an oxygen atom-containing component and / or a metal carbonate contained in the sulfide crude product into a sulfide, comprising a step of heating the sulfide that has been subjected to the mixing step in a hydrogen gas stream. (6) mixing the crude sulfide product with sulfur and acrylonitrile; A method for improving the function of a sulfide-carbon composite, comprising the step of heating the sulfide that has been subjected to the mixing step in a hydrogen gas flow. In these embodiments, the conditions for the mixing step, the heating step, etc. can be the same as those described above for the method for producing a sulfide with reduced oxygen atom-containing components. The type of the crude sulfide product in these embodiments is also the same as that described above for the method for producing a sulfide with reduced oxygen atom-containing components, and preferably lithium sulfide.

[0059] Specifically, lithium sulfide is unstable in air and decomposes in the presence of moisture to produce hydrogen sulfide (Li 2 S+2H 2 O = 2LiOH + H 2 S). This makes it easy for lithium hydroxide to form on the surface, which is thought to make the material lose its activity as an electrode material. In fact, lithium sulfide, which is commercially available as a reagent, can lose its properties as an electrode material during storage. For this reason, there is a need for a method to remove the lithium hydroxide and lithium oxide that form on the surface and increase the surface activity of lithium sulfide.

[0060] In addition, because lithium sulfide is an insulator, when it is used as an electrode, it is necessary to mix a large amount of a conductive assistant with it to impart conductivity, as is conventionally done. Lithium sulfide, a positive electrode material, has been reported to be a high-capacity positive electrode material for lithium-ion secondary batteries (Non-Patent Documents 1, 2). However, only about 60% of the theoretical capacity has been achieved. This is due to the aforementioned Li 2 This is thought to be due to the size of the S particles and the activity of the surface. In addition, when charging, lithium sulfide is oxidized, lithium ions are released, and sulfide ions (S 2- ) combine to form polysulfide ions ( - SSS- , - SSSS - , - SSSSSSSS - etc.) When charging is complete, sulfur (but cyclic S 8 The polysulfide ions (S x 2- ) is easily dissolved in the electrolyte, and the dissolved ions diffuse to the negative electrode and are reduced to form lithium sulfide at the negative electrode, which precipitates and reduces capacity, resulting in poor charge-discharge cycle characteristics. There is a need for a composite technology that can protect the surface of lithium sulfide, which is prone to losing activity, suppress the elution of polysulfides that occur during charging, improve the related cycle characteristics, and even create a structure for the sulfur generated at the end of charging that is suitable for battery materials (it has been reported that a gamma structure is good for cycle characteristics).

[0061] In the preferred embodiment of the present invention, the sulfide (preferably lithium sulfide) is composited with carbon and exhibits good electrode properties, and therefore such problems can be solved and the present invention is extremely useful.

[0062] Specific embodiments of the present invention will be described in more detail below using examples, but the present invention is not limited to these examples. EXAMPLES

[0063] Glove box In the following examples and comparative examples, lithium sulfide is H 2 O<1ppm, O 2 The samples were handled in a purged glove box (Miwa Manufacturing) equipped with a gas circulation purification device of <1 ppm and stored in a sealed glass bottle. Items and samples were taken in and out through a side box, and when samples were brought into the glove box from the outside, vacuum degassing and dry argon gas injection were performed three times.

[0064] <Mixing process> In the following examples, lithium sulfide (crude product) and sulfur (and organic matter) were mixed by ball milling. For the ball milling, a planetary ball mill (Fritsch, P-6) was used, along with a zirconia pot with an internal volume of 80 mL and five zirconia balls with a diameter of 20 mm.

[0065] <Heating and baking process> In the following examples and comparative examples, a programmable electric tubular furnace (AS ONE, TMF-500N, alumina 99% core tube) was used for the heating and sintering process of the mixture of lithium sulfide (crude product) and sulfur (and organic matter). The mixture was placed in an alumina Tamman tube (Nikkato, NC Tamman No. 6 40 x 34 x 150, material SSA-S). A gas replacement unit (AS ONE, 1-7555-44) was used to introduce the flow gas into the core tube, and the gas inlet was connected to a gas line and the exhaust port was connected to a trap bubbler. Argon was used for gas replacement inside the core tube before and after operation, and hydrogen was used as the flow gas during operation at a flow rate of 100 mL per minute.

[0066] <X-ray crystal structure analysis (XRD measurement) conditions> XRD of the lithium sulfide sample was measured by a conventional method, except for the following conditions. X-ray diffraction measurement equipment: Rigaku Corporation, model: UltimaIV. ·X-ray source: CuKα Output: 50kV / 40mA Sampling width: 0.0200° Scan speed: 20.0° / min 2θ:20~80° Specimen holder: Airtight specimen holder

[0067] <Evaluation conditions for impurity (lithium oxide) content> In the XRD measurement, the peak intensity (β) was defined as the difference between the maximum and minimum intensity values ​​in the diffraction angle (2θ) region of 27.0±0.5°, where lithium sulfide has its first peak, and the peak intensity (γ) was defined as the difference between the maximum and minimum intensity values ​​in the diffraction angle (2θ) region of 33.5±1.0°, where lithium oxide, an oxygen-containing impurity, has its first peak, to calculate the peak intensity ratio (γ / β).

[0068] <SEM-EDS analysis> The lithium sulfide and lithium sulfide-carbon composites of the examples and comparative examples were analyzed using a field emission scanning electron microscope (Hitachi High-Technologies, S-5500) and an energy-dispersive X-ray spectroscopy system (EDAX EDS). Silver-based tape was used to fix the samples to the sample stage.

[0069] Evaluation of charge / discharge characteristics A coin-type lithium ion battery cell (CR2032) was prepared using the lithium sulfide and lithium sulfide-carbon composite of each Example and Comparative Example as the positive electrode active material. Specifically, the positive electrode active material of each Example: conductive additive (Ketjen Black, Lion, EC600JD): binder (Polyflon PTFE, Daikin, F-104) were weighed out in a weight ratio of 60:30:10 and mixed in an agate mortar to prepare a positive electrode body.

[0070] The electrolyte was prepared by dissolving LiTFSI (lithium bis(trifluoromethanesulfonyl)imide, CAS registration number 90076-65-6, Fujifilm Wako Pure Chemical Industries, Ltd.) at a concentration of 1 mol / L in a mixed solvent of DME (dimethoxyethane, CAS registration number 110-71-4, Fujifilm Wako Pure Chemical Industries, Ltd.) and DOL (1,3-dioxolane, CAS registration number 646-06-0, Fujifilm Wako Pure Chemical Industries, Ltd.) at a volume ratio of 1:1. Metallic Li foil (Honjo Metals, 0.5 mmt, 12 mmφ) and a polypropylene separator (16 mmφ) were used as the negative electrode body.

[0071] The charge / discharge characteristics were evaluated in a thermostatic chamber maintained at 30°C, with the initial voltage range being 1.5-4.2V, and the voltage range from the second time onwards being 1.5-3.0V, at a rate of 0.1C (1C=1166mA / g).

[0072] Example 1 <Step of obtaining crude lithium sulfide from lithium sulfate and sucrose> 6.88 g of lithium sulfate (CAS registration number 10377-48-7, Tokyo Chemical Industry Co., Ltd., 98.0%) and 5.36 g of sucrose (CAS registration number 57-50-1, Kishida Chemical Co., Ltd.) were weighed out in order, put into a mortar, and mixed for 5 minutes. The mixture was fired in the SV timer mode of a programmable tubular furnace at a set temperature of 960°C and an OFF timer time of 30 minutes. The fired product was pulverized in an agate mortar to recover a crude product of lithium sulfide.

[0073] (Step of mixing the crude lithium sulfide product with sulfur and heating in a hydrogen stream) 1.00 g of the lithium sulfide crude product and 1.00 g of sulfur (CAS registration number 7704-34-9, Hosoi Chemicals, 99.9%) were added to an agate mortar, mixed, and ground and mixed in a planetary ball mill at a rotation speed of 500 rpm and an operating time of 60 minutes. The resulting mixture was heated in a program operation mode of a program tube furnace under the following conditions: step number 4, step 1 set temperature 340°C, step 1 set time 30 minutes, step 2 set temperature 340°C, step 2 set time 30 minutes, step 3 set temperature 400°C, step 3 set time 10 minutes, step 4 set temperature 400°C, and step 4 set time 60 minutes. The fired product was ground in an agate mortar to recover 1.01 g of lithium sulfide. The XRD measurement results of the synthesized lithium sulfide are shown in Figure 1. In the obtained X-ray diffraction spectrum, the intensity ratio (γ / β) of the peak (β) in the region of diffraction angle (2θ) 27.0±0.5° to the peak (γ) in the region of diffraction angle (2θ) 33.5±1.0° was 0.0092. The half-width (2θ) of the peak (β) in the region of diffraction angle (2θ) 27.0±0.5° was 0.174°.

[0074] Comparative Example 1 A crude lithium sulfide product was prepared in the same manner as in Example 1 (step of obtaining a crude lithium sulfide product from lithium sulfate and sucrose), and the step of mixing the crude lithium sulfide product with sulfur and heating in a hydrogen stream was not performed. The XRD measurement results of the synthesized lithium sulfide are shown in FIG. 2. The intensity ratio (γ / β) of the peak (β) in the region of a diffraction angle (2θ) of 27.0±0.5° and the peak (γ) in the region of 33.5±1.0° in the obtained X-ray diffraction spectrum was 0.0366. The half-width of the peak (β) in the region of a diffraction angle (2θ) of 27.0±0.5° was 0.145°.

[0075] Example 2 <Step for obtaining crude lithium sulfide from lithium sulfate and cellulose> 6.84g of lithium sulfate, 5.44g of cellulose (CAS registration number 9004-34-6, Fujifilm Wako Pure Chemical Industries, Ltd.), and 5.05g of distilled water were weighed out in order, put into a mortar, mixed until uniform, heated in a hot air circulation oven (Koyo Thermo Systems, KLO-45M) set at 150°C, and dried until all moisture was removed. The solid matter was crushed in the mortar and heated for an additional 4 hours to recover a dry powder. The dry powder was fired in a programmable tubular furnace in SV timer mode with a set temperature of 960°C and an OFF timer time of 30 minutes. The fired matter was crushed in an agate mortar to recover a crude product of lithium sulfide.

[0076] (Step of mixing the crude lithium sulfide product with sulfur and heating in a hydrogen stream) 1.00 g of the lithium sulfide crude product and 1.00 g of sulfur were added in turn to an agate mortar, mixed, and ground and mixed in a planetary ball mill at a rotation speed of 500 rpm and an operating time of 60 minutes. The obtained mixture was heated in a program operation mode of a program tube furnace under the conditions of step number 4, step 1 set temperature 340°C, step 1 set time 30 minutes, step 2 set temperature 340°C, step 2 set time 30 minutes, step 3 set temperature 400°C, step 3 set time 10 minutes, step 4 set temperature 400°C, and step 4 set time 60 minutes. The fired product was ground in an agate mortar to recover 0.98 g of lithium sulfide. XRD of the synthesized lithium sulfide was measured. The intensity ratio (γ / β) of the peak (β) in the region of diffraction angle (2θ) 27.0±0.5° and the peak (γ) in the region of 33.5±1.0° in the obtained X-ray diffraction spectrum was 0.0071. The half-width of the peak (β) in the region of diffraction angle (2θ) 27.0±0.5° was 0.162°.

[0077] Comparative Example 2 A crude lithium sulfide product was prepared in the same manner as in Example 2 (step of obtaining a crude lithium sulfide product from lithium sulfate and cellulose), and the step of mixing the crude lithium sulfide product with sulfur and heating in a hydrogen stream was not performed. The XRD of the synthesized lithium sulfide was measured. The intensity ratio (γ / β) of the peak (β) in the region of the diffraction angle (2θ) of 27.0±0.5° and the peak (γ) in the region of 33.5±1.0° in the obtained X-ray diffraction spectrum was 0.0353. The half-width of the peak (β) in the region of the diffraction angle (2θ) of 27.0±0.5° was 0.144°.

[0078] Example 3 <Step for obtaining crude lithium sulfide from lithium sulfate and starch> 6.87g of lithium sulfate and 6.87g of starch (CAS registration number 9005-25-8, King Foods) were weighed out in order, put into a mortar, and mixed for 5 minutes. The mixture was fired in a programmable tubular furnace in SV timer mode at a set temperature of 960°C and an OFF timer time of 30 minutes. The fired product was pulverized in an agate mortar to recover a crude product of lithium sulfide.

[0079] (Step of mixing the crude lithium sulfide product with sulfur and heating in a hydrogen stream) 1.00 g of the lithium sulfide crude product and 1.00 g of sulfur were added in turn to an agate mortar, mixed, and ground and mixed in a planetary ball mill at a rotation speed of 500 rpm and an operating time of 60 minutes. The obtained mixture was heated in a program operation mode of a program tube furnace under the conditions of step number 4, step 1 set temperature 340°C, step 1 set time 30 minutes, step 2 set temperature 340°C, step 2 set time 30 minutes, step 3 set temperature 400°C, step 3 set time 10 minutes, step 4 set temperature 400°C, and step 4 set time 60 minutes. The fired product was ground in an agate mortar to recover 0.97 g of lithium sulfide. XRD of the synthesized lithium sulfide was measured. The intensity ratio (γ / β) of the peak (β) in the region of diffraction angle (2θ) 27.0±0.5° and the peak (γ) in the region of 33.5±1.0° in the obtained X-ray diffraction spectrum was 0.0117. The half-width of the peak (β) in the region of diffraction angle (2θ) 27.0±0.5° was 0.175°.

[0080] Comparative Example 3 A crude lithium sulfide product was prepared in the same manner as in Example 3 (step of obtaining a crude lithium sulfide product from lithium sulfate and starch), and the step of mixing the crude lithium sulfide product with sulfur and heating in a hydrogen stream was not performed. The XRD of the synthesized lithium sulfide was measured. The intensity ratio (γ / β) of the peak (β) in the region of a diffraction angle (2θ) of 27.0±0.5° and the peak (γ) in the region of 33.5±1.0° in the obtained X-ray diffraction spectrum was 0.0287. The half-width of the peak (β) in the region of a diffraction angle (2θ) of 27.0±0.5° was 0.148°.

[0081] Example 4 (Step of mixing lithium sulfide and sulfur as reagents and heating in a hydrogen stream) 1.04g of the reagent lithium sulfide (CAS registration number 12136-58-2, Mitsuwa Chemicals, 99.9%) and 0.15g of sulfur (CAS registration number 7704-34-9, Hosoi Chemicals, 99.9%) were added to the agate mortar, mixed, and ground and mixed in a planetary ball mill at a rotation speed of 500 rpm and an operating time of 60 minutes. The resulting mixture was heated in the program operating mode of a program tube furnace under the following conditions: step number 4, step 1 set temperature 340°C, step 1 set time 30 minutes, step 2 set temperature 340°C, step 2 set time 30 minutes, step 3 set temperature 400°C, step 3 set time 10 minutes, step 4 set temperature 400°C, and step 4 set time 60 minutes. The fired product was ground in an agate mortar to recover 1.01g of lithium sulfide. XRD of the lithium sulfide after treatment was measured. The half-width of the peak (β) in the region of diffraction angle (2θ) 27.0±0.5° was 0.227°.

[0082] Comparative Example 4 XRD was measured using lithium sulfide as a reagent in Comparative Example 4. The half-width of the peak (β) in the region of a diffraction angle (2θ) of 27.0±0.5° was 0.141°.

[0083] SEM-EDS analysis was carried out on the lithium sulfide of Example 4 and Comparative Example 4. FIG. 3 shows the SEM observation results of Example 4, and FIG. 4 shows the SEM observation results of Comparative Example 4. The SEM observation was carried out at an electron beam acceleration voltage of 1 kV. The particles of Comparative Example 4 were in the form of angular fragments, whereas the particles of Example 4 had a fine structure on the surface. Table 1 also shows the weight percent ratio of oxygen (O) and sulfur (S) of each sample in the EDS analysis. The EDS analysis values ​​in Table 1 are the average values ​​of five measurements carried out at an electron beam acceleration voltage of 10 kV. The results of carbon (C) and silver (Ag) derived from the sample tape are also shown. From the results in Table 1, the ratio of oxygen (O) to sulfur (S) in lithium sulfide was reduced from 0.128 to 0.028 by carrying out the treatment of the present invention.

[0084] [Table 1]

[0085] The charge / discharge characteristics were evaluated using the lithium sulfide of Example 4 and Comparative Example 4 as the positive electrode active material. The initial charge / discharge curves are shown in Figure 5. By carrying out the treatment of the present invention, the voltage at the beginning of charging was reduced, and the discharge capacity and coulombic efficiency were improved.

[0086] Comparative Example 5 <Process for obtaining crude lithium sulfide from lithium sulfate monohydrate and sucrose> Lithium sulfate monohydrate (CAS registration number 10102-25-7, Kishida Chemical, 99.0%) 16.11 g, sucrose (CAS registration number 57-50-1, Kishida Chemical), and distilled water 8.05 g were weighed out in order, put into a mortar, and mixed for 5 minutes. Acetic acid (CAS registration number 64-19-7, Kishida Chemical, 99%) 2.01 g was weighed out, put into a mortar, mixed until uniform, heated in a hot air circulating oven set at 150 °C, and dried until all moisture was removed. The solid matter was crushed in a mortar and heated for another 4 hours to recover a dried powder. The dried powder was fired in a program tube furnace in SV timer mode with a set temperature of 960 °C and an OFF timer time of 30 min. The fired matter was crushed in an agate mortar to recover a crude product of lithium sulfide. The XRD measurement results of the synthesized lithium sulfide are shown in Figure 6. In the obtained X-ray diffraction spectrum, peaks derived from lithium carbonate were confirmed before and after the peak (β) in the region of a diffraction angle (2θ) of 27.0±0.5°.

[0087] Example 5 <A process of mixing lithium sulfide, sulfur, and acrylonitrile and heating in a hydrogen stream> 1.89 g of the lithium sulfide crude product in Comparative Example 5, 0.31 g of sulfur, and 0.30 g of acrylonitrile (CAS registration number 107-13-1, Fujifilm Wako Pure Chemical Industries, 97.0%) were added in turn to an agate mortar, mixed, and ground and mixed in a planetary ball mill at a rotation speed of 500 rpm and an operation time of 60 minutes. The obtained mixture was heated in a program operation mode of a program tube furnace under the following conditions: step number 6, step 1 set temperature 340°C, step 1 set time 30 minutes, step 2 set temperature 340°C, step 2 set time 30 minutes, step 3 set temperature 400°C, step 3 set time 10 minutes, step 4 set temperature 400°C, step 4 set time 60 minutes, step 5 set temperature 600°C, step 5 set time 30 minutes, step 6 set temperature 600°C, step 6 set time 60 minutes. The fired product was ground in an agate mortar to recover 1.91 g of lithium sulfide-carbon composite. The XRD measurement result of the synthesized lithium sulfide is shown in Figure 7. The peaks derived from lithium carbonate, which were seen before and after the peak (β) in the region of the diffraction angle (2θ) of 27.0±0.5° in the obtained X-ray diffraction spectrum, disappeared. By the process of the present invention of mixing lithium sulfide, sulfur, and acrylonitrile and heating in a hydrogen stream, it was possible to reduce not only the lithium oxide content in lithium sulfide but also the lithium carbonate content.

[0088] Example 6 A crude lithium sulfide product was prepared in the same manner as in the <step of obtaining a crude lithium sulfide product from lithium sulfate and sucrose> in Example 1, and the <step of mixing lithium sulfide, sulfur, and an organic substance and heating in a hydrogen stream> of the present invention was carried out. 3.89 g of the crude lithium sulfide product, 0.55 g of sulfur, 0.55 g of sucrose, and 0.10 g of Ketjen Black were added in order to an agate mortar, mixed, and ground and mixed in a planetary ball mill at a rotation speed of 500 rpm, an operating time of 60 minutes, and an inversion repetition rate of 9 times. The obtained mixture was heated in a programmed operation mode of a programmed tubular furnace under the following conditions: number of steps 6, step 1 set temperature 340°C, step 1 set time 30 minutes, step 2 set temperature 340°C, step 2 set time 30 minutes, step 3 set temperature 400°C, step 3 set time 10 minutes, step 4 set temperature 400°C, step 4 set time 60 minutes, step 5 set temperature 600°C, step 5 set time 30 minutes, step 6 set temperature 600°C, step 6 set time 60 minutes. The fired product was pulverized in an agate mortar to recover 4.17 g of lithium sulfide-carbon composite.

[0089] Evaluation of charge / discharge characteristics of lithium sulfide-carbon composite of Example 6 The lithium sulfide-carbon composite of Example 6 was used as a positive electrode active material to evaluate the charge-discharge characteristics. In addition to the 1M LiTFSI (DME-DOL) electrolyte, a sulfur-insoluble electrolyte SL (sulfolane, CAS registration number 126-33-0, Kishida Chemical, 99%): LiTFSA (lithium bis(trifluoromethanesulfonyl)amide, CAS registration number 90076-65-6, Solvay Japan): HFE (1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether, CAS registration number 16627-68-2, Daikin) = 2:1:2 (molar ratio) was used. The results are shown in Table 2. In the case of the 1M LiTFSI (DME-DOL) electrolyte, the initial discharge capacity was 808 mAh / g. Gravimetric analysis revealed that the lithium sulfide content in the lithium sulfide-carbon composite of Example 6 was 90 wt %, and the capacity per unit weight of lithium sulfide was 898 mAh / g. Li2SIt was found that the discharge capacity was higher than the results in non-patent papers 1 and 2. In the case of the SL-LiTFSA-HFE electrolyte, the initial discharge capacity was 717 mAh / g, and the 20th discharge capacity was 603 mAh / g.

[0090] [Table 2]

[0091] Examples 7 to 9 The <lithium sulfide-carbon composite> of the present invention was prepared using lithium sulfide as a reagent and N-acetylglucosamine (CAS registration number 7512-17-6, Kishida Chemical, 98%) as a nitrogen-containing organic material in the same manner as in the <step of mixing lithium sulfide, sulfur, and an organic material and heating in a hydrogen stream> of Example 6. The lithium sulfide-carbon composite of Example 7 was prepared under the same conditions except that sucrose in the step of mixing lithium sulfide, sulfur, and an organic material was changed to N-acetylglucosamine. The lithium sulfide-carbon composite of Example 8 was prepared under the same conditions except that the set temperatures of steps 5 and 6 of the program operation mode in the heating step of the mixture from Example 7 were changed from 600°C to 700°C. The lithium sulfide-carbon composite of Example 9 was prepared under the same conditions except that the set temperatures of steps 5 and 6 of the program operation mode in the heating step of the mixture from Example 7 were changed from 600°C to 500°C.

[0092] Evaluation of charge / discharge characteristics of lithium sulfide-carbon composites of Examples 7 to 9 The lithium sulfide-carbon composites of Examples 7 to 9 were used as a positive electrode active material to evaluate the charge and discharge characteristics. The results are shown in Table 3. In the case of Example 7, the initial discharge capacity was 767 mAh / g, and the 20th discharge capacity was 569 mAh / g. In the case of Example 8, the initial discharge capacity was 725 mAh / g, and the 20th discharge capacity was 485 mAh / g. In the case of Example 9, the initial discharge capacity was 717 mAh / g, and the 20th discharge capacity was 539 mAh / g. The lithium sulfide-carbon composites of Examples 7 to 9 showed higher discharge capacity and cycle characteristics than the lithium sulfide of the reagent of Comparative Example 4.

[0093]

Table 3

Claims

1. mixing the crude sulfide product with sulfur; A method for producing a sulfide having a reduced content of oxygen atom-containing components contained in a crude sulfide product, comprising the step of heating the sulfide that has been subjected to the mixing step in a hydrogen gas flow.

2. The sulfide is lithium sulfide and the oxygen atom-containing component is lithium oxide. The method of claim 1.

3. 3. The method of claim 2, wherein the lithium sulfide is obtained by calcining lithium sulfate and a carbon source in a non-oxidizing atmosphere.

4. The method of claim 3 , wherein the carbon source is a sugar.

5. The method according to any one of claims 1 to 4, wherein the heating step is carried out at 250 to 700 °C.

6. The method according to claim 3 or 4, wherein the calcination step is carried out at 700 to 1000°C.

7. Obtained by the method according to any one of claims 2 to 4, Lithium sulfide having a half-width (2θ) of a diffraction peak at a diffraction angle (2θ) of 27±0.5° as measured by X-ray diffraction measurement using CuKα rays of 0.15° or more and 0.50° or less.

8. mixing a sulfide, an organic substance, and sulfur; The mixture obtained in the mixing step is heated in a hydrogen stream. A method for producing a sulfide-carbon composite.

9. The sulfide is lithium sulfide; 9. The method of claim 8 for producing a lithium sulfide-carbon composite.

10. The organic substance is a nitrogen-containing organic substance.

10. The method of claim 9 for producing a lithium sulfide-carbon composite.

11. 5. The method according to any one of claims 1 to 4 for producing a positive electrode material for a lithium ion battery.

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