TECHNOLOGY FOCUS - ORGANIC CHEMISTRY - Preferred Composition: The isocyanate is at least one selected from tolylene 2,4-diisocyanate, tolylene 2,6-diisocyanate, diphenylmethane 4,4'-diisocyanate, hexamethylene diisocyanate, diphenylmethane diisocyanate, isophoprone diisocyanate, triphenylmethane diisocyanate , tris-(isocyanatephenyl)thiophosphate, lysine ester triisocyanate, 1,8-diisocyanate-4-isocyanatemethylocartane, undecane 1,6,11-triisocyanate, hexamethylene 1,3,6-triisocyanate and bicycloheptane triisocyanate. The cross-linking agent is at least one selected from glycerol ethoxylate, glycerol propoxylate, 3-methyl-1,3,5-pentanetriol and caprolactone. The organic solvent is at least one solvent selected from propylene carbonate, ethylene carbonate, dimethyl carbonate, methylethyl carbonate, diethyl carbonate and vinylene carbonate. The total weight of the organic solvent and the lithium salt is 3-30 times of the pre-polymer.    TECHNOLOGY FOCUS - INORGANIC CHEMISTRY - Preferred Lithium Salt: The lithium salt is at least one selected from lithium perchlorate (LiClO4), lithium tetrafluoroborate (LiBF4), lithium hexafluorophosphate (LiPF6), lithium trifluoromethanesulfonate (LiCF3SO3) and lithium bistrifluoromethanesulfonyl amide (LiN(CF3SO2)2). The amount of lithium salt in the cross-linked polyether urethane, is 0.5-2 M.    TECHNOLOGY FOCUS - POLYMERS - Preferred Battery: The battery comprises a separator having a network structure made of an insulating resin, provided between cathode and anode. The separator is formed from polypropylene and/or polyethylene. The cathode or anode comprises a current collector, and an anode active material layer formed on the current collector. The electrode active layer comprises an electrode active material, a binder, a conductive agent and a solvent.Preferred Method: The pre-polymer is obtained by reacting isocyanate with a glycol selected from polyethylene glycol and/or polypropylene glycol. The separator is interposed between the cathode and anode to form an electrode assembly. The electrode assembly is accommodated into a battery case. The mixture of pre-polymer, cross-linking agent, organic solvent and lithium salt, is injected into the battery case and cross-linked the resultant product by heating at 25-65 degrees C. The electrode active layer formed by mixing electrode active material, binder, conductive agent and solvent, is coated on the current collector to form cathode or anode. Alternatively, the cathode or anode is formed by coating mixture of electrode active layer formed by mixing electrode active material, binder, conductive agent and solvent on the support, peeling the electrode active material from the support, and laminating the peeled electrode active material on the current collector to form cathode or anode. EXAMPLE - Polyethylene glycol (in g) (4) having molecular weight of 400, and hexamethylene diisocyanate (4.205), were reacted at 65 degrees C in the presence of 1 weight% of dibutyltin dilaurate catalyst (0.092), to form a pre-polymer having a polyethylene backbone and terminated with isocyanate. Subsequently, the pre-polymer (0.085) was mixed with glycerol ethoxylate cross-linking agent (0.077), and mixed solution (2.92) containing 1.3 M lithium phosphorous hexafluoride, and ethylene carbonate/propylene carbonate/diethyl carbonate in ratio of 41:49:10, and dibutyl dilaurate (0.0235). The mixture (3) was added into a pouch having a windable gelly roll, sealed and maintained for 2 days. Then, the resultant product was thermally cross-linked 65 degrees C for 4 hours and a polyether urethane polymeric electrolyte was obtained. The polymeric electrolyte was electrochemically stable even at 5 V or higher, and used for lithium secondary battery. Reliability and safety of the battery were improved.