This paper describes the fabrication and performance of microporous gel electrolyte (MGE) Li-ion batteries. Fabrication of the MGE battery includes three steps: (1) making a microporous membrane separator by the phase inversion method, (2) activating the battery with a liquid electrolyte, and (3) forming MGE in situ by cycling the battery. The MGE contains three phases: liquid electrolyte, gel electrolyte, and a polymer matrix. MGE has advantages of high ionic conductivity, good adhesion to the electrodes, and good mechanical strength. In this work, poly(vinylidene fluoride-co-hexafluoropropylene) copolymer was used as the polymer matrix, and a solution of 1.0 m LiBF4 dissolved in 1:3 (wt.) mixture of ethylene carbonate and gamma-butyrolactone was used as liquid electrolyte. Typically, a MGE gelled with 120 wt.% of liquid electrolyte shows an ionic conductivity of 3.4 mS/cm at 20degreesC and the resulting battery can withstand high temperature (60degreesC) operation with low capacity fading. (C) 2003 Elsevier B.V. All rights reserved.