Sulfur-containing polymer

Abstract

To provide a high-sulfur-content polymer that is applicable to a positive electrode for a secondary battery, particularly a metal-lithium secondary battery, and is soluble in an organic solvent, and to provide a positive electrode active material for a secondary battery that exhibits superior electrochemical characteristics over common elemental sulfur (S8).SOLUTION: A sulfur-containing polymer of the present invention has a repeating unit represented by the following formula (I). (In the formula, Y represents S(O)m, -O-, -COO-, -OCO-, -NH-, -NHCO-, or -CONH-; m represents an integer of 0 to 2; Z represents a C1-C6 alkyl group optionally having a halogeno group as a substituent; R represents a hydrogen atom or a methyl group; X represents a positive integer of 1 or more, where the number of X in each unit may be different; and n represents an integer of 0 to 10.)SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a sulfur-containing polymer with a high content obtained by reacting molecular sulfur (S8) with an alkenyl compound, and to a positive electrode and a secondary battery containing the sulfur-containing polymer. [Background technology]

[0002] Sulfur (S8) is a natural resource that exists in abundance as a ring of eight sulfur atoms, but it is also a by-product that is produced in large quantities during the desulfurization process in petroleum refining. For this reason, there is a global demand, including in Japan, for the creation of technologies and fields that can convert sulfur into a more useful resource. One sulfur molecule can utilize 16 electrons electrochemically (16-electron reaction), enabling the storage of 1675 mAh / g of electrical energy (high capacity). Therefore, in recent years, research has been actively conducted on lithium-sulfur batteries (Li-S batteries), which use sulfur as the positive electrode active material and lithium metal as the negative electrode. These batteries are expected to have a higher energy density than lithium-ion secondary batteries (LIBs, energy density 200 Wh / kg). However, there are several remaining challenges to their practical application: the leaching of sulfur components into the electrolyte, the insulating properties of sulfur, the high cost of lithium, and the limited (short) cycle life due to the formation of dendrites (needle-shaped precipitates) at the Li-anode interface. Against this background, functional materials based on sulfur have been reported in recent years. The common technology in these reports is "inverse vulcanization," which involves adding a small amount of vinyl monomer to sulfur and then radically copolymerizing it by heating at 160°C. This method makes it possible to synthesize functional high-sulfur polymers by imparting the properties of vinyl monomers to sulfur. Much research has been conducted on their application as cathode materials for Li-S batteries, taking advantage of their excellent molding and processing properties (Non-Patent Documents 1-5).

[0003] Non-Patent Documents 1 and 2 describe sulfur-containing network polymers synthesized by inverse vulcanization of 1,3-diisopropenylbenzene (DIB), which has two double bonds, with sulfur. Unless the sulfur content is low (50 wt% or less), the polymers are not soluble in organic solvents (chloroform). Non-Patent Document 3 describes a polymer with pendant sulfur molecules produced by inverse vulcanization of a main chain polymer (TAR) composed of thiourea and formaldehyde with sulfur. The polymer contains 85 wt% or more of sulfur. By creating a sheet structure with sulfur pendant on the side chains, the reactivity with metal ions such as lithium ions is improved, achieving a discharge capacity of nearly 600 mAh / g at 5C. While the inverse vulcanization method is the same as that of the present invention, the polymer is not soluble in organic solvents and the electrode is produced in a heterogeneous system (slurry). Non-Patent Document 4 describes a network polymer (S-DVB) synthesized by reverse vulcanization of 3,5-divinylbenzene (DVB) with double bonds and sulfur. An all-solid-state Li-S battery is produced by using a solidified electrolyte made of polyethylene glycol containing lithium salt. The sulfur content in S-DVB is approximately 40 wt%, and electrodes are prepared as a slurry solution. Non-Patent Document 5 describes a network polymer (S-TVTCSi) formed by the inverse vulcanization of sulfur and 2,4,6,8-tetravinyl-2,4,6,8-tetramethylcyclo tetrasiloxane (TVTCSi). Although the sulfur content is 75 wt% or more, it is not soluble in organic solvents. Operation at 0.5C for 500 cycles has been confirmed, and the capacity retention rate is also high. However, since the electrode is prepared in a slurry state, it is thin.

[0004] The present inventors have also previously invented sulfur-containing polymers (Patent Documents 1-3). Patent Document 1 describes a polymer synthesized by inverse vulcanization of sulfur with a bisalkenyl compound having a cyclic or linear oligoethylene oxide skeleton or a monoalkenyl compound having a long-chain aliphatic hydrocarbon. The concept of imparting solubility in organic solvents and impregnating the porous carbon is common to both documents, but the sulfur content must be significantly reduced (50 wt% or less) to dissolve in organic solvents. Patent Document 2 describes a polymer synthesized by inverse vulcanization of sulfur with a monoalkenyl compound having an ionic group (mainly sulfonic acid or its salt). While both have a sulfonyl (O=S=O) skeleton, this polymer differs from the present invention in that it dissociates into ions, such as sulfonic acid. Furthermore, sulfur polymers with ionic groups are not soluble in organic solvents. Patent Document 3 describes a solid polymer obtained by inverse vulcanization of a vinyl (allyl) compound with alkoxysilane side chains and sulfur. It dissolves in organic solvents even with a sulfur content of 80 wt% or more. Furthermore, impregnation of carbon with the polymer causes hydrolysis by acid treatment, immobilizing the sulfur component on the carbon and making it insoluble. However, because it does not have a skeleton that interacts with metal ions such as lithium ions, its charge / discharge rate is extremely slow (0.1C). [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Laid-Open No. 2015-227438 [Patent Document 2] Japanese Patent Application Laid-Open No. 2016-53159 [Patent Document 3] JP 2018-65980 A [Non-patent literature]

[0006] [Non-Patent Document 1] J. Pyun et al., The use of elemental sulfur as an alternative feedstock for polymeric materials, Nature Chemistry, 5, 518-524 (2013) [Non-patent document 2] J. Pyun et al., Inverse Vulcanization of Elemental Sulfur to Prepare Polymeric Electrode Materials for Li-S Batteries, ACS Macro Letters, 3, 229-232 (2014) [Non-patent document 3] S. Chen et al., Highly crosslinked organosulfur copolymer nanosheets with abundant mesopores as cathode materials for efficient lithium-sulfur batteries, Electrochimica Acta, 263, 53-59 (2018) [Non-patent document 4] M. Armand et al., S-containing copolymer as cathode material in poly(ethylene oxide)-based all-solid-state Li-S batteries, Journal of Power Sources, 390, 148-152 (2018) [Non-Patent Document 5] C. Yang et al., Sulfur-rich polymer / Ketjen Black composites as lithium-sulfur battery cathode with high cycling stability, Journal of Alloys and Compounds, 962, 171177 (2023) Summary of the Invention [Problem to be solved by the invention]

[0007] The present invention has been made in view of the above circumstances, and the problem to be solved by the present invention is to provide a sulfur-containing polymer having a high sulfur content that is soluble in organic solvents and applicable to the positive electrodes of secondary batteries, particularly metal-lithium secondary batteries, and further to provide a positive electrode active material for secondary batteries that has high energy density and excellent electrochemical properties. [Means for solving the problem]

[0008] As a result of extensive research, the present inventors have found that a sulfur-containing polymer having a high sulfur content and soluble in a specific organic solvent can be synthesized by reacting an allyl compound or vinyl compound having a group such as a sulfonyl (-SO-) group in the side chain with molecular sulfur (S) using an inverse vulcanization method.

[0009] That is, the present invention provides: (1) Formula (I) [ka] (In the formula, Y represents S(O)m, -O-, -COO-, -OCO-, -NH-, -NHCO- or -CONH-; m represents an integer of 0 to 2, Z is a C1-C6 alkyl group which may have a halogeno group as a substituent; a C6 to C10 aryl group optionally having a C1 to C6 alkyl group or a halogeno group as a substituent; a C6-C10 aryl C1-C6 alkyl group optionally having a C1-C6 alkyl group or a halogeno group as a substituent; a heteroaryl group optionally having a C1-C6 alkyl group or a halogeno group as a substituent; or represents a halogeno group, R represents a hydrogen atom or a methyl group; X represents a positive integer of 1 or more, and the number of X in each unit may be different; n represents an integer between 0 and 10. A sulfur-containing polymer having a repeating unit represented by the formula: (2) The sulfur-containing polymer according to (1), wherein Y is SO2, Ar is a phenyl group, and n is 1. (3) The sulfur-containing polymer according to (1) or (2), wherein the sulfur content in the sulfur-containing polymer (excluding sulfur derived from S(O)m) is 50 to 95 mass %.

[0010] The present invention also provides (4) A positive electrode for a secondary battery containing the sulfur-containing polymer according to (1) or (2). (5) A secondary battery including the positive electrode for secondary batteries according to (4).

[0011] Furthermore, the present invention provides (6) Formula (II) below CH2=CR-(CH2) n -YZ (II) (In the formula, Y represents S(O)m, -O-, -COO-, -OCO-, -NH-, -NHCO- or -CONH-; m represents an integer of 0 to 2, Z is a C1-C6 alkyl group which may have a halogeno group as a substituent; a C6 to C10 aryl group optionally having a C1 to C6 alkyl group or a halogeno group as a substituent; a C6-C10 aryl C1-C6 alkyl group optionally having a C1-C6 alkyl group or a halogeno group as a substituent; a heteroaryl group optionally having a C1-C6 alkyl group or a halogeno group as a substituent; or represents a halogeno group, R represents a hydrogen atom or a methyl group; and n represents an integer of 0 to 10) with molecular sulfur (S8). [Effects of the Invention]

[0012] The sulfur-containing polymer obtained by the present invention has a high sulfur content, a high energy density, and the property of dissolving in a specific organic solvent makes it easy to uniformly impregnate the porous carbon with the sulfur component at a lower temperature. Furthermore, if the polymer contains polar groups such as SO groups, electrostatic interactions with metal ions also occur, accelerating the chemical reaction with sulfur, resulting in a positive electrode active material for secondary batteries with excellent electrochemical properties. [Brief explanation of the drawings]

[0013] [Figure 1] Schematic diagrams of lithium-sulfur (Li-S) and magnesium-sulfur (Mg-S) batteries are shown. [Figure 2] 1 shows 1H NMR spectra of APS (allyl phenyl sulfone) and S-APS (a sulfur-containing polymer derived from the APS of the present invention). [Figure 3] 1 shows DSC charts of molecular sulfur (S8) and S-APS (a sulfur-containing polymer derived from the APS of the present invention). [Figure 4] The manufacturing process of S-APS electrodes and Li-S batteries is shown. [Figure 5] The graph shows the charge-discharge measurement results (0.5C) of a Li-S battery using molecular sulfur (S8) and S-APS as the positive electrode. The upper graph shows the results after 20 charge-discharge cycles, and the lower graph shows the results after 100 charge-discharge cycles. [Figure 6] The image shows the coordination of lithium ions with S-APS. [Figure 7] The manufacturing process for an Mg-S battery using an S-APS electrode is shown. [Figure 8] The results of charge / discharge measurements of an Mg-S battery using an S-APS electrode are shown. DETAILED DESCRIPTION OF THE INVENTION

[0014] (sulfur-containing polymer) The sulfur-containing polymer of the present invention is represented by the following formula (I): [ka] It is a sulfur-containing polymer having a repeating unit represented by the formula:

[0015] In the repeating unit, Sx is derived from molecular sulfur (S8), which is a raw material for production. When molecular sulfur (S8) reacts with the alkenyl compound represented by formula (II), which is another raw material for production, it is thought that not only are eight sulfur atoms connected, but also that one to seven, or nine or more sulfur atoms are involved through decomposition or bonding. Therefore, the number of sulfur atoms, X, is a positive integer of 1 or more, and the number of sulfur atoms, X, in each repeating unit may be the same or different.

[0016] In the repeating unit, the ethylene group moiety having a side chain is derived from the alkenyl compound represented by formula (II), which is the raw material for production. Y represents S(O)m, -O-, -COO-, -OCO-, -NH-, -NHCO-, or -CONH-, and m represents an integer of 0 to 2. When used in the positive electrode of a lithium-sulfur battery or a magnesium-sulfur battery, any of these groups can provide charge interaction with cations such as Li ions and Mg ions. SO2 is preferred. Z is a C1-C6 alkyl group which may have a halogeno group as a substituent; a C6 to C10 aryl group optionally having a C1 to C6 alkyl group or a halogeno group as a substituent; a C6-C10 aryl C1-C6 alkyl group optionally having a C1-C6 alkyl group or a halogeno group as a substituent; a heteroaryl group optionally having a C1-C6 alkyl group or a halogeno group as a substituent; or Represents a halogeno group.

[0017] Examples of the "C1 to C6 alkyl group" in the above "C1 to C6 alkyl group which may have a halogeno group as a substituent" include a methyl group, an ethyl group, an n-propyl group, an i-propyl group, an n-butyl group, an s-butyl group, an i-butyl group, a t-butyl group, an n-pentyl group, an n-hexyl group, etc. The "C6-C10 aryl group" in the above "C6-C10 aryl group which may have a C1-C6 alkyl group or a halogeno group as a substituent" and "C6-C10 aryl-C1-C6 alkyl group which may have a C1-C6 alkyl group or a halogeno group as a substituent" refers to a monocyclic or polycyclic aryl group. Here, polycyclic aryl groups include not only fully unsaturated groups but also partially saturated groups. Specific examples include a phenyl group, a naphthyl group, an azulenyl group, an indenyl group, an indanyl group, and a tetralinyl group. Examples of the C6-C10 aryl C1-C6 alkyl group include a benzyl group and a phenethyl group. The "heteroaryl group" in the above "heteroaryl group which may have a C1-C6 alkyl group or a halogeno group as a substituent" includes a 5- to 7-membered monocyclic or polycyclic aromatic heterocycle having 1 to 4 nitrogen atoms, oxygen atoms or sulfur atoms as heteroatoms, and a fused ring formed by condensing a benzene ring with a 5- to 7-membered heterocycle having 1 to 4 nitrogen atoms, oxygen atoms or sulfur atoms as heteroatoms. Specific examples include pyridinyl, imidazolyl, pyrimidinyl, pyrazolyl, triazolyl, pyrazinyl, tetrazolyl, furyl, thienyl, isoxazolyl, thiazolyl, oxazolyl, isothiazolyl, pyrrolyl, quinolinyl, isoquinolinyl, indolyl, benzimidazolyl, benzofuranyl, cinnolinyl, indazolyl, indolizinyl, phthalazinyl, pyridazinyl, triazinyl, isoindolyl, pteridinyl, purinyl, oxadiazolyl, thiadiazolyl, furazanyl, benzofurazanyl, benzothiophenyl, benzothiazolyl, benzoxazolyl, quinazolinyl, quinoxalinyl, naphthyridinyl, etc. A phenyl group is preferred. The above-mentioned "halogeno group" includes a fluoro group, a chloro group, a bromo group, and the like. Examples of the "C1-C6 alkyl group" and "halogeno group" as the substituent include the same as those mentioned above. One or more of these substituents may be present.

[0018] R represents a hydrogen atom or a methyl group. n is the number of CH2 groups and is an integer of 0 to 10. To increase the sulfur content, it is preferable that the number n of CH2 groups is small.

[0019] The sulfur content in the sulfur-containing polymer of the present invention is preferably from 50 to 95% by mass, more preferably from 55 to 95% by mass, and even more preferably from 60 to 95% by mass.

[0020] In addition to having a high sulfur content, the sulfur-containing polymer of the present invention is soluble in organic solvents such as halogenated hydrocarbon solvents such as chloroform and dichloromethane, and aprotic polar solvents such as tetrahydrofuran, dimethylformamide and N-methyl-2-pyrrolidone. In addition, when Y is SO2, the crystallinity is low and it exists as a viscous substance at room temperature.

[0021] The number average molecular weight (Mn) of the sulfur-containing polymer of the present invention is not particularly limited as long as the object of the present invention can be achieved, and can be set, for example, to 1,000 to 20,000, 2,000 to 15,000, 5,000 to 10,000, etc. These ranges of average molecular weight can be appropriately set by adjusting the amounts of raw materials charged for production, etc. The sulfur-containing polymer of the present invention can be confirmed by 1H-NMR, 13H-NMR, differential scanning calorimetry (DSC), GPC, and the like.

[0022] (Method of producing sulfur-containing polymer) The method for producing the sulfur-containing polymer of the present invention is not limited to a particular method, but a specific example of the production method is as follows. The following formula (II) CH2=CR-(CH2) n -YZ (II) (wherein Y, m, Z, R and n are the same as defined in formula (I)) is reacted with molecular sulfur (S8).

[0023] The compound represented by formula (II) (monoalkenyl compound) is not particularly limited as long as it corresponds to the above formula. To be soluble in organic solvents, the compound has a melting point of 150°C or less, preferably 50°C or less, and more preferably 30°C or less. In order to increase the sulfur content, a compound having a molecular weight of 200 or less is preferred. The above compounds can be produced by known methods, but commercially available compounds may also be used.

[0024] For example, S(O) m Examples of compounds having a group include the following compounds. Compounds with an SO2 group: alkyl group-containing sulfone compounds such as vinyl methyl sulfone, allyl methyl sulfone, allyl ethyl sulfone, and 5-hexenyl methyl sulfone; aryl group-containing sulfone compounds such as vinyl phenyl sulfone, allyl phenyl sulfone, 3-butenyl phenyl sulfone, and 5-hexenyl phenyl sulfone; Alkylaryl group-containing sulfone compounds such as vinyltolyl sulfone and allyltolyl sulfone; arylalkyl group-containing sulfone compounds such as vinylbenzyl sulfone and allylbenzyl sulfone; heteroaryl group-containing sulfone compounds such as vinylpyridinyl sulfone, allylfuryl sulfone, allylpyridinyl sulfone, and allylmethylpyridinyl sulfone; halogeno group-containing sulfone compounds such as vinyl fluorosulfone; Compounds containing SO or S groups: Vinyl phenyl sulfoxide, allyl phenyl sulfoxide, vinyl phenyl sulfide, allyl phenyl sulfide, and the like.

[0025] Molecular sulfur (S8) is insoluble or poorly soluble in organic solvents, so it must be melted by heating in a sulfur reaction vessel. Molecular sulfur (S8) has three crystalline forms (α-sulfur, β-sulfur, and γ-sulfur), with melting points of 112.8°C, 119.6°C, and 106.8°C, respectively. Therefore, heating to temperatures above 120°C is necessary to melt molecular sulfur (S8). Furthermore, molecular sulfur (S8) transforms from the stable α-sulfur structure to β-sulfur, λ-sulfur, and μ-sulfur as the temperature increases. At temperatures above 159.4°C, radical cleavage of the cyclic sulfur occurs, producing divalent radicals. The reaction temperature of the compound represented by formula (II) with molecular sulfur (S8) is preferably within a temperature range in which molecular sulfur (S8) generates radicals and in which the compound represented by formula (II) becomes liquid, and is preferably 160°C to 175°C. The charge ratio (molar ratio) of molecular sulfur (S8) to the compound represented by the above formula (II) is preferably 1:0.05 to 1:1.4, and more preferably 1:0.1 to 1:1.1.

[0026] (Cathode materials for secondary batteries) The sulfur-containing polymer of the present invention can be used as a positive electrode material for secondary batteries. Schematic diagrams of a lithium-sulfur (Li-S) battery and a magnesium-sulfur (Mg-S) battery are shown in Figure 1. The positive electrode material for a secondary battery is not particularly limited as long as it contains the sulfur-containing polymer, but it preferably contains a conductive additive, a binder, and a solvent. Furthermore, the positive electrode for a secondary battery of the present invention can be produced by applying the positive electrode material for a secondary battery to a current collector.

[0027] Specific examples of the conductive additive include carbon powders such as vapor grown carbon fiber (VGCF), carbon black (CB), activated carbon for capacitors, carbon nanotubes, graphene, fullerene, and graphite; and fine powders of metals stable at the positive electrode potential, such as aluminum and titanium. Preferred carbon blacks are acetylene black (AB, manufactured by Denka Co., Ltd.), Ketjenblack (KB, manufactured by Lion Corporation), and Knobel (manufactured by Toyo Tanso Co., Ltd.). Porous carbon is particularly preferred. In the present invention, the sulfur-containing polymer can be uniformly impregnated into the porous carbon at room temperature (for example, 25° C.) under reduced pressure. The blending ratio of the sulfur-containing polymer and conductive aid contained in the positive electrode for secondary battery of the present invention is preferably 1 to 50:1, more preferably 5 to 20:1, in terms of sulfur-containing polymer of the present invention:conductive aid (mass ratio).

[0028] Specific examples of the binder include polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), styrene-butadiene rubber (SBR), polyimide (PI), polyamide-imide (PAI), carboxymethyl cellulose (CMC), polyvinyl chloride (PVC), methacrylic resin (PMA), polyacrylonitrile (PAN), modified polyphenylene oxide (PPO), polyethylene oxide (PEO), polyethylene (PE), polypropylene (PP), and alginic acid (Alg).

[0029] A preferred material for the negative electrode of the secondary battery of the present invention comprising the positive electrode for the secondary battery of the present invention is any one selected from lithium, magnesium, aluminum, sodium, silicon, and carbon materials such as artificial graphite, and more preferred materials are lithium and magnesium.

[0030] The current collector of the secondary battery of the present invention is a current collector generally used in a positive electrode for a secondary battery. Specific examples of the current collector of the secondary battery of the present invention include stainless steel foil, stainless steel mesh, aluminum foil, aluminum mesh, punched aluminum sheet, expanded aluminum sheet, stainless steel foil, stainless steel mesh, punched stainless steel sheet, expanded stainless steel sheet, foamed nickel, nickel nonwoven fabric, copper foil, copper mesh, punched copper sheet, expanded copper sheet, titanium foil, titanium mesh, nonwoven carbon fabric, and woven carbon fabric. A preferred current collector is stainless steel mesh.

[0031] The electrolyte of the secondary battery of the present invention is an electrolyte commonly used in secondary batteries. Specific examples of solvents for the electrolyte of the secondary battery of the present invention include 1,2-dimethoxyethane, acetonitrile, propylene carbonate, ethylene carbonate, 3-methoxypropionitrile, methoxyacetonitrile, ethylene glycol, propylene glycol, diethylene glycol, triethylene glycol, butylolactone, dimethoxyethane, dimethyl carbonate, 1,3-dioxolane, methyl formate, 2-methyltetrahydrofuran, 3-methoxyoxazoliden-2-one, sulfolane, tetrahydrofuran, triethylene glycol dimethyl ether, and water. These may be used alone or in combination.

[0032] The solvent of the electrolyte solution of the secondary battery of the present invention may be an ionic liquid. An ionic liquid is a liquid composed of ions formed by combining organic cations and anions. Specific examples of the organic cation include imidazolium ions such as dialkylimidazolium cations and trialkylimidazolium cations, tetraalkylammonium ions, alkylpyridinium ions, dialkylpyrrolidinium ions, and dialkylpiperidinium ions.

[0033] Specific examples of anions that serve as counter ions for these organic cations include PF6 anion, PF3(C2F5)3 anion, PF3(CF3)3 anion, BF4 anion, BF2(CF3)2 anion, BF3(CF3) anion, bisoxalate borate anion, Tf (trifluoromethanesulfonyl) anion, Nf (nonafluorobutanesulfonyl) anion, bis(fluorosulfonyl)imide anion, bis(trifluoromethanesulfonyl)imide anion, bis(pentafluoroethanesulfonyl)imide anion, and dicyanoamine anion.

[0034] Specific examples of the electrolyte for the electrolytic solution of the secondary battery of the present invention include sodium salts such as NaPF6, NaBF4, NaClO4, NaCF3CONSO2CF3, NaN(SO2C2F5)2, NaN(SO2CF3)2, NaCF3SO3, and NaC(CF3SO2)3; lithium salts such as LiCl, LiClO4, LiAsF6, LiPF6, LiBF4, LiB(C6H5)4, LiCH3SO3, LiCF3SO3, Li(C2F5SO2)2N, Li(CF3SO2)2N, LiC(CF3SO2)3, and LiBr; magnesium salts such as Mg(ClO4)2 and Mg(CF3SO3)2; and aluminum salts of phosphate compounds such as orthophosphoric acid, pyrophosphoric acid, hypophosphorous acid, hypodiphosphoric acid, phosphorous acid, diphosphorous acid, pyrophosphorous acid, isohypophosphoric acid, and hypophosphoric acid.

[0035] The shape of the secondary battery of the present invention is not limited, and may be a coin type, button type, sheet type, laminated type, cylindrical type, flat type, prismatic type, or a large secondary battery for use in an electric vehicle or the like.

[0036] The sulfur-containing polymer of the present invention has low crystallinity and viscosity when it has SO2 groups, etc. Therefore, when used as an electrode, it is less likely to crumble. This enables the fabrication of uniform sulfur-carbon composite materials with high energy density, which may contribute to efficient electrode reactions and is useful as a battery material, particularly as a positive electrode material for secondary batteries.

[0037] In the background art of Patent Document 3 (JP 2018-65980 A), hydroxyl groups present on the surface of porous carbon are bonded (immobilized) with alkoxysilanes on the side chains of sulfur-containing polymers through a condensation reaction caused by hydrolysis, which requires the use of a strong acid. Therefore, usable carbon must have hydroxyl groups, and the composite with carbon requires not only an impregnation operation but also an acid treatment, which increases the number of work steps. In contrast, in the present invention, impregnation can be achieved in one step at room temperature regardless of the type of porous carbon.

[0038] Furthermore, the sulfur-containing polymer of the present invention can suppress the elution of polysulfide ions (sulfur components) even after many reaction cycles in electrochemical evaluation. Furthermore, when the sulfur-containing polymer of the present invention is used together with porous carbon as a positive electrode material, the sulfur-containing polymer penetrates deep into the porous carbon, suppressing the elution of sulfur components into the electrolyte after repeated electrochemical reactions, and reducing the risk of battery deterioration due to the passage of charge-discharge cycles. [Example]

[0039] The present invention will be described in more detail below with reference to examples, but the technical scope of the present invention is not limited to these examples. Not limited. The analytical methods for the raw materials and the synthesized sulfur-containing polymers are as follows. (1) 1 HNMR analysis The sample was dissolved in deuterated chloroform as a solvent to a concentration of 0.1 M, and the sample was analyzed using a JNM-ECA ( 1 HNMR) using trimethylsilane as an internal standard. (2) Differential scanning calorimetry (DSC) analysis A DSC7020 manufactured by Hitachi High-Technologies Corporation was used, and the sample was heated at a heating rate of 10°C / min and analyzed.

[0040] [Polymer Example 1] One part by mass of molecular sulfur (S8) (Kishida Chemical Co., Ltd.) and 0.36 parts by mass of allyl phenyl sulfone (APS; Tokyo Chemical Industry Co., Ltd.) (equivalent to 0.5 times the molar amount of molecular sulfur) were added to a sample bottle and stirred at 160°C for 3 hours. The reaction product was then allowed to cool to room temperature, and the solidified polymer (S-APS) was collected. The yield was nearly 100%, and the sulfur content was estimated to be 74 mass%.

[0041] [Comparative Polymer Example 1] One part by mass of molecular sulfur (S8) (Kishida Chemical Co., Ltd.) and 0.21 parts by mass of vinyl sulfonic acid (Tokyo Chemical Industry Co., Ltd.) (equivalent to 0.5 times the molar amount of molecular sulfur) were added to a sample bottle and stirred at 160°C for 3 hours. The reaction product was then allowed to cool to room temperature, and the solidified polymer was collected.

[0042] (Appearance of the product) The product obtained in Polymer Example 1 (polymer of the present invention) was a viscous / sticky, highly homogeneous brownish-red substance, although no flowability due to its own weight was observed. In contrast, the product obtained in Comparative Polymer Example 1 remained in a two-layer separated state throughout the reaction, and it was observed that the final substance also had a biased distribution of sulfur-derived and vinyl sulfonic acid-derived regions. This suggests that the large difference in polarity between sulfur and vinyl sulfonic acid prevented the reaction from proceeding without homogenization.

[0043] (Solubility of the product in organic solvents) The product obtained in Polymer Example 1 (the polymer of the present invention) was found to be soluble in a given amount of tetrahydrofuran (THF). In contrast, the product obtained in Comparative Polymer Example 1 precipitated.

[0044] (Product Analysis) The raw material allyl phenyl sulfone (APS) and the product (S-APS) obtained in Polymer Example 1 were subjected to 1H NMR measurement in deuterated chloroform (FIG. 2). It was observed that the proton peaks a and b derived from the double bond, which appear at 5-6 ppm in the APS spectrum (A), completely disappeared in the post-reaction spectrum (B). In addition, the methylene proton peak c adjacent to the double bond of APS also disappeared after the reaction, confirming the formation of a polymer (also known as inverse vulcanization) through the reaction of APS with molecular sulfur. These lost protons were observed as new peaks (e.g., near 2.0-4.6 ppm) in the spectrum of (B). These peaks appeared multiple and broad, suggesting that the different lengths of the sulfur chains copolymerized with APS had an effect. Figure 3 shows the results of sulfur (S8) and S-APS measurements by differential scanning calorimetry (DSC). As shown in Figure 3, the DSC chart of sulfur shows three typical endothermic peaks due to phase transition in the range of 100-180°C. On the other hand, an endothermic peak due to the melting point was observed at 95°C in S-APS, suggesting that the raw material sulfur was not present and that a linear sulfur polymer was produced.

[0045] [Battery Example 1] (Li-S battery) In a vessel, 1 part by mass of the sulfur-containing polymer (S-APS) produced in Polymer Example 1 was dissolved in 40 parts by mass of tetrahydrofuran. Next, 0.5 parts by mass of carbon black (Knobel (registered trademark), manufactured by Toyo Tanso Co., Ltd.) was added to the solution, and the S-APS was then impregnated into the porous carbon under reduced pressure (10 kPa). Then, 9 parts by mass of the S-APS-impregnated carbon black and 1 part by mass of PTFE (Polyflon PTFE fine powder manufactured by Daikin Corporation) were mixed together to form a positive electrode plate (Figure 4). Next, a simple coin cell (R2032 type) was fabricated using this positive electrode, a lithium metal foil as the negative electrode, and an electrolyte solution of lithium (trifluoromethylsulfonyl)imide (LiTFSI) dissolved in a 1:1 volumetric mixture of dioxolane (DOX) and dimethoxymethane (DME) (1M LiTFSI-DOX / DME) (Figure 4).

[0046] Figure 5 shows the results of charge / discharge measurements at 0.5C for a Li-S battery consisting of molecular sulfur (S8) and an S-APS cathode. The horizontal axis represents the cycle number, while the vertical axis represents the discharge capacity (mAh / g) and coulombic efficiency. The coulombic efficiency was calculated as [(discharge capacity / charge capacity) × 100(%)]. As a result, batteries using a positive electrode made of S-APS showed a higher capacity retention rate than batteries using a conventional sulfur positive electrode. Furthermore, batteries using S-APS electrodes tend to have smaller overvoltages during discharge and charge than those using molecular sulfur (S8) electrodes, which is expected to enable faster charging. These results suggest that the sulfonyl units in S-APS attract lithium ions in the electrolyte via ion-dipole interactions, enhancing the reactivity of sulfur with lithium ions (Fig. 6).

[0047] [Battery Example 2] (Mg-S battery) Using the positive electrode plate prepared in Battery Example 1, a simple coin cell (R2032 type) was fabricated as shown in FIG. The electrolyte used was a commercially available magnesium salt (magnesium bistrifluoromethanesulfonylamide, Mg(TFSAI)2) dissolved in triethylene glycol dimethyl ether (G3), known as Mg(TFSAI)2 / G3. The magnesium anode was a cast MgBi alloy containing 50 wt% bismuth (Bi) relative to magnesium (Mg). The charge-discharge measurement results of the fabricated Mg-S battery are shown in Figure 8. A reversible discharge-charge profile was observed, suggesting that the material can be used as a positive electrode material for Mg-S batteries.

Claims

1. The following formula (I) 【Chemical 1】 (In the formula, Y represents S(O)m, —O—, —COO—, —OCO—, —NH—, —NHCO—, or —CONH—; m represents an integer of 0 to 2; Z is a C1-C6 alkyl group which may have a halogeno group as a substituent; a C6 to C10 aryl group optionally having a C1 to C6 alkyl group or a halogeno group as a substituent; a C6-C10 aryl C1-C6 alkyl group optionally having a C1-C6 alkyl group or a halogeno group as a substituent; a heteroaryl group optionally having a C1-C6 alkyl group or a halogeno group as a substituent; or represents a halogeno group, R represents a hydrogen atom or a methyl group; X represents a positive integer of 1 or more, and the number of X in each unit may be different. n represents an integer of 0 to 10. A sulfur-containing polymer having a repeating unit represented by the formula:

2. Y is SO ２ 2. The sulfur-containing polymer according to claim 1, wherein Z is a phenyl group and n is 1.

3. The sulfur-containing polymer according to claim 1 or 2, wherein the sulfur content in the sulfur-containing polymer (excluding sulfur derived from S(O)m) is 50 to 95% by mass.

4. A positive electrode for a secondary battery, comprising the sulfur-containing polymer according to claim 1 or 2.

5. A secondary battery comprising the positive electrode for secondary batteries according to claim 4 .

6. The following formula (II) CH ２ =CR-(CH ２ ) ｎ -Y-Z (II) (In the formula, Y represents S(O)m, —O—, —COO—, —OCO—, —NH—, —NHCO—, or —CONH—; m represents an integer of 0 to 2; Z is a C1-C6 alkyl group which may have a halogeno group as a substituent; a C6 to C10 aryl group optionally having a C1 to C6 alkyl group or a halogeno group as a substituent; a C6-C10 aryl C1-C6 alkyl group optionally having a C1-C6 alkyl group or a halogeno group as a substituent; a heteroaryl group optionally having a C1-C6 alkyl group or a halogeno group as a substituent; or represents a halogeno group, R represents a hydrogen atom or a methyl group; n represents an integer of 0 to 10. and molecular sulfur (S ８ 2. The method for producing a sulfur-containing polymer according to claim 1, wherein the reactant is a carboxylic acid or a carboxylic acid.

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