A composite electrolyte comprises (a) surface modified fumed silica filler, wherein the surface modified fumed silica comprises polymerizable groups on the surface thereof, the polymerizable groups being bonded to each other such that the surface modified fumed silica filler is crosslinked in a three-dimensional structure; (b) a dissociable lithium salt; and (c) a bulk medium which contains the surface modified fumed silica filler and the dissociable lithium salt. An electrochemical cell comprises an anode, a cathode, and a composite electrolyte dispersed between the anode and cathode.