Sulfur is one of the candidates for cathode of Li battery to realize improved capacity of the cell. The issues to be solved are selection of electrolyte which is compatible to both of sulfur cathode and anode, and high solubility of polysulfides generated during the charge-discharge operation of cathode. The dissolved polysulfide into the electrolyte phase inside the battery leads chemical shortage between the cathode and the anode, i.e. the dissolved redox active polysulfide works as redox shuttle between the electrodes, and causes difficulty of charging and self-discharge. Some ideas to stop the dissolution of polysulfide are proposed, such as (1) apply sulfide organic  compound with high molecular weight or polymer as the cathode active material, (2) use insoluble solvent for electrolyte solution, (3) control the mass transfer of Li⁺ and polysurfide, and so on. For blocking the transfer of dissolved polysulfide, cell structure with the perm-selective membrane inserted the between the electrodes by placing in the electrolyte phase or by covering one of the electrode can be considered. In those cases, the perm-selective membrane should have the property of well-transporting of Li⁺ with low impedance beside the non-transfer of polysulfide. Since polysurfide has negative charge, cation exchange membrane would be the one of the candidates for the perm-selective membrane.

Ion exchange membrane is well known to be designed with polymer having anionic groups inside the matrix. Polypyrrole, which is known as a redox active polymer and is able to be formed as a sheet form on conducting substrate by electro-oxidative polymerization as anion doped state. Polypyrrole doped with anion having large size exhibit an interesting properties, such as cation exchange properties. For example, Polypyrrole doped with polyanion, such as polystyrenesulfonate acts as cation exchange membrane.

In this work, we prepared polypyrrole doped with soft/large anion by electropolymerization method forming directly onto the surfur-carbone composite cathode. The prepared polypyrrole film was confirmed to exhibit cation exchange properties, and the film prepared on S-C composite electrode was found to be formed with covering the S and C phase. The electrtode of S-C composite covered by polypyrrole exhibits discharge capacity of over 800 mAh/g(sulfur) at 0.1C-rate with the current efficiency of above 95% during the continuous charge-discharge cycling. The detailed battery performance of the polypyrrole coated S cathode and effect of polypyrrole formation condition to the the battery perrormance will be introduced.