Abstract: A reversible high energy capacity battery and method of making that battery. A cathode is formed by packing a mixture of from about 10 to 90 weight percent finely divided sulfur and from about 90 to 10 weight percent finely divided graphite about an electrically conductive electrode, preferably in a porous enclosure. This cathode and a reactive metal anode are placed in a case of suitable configuration which is non-reactive with other components. An electrolyte is prepared by mixing particles of an ion exchange resin system including a polyalkyl sulfide, a resin substrate and agent selected from the group consisting of H.sub.2 S, HS.sup.-, S.sub.2.sup.- and mixtures thereof in a polar solvent, such as water. Buffering agents, conditioners and complexing agents may be added to the electrolyte to improve battery life and performance.