We describe the development of a class of ambient temperature solid electrolytes based on the incorporation of a small mole fraction, 5-10% repeat units, of high molecular weight polymer into a supercooled lithium salt mixture of sub-ambient glass transition temperature. Such materials constitute a new class of rubber in which the majority components are simple ions and, as such, offer a type of "solid" electrolyte which has many advantages over the current "superionic glass" and "salt-in-polymer" electrolytes. Indeed this class combines the best features of the other two, cationic conduction and rubbery compliances respectively, into a single material.