Sulfur-based positive-electrode active material, positive electrode and lithium-ion secondary battery

Abstract

An object of the present invention is to provide a novel sulfur-based positive-electrode active material which can largely improve cyclability of a lithium-ion secondary battery, a positive electrode comprising the positive-electrode active material and a lithium-ion secondary battery comprising the positive electrode. The sulfur-based positive-electrode active material is one comprising: a carbon skeleton derived from a polymer composed of a monomer unit having at least one hetero atom-containing moiety, and sulfur incorporated into the carbon skeleton as the carbon skeleton is formed from the polymer by heat treatment.

1 . A sulfur-based positive-electrode active material comprising:
 a carbon skeleton derived from a polymer composed of a monomer unit having at least one hetero atom-containing moiety, and   sulfur incorporated into the carbon skeleton as the carbon skeleton is formed from the polymer by heat treatment,   wherein the hetero atom-containing moiety is a moiety having a group selected from the group consisting of a monovalent functional group having at least one hetero atom selected from the group consisting of O, S, P and N, a heterocyclic group having at least one hetero atom selected from the group consisting of O, S, P and N, and a group represented by —S a — (“a” is an integer of 2 to 4).   
     
     
         2 . The sulfur-based positive-electrode active material of  claim 1 , wherein the polymer composed of a monomer unit having at least one hetero atom-containing moiety is one represented by the following formula (1) or (2). 
       
         
           
           
               
               
           
         
         wherein R 1  represents a hydrogen atom or an alkyl group, X 1  represents a group having a monovalent functional group having a hetero atom selected from the group consisting of O, S, P and N, or a group having a heterocyclic group having a hetero atom selected from the group consisting of O, S, P and N, “n” represents an integer. 
       
       
         
           
           
               
               
           
         
         wherein R 2  represents an alkyl group, “a” represents an integer of 2 to 4, “m” represents an integer of 2 to 12. 
       
     
     
         3 . The sulfur-based positive-electrode active material of  claim 1 , wherein the heterocyclic group is a 5- to 14-membered heterocyclic group having 1 to 3 hetero atoms selected from the group consisting of O, S, P and N. 
     
     
         4 . The sulfur-based positive-electrode active material of  claim 1 , wherein the monovalent functional group is at least one selected from the group consisting of a hydroxyl group, a sulfo group, a carboxyl group, a phosphate group and an ammonium group, and
 the heterocyclic group is one selected from the group consisting of pyrrolidine, pyrrole, pyridine, imidazole, pyrolidone, tetrahydrofuran, triazine, thiophene, oxazole, thiazole, phosphole, indole, benzimidazole, quinoline, carbazole, thianthrene, phenoxazine, phenothiazine, xanthene, thieno[3,2-b]thiophene, benzothiophene and phosphindole.   
     
     
         5 . The sulfur-based positive-electrode active material of  claim 1 , wherein the polymer is at least one selected from the group consisting of polyvinylpyridine, phosphorylcholine polymer, alkylphenol-sulfur chloride condensate, and polystyrene sulfonic acid. 
     
     
         6 . The sulfur-based positive-electrode active material of  claim 1 , wherein a weight average molecular weight of the polymer is from 2000 to 1500000. 
     
     
         7 . The sulfur-based positive-electrode active material of  claim 1 , wherein at the heat treatment, an electrically conductive carbon material is further mixed in addition to the polymer and the sulfur. 
     
     
         8 . The sulfur-based positive-electrode active material of  claim 7 , wherein the electrically conductive carbon material is a carbon material having a graphite structure. 
     
     
         9 . The sulfur-based positive-electrode active material of  claim 1 , wherein a total content of the sulfur is not less than 50% by mass. 
     
     
         10 . The sulfur-based positive-electrode active material of  claim 1 , wherein the sulfur-based positive-electrode active material is one prepared by a preparation process comprising a step of heat-treating the polymer composed of a monomer unit having at least one hetero atom-containing moiety and sulfur under a non-oxidizing atmosphere, and a heat-treating temperature is from 250° C. to 550° C. 
     
     
         11 . A sulfur-based positive-electrode active material prepared by a preparation process comprising a step of heat-treating a polymer composed of a monomer unit having at least one hetero atom-containing moiety, and sulfur under a non-oxidizing atmosphere,
 wherein the hetero atom-containing moiety is a moiety having a group selected from the group consisting of a monovalent functional group having at least one hetero atom selected from the group consisting of O, S, P and N, a heterocyclic group having at least one hetero atom selected from the group consisting of O, S, P and N, and a group represented by —S a — (“a” is an integer of 2 to 4), and   a heat-treating temperature is from 250° C. to 550° C.   
     
     
         12 . A positive-electrode comprising the sulfur-based positive-electrode active material of  claim 1 . 
     
     
         13 . A lithium-ion secondary battery comprising the positive electrode of  claim 12 .