In the preferred embodiment, the separator comprises a mixture of an ionically conductivepolymeric material, such as, for example, polyethers such as polyethylene oxide (PEO), polypropylene oxide (PPO), copolymers of the above polymers, or amorphous polyethylene oxide such as oxymethylene-linked polyethylene oxides; and a stable, conductivealkali metal salt such as sodium perchlorate or lithium pe