NOVELTY - A polyelectrolyte contains a boron-containing compound.    USE - Polyelectrolyte is used for lithium secondary battery (claimed).    ADVANTAGE - The polyelectrolyte has excellent electrochemical stability. The lithium secondary battery formed using the polyelectrolyte has excellent performance characteristics.    DETAILED DESCRIPTION - A polyelectrolyte contains a boron-containing compound (BY'), where Y' is phosphate or nitrogen. The boron-containing compound is added to a polymer aggregate. The polyelectrolyte further contains a polymer matrix, lithium salt, plasticizer, a water-insoluble organic solvent and non-woven fabric. The polymer matrix contains polypropylene, polyimide, polysulfone, polyurethane, polyvinyl chloride, cellulose, nylon, poly acrylonitrile, polyvinylidene fluoride, poly tetrafluoro ethylene, vinylidene fluoride, polymethylacrylate, polymethylacrylate, polymethyl methacrylate, polyethylmethacrylate, polybutylacrylate, polybutylmethacrylate, polyethylene glycol acrylate, polyethylene glycol methyl acrylate, polyethylene glycol dimethylacrylate, polyvinylacetate, polyethylene oxide, polypropylene oxide, polysiloxane, copolymer of vinylidene fluoride and tetrafluoroethylene, copolymer of trifluoroethylene and vinylidene fluoride and/or copolymer of hexafluoropropylene and polyethylene. The water-insoluble organic solvent is ethylene carbonate, polypropylene carbonate, butylene carbonate, dimethyl carbonate, tetrahydrofuran, 2-methyl tetrahydrofuran, 1,3-dioxirane, 4,4-dimethyl-1,3-dioxirane, gamma -butylolactone, acetonitrile, polyethylene glycol dimethyl ether, polyethylene glycol diethylether, polyethylene glycol dipropyl ether, polyethylene glycol dibutyl ether, polyethylene glycol diglycidyl ether, polypropylene glycol dimethyl ether or polypropylene glycol diglycidyl ether. The lithium salt is lithium perchlorate, lithium tetrafluoroborate, lithium trifluoromethane sulfonate, lithium hexaflurophosphate, lithium hexafluoroarsenate, lithium bis(trifluoromethylsulfonyl)imide or lithium bis(oxalato)borate.