TECHNOLOGY FOCUS - POLYMERS - Preferred Components: The thermoplastic resin contains units of formula (I)-(C(=O)-(CH2)m-O)n- (I)m = 3-5;n = at least 5.The separator is composed of separator base impregnated with electrolyte solution, or gel electrolyte.Preferred Composition: The thermoplastic resin contains (in wt.%, based on the overall thermoplastic resin) thermoplastic polyurethane resin (1-100), prepared by reacting a polyol compound with a polyisocyanate compound and a chain extender.Preferred Compound: The polyol compound is polyester polyol, polyester polyether polyol, polyester polycarbonate polyol, and/or polycaprolactone polyol.Preferred Property: The gel electrolyte has an ionic conductivity approximately s1 (S/cm) and an ionic conductivity approximately s2 (S/cm), (both measured by alternating current impedance method at 25 degreesC). The ratio of approximately s1/approximately s2 is 1 to 10. The thermoplastic resin in the binder composition has a glass transition temperature lower than the freezing point of the electrolyte solution. EXAMPLE - Praccel 220N (preheated and dehydrated polycaprolactone diol) (64.34 pbw) and 4,4'-diphenylmethane diisocyanate (28.57 pbw) were charged to a reactor. The reactor contents were stirred and mixed for 2 hours at 120 degreesC under a stream of nitrogen, and further added with 1,4-butanediol (7.09 pbw). The reaction was further effected at 120 degreesC under a stream of nitrogen. When the reaction product became rubbery, the reaction was stopped. The reaction product was removed and then heated at 100 degrees C for 12 hours. The resulting polyurethane resin, having a weight average molecular weight of 100a, was coated with a doctor blade, and then dried under reduced pressure at 120 degrees C for 2 hours to form polyurethane film. The film was immersed in electrolyte solution prepared by dissolving lithium hexafluorophosphate to a mixed solvent comprising ethylene carbonate, dimethyl carbonate and methyl carbonate, for 24 hours at 20 degrees C. The film had a swelling ratio of 320%.