Abstract: A polymer-ceramic composite electrolyte is provided which may be formed into a thin film having a room temperature conductivity of from 10.sup.-5 S cm.sup.-1 to 10.sup.-3 S cm.sup.-1. In one embodiment, the composite electrolyte comprises from about 30 to 60% by weight poly(ethylene oxide), from about 10 to 20% by weight lithium tetrafluoroborate, and from about 25 to 60% by weight lithium nitride. The film is preferably produced by mixing and grinding the components, then placing the ground mixture in a die and compacting the mixture to form a disc which is then flattened. The resulting film is annealed to ensure high conductivity at room temperature.