Vinyl-functionalized SiO2 particles of different sizes are synthesized and coated onto both sides of a polyethylene separator to prepare a reactive composite separator for lithium-ion polymer cells. The SiO2-coated composite separators exhibit excellent thermal stability due to the presence of heat-resistant silica particles. By using these reactive composite separators and a gel electrolyte precursor, lithium-ion polymer cells composed of a graphite negative electrode and a LiNi1/3Co1/3Mn1/3O2 positive electrode are assembled by in-situ chemical cross-linking, and their cycling performance is evaluated. The cells assembled with a reactive composite separator exhibit superior cycling performance to cell prepared with a conventional polyethylene separator due to the strong interfacial adhesion between the electrodes and separator, as well as suppression of deleterious reactions during cycling.