The present invention relates to a method of making a polymeric porous separation membrane for a lithium ion polymer battery, the method comprising: providing a porous polyolefin membrane and a polymer solution of the concentration of 1 20 weight % in an organic solvent fabricated by dissolving in an organic solvent a polymer compound that is stable in an electrolyte containing a lithium salt and can be gelled in the electrolyte, at the temperature of 10 40.degree. C.; and fabricating a polymeric porous separation membrane by coating the polymeric porous polyolefin membrane with the polymer solution. Thus, the porous separation membrane for a lithium ion polymer battery is fabricated by using the polymer solution fabricated at the low temperatures of 10 40.degree. C., and the size of the pores of the coated is proper, so that the containing capacity of the electrolyte and the ion conductivity are good. Further gelling at low pressure is enabled and cohesive force to the electrodes is improved. In the lithium ion battery containing the porous separation membrane, the gelling is accomplished at atmospheric pressure or at low pressure for a few hours, so that problems caused by gelling for a short period at high pressure and high temperature such as the non uniform gelling of the battery, the deformation of the separation membrane and the weakened cohesion can be avoided. Moreover, the high-rate discharge, the cycle life becomes excellent.

 
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