The polyvinyl acetal based polymer membranes are prepared via phase inversion method using polyvinyl formal (PVFM) as the polymer matrix, N-methyl pyrrolidone (NMP) as the solvent, 4,4'- diphenyl-methane diisocyanate (MDI) as the chemical crosslinking agent and deionized water as the nonsolvent, respectively. Three-dimensional network structure has been formed due to the polyurethane reaction between the hydroxyl group of PVFM and MDI, which contributes to the chemical stability of PVFM based membrane in LiFP6-based liquid electrolyte. The morphologies of the polymer membranes can be controlled in thermodynamic isothermal process to access porous and dense membranes. The dense membranes have good mechanical strength and thermal-resistant characteristics. The membranes absorb and swell liquid electrolyte to form stable gel polymer electrolytes (GPEs) with ionic conductivity of 1.77 x 10(-4) S/cm at room temperature and wide electrochemical window of 1.8-5.0 V (vs. Li/Li+). (C) 2013 Elsevier Ltd. All rights reserved.