Positive electrode mixture for all-solid-state lithium-sulfur secondary battery, positive electrode for all-solid-state lithium-sulfur secondary battery, and all-solid-state lithium-sulfur secondary battery

Abstract

The present invention pertains to a positive electrode mixture for an all-solid-state lithium-sulfur secondary battery, the positive electrode mixture including a sulfur compound synthesized by an inverse vulcanization method, a solid electrolyte, and a conductive additive, and optionally including elemental sulfur, wherein the total contents of the sulfur compound synthesized by the inverse vulcanization method and the elemental sulfur with respect to the total mass of the positive electrode mixture for an all-solid-state lithium-sulfur secondary battery are 20-70 mass %, the content of the solid electrolyte is 10-60 mass %, and the content of the conductive additive is 20 mass % or less.

Description

Positive electrode composite material for all-solid-state lithium-sulfur secondary batteries, positive electrode for all-solid-state lithium-sulfur secondary batteries, and all-solid-state lithium-sulfur secondary batteries

[0001] The present invention relates to a cathode composite material for all-solid-state lithium-sulfur secondary batteries, a cathode for all-solid-state lithium-sulfur secondary batteries, and an all-solid-state lithium-sulfur secondary battery. This application claims priority based on Japanese Patent Application No. 2025-049361, filed in Japan on March 25, 2025, the contents of which are incorporated herein by reference.

[0002] Lithium-sulfur secondary batteries, which use sulfur as the positive electrode active material and metallic lithium as the negative electrode active material, have a theoretical energy density of 2,800 Wh / kg and a theoretical sulfur capacity of 1,672 mAh / g. These theoretical energy densities and capacities are significantly higher than those of other secondary batteries. Furthermore, because sulfur, the main raw material, is abundant and inexpensive, it is expected to be a next-generation storage battery.

[0003] However, liquid-type lithium-sulfur secondary batteries that use an electrolyte have the problem of short cycle life due to the leaching and diffusion of lithium polysulfide into the electrolyte during the charge-discharge process. To solve this problem, all-solid-state lithium-sulfur secondary batteries are being investigated. For example, Patent Document 1 discloses an all-solid-state lithium-sulfur secondary battery that contains elemental sulfur as the positive electrode active material. In all-solid-state lithium-sulfur secondary batteries, a solid electrolyte is used instead of an electrolyte, so it is thought that cycle degradation due to the leaching of lithium polysulfide can not be caused, and a long cycle life can be maintained.

[0004] On the other hand, in all-solid-state lithium-sulfur secondary batteries, unlike liquid-type lithium-sulfur secondary batteries that use an electrolyte, an electrochemical reaction occurs at the solid-solid interface between the positive electrode active material and the solid electrolyte. Therefore, it is considered important to impart flexibility to the positive electrode active material in order to promote interfacial adhesion. It is known that elemental sulfur contained in the positive electrode active material of the all-solid-state lithium-sulfur secondary battery described in Patent Document 1 undergoes a 1.8-fold volume change when it reacts with lithium, and this volume change of the positive electrode active material due to charging and discharging can cause cracks in the positive electrode. If cracks occur in the positive electrode, it is a concern that the interfacial resistance will increase, leading to cycle degradation. Cycle degradation refers to a decrease in discharge capacity due to the charge-discharge cycle. Therefore, the positive electrode active material of an all-solid-state lithium-sulfur secondary battery is required to have flexibility from the viewpoint of promoting interfacial adhesion and absorbing stress changes due to volume changes.

[0005] Studies are underway to use sulfur polymers instead of pure sulfur. For example, Patent Document 2 discloses an all-solid-state lithium-sulfur secondary battery comprising a positive electrode containing sulfur-modified polyacrylonitrile and a sulfide-based solid electrolyte, and an electrolyte layer containing a sulfide-based solid electrolyte. By using sulfur-modified polyacrylonitrile as the positive electrode active material, it is thought that cracking of the positive electrode is less likely to occur even if the volume of the positive electrode active material changes due to charging and discharging, the increase in interfacial resistance is suppressed, and cycle degradation is suppressed.

[0006] Patent No. 6108267 Patent No. 6090895

[0007] However, when the inventors of the present application conducted an investigation using sulfur-modified polyacrylonitrile described in Patent Document 2, they found that the suppression of cyclic degradation was insufficient.

[0008] The present invention has been made in view of the above circumstances, and aims to provide a positive electrode composite material for an all-solid-state lithium-sulfur secondary battery that can be obtained with a longer cycle life than conventional all-solid-state lithium-sulfur secondary batteries, a positive electrode for an all-solid-state lithium-sulfur secondary battery containing the positive electrode composite material for an all-solid-state lithium-sulfur secondary battery, and an all-solid-state lithium-sulfur secondary battery containing the positive electrode for an all-solid-state lithium-sulfur secondary battery.

[0009] To solve the above problems, the present invention has the following embodiments: [1] A positive electrode material for an all-solid-state lithium-sulfur secondary battery comprising a sulfur compound synthesized by a reverse vulcanization method, a solid electrolyte, and a conductive additive, and which may also contain elemental sulfur, wherein the total content of the sulfur compound synthesized by the reverse vulcanization method and the elemental sulfur relative to the total mass of the positive electrode material for an all-solid-state lithium-sulfur secondary battery is 20 to 70% by mass, the content of the solid electrolyte is 10 to 60% by mass, and the content of the conductive additive is 20% by mass or less. [2] The positive electrode material for an all-solid-state lithium-sulfur secondary battery according to [1], wherein the crosslinking agent used in the reverse vulcanization method has 2 to 4 reaction sites. [3] The positive electrode material for an all-solid-state lithium-sulfur secondary battery according to [1], wherein the crosslinking agent used in the reverse vulcanization method contains either or both oxygen atoms and nitrogen atoms in its molecule. [4] The positive electrode material for an all-solid-state lithium-sulfur secondary battery according to [2] or [3], wherein the crosslinking agent comprises at least one crosslinking agent selected from the group consisting of 2,4,6-tris(allyloxy)-1,3,5-triazine, 1,3,5-trialyl-1,3,5-triazine-2,4,6(1H,3H,5H)-trione, 1,3,4,6-tetraallyl glycoluryl, and 1,3-diallyl-5-methyl-1,3,5-triazine-2,4,6-trione. [5] A positive electrode for an all-solid-state lithium-sulfur secondary battery comprising the positive electrode material for an all-solid-state lithium-sulfur secondary battery according to any one of [1] to [4]. [6] An all-solid-state lithium-sulfur secondary battery comprising the positive electrode for an all-solid-state lithium-sulfur secondary battery according to [5].

[0010] According to the present invention, it is possible to provide a positive electrode composite material for an all-solid-state lithium-sulfur secondary battery that can be obtained with a longer cycle life compared to conventional all-solid-state lithium-sulfur secondary batteries, a positive electrode for an all-solid-state lithium-sulfur secondary battery containing the positive electrode composite material for an all-solid-state lithium-sulfur secondary battery, and an all-solid-state lithium-sulfur secondary battery containing the positive electrode for an all-solid-state lithium-sulfur secondary battery.

[0011] This is a schematic diagram showing how to determine the Hansen solubility parameter value and the interaction radius. This is a schematic cross-sectional view showing an example of a stacked configuration in an all-solid-state lithium-sulfur secondary battery according to one embodiment of the present invention.

[0012] The embodiments of the present invention will be described in detail below, but the following description is merely one example of an embodiment of the present invention, and the present invention is not limited to these contents and can be modified and implemented within the scope of its gist.

[0013] ≪Positive Electrode Mixture for All-Solid-State Lithium-Sulfur Secondary Batteries≫ The positive electrode mixture for all-solid-state lithium-sulfur secondary batteries of this embodiment (hereinafter also simply referred to as "positive electrode mixture") comprises a sulfur compound synthesized by reverse vulcanization (hereinafter also simply referred to as "sulfur compound"), a solid electrolyte, and a conductive additive. The positive electrode mixture may also contain elemental sulfur. "The positive electrode mixture may contain elemental sulfur" means that the positive electrode mixture may or may not contain elemental sulfur. The total content of the sulfur compound and elemental sulfur relative to the total mass of the positive electrode mixture is 20 to 70% by mass, the content of the solid electrolyte is 10 to 60% by mass, and the content of the conductive additive is 20% by mass or less.

[0014] <Sulfur Compounds> Sulfur compounds are synthesized by the reverse vulcanization method. That is, the main component of sulfur compounds is a sulfur polymer. In addition to sulfur polymers, sulfur compounds may also contain low-molecular-weight sulfur oligomers produced by the reverse vulcanization reaction. Sulfur polymers contained in sulfur compounds synthesized by the reverse vulcanization method have the advantage of being able to easily increase the sulfur atom content compared to the sulfur-modified polyacrylonitriles mentioned above. A higher sulfur atom content tends to increase the energy density of the resulting all-solid-state lithium-sulfur secondary battery.

[0015] The reverse vulcanization method is a technique for obtaining polymers by adding a small amount of crosslinking agent to a large amount of molten sulfur to form continuous crosslinks. By using the reverse vulcanization method, it is easier to increase the sulfur atom content relative to the total mass of the sulfur compound. As a result, the energy density of the resulting all-solid-state lithium-sulfur secondary battery tends to be higher. However, in sulfur compounds synthesized by the reverse vulcanization method, it is difficult to determine how elemental sulfur is crosslinked by the crosslinking agent. In other words, it is impossible or impractical to directly identify sulfur compounds synthesized by the reverse vulcanization method based on their structure or properties.

[0016] (Elemental Sulfur) In the reverse vulcanization process, elemental sulfur used as a raw material usually exists as a cyclic molecule of S8. By heating, or by heating and the action of a catalyst, a portion of the molecular chain of elemental sulfur is cleaved, producing an 8-atom chain of sulfur. Furthermore, the 8-atom chain of sulfur may be further cleaved to produce a chain of sulfur with 7 or fewer atoms. The elemental sulfur used as a raw material is not particularly limited and may be produced from any raw material and by any manufacturing method. The crystalline form of sulfur is not particularly limited and may be α-sulfur (orthorhombic sulfur), β-sulfur (monoclinic sulfur), or γ-sulfur (monoclinic sulfur), or a mixture thereof.

[0017] (Crosslinking agent) The crosslinking agent has two or more reaction sites. The reaction sites of the crosslinking agent are, for example, reactive carbon-carbon double bonds and carbon-carbon triple bonds. A reactive carbon-carbon double bond refers to a carbon-carbon double bond located at the end of the molecule. The number of reaction sites (reactive carbon-carbon double bonds, etc.) of the crosslinking agent is two or more, preferably 2 to 5, more preferably 2 to 4, even more preferably 2 or 3, and particularly preferably 3. In one embodiment, the number of reaction sites of the crosslinking agent is preferably 3 or 4, and more preferably 4. Note that one carbon-carbon triple bond has two reaction sites. If the number of reaction sites of the crosslinking agent is above the lower limit, crosslinking proceeds easily. If the number of reaction sites of the crosslinking agent is below the upper limit, the flexibility of the resulting sulfur compound tends to improve.

[0018] In sulfur compounds synthesized by reverse vulcanization, a polymer structure is formed by the reaction of a linear sulfur chain with a crosslinking agent having two or more reaction sites. Therefore, it is presumed that the number of reaction sites of the crosslinking agent affects the flexibility of the resulting sulfur compound. For example, it is presumed that if there are many reaction sites in the crosslinking agent, the crosslinking density will be high, making the polymer more rigid, while conversely, if there are few reaction sites, the crosslinking density will be low, making the polymer more flexible. As described above, if the flexibility of the sulfur compound is improved, the sulfur compound in the cathode material will play a role in absorbing the volume change of the cathode material that occurs during charging and discharging, making the cathode material less prone to cracking, and thus extending the cycle life of the all-solid-state lithium-sulfur secondary battery. Therefore, it is presumed that if the number of reaction sites of the crosslinking agent is below the above upper limit, the cycle life of the all-solid-state lithium-sulfur secondary battery will be even longer.

[0019] The crosslinking agent is not particularly limited as long as it has two or more reaction sites, but in one embodiment, it may be a crosslinking agent having heteroatoms other than carbon atoms and hydrogen atoms in its molecule. Examples of heteroatoms include nitrogen atoms, oxygen atoms, and sulfur atoms, with nitrogen atoms and oxygen atoms being preferred. The number of heteroatoms in one molecule of the crosslinking agent is preferably 2 to 10, more preferably 3 to 8, and even more preferably 4 to 8. The number of nitrogen atoms in one molecule of the crosslinking agent is preferably 2 to 8, more preferably 2 to 6, and even more preferably 2 to 4. The number of oxygen atoms in one molecule of the crosslinking agent is preferably 2 to 8, more preferably 2 to 6, and even more preferably 2 to 4.

[0020] The molecular weight of the crosslinking agent is preferably 80 to 400, more preferably 100 to 350, and even more preferably 120 to 320. The molecular weight of the crosslinking agent is expressed as formula weight.

[0021] [Hansen Solubility Parameter] The value of the energy (δp) derived from intermolecular dipole interactions in the Hansen solubility parameter (hereinafter also simply referred to as "HSP") of crosslinking agents is 9 to 14 MPa. １／２ Preferably, 9.3 to 12 MPa １／２ More preferably, 9.5 to 10 MPa １／２This is particularly preferable. The crosslinking agent is incorporated into the sulfur compound by reacting with elemental sulfur, and its structure changes. However, the only part of the crosslinking agent that changes is basically the crosslinked portion, and the main skeleton is maintained. Therefore, the δp of the crosslinking agent is considered to reflect the δp of the crosslinking agent-derived structure in the sulfur compound.

[0022] The energy (δp) derived from the intermolecular dipole interaction of a specific crosslinking agent is calculated from the HSP. The Hansen solubility parameter is determined as follows:

[0023] The Hansen solubility parameter is based on the idea that two substances with similar intermolecular interactions readily dissolve in each other. The HSP consists of energy derived from intermolecular dispersion forces (δd), energy derived from intermolecular dipole interactions (δp), and energy derived from intermolecular hydrogen bonding (δh). These three parameters can be considered as coordinates in three-dimensional space (Hansen space).

[0024] The HSP value of an evaluation sample with an unknown HSP value can be calculated using the following method. By plotting the HSP values ​​(δdm, δpm, δhm) in three-dimensional space, the HSP value of the evaluation sample can be calculated by plotting multiple pure substances (substances consisting of one compound) with known HSP values ​​in the Hansen solubility parameter space, identifying the Hansen spheres based on the solubility of the evaluation sample in relation to the pure substances, and determining the center value of the Hansen sphere (Hansen sphere method). Alternatively, the HSP value of the evaluation sample can also be calculated using the group contribution method from information on the average molecular structure. In both the Hansen sphere method and the group contribution method, the HSP value of the evaluation sample can be calculated using, for example, the computer software Hansen Solubility Parameters in Practice (HSPiP). In the case of the Hansen sphere method, the evaluation sample may be a pure substance or a mixture.

[0025] The method for determining the center value of the Hansen sphere, i.e., the HSP value (δdm, δpm, δhm), will be explained using Figure 1. First, the HSP values ​​of approximately 15 to 30 pure substances with known HSP values ​​are plotted in a three-dimensional space (with the dispersion force term δd, the inter-dipole force term δp, and the hydrogen bonding force term δh as the coordinate axes) as illustrated in Figure 1. At this time, as shown in Figure 1, pure substances that show solubility in the evaluation sample are marked with a circle (○), and pure substances that do not show solubility in the evaluation sample are marked with an "x" (×). Next, based on the solubility of the plotted evaluation sample, the Hansen sphere S (shown as a sphere in Figure 1) is determined from among the virtual spheres that include the pure substances that showed solubility (shown with a circle in Figure 1) and do not include the pure substances that did not show solubility (shown with an "x" in Figure 1), and which has the smallest radius. The radius of the Hansen sphere S (the smallest radius above) is the interaction radius R, which dissolves the pure substances shown with a circle in the figure and shows compatibility. ０ The resulting Hansen sphere S center values ​​(δdm, δpm, δhm) become the HSP values ​​of the evaluation sample. For example, the HSP value of the pure substance used to determine the Hansen spheres is such that the dispersion force term δd is 10 to 25 MPa. １／２ The dipole force term δp is approximately 0 to 20 MPa. １／２ The hydrogen bonding force term δh is approximately 0 to 20 MPa. １／２ This is the extent of the problem. Examples of the above-mentioned pure substances include acetone, acetonitrile, aniline, benzyl alcohol, γ-butyrolactone, ethanol, n-hexane, toluene, N,N-dimethylacetamide, N-methylformamide, pyridine, quinoline, 1,1,2,2-tetrabromoethane, salicylaldehyde, N-methyl-2-pyrrolidone, N,N-dimethylformamide, dimethyl sulfoxide, 1-methylimidazole, and nitrobenzene. For example, 1 mL of the above-mentioned pure substance is added to 1 g of the crosslinking agent, left to stand at a predetermined temperature for 48 hours, and then the solubility is confirmed visually. Furthermore, since solubility depends on temperature, it is preferable to perform the solubility test at the temperature at which the crosslinking agent is actually dissolved when determining the above-mentioned Hansen spheres. For example, the dissolution temperature is 25°C.

[0026] [Structure of Crosslinking Agent] In one embodiment, the crosslinking agent preferably has an aromatic ring. The aromatic ring means a ring that satisfies Hückel's rule. For the number of aromatic rings, one closed ring is counted as one. For example, naphthalene is a compound having two aromatic rings. The number of aromatic rings in one molecule of the crosslinking agent is preferably 1 to 4, more preferably 1 to 3, and still more preferably 1 to 2.

[0027] The aromatic ring may be an aromatic ring containing a heteroatom in the ring structure. Examples of the aromatic ring containing a heteroatom in the ring structure include: aromatic rings containing a nitrogen atom in the ring structure such as pyridine, pyridazine, pyrimidine, pyrazine, pyrrole, pyrazole, imidazole, and 1,3,5-triazine; aromatic rings containing an oxygen atom in the ring structure such as furan; aromatic rings containing a sulfur atom in the ring structure such as thiophene; aromatic rings containing a nitrogen atom and a sulfur atom in the ring structure such as thiazole; and aromatic rings containing a nitrogen atom and an oxygen atom in the ring structure such as oxazole. Among these, an aromatic ring containing a nitrogen atom in the ring structure is preferred, and 1,3,5-triazine is more preferred. As a crosslinking agent having 1,3,5-triazine as an aromatic ring, a crosslinking agent represented by the following formula 1 is preferred. The crosslinking agent represented by the following formula 1 may be used alone, or two or more thereof may be used in combination.

[0028] In the above formula 1, R １ to R ３ each independently represent an alkenyloxy group having a carbon-carbon double bond at the molecular terminal with 2 to 15 carbon atoms, an alkyl group having 1 to 15 carbon atoms, an alkenyl group having 2 to 15 carbon atoms, an alkynyl group having 2 to 15 carbon atoms, an alkoxy group having 2 to 15 carbon atoms, an aryl group having 6 to 20 carbon atoms, an amino group, an amide group, an imino group, a carboxy group, a hydroxy group, an ester group, a glycidyl group, a nitro group, a nitrile group, a sulfide group, a mercapto group, a sulfonyl group, a silyl group, or a hydrogen atom. The alkenyloxy group, the alkyl group, the alkenyl group, the alkynyl group, and the alkoxy group may each independently be linear or branched, and may have a substituent. The aryl group may have a substituent. Provided that R １ to R ３Of these, two or more are alkenyloxy groups having a carbon-carbon double bond at the molecular end, with 2 to 15 carbon atoms. １ ~R ３ These may be the same or different.

[0029] The number of carbon atoms in the above alkenyloxy group is preferably 2 to 10, more preferably 2 to 8, and even more preferably 2 to 6. The above alkenyloxy group is preferably a straight chain. Examples of the above alkenyloxy group include vinyloxy group and allyloxy group, with allyloxy group being preferred.

[0030] Examples of substituents that the above-mentioned alkenyloxy group may have include aryl groups having 6 to 10 carbon atoms and halogen atoms. Note that the number of carbon atoms in the alkenyloxy group includes the number of carbon atoms in the substituents. Examples of aryl groups having 6 to 10 carbon atoms include phenyl groups and naphthyl groups.

[0031] The alkyl group, alkenyl group, alkynyl group, and alkoxy group described above are preferably linear. Examples of substituents that these groups may have include halogen atoms. Examples of alkyl groups include methyl group, ethyl group, propyl group, butyl group, pentyl group, and hexyl group. Examples of alkenyl groups include vinyl group, allyl group, propenyl group, isopropenyl group, 2-methyl-1-propenyl group, 2-methylallyl group, and 2-butenyl group. Examples of alkynyl groups include ethynyl group, 2-propynyl group, and 2-butynyl group. Examples of alkoxy groups include methoxy group, ethoxy group, propoxy group, and butoxy group.

[0032] Examples of substituents that the above aryl group may have include halogen atoms. Examples of the above aryl group include phenyl groups and naphthyl groups.

[0033] R １ ~R ３ Preferably, two or more of these are alkenyloxy groups having 2 to 15 carbon atoms and a carbon-carbon double bond at the molecular end, and all three are alkenyloxy groups having 2 to 15 carbon atoms and a carbon-carbon double bond at the molecular end.

[0034] Examples of crosslinking agents represented by the above formula 1 include 2,4,6-tris(allyloxy)-1,3,5-triazine, 2-methyl-4,6-bis(2-propene-1-yloxy)-1,3,5-triazine, 2,4-bis(2-propene-1-yloxy)-6-(2-propyne-1-yloxy)-1,3,5-triazine, 2-[[4,6-bis(2-propene-1-yloxy)-1,3,5-triazine-2-yl]oxy]ethanol, 2-phenyl-4,6-bis(2-propene-1-yloxy)-1,3,5-triazine, and 2-(2-oxyranylmethoxy)-4,6-bis(2-propene-1-yloxy)-1,3,5-triazine, with 2,4,6-tris(allyloxy)-1,3,5-triazine being preferred.

[0035] Furthermore, a crosslinking agent having an aromatic ring that does not contain heteroatoms in its ring structure may be used. Examples of such crosslinking agents include those in which an alkenyl group having a carbon-carbon double bond at the molecular terminus is bonded to an aromatic ring that does not contain heteroatoms in its ring structure. Examples of aromatic rings that do not contain heteroatoms include benzene rings and naphthalene rings. An example of an alkenyl group having a carbon-carbon double bond at the molecular terminus is the above R １ ~R ３ Among the alkenyl groups described above, alkenyl groups having a carbon-carbon double bond at the molecular terminus are exemplified, with vinyl groups, allyl groups, and isopropenyl groups being preferred. Examples of such crosslinking agents include divinylbenzene, diisopropenylbenzene, trivinylbenzene, and triisopropenylbenzene.

[0036] In one embodiment, the crosslinking agent preferably has an isocyanuric acid skeleton. A crosslinking agent having an isocyanuric acid skeleton means a crosslinking agent in which the hydrogen atom bonded to the nitrogen atom of isocyanuric acid is replaced by a substituent. As the crosslinking agent having an isocyanuric acid skeleton, the crosslinking agent represented by the following formula 2 is preferred. The crosslinking agent represented by the following formula 2 may be used alone or in combination of two or more types.

[0037] In the above formula 2, R ４ ~R ６Each of these is independently a substituent that can react with sulfur, an alkyl group having 1 to 15 carbon atoms, an alkoxy group having 2 to 15 carbon atoms, an aryl group having 6 to 20 carbon atoms, an amino group, an amide group, an imino group, a carboxyl group, a hydroxyl group, an ester group, a nitro group, a nitrile group, a sulfonyl group, a silyl group, or a hydrogen atom. The alkyl group and the alkoxy group may each independently be linear or branched and may have substituents. The aryl group may have substituents. However, R ４ ~R ６ Of these, two or more are substituents that can react with sulfur (however, if the substituent that can react with sulfur is an alkynyl group having a carbon-carbon triple bond at the molecular terminus, R ４ ~R ６ (It is sufficient if one or more of these are the above-mentioned alkynyl groups.) ４ ~R ６ These may be the same or different.

[0038] Examples of substituents that can react with sulfur include alkenyl groups having 2 to 15 carbon atoms and a carbon-carbon double bond at the molecular terminus, alkynyl groups having 2 to 15 carbon atoms, sulfide groups, mercapto groups, glycidyl groups, cyano groups, and carbonyl groups. Note that the above alkenyl groups and alkynyl groups may independently be linear or branched chains and may have substituents.

[0039] The number of carbon atoms in the above alkenyl group is preferably 2 to 10, more preferably 2 to 8, and even more preferably 2 to 6. The above alkenyl group is preferably a straight chain. Examples of the above alkenyl group include vinyl group and allyl group, with allyl group being preferred.

[0040] The number of carbon atoms in the alkynyl group is preferably 2 to 10, more preferably 2 to 8, and even more preferably 2 to 6. The alkynyl group is preferably in a straight chain. Furthermore, the carbon-carbon triple bond is preferably located at the molecular terminal. Examples of the alkynyl group include the ethynyl group and the propargyl group.

[0041] Examples of substituents that the above-mentioned alkenyl group and alkynyl group may have include aryl groups having 6 to 10 carbon atoms and halogen atoms. Note that the number of carbon atoms in the alkenyl group and alkynyl group includes the number of carbon atoms of the substituents. Examples of aryl groups having 6 to 10 carbon atoms include phenyl groups and naphthyl groups.

[0042] The alkyl group and alkoxy group described above are preferably linear. Examples of substituents that these groups may have include halogen atoms. Examples of substituents that the aryl group may have include halogen atoms. The alkyl group, alkoxy group, aryl group and glycidyl group described above are R １ ~R ３ The examples listed above are similarly illustrated.

[0043] Examples of crosslinking agents represented by the above formula 1 include 1,3-diallyl-5-methyl-1,3,5-triazine-2,4,6-trione, 1,3,5-trialyl-1,3,5-triazine-2,4,6(1H,3H,5H)-trione, 1-(2-methyl-2-propen-1-yl)-3,5-di-2-propen-1-yl-1,3,5-triazine-2,4,6(1H,3H,5H)-trione, and 1-methyl-3 ,5-di-2-propen-1-yl-1,3,5-triazine-2,4,6(1H,3H,5H)-trione,1,3-di-2-buten-1-yl-5-methyl-1,3,5-triazine-2,4,6(1H,3H,5H)-trione,1-(chloromethyl)-3,5-di-2-propen-1-yl-1,3,5-triazine-2,4,6(1H,3H,5H)-trione,1,3-di-2-buten- 1-yl-5-methyl-1,3,5-triazine-2,4,6(1H,3H,5H)-trione, 1,3,5-triethinyl-1,3,5-triazine-2,4,6(1H,3H,5H)-trione, 1-(2-hydroxyethyl)-3,5-di-2-propen-1-yl-1,3,5-triazine-2,4,6(1H,3H,5H)-trione, 1-(2-mercaptoethyl)-3,5-di-2-propen Ropin-1-yl-1,3,5-triazine-2,4,6(1H,3H,5H)-trione is an example, 1,3-diallyl-5-methyl-1,3,5-triazine-2,4,6-trione and 1,3,5-trialyl-1,3,5-triazine-2,4,6(1H,3H,5H)-trione are preferred, and 1,3-diallyl-5-methyl-1,3,5-triazine-2,4,6-trione is particularly preferred.

[0044] In one embodiment, the crosslinking agent preferably has a glycoluryl skeleton. A crosslinking agent having a glycoluryl skeleton means a crosslinking agent in which the hydrogen atom bonded to the nitrogen atom of glycoluryl is substituted with a substituent. As the crosslinking agent having a glycoluryl skeleton, the crosslinking agent represented by the following formula 3 is preferred. The crosslinking agent represented by the following formula 3 may be used alone or in combination of two or more types.

[0045] In the above formula 3, R ７ ~R１０ Each of these is independently a substituent that can react with sulfur, an alkyl group having 1 to 15 carbon atoms, an alkoxy group having 2 to 15 carbon atoms, an aryl group having 6 to 20 carbon atoms, an amino group, an amide group, an imino group, a carboxyl group, a hydroxyl group, an ester group, a nitro group, a nitrile group, a sulfonyl group, a silyl group, or a hydrogen atom. The alkyl group and the alkoxy group may each independently be linear or branched and may have substituents. The aryl group may have substituents. However, R ７ ~R １０ Of these, two or more are substituents that can react with sulfur (however, if the substituent that can react with sulfur is an alkynyl group having a carbon-carbon triple bond at the molecular terminus, R ７ ~R １０ (It is sufficient if one or more of these are the above-mentioned alkynyl groups.) ７ ~R １０ These may be the same or different.

[0046] Examples of substituents that can react with sulfur include alkenyl groups having 2 to 15 carbon atoms and a carbon-carbon double bond at the molecular terminus, alkynyl groups having 2 to 15 carbon atoms, sulfide groups, mercapto groups, glycidyl groups, cyano groups, and carbonyl groups. Note that the above alkenyl groups and alkynyl groups may independently be linear or branched chains and may have substituents.

[0047] The number of carbon atoms in the above alkenyl group is preferably 2 to 10, more preferably 2 to 8, and even more preferably 2 to 6. The above alkenyl group is preferably a straight chain. Examples of the above alkenyl group include vinyl group and allyl group, with allyl group being preferred.

[0048] The number of carbon atoms in the alkynyl group is preferably 2 to 10, more preferably 2 to 8, and even more preferably 2 to 6. The alkynyl group is preferably in a straight chain. Furthermore, the carbon-carbon triple bond is preferably located at the molecular terminal. Examples of the alkynyl group include the ethynyl group and the propargyl group.

[0049] Examples of substituents that the above-mentioned alkenyl group and alkynyl group may have include aryl groups having 6 to 10 carbon atoms and halogen atoms. Note that the number of carbon atoms in the alkenyl group and alkynyl group includes the number of carbon atoms of the substituents. Examples of aryl groups having 6 to 10 carbon atoms include phenyl groups and naphthyl groups.

[0050] The alkyl group and alkoxy group described above are preferably linear. Examples of substituents that these groups may have include halogen atoms. Examples of substituents that the aryl group may have include halogen atoms. The alkyl group, alkoxy group, aryl group and glycidyl group described above are R １ ~R ３ The examples listed above are similarly illustrated.

[0051] Examples of crosslinking agents represented by formula 3 above include 1,3,4,6-tetravinyl glycol uryl, the crosslinking agent represented by formula 4 below (1,3,4,6-tetraallyl glycol uryl), 1,3,4-trivinyl glycol uryl, 1,3,4-trivinyl-6-alkyl glycol uryl, 1,3,4-trialyl glycol uryl, 1,3,4-trialyl-6-alkyl glycol uryl, 1,3-divinyl glycol uryl, 1,4-divinyl glycol uryl, 1,6-divinyl glycol uryl, and 1,3-divinyl-4,6-dialkyl glycol Preferred crosslinking agents include choluryl, 1,4-divinyl-3,6-dialkylglycoluryl, 1,6-divinyl-3,4-dialkylglycoluryl, 1,3-diallylglycoluryl, 1,4-diallylglycoluryl, 1,6-diallylglycoluryl, 1,3-diallyl-4,6-dialkylglycoluryl, 1,4-diallyl-3,6-dialkylglycoluryl, and 1,6-diallyl-3,4-dialkylglycoluryl, with the crosslinking agent represented by formula 4 below (1,3,4,6-tetraarylglycoluryl) being particularly preferred. The alkyl group is preferably one of the alkyl groups mentioned above.

[0052]

[0053] (Method for producing sulfur compounds) The sulfur compounds of this embodiment can be produced by crosslinking sulfur and the above-mentioned crosslinking agent in a reactor. A catalyst may be used if necessary.

[0054] The catalyst is not particularly limited as long as it promotes the reaction between sulfur and the crosslinking agent, but examples include dithiocarbamate-based catalysts, which are salts of an anion represented by the following formula 5 and a metal cation. １１ R １２ NC(=S)S － Equation 5 In the above Equation 5, R １１ and R １２ Each of these is independently a linear or branched alkyl group having 1 to 6 carbon atoms. １１ and R １２ Each of these is preferably a linear alkyl group having 1 to 3 carbon atoms. Specifically, a methyl group, an ethyl group, or an n-propyl group is preferred. Examples of metal cations that are countercations of the anion represented by formula 5 include zinc ions, sodium ions, silver ions, iron ions, ammonium ions, cobalt ions, nickel ions, copper ions, lithium ions, and manganese ions. The valence of the metal cation is the same as the number of anions represented by formula 5. For example, when using a zinc ion with a valence of 2, two anions represented by formula 5 bond to the zinc ion. Examples of dithiocarbamate catalysts include zinc diethyldithiocarbamate, zinc dimethyldithiocarbamate, copper diethyldithiocarbamate, silver diethyldithiocarbamate, and sodium dimethyldithiocarbamate, with zinc diethyldithiocarbamate and copper diethyldithiocarbamate being preferred.

[0055] The amount of sulfur used is preferably 55 to 95% by mass, more preferably 60 to 90% by mass, and even more preferably 65 to 90% by mass, relative to the total mass of sulfur and crosslinking agent. If the amount of sulfur used is above the lower limit, the sulfur content of the resulting sulfur compound tends to be higher. If the amount of sulfur used is below the upper limit, the flexibility of the resulting sulfur compound tends to be higher.

[0056] The amount of crosslinking agent used is preferably 5 to 90 parts by mass, and more preferably 10 to 80 parts by mass, per 100 parts by mass of sulfur. If the amount of crosslinking agent used is above the lower limit, the flexibility of the resulting sulfur compound tends to increase. If the amount of crosslinking agent used is below the upper limit, the sulfur content of the resulting sulfur compound tends to increase.

[0057] As a crosslinking agent, the crosslinking agent represented by formula 1, the crosslinking agent represented by formula 2, and the crosslinking agent represented by formula 3 may be used in combination. When used in combination, the content of the crosslinking agent represented by formula 1 relative to the total mass of the crosslinking agents represented by formula 1, formula 2, and formula 3 is preferably 50% by mass or more and less than 100% by mass, and more preferably 80% by mass or more and less than 100% by mass.

[0058] The amount of catalyst used is preferably 0.1 to 5 parts by mass, and more preferably 0.5 to 3 parts by mass, per 100 parts by mass of the total amount of sulfur and crosslinking agent. When the amount of catalyst added is within the above range, the yield of the sulfur compound tends to improve.

[0059] The method of adding sulfur, crosslinking agent, and catalyst to the reaction vessel is not particularly limited. For example, sulfur, crosslinking agent, and catalyst may be added to the reaction vessel separately. Alternatively, sulfur, crosslinking agent, and catalyst may be mixed in a separate container beforehand and then added to the reaction vessel. Furthermore, these reaction materials may be stirred before heating, or heating may be started without stirring these reaction materials. It is preferable to stir after heating has started.

[0060] The reaction temperature is not particularly limited as long as it is the temperature at which sulfur and the crosslinking agent react, but is preferably 120 to 220°C, more preferably 140 to 190°C, and even more preferably 140 to 180°C. When the reaction temperature is within the above range, the reaction between sulfur and the crosslinking agent proceeds smoothly. In particular, when the reaction temperature is above the lower limit, the reaction between sulfur and the crosslinking agent proceeds smoothly. Furthermore, when the reaction temperature is below the upper limit, hydrogen atom abstraction reactions by sulfur radical species are less likely to occur, and decomposition reactions accompanied by the generation of sulfides such as hydrogen sulfide are suppressed.

[0061] The reaction time is preferably 0.5 to 24 hours, and more preferably 2 to 8 hours. The reaction is preferably carried out in air or in an inert atmosphere such as nitrogen, helium, or argon.

[0062] The sulfur compound obtained by the reaction may be washed if necessary. Examples of liquids that can be used for washing include water, methanol, ethanol, diethyl ether, tetrahydrofuran, dimethylformamide, dichloromethane, ethyl acetate, hexane, carbon disulfide, and toluene. When washing the sulfur compound, the process may be carried out under heating and reflux of an organic solvent.

[0063] <Solid Electrolyte> As the solid electrolyte, a solid electrolyte used in known all-solid-state lithium secondary batteries can be used. Examples of such solid electrolytes include inorganic electrolytes and organic electrolytes. Examples of inorganic electrolytes include oxide-based solid electrolytes, sulfide-based solid electrolytes, halide-based solid electrolytes, and hydride-based solid electrolytes. Examples of organic electrolytes include polymer-based solid electrolytes. Among these, sulfide-based solid electrolytes are preferred.

[0064] (Sulfide-based solid electrolytes) Examples of sulfide-based solid electrolytes include Li ２ S-P ２ S ５ Solid electrolytes containing Li ２ S-SiS ２ Solid electrolytes containing Li ２ S-GeS ２ Solid electrolytes containing Li ２ S-B ２ S ３ Solid electrolyte containing LiI-Si ２ S-P ２ S ５ Solid electrolyte containing LiI-Li ２ S-P ２ O ５ Solid electrolyte containing LiI-Li ３ PO ４ -P ２ S ５ Solid electrolytes containing Li ２ S-SnS ２ -P ２ S５ a solid electrolyte containing, and Li １０ GeP ２ S １２ a solid electrolyte containing, and argyrodite is exemplified. Among them, Li ２ S and P ２ S ５ a solid electrolyte containing is preferable.

[0065] Li ２ S-P ２ S ５ based on the total mass of the solid electrolyte containing, the content of Li ２ S and P ２ S ５ the total content is preferably 50 to 100% by mass, more preferably 70 to 100% by mass, and may be 100% by mass. Li ２ S-P ２ S ５ based on the total mass of the solid electrolyte containing, the content of Li ２ S is preferably 20 to 50% by mass, more preferably 25 to 45% by mass, and still more preferably 30 to 40% by mass. Li ２ S-P ２ S ５ based on the total mass of the solid electrolyte containing, the content of P ２ S ５ is preferably 50 to 80% by mass, more preferably 55 to 75% by mass, and still more preferably 60 to 70% by mass.

[0066] Li ２ S-P ２ S ５ as the solid electrolyte containing, Li ２ S-P ２ S ５ , Li ２ S-P ２ S ５ -LiI, Li ２ S-P ２ S ５ -LiCl, Li ２ S-P ２ S ５ -LiBr, Li ２ S-P ２ S ５ -LiI-LiBr, and the like.

[0067] <Conductive additives> Carbon materials are preferred as conductive additives. Examples of carbon materials include conductive carbon powders such as Ketjenblack and acetylene black, carbon nanofibers, carbon nanotubes, and graphene. Among these, Ketjenblack, acetylene black, and carbon nanofibers are preferred as carbon materials.

[0068] The primary particle size of the conductive additive is preferably 0.01 to 100 μm, and more preferably 0.01 to 10 μm.

[0069] The conductive additive may be formulated by mixing it with a sulfur compound. If the conductive additive is a carbon material, the conductive additive may be added during the manufacturing process of the sulfur compound to produce a carbon-compound sulfur compound.

[0070] (Method for producing carbon-complex sulfur compounds) Carbon-complex sulfur compounds can be produced in the above-mentioned method for producing sulfur compounds by heating raw materials containing sulfur, a crosslinking agent, and a catalyst, and adding a conductive additive to the raw materials once they have become fluid. Except for the addition of the conductive additive, the method can be carried out in substantially the same manner as the above-mentioned method for producing sulfur compounds.

[0071] The temperature of the raw materials when adding the conductive additive should be above the temperature at which the raw materials become fluid due to the melting of sulfur, preferably 120 to 220°C, more preferably 140 to 180°C, and even more preferably 140 to 160°C. It should be noted that at the time of adding the conductive additive, some reaction between sulfur and the crosslinking agent may have already occurred. However, as long as this does not hinder the dispersion of the conductive additive in the raw materials, some reaction between sulfur and the crosslinking agent in the raw materials before the carbon material is added is acceptable.

[0072] The conductive additive may be added all at once or in multiple additions. To achieve good dispersion, it is preferable to continue stirring after adding a predetermined amount of the conductive additive to further disperse it. Since the raw material is in a fluid state when the conductive additive is added, by adding the conductive additive to the raw material while stirring the fluid raw material, the conductive additive is well dispersed in the raw material. As a result, the raw material enters the pores of the conductive additive while maintaining uniformity and polymerizes, achieving both high yield and good dispersibility with the conductive additive. The raw material being in a fluid state means that the viscosity of the dissolved raw material is low and it can be easily stirred with a stirring bar using a magnetic stirrer or the like. An example of the viscosity of the fluid raw material is 10,000 mPa·s or less.

[0073] The amount of conductive additive added is not particularly limited, but is preferably 1 to 30 parts by mass, and more preferably 3 to 15 parts by mass, per 100 parts by mass of the total of sulfur and crosslinking agent. When the amount of conductive additive is within the above range, a uniform carbon-compound sulfur compound with a high sulfur content is easily obtained.

[0074] The addition of the conductive additive to the raw materials is carried out while the raw materials are in a flow state. The timing for ending the addition of the conductive additive to the raw materials varies depending on the reaction scale, and is appropriately selected while the raw materials are in a flow state. The temperature of the raw materials when the addition of the conductive additive to the raw materials is completed is preferably 120 to 220°C, more preferably 140 to 180°C, and even more preferably 140 to 160°C. If the temperature of the raw materials is below the above upper limit, the reaction between sulfur and the crosslinking agent will not proceed too far, the fluidity of the raw materials will not decrease easily, and the dispersibility of the conductive additive to the raw materials will not decrease easily.

[0075] The time from the start of adding the conductive additive to the raw material to the end of adding the conductive additive to the raw material is preferably 0 to 15 minutes, and more preferably 1 to 10 minutes. When the time is within the above range, a carbon-compound sulfur compound with high dispersibility of the conductive additive is more likely to be obtained.

[0076] It is preferable to add at least 50% by mass of the conductive additive to the raw material within 10 minutes from the start of adding the conductive additive to the raw material, more preferably at least 80% by mass, and even more preferably the entire amount. As a result, a carbon-compound sulfur compound with high dispersibility of the conductive additive is easily obtained.

[0077] It is preferable to add at least 50% by mass of the conductive additive to the raw material while the temperature of the raw material is 220°C or lower, more preferably at least 80% by mass, and even more preferably the entire amount. As a result, a carbon-compound sulfur compound with high dispersibility of the conductive additive is easily obtained.

[0078] When the viscosity of the raw materials is 10,000 mPa·s or less, it is preferable to add the entire amount of the conductive additive to the raw materials. As a result, a carbon-compound sulfur compound with high dispersibility of the conductive additive is easily obtained.

[0079] After adding a conductive additive to the raw materials, the reaction between sulfur and the crosslinking agent is carried out. The reaction temperature is preferably 120 to 220°C, and more preferably 130 to 200°C. The reaction time, reaction atmosphere, etc., can be set as appropriate and can be the same as in the method for producing the sulfur compound described above. Furthermore, the carbon-complexed sulfur compound may be washed after the reaction, similar to the method for producing the sulfur compound.

[0080] <Elemental Sulfur> The cathode mixture of this embodiment may also contain elemental sulfur. Examples of elemental sulfur included in the cathode mixture include elemental sulfur that remains unreacted in the synthesis of the above-mentioned sulfur compound. Elemental sulfur may also be added to the cathode mixture. According to the inventors of this application, the cycle life tends to be longer when the cathode mixture contains elemental sulfur in addition to the sulfur compound.

[0081] <Other Components> Other components other than sulfur compounds, solid electrolytes, conductive additives, and elemental sulfur contained in the positive electrode composite include, for example, the catalyst used in the reverse vulcanization method described above, unreacted crosslinking agents, binders, etc.

[0082] <Composition> The total content of sulfur compounds and elemental sulfur relative to the total mass of the positive electrode composite material is 20 to 70% by mass, more preferably 25 to 60% by mass, and even more preferably 30 to 50% by mass. When a carbon-compounded sulfur compound is used, the content of the sulfur compound refers to the amount obtained by subtracting the conductive additive in the carbon-compounded sulfur compound from the carbon-compounded sulfur compound. If the total content of sulfur compounds and elemental sulfur is above the lower limit, the proportion of positive electrode active material increases, and the energy density of the battery improves. If the total content of sulfur compounds and elemental sulfur is below the upper limit, the cycle characteristics during battery charging and discharging improve.

[0083] The solid electrolyte content relative to the total mass of the positive electrode composite material is 10 to 60% by mass, more preferably 25 to 55% by mass, and even more preferably 40 to 50% by mass. If the solid electrolyte content is above the lower limit, the cycle characteristics during battery charging and discharging are improved. If the solid electrolyte content is below the upper limit, the proportion of positive electrode active material increases, and the energy density of the battery is improved.

[0084] The content of the conductive additive relative to the total mass of the positive electrode composite material is 20% by mass or less, more preferably 1 to 15% by mass, and even more preferably 3 to 10% by mass. When a carbon-compound sulfur compound is used, the content of the conductive additive includes the amount of conductive additive in the carbon-compound sulfur compound. If the content of the conductive additive is above the lower limit, the cycle characteristics during battery charging and discharging improve. If the content of the conductive additive is below the upper limit, the proportion of positive electrode active material increases, improving the energy density of the battery.

[0085] When a carbon-compound sulfur compound is used as the sulfur compound, the content of the conductive additive contained in the carbon-compound sulfur compound relative to the total mass of the conductive additive contained in the carbon-compound sulfur compound and the conductive additive added to the positive electrode composite material is preferably 1 to 10% by mass, more preferably 2 to 10% by mass, and even more preferably 3 to 10% by mass.

[0086] The sulfur atom content relative to the total mass of the sulfur compound and elemental sulfur is preferably 55 to 95% by mass, more preferably 60 to 90% by mass, and even more preferably 65 to 85% by mass. If the sulfur atom content is above the lower limit, the energy density of the resulting all-solid-state lithium-sulfur secondary battery tends to increase. If the sulfur atom content is below the upper limit, the flexibility of the sulfur compound tends to improve. As a result, cracking of the sulfur compound due to volume changes during charging and discharging becomes less likely, and the cycle life of the all-solid-state lithium-sulfur secondary battery tends to be longer.

[0087] When the positive electrode mixture contains elemental sulfur, the content of elemental sulfur relative to the total mass of sulfur compounds and elemental sulfur is preferably 45% by mass or less, more preferably 42% by mass or less, and even more preferably 40% by mass or less. The content of elemental sulfur may be 20% by mass or more, 25% by mass or more, 30% by mass or more, or 35% by mass or more. The content of elemental sulfur is preferably 25 to 45% by mass, more preferably 30 to 42% by mass, and even more preferably 35 to 40% by mass. If the content of elemental sulfur is above the lower limit, the sulfur content in the positive electrode mixture increases, and as a result, the capacity of the all-solid-state lithium-sulfur secondary battery tends to increase. If the content of elemental sulfur is below the upper limit, the content of sulfur compounds in the positive electrode mixture increases, and as a result, the cycle life of the all-solid-state lithium-sulfur secondary battery tends to be longer.

[0088] The content of components other than sulfur compounds, conductive additives, solid electrolytes, and elemental sulfur, relative to the total mass of the positive electrode composite material, is preferably 5% by mass or less, more preferably 3% by mass or less, and even more preferably 1% by mass or less.

[0089] ≪Positive Electrode for All-Solid-State Lithium-Sulfur Secondary Battery≫ The positive electrode for the all-solid-state lithium-sulfur secondary battery of this embodiment (hereinafter also simply referred to as "positive electrode") includes the positive electrode composite material described above. The positive electrode composite material may form a positive electrode composite layer together with a binder in the positive electrode. The positive electrode composite layer may be in the form of a thin sheet. The positive electrode may consist only of the positive electrode composite layer, or it may consist of a positive electrode current collector and the positive electrode composite layer. If the positive electrode includes a positive electrode current collector, the positive electrode composite layer may be provided on only one side of the positive electrode current collector, or it may be provided on both sides.

[0090] <Positive electrode current collector> Examples of positive electrode current collectors include metal foil. Examples of metal foils include aluminum foil, stainless steel foil, and nickel foil. A carbon coating layer may also be formed on the surface of the positive electrode current collector. Furthermore, the positive electrode current collector may be processed into a mesh shape.

[0091] <Binding Agent> The positive electrode may contain a binding agent to bind the positive electrode active material, solid electrolyte, conductive additive, and positive electrode current collector, or to bind the positive electrode composite layer, which consists of the positive electrode active material, solid electrolyte, and conductive additive, to the positive electrode current collector. The binding agent can be contained within the positive electrode composite layer or applied between the positive electrode composite layer and the positive electrode current collector. Examples of such binding agents include carboxymethylcellulose, polyvinylidene fluoride (hereinafter also referred to as "PVDF"), polytetrafluoroethylene (hereinafter also referred to as "PTFE"), polyacrylic acid, fluororubber, and diene rubbers such as styrene-butadiene rubber. It is preferable that the binding agent contains a crystalline polymer with a melting point. It is preferable that the binding agent is a polymer containing fluorine. Examples of polymers containing fluorine include PVDF, PTFE, and fluororubber. The binder content relative to the total mass of the positive electrode composite material is 20% by mass or less, more preferably 1 to 15% by mass, and even more preferably 3 to 10% by mass.

[0092] ≪All-Solid-State Lithium-Sulfur Secondary Battery≫ The all-solid-state lithium-sulfur secondary battery of this embodiment (hereinafter also simply referred to as "secondary battery") includes the positive electrode described above. The secondary battery further includes a solid electrolyte and a negative electrode. Figure 2 is a schematic cross-sectional view showing an example of a stacked configuration in a secondary battery according to one embodiment. The stacked body 1 consists of a positive electrode 13, a solid electrolyte layer 17, and a negative electrode 16 stacked in this order. The positive electrode 13 consists of a positive electrode current collector 11 and a positive electrode composite material layer 12 provided on the surface of the positive electrode current collector 11. In Figure 2, the positive electrode composite material layer 12 is provided on only one side of the positive electrode current collector 11, but it may be provided on both sides. The positive electrode 13 may also consist only of the positive electrode composite material layer 12. The negative electrode 16 consists of a negative electrode current collector 14 and a negative electrode composite material layer 15 provided on the surface of the negative electrode current collector 14. In Figure 2, the negative electrode composite layer 15 is provided on only one side of the negative electrode current collector 14, but it may be provided on both sides. Also, in Figure 2, only one positive electrode 13 and one negative electrode 16 are included, but an electrode group in which multiple positive electrodes 13 and negative electrodes 16 are stacked alternately may also be used. In that case as well, an electrolyte layer 17 is provided between the positive electrode 13 and the negative electrode 16.

[0093] <Negative Electrode Current Collector> As described above, the sulfur compound, which is the positive electrode active material contained in the positive electrode composite layer 12 of the positive electrode 13 in this embodiment, does not contain lithium. Therefore, in the laminate 1, lithium must be included in the negative electrode 16. Accordingly, the negative electrode current collector 14 or the negative electrode active material 15 must contain lithium. Examples of negative electrode current collectors 14 include metal foil. Examples of metal foils include lithium foil, lithium alloy foil, and copper foil. The negative electrode current collector 14 may also be processed into a mesh shape.

[0094] <Negative Electrode Mixture Layer> The negative electrode mixture layer 15 contains a negative electrode active material. The negative electrode mixture layer 15 may also contain a conductive additive and a binder as needed.

[0095] The negative electrode active material is not particularly limited as long as it is capable of intercalating and releasing lithium ions. Examples of negative electrode active materials include lithium metal and lithium alloy. Alternatively, lithium-free materials that are pre-doped with lithium may also be used. Examples of lithium-free materials include graphite (artificial graphite, natural graphite), amorphous carbon (hard carbon), mesocarbon microbeads, carbon fibers, and Si materials (silicon, Si alloys, Si oxides).

[0096] As a conductive additive, the conductive additive described above for the positive electrode composite material can be used.

[0097] The binder binds the negative electrode active material, solid electrolyte, conductive additive, and negative electrode current collector 14, respectively. Alternatively, it binds the negative electrode composite material, consisting of the negative electrode active material, solid electrolyte, and conductive additive, to the negative electrode current collector 14. Examples of binders include carboxymethylcellulose, PVDF, PTFE, polyacrylic acid, fluororubber, and diene rubbers such as styrene-butadiene rubber. It is preferable that the binder contains a crystalline polymer with a melting point. It is preferable that the binder is a polymer containing fluorine. Examples of polymers containing fluorine include PVDF, PTFE, and fluororubber. The binder content relative to the total mass of the negative electrode composite material is 20% by mass or less, more preferably 1 to 15% by mass, and even more preferably 3 to 10% by mass.

[0098] (Electrode Tabs) In order to extract current to the outside of the battery, the positive electrode current collector 11 and the negative electrode current collector 14 may each be connected to electrode tabs. The electrode tabs are electrically connected to these current collectors and are extracted to the outside of the outer casing of the laminate 1, for example.

[0099] The material constituting the electrode tab is not particularly limited, but known highly conductive materials conventionally used as electrode tabs are preferred. Preferred materials for the electrode tab include metallic materials such as aluminum, copper, titanium, nickel, stainless steel, and alloys thereof, and more preferably aluminum, copper, etc., from the viewpoint of lightness, corrosion resistance, and high conductivity. Another method involves processing the current collector into a tab shape by laser cutting and using it as an electrode tab.

[0100] (Outer casing) The laminated body 1 is housed in an outer casing (not shown). As the outer casing, a known metal can case can be used, or a bag-shaped case made of a laminate film containing aluminum may be used. For the laminate film, for example, a three-layer laminate film in which polypropylene, aluminum, and nylon are laminated in that order may be used.

[0101] Positive and negative electrode terminal leads (neither shown) connected to the electrode tabs may also be used as needed. The materials for the positive and negative electrode terminal leads can be those of known origin. It is preferable that the portions removed from the outer casing are covered with heat-resistant, heat-shrinkable tubing or the like. Furthermore, in wound-type all-solid-state lithium-sulfur secondary batteries, instead of electrode tabs, for example, cylindrical cans (metal cans) may be used to form the terminals.

[0102] (Solid Electrolyte) As the solid electrolyte of the solid electrolyte layer 17, the solid electrolyte described in the positive electrode composite material section can be used.

[0103] The present invention will be described in more detail below with reference to examples and comparative examples, but the present invention is not limited to the following examples.

[0104] <Cycle Characteristics> (Preparation of Solid Electrolyte) 75 mol% lithium sulfide (manufactured by Mitsuwa Chemical Co., Ltd.) and 25 mol% phosphorus pentasulfide (manufactured by Sigma-Aldrich) were measured into a mortar in a glove box and mixed for 20 minutes. The mixture and an organic solvent (heptane, etc.) were placed in a zirconia pot and mixed repeatedly 60 times using a planetary ball mill (240 rpm, 30 min). The organic solvent (heptane, etc.) was then removed by heating in a glove box at 140°C for 5 hours. The mixture was then sieved sequentially using an ultrasonic vibrating sieve (manufactured by Tokyo Screen Co., Ltd.) with a sieve with a mesh size of 45 μm and then with a mesh size of 25 μm, and the powder that passed through the sieve with a mesh size of 25 μm was obtained as the LPS-based solid electrolyte.

[0105] (Preparation of Cathode Mixture) The preparation of the cathode mixture shall be carried out based on either (Preparation of Cathode Mixture 1) or (Preparation of Cathode Mixture 2) below.

[0106] (Preparation of Cathode Composition Material 1) The carbon-compound sulfur pulverized product of Production Example 2 described below, or the carbon-compound sulfur compound pulverized products of Production Examples 1, 4, and 5, the above-mentioned LPS-based solid electrolyte, and the conductive additive Ketjenblack were measured in glove boxes in the proportions shown in Table 1. Note that the proportion of conductive additive in Table 1 includes the conductive additive contained in the carbon-compound sulfur pulverized product or carbon-compound sulfur compound pulverized product. The proportion of cathode active material is the carbon-compound sulfur compound pulverized product minus the conductive additive contained in the carbon-compound sulfur compound pulverized product. The carbon-compound sulfur pulverized product or carbon-compound sulfur compound pulverized product and the conductive additive were placed in a zirconia pot and mixed in a planetary ball mill at 370 rpm for 180 minutes. Then, the LPS-based solid electrolyte was added and mixed further at 370 rpm for 120 minutes to prepare the cathode composition material.

[0107] (Preparation of Cathode Mixture 2) The sulfur-modified polyacrylonitrile from Production Example 3 described below, the above-mentioned LPS-based solid electrolyte, and the conductive additive Ketjenblack were measured into an agate mortar in a glove box in the proportions shown in Table 1, mixed approximately 200 times, and then stirred in a vortex mixer for 3 minutes to prepare the cathode mixture. When the cathode mixture was prepared using the sulfur-modified polyacrylonitrile from Production Example 3 in (Preparation of Cathode Mixture 1) above, the initial discharge capacity was extremely small, so the preparation conditions were reviewed and (Preparation of Cathode Mixture 2) was adopted.

[0108] (Cycle Test 1) The cycle life of Examples 1 and 2, and Comparative Examples 1 and 2, described below, was evaluated by Cycle Test 1. Specifically, 5 mg of positive electrode composite material, 80 mg of the above-mentioned LPS-based solid electrolyte, indium foil (thickness: 0.3 mm), and lithium foil (thickness: 0.2 mm) were placed in this order on an all-solid-state measurement cell manufactured by Osaka Glass Co., Ltd., and an all-solid-state cell was obtained by uniaxial press. Electrochemical measurements were performed using an ELF1024-VC001 manufactured by Electrofield Co., Ltd. The cell was charged from 0.4 V to 2.4 V with a constant current of 0.05 C (1 C = 1672 mAh / g), and then discharged from 2.4 V to 0.4 V with a constant current of 0.05 C. One charge-discharge cycle was defined as one cycle. The charging and discharging were performed at 30°C. The discharge capacity at the 200th cycle is shown in Table 1. Note that the discharge capacity in Table 1 is the discharge capacity per unit mass of sulfur, obtained by dividing the discharge capacity by the mass of sulfur.

[0109] (Cycle Test 2) The cycle life of Examples 3 to 5 and Comparative Examples 3 and 4, described below, was evaluated by Cycle Test 2. Specifically, 2.5 mg of positive electrode composite material, 80 mg of the above-mentioned LPS-based solid electrolyte, indium foil (thickness: 0.3 mm), and lithium foil (thickness: 0.2 mm) were placed in this order on an all-solid-state measurement cell manufactured by Osaka Glass Co., Ltd., and an all-solid-state cell was obtained by uniaxial press. Electrochemical measurements were performed using an ELF1024-VC001 manufactured by Electrofield Co., Ltd. For Examples 3 to 5 and Comparative Example 3, the cells were charged from 0.4 V to 2.4 V with a constant current of 0.1 C (1 C = 1672 mAh / g), and then discharged from 2.4 V to 0.4 V with a constant current of 0.1 C. This charge-discharge cycle was considered one cycle. For Comparative Example 4, the battery was charged from 0.4V to 2.4V with a constant current of 0.05C (1C = 1672mAh / g), and then discharged from 2.4V to 0.4V with a constant current of 0.05C. This charge-discharge cycle was considered one cycle. The charging and discharging were performed at 30°C. The discharge capacity at the 150th cycle is shown in Table 2. Note that the discharge capacity in Table 2 is the discharge capacity per unit sulfur mass, obtained by dividing the discharge capacity by the mass of sulfur.

[0110] <Measurement of Composite Elastic Modulus> The carbon-compound sulfur pulverized product from Manufacturing Example 2 (described below), and the carbon-compound sulfur compound pulverized products from Manufacturing Examples 1, 4, and 5 were embedded in epoxy resin and cross-sectioned using an ultramicrotome. The composite elastic modulus was measured using a HYSITRON TI Premier Multi Scale under the following conditions: Berkovich indenter (diamond), maximum load: 250 μN, loading / unloading time: 5 seconds, maximum load holding time: 2 seconds. A higher composite elastic modulus indicates greater hardness. Five measurements were taken, and the averages are shown in Tables 1 and 2.

[0111] <Measurement of energy (δp) derived from intermolecular dipole interactions> For each crosslinking agent, 1 mL each of 19 pure substances with known HSP values ​​was added at a temperature of 25°C, and the mixture was allowed to stand for 48 hours. Solubility was then evaluated visually. The 19 pure substances used were acetone, acetonitrile, aniline, benzyl alcohol, γ-butyrolactone, ethanol, n-hexane, toluene, N,N-dimethylacetamide, N-methylformamide, pyridine, quinoline, 1,1,2,2-tetrabromoethane, salicylaldehyde, N-methyl-2-pyrrolidone, N,N-dimethylformamide, dimethyl sulfoxide, 1-methylimidazole, and nitrobenzene. Based on the obtained solubility evaluation results, the center value (HSP value) of Hansen spheres for each crosslinking agent was determined using the computer software Hansen Solubility Parameters in Practice (HSPiP), and the energy (δp) derived from intermolecular dipole interactions is shown in Table 2.

[0112] [Manufacturing Example 1] 11.9 g of sulfur (manufactured by Kanto Chemical Co., Ltd.), 2.1 g of 2,4,6-tris(allyloxy)-1,3,5-triazine (manufactured by Sigma-Aldrich), and 0.14 g of zinc diethyldithiocarbamate (manufactured by Tokyo Chemical Industry Co., Ltd.) were weighed and added to a polyfluoroalkoxy resin container to serve as the reaction raw materials. The reaction raw materials were then lightly mixed with a spatula, a stirring bar was placed in the container, and the mixture was heated in an oil bath at 170°C while stirring with a magnetic stirrer. Time was measured starting from the point when the thermocouple in the reaction raw materials showed 140°C. After heating for 3 minutes, the stirring bar was removed. The temperature at this time was 152°C. Next, 0.74 g of Ketjenblack (porous hollow carbon black, manufactured by Lion Specialty Chemicals, particle size: 34 nm) was added and quickly mixed with a spatula. Next, the mixture was kneaded by hand using the flat part of a spatula to ensure uniform mixing, and preheated at 170°C for 10-15 minutes. Then, the container was removed from the oil bath and heated at 140°C for 6 hours in a constant temperature oven under a nitrogen atmosphere. After heating, it was allowed to cool naturally to room temperature under a nitrogen atmosphere to obtain the carbon-compound sulfur compound. The obtained carbon-compound sulfur compound was placed in an agate mortar and pre-ground with a pestle, then ground under liquid nitrogen freezing conditions using a freeze grinder (AS ONE Corporation, HTPT-01). The ground product was then classified using a sieve with a mesh size of 100 μm, and particles that did not pass through the sieve were removed to obtain the carbon-compound sulfur compound pulverized product, which is the positive electrode active material. The particle size distribution of the obtained carbon-compound sulfur compound pulverized product was measured using a particle size distribution analyzer (Shimadzu Corporation, SALD-7500) by laser diffraction and scattering method, and the volume-based median diameter was 30.0 μm. In Tables 1 and 2, carbon-complex sulfur compounds are denoted as "C+S compound (1)". Note that in Tables 1 and 2, sulfur content 1 represents the sulfur content of the raw materials relative to the total mass of the raw materials used in the production of the carbon-complex sulfur compound, while sulfur content 2 represents the sulfur content of the raw materials relative to the total mass obtained by subtracting Ketjenblack from the carbon-complex sulfur compound.

[0113] [Manufacturing Example 2] 14.0 g of sulfur (manufactured by Kanto Chemical Co., Ltd.) was weighed and added to a polyfluoroalkoxy resin container as the reaction material. Next, a stirring bar was placed in the container and heated in an oil bath at 170°C while stirring with a magnetic stirrer. The time was measured starting from when the thermocouple in the reaction material showed 140°C. After heating for 3 minutes, the stirring bar was removed. Next, 0.74 g of Ketjenblack (porous hollow carbon black, manufactured by Lion Specialty Chemicals, primary particle size 34 nm) was added and quickly mixed with a spatula. Next, preheating was performed at 170°C for 10 to 15 minutes while kneading by hand with the flat part of the spatula to ensure uniform mixing. Next, the container was removed from the oil bath and heated in a constant temperature oven under a nitrogen atmosphere for 6 hours. After heating was complete, it was allowed to cool naturally to room temperature under a nitrogen atmosphere to obtain carbon-compounded sulfur. The obtained carbon-compound sulfur was crushed and classified in the same manner as in Production Example 1, and the particle size distribution was measured. A crushed carbon-compound sulfur product, which is a positive electrode active material with a volume-based median diameter of 12.6 μm, was obtained. In Tables 1 and 2, the crushed carbon-compound sulfur product is referred to as "C + S elemental". In Tables 1 and 2, sulfur content 1 is the sulfur content of the raw materials relative to the total mass of the raw materials used in the production of the carbon-compound sulfur compound, and sulfur content 2 is the sulfur content of the raw materials relative to the total mass obtained by subtracting Ketjenblack from the carbon-compound sulfur compound.

[0114] [Manufacturing Example 3] 0.5 g of polyacrylonitrile (manufactured by Aldrich) and 1.0 g of sulfur (manufactured by Hosoi Chemical Co., Ltd.) were measured into an agate mortar and mixed approximately 100 times to obtain a mixture. The mixture was placed in a graphite crucible, a carbon lid was placed over the graphite crucible, and the mixture was placed in a quartz Tamman tube. A silicone stopper with an inlet and outlet for displacement gas was then attached. The quartz Tamman tube was placed in an electric furnace and heated to 340°C over 40 minutes while flowing argon gas, and then heated to 400°C over a further 20 minutes. The heating was maintained at 400°C for 1 hour, and then cooled to room temperature to obtain sulfur-modified polyacrylonitrile, a black-green powder that served as the positive electrode active material. In Tables 1 and 2, sulfur-modified polyacrylonitrile is denoted as "SPAN". The sulfur content of the obtained sulfur-modified polyacrylonitrile was measured using LECO S632. The results are shown in Table 1. Note that in Tables 1 and 2, the sulfur content 1 represents the sulfur content relative to the total mass of sulfur-modified polyacrylonitrile. Since sulfur-modified polyacrylonitrile does not contain Ketjenblack, the sulfur content 2 in Tables 1 and 2 is the same as the sulfur content 1.

[0115] [Examples 1 and 2, Comparative Examples 1 and 2] Following the above (Preparation of positive electrode composite), positive electrode composites with the configurations shown in Table 1 were prepared, and the above (Cycle Test 1) was carried out. The discharge capacity per unit sulfur mass at 200 cycles is shown in Table 1.

[0116] As shown in Table 1, the discharge capacity after 200 cycles was higher in Examples 1 and 2, which used sulfur compounds synthesized by reverse vulcanization, compared to Comparative Example 1, which used only elemental sulfur as the positive electrode active material. This result was thought to be due to the fact that the sulfur compounds synthesized by reverse vulcanization are softer than elemental sulfur. In Comparative Example 2, which used a polymer (SPAN) similar to Examples 1 and 2, the discharge capacity after 200 cycles was actually lower than in Comparative Example 1, which used only elemental sulfur.

[0117] [Manufacturing Example 4] 11.9 g of sulfur (manufactured by Kanto Chemical Co., Ltd.), 2.1 g of 1,3,4,6-tetraallyl glycoluryl (manufactured by Shikoku Chemicals Co., Ltd.), and 0.14 g of zinc diethyldithiocarbamate (manufactured by Tokyo Chemical Industry Co., Ltd.) were weighed and added to a polyfluoroalkoxy resin container to form the reaction raw materials. The reaction raw materials were then lightly mixed with a spatula, a stirring bar was placed in the container, and the mixture was heated in an oil bath at 170°C while stirring with a magnetic stirrer. The time was measured starting from when the thermocouple in the reaction raw materials showed 140°C. After heating for 3 minutes, the stirring bar was removed. Next, 0.74 g of Ketjenblack (porous hollow carbon black, manufactured by Lion Specialty Chemicals, particle size: 34 nm) was added and quickly mixed with a spatula. Next, the mixture was kneaded by hand using the flat part of a spatula to ensure uniform mixing, and preheated at 170°C for 10-15 minutes. Then, the container was removed from the oil bath and heated in a constant temperature oven at 140°C for 6 hours under a nitrogen atmosphere. After heating, it was allowed to cool naturally to room temperature under a nitrogen atmosphere to obtain the carbon-compound sulfur compound. The obtained carbon-compound sulfur was crushed and classified in the same manner as in Production Example 1, and the particle size distribution was measured to obtain a crushed carbon-compound sulfur product, which is a positive electrode active material with a volume-based median diameter of 26 μm. In Table 2, the carbon-compound sulfur compound is denoted as "C+S compound (2)". The sulfur content of the obtained crushed carbon-compound sulfur product is shown in Table 2. Note that Sulfur content 1 in Table 2 is the sulfur content of the raw material relative to the total mass of the raw materials used in the production of the carbon-compound sulfur compound, and Sulfur content 2 in Table 2 is the sulfur content of the raw material relative to the total mass obtained by subtracting Ketjenblack from the carbon-compound sulfur compound.

[0118] [Manufacturing Example 5] 11.9 g of sulfur (manufactured by Kanto Chemical Co., Ltd.), 2.1 g of 1,3-diallyl-5-methyl-1,3,5-triazine-2,4,6-trione (manufactured by Shikoku Chemicals Co., Ltd.), and 0.14 g of zinc diethyldithiocarbamate (manufactured by Tokyo Chemical Industry Co., Ltd.) were weighed and added to a polyfluoroalkoxy resin container to form the reaction raw materials. The reaction raw materials were then lightly mixed with a spatula, a stirring bar was placed in the container, and the mixture was heated in an oil bath at 170°C while stirring with a magnetic stirrer. The time was measured starting from when the thermocouple in the reaction raw materials showed 140°C. After heating for 3 minutes, the stirring bar was removed. Next, 0.74 g of Ketjenblack (porous hollow carbon black, manufactured by Lion Specialty Chemicals, particle size: 34 nm) was added and quickly mixed with a spatula. Next, the mixture was kneaded by hand using the flat part of a spatula to ensure uniform mixing, and preheated at 170°C for 10-15 minutes. Then, the container was removed from the oil bath and heated at 140°C for 6 hours in a constant temperature oven under a nitrogen atmosphere. After heating, it was allowed to cool naturally to room temperature under a nitrogen atmosphere to obtain the carbon-compound sulfur compound. The obtained carbon-compound sulfur compound was placed in an agate mortar and pre-ground with a pestle, then ground using a small grinder (M-Tech Chemical Co., Ltd., MJM1), and the ground product was classified using a sieve with a mesh size of 45 μm to remove particles that did not pass through the sieve, thereby obtaining the carbon-compound sulfur compound pulverized product, which is the positive electrode active material. The particle size distribution of the obtained carbon-compound sulfur compound pulverized product was measured using a particle size distribution analyzer (Shimadzu Corporation, SALD-7500) by laser diffraction and scattering method, and the volume-based median diameter was 17 μm. The pulverized carbon-compound sulfur compound described above was further washed with methanol and then dried under reduced pressure. In Table 2, the carbon-compound sulfur compound from which the unreacted crosslinking agent has been removed is denoted as "C+S compound (3)". The sulfur content of the obtained pulverized carbon-compound sulfur compound is shown in Table 2. Note that Sulfur content 1 in Table 2 is the sulfur content of the raw materials relative to the total mass of the raw materials used in the production of the carbon-compound sulfur compound, and Sulfur content 2 in Table 2 is the sulfur content of the raw materials relative to the total mass obtained by subtracting Ketjenblack from the carbon-compound sulfur compound.

[0119] [Examples 3-5, Comparative Examples 3 and 4] Following the above (Preparation of positive electrode composite), positive electrode composites with the configurations shown in Table 2 were prepared, and the above (Cycle Test 2) was carried out. The discharge capacity per unit sulfur mass at 150 cycles is shown in Table 2.

[0120] As shown in Table 2, the discharge capacity after 150 cycles was higher in Examples 3-5, which used sulfur compounds synthesized by reverse vulcanization, compared to Comparative Example 3, which used only elemental sulfur as the positive electrode active material. This result was thought to be due to the fact that sulfur compounds synthesized by reverse vulcanization are softer than elemental sulfur. In addition, in the comparison of Examples 3-5, the capacity after 150 cycles was higher when a crosslinking agent with a higher δp was used. This is thought to be because when a crosslinking agent with a higher δp is used, Li ions are more strongly attracted during the initial discharge, forming a stable structure. Furthermore, the discharge capacity after 150 cycles for Examples 3-5 was higher than that of Comparative Example 4, which also used a polymer (SPAN). Comparing Comparative Examples 1 and 3, which used only C+S, Comparative Example 3 had a significantly smaller discharge capacity. This was thought to be partly due to the fact that charging and discharging was performed at 0.1C in Comparative Example 3, while charging and discharging was performed at 0.05C in Comparative Example 1. In other words, it was suggested that the use of only C+S is greatly affected by the charge-discharge rate.

[0121] The positive electrode composite material for all-solid-state lithium-sulfur secondary batteries of the present invention is useful because it allows for the production of all-solid-state lithium-sulfur secondary batteries with a longer cycle life compared to conventional materials.

[0122] DESCRIPTION OF SYMBOLS 1... Secondary battery, 11... Positive electrode current collector, 12... Positive electrode composite material layer, 13... Positive electrode, 14... Negative electrode current collector, 15... Negative electrode composite material layer, 16... Negative electrode, 17... Solid electrolyte layer

Claims

1. A positive electrode material for an all-solid-state lithium-sulfur secondary battery comprising a sulfur compound synthesized by reverse vulcanization, a solid electrolyte, and a conductive additive, which may also contain elemental sulfur, wherein the total content of the sulfur compound synthesized by reverse vulcanization and the elemental sulfur relative to the total mass of the positive electrode material for an all-solid-state lithium-sulfur secondary battery is 20 to 70% by mass, the content of the solid electrolyte is 10 to 60% by mass, and the content of the conductive additive is 20% by mass or less.

2. The crosslinking agent used in the reverse vulcanization method has 2 to 4 reaction sites, as described in claim 1, for a positive electrode composite material for an all-solid-state lithium-sulfur secondary battery.

3. The crosslinking agent used in the reverse vulcanization method comprises either or both oxygen atoms and nitrogen atoms in its molecule, as described in claim 1, for a positive electrode composite for an all-solid-state lithium-sulfur secondary battery.

4. The positive electrode composite material for an all-solid-state lithium-sulfur secondary battery according to claim 2, wherein the crosslinking agent comprises at least one crosslinking agent selected from the group consisting of 2,4,6-tris(allyloxy)-1,3,5-triazine, 1,3,5-trialyl-1,3,5-triazine-2,4,6(1H,3H,5H)-trione, 1,3,4,6-tetraallyl glycoluryl, and 1,3-diallyl-5-methyl-1,3,5-triazine-2,4,6-trione.

5. The positive electrode composite material for an all-solid-state lithium-sulfur secondary battery according to claim 3, wherein the crosslinking agent comprises at least one crosslinking agent selected from the group consisting of 2,4,6-tris(allyloxy)-1,3,5-triazine, 1,3,5-trialyl-1,3,5-triazine-2,4,6(1H,3H,5H)-trione, 1,3,4,6-tetraallyl glycoluryl, and 1,3-diallyl-5-methyl-1,3,5-triazine-2,4,6-trione.

6. A positive electrode for an all-solid-state lithium-sulfur secondary battery, comprising the positive electrode composite material for an all-solid-state lithium-sulfur secondary battery described in any one of claims 1 to 5.

7. An all-solid-state lithium-sulfur secondary battery comprising the positive electrode for an all-solid-state lithium-sulfur secondary battery as described in claim 6.

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