TECHNOLOGY FOCUS - POLYMERS - Preferred Components: Each of the segments A is a polymer consisting of polystyrene, polyethylene or polypropylene and having a weight average molecular weight of 10,000-500,000 and a content of the segments A in the block copolymer is 0.5-70 wt.% and each of the segments B is a polymer consisting of polybutadiene, polychloroprene or polyisoprene and having a weight average molecular weight of 10,000-300,000. The solidifying material is in the form of a film or sheet of 0.0001-2 mm in thickness. It may be a graft copolymer. Each of segments A is a polymer consisting of polystyrene, polyethylene, polypropylene, polyacrylonitrile or poly (meth)acrylate ester having a weight average molecular weight of 3,000-20,000, and a content of the segments A in the graft copolymer is 0.5-70 wt.%. The solidifying material comprises a film or sheet of a polymer having properties that the polymer is insoluble in the cell electrolyte solution but the polymer absorbs and solidifies the cell electrolyte solution, and a backing reinforcing the film or sheet. The backing is a woven fabric, nonwoven fabric or porous film. The polymer comprises as principal component, polyacrylic acid, poly (N-vinylacetamide), poly((2-oxo-1,3-dioxoran-4-yl)methyl (meth)acrylate) or polyacrylamide. The solidifying material is in the form of particles having an average particle size not greater than 100 microns. The backing is made of polyethylene or polypropylene. The backing is a film or sheet of 1-1200 microns thickness and 95-100% porosity.Preferred Composition: The solidifying material further comprises not greater than 85 wt.% based on the block copolymer, of an elastomer non-compatible with the cell electrolyte solution. EXAMPLE - A block copolymer (15 parts) composed of polystyrene, polybutadiene and polystyrene was dissolved in a mixed solvent formed of toluene (45 parts), cyclohexane (75 parts) and methyl ethyl ketone (35 parts), and the resulting solution was heated to 70 degrees C. Into the solution, thioglycolic acid (20 parts) and azobisisobutyronitrile (0.3 part) were added and reacted for 12 hours. The mixture was washed with a saturated aqueous solution of sodium sulfate, and 15% solution of potassium hydroxide in methanol was added. The solvent was distilled off. The particles had a particle size of 100 nm. The solidifying material taken out of the solution had a swelling index of 3000% in deionized water.