Sodium metal-halide energy storage devices utilizing a substituting salt in its secondary electrolyte can operate at temperatures lower than conventional ZEBRA batteries while maintaining desirable performance and lifetime characteristics. According to one example, a sodium metal-halide energy storage device operates at a temperature less than or equal to 200.degree. C. and has a liquid secondary electrolyte having M.sub.xNa.sub.1-yAlCl.sub.4-yH.sub.y, wherein M is a metal cation of a substituting salt, H is an anion of the substituting salt, y is a mole fraction of substituted Na and Cl, and x is a ratio of y over r, where r is the oxidation state of M. The melting temperature of the substituting salt is less than that of NaCl.
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
 This invention was made with Government support under Contract DE-AC0576RLO1830 awarded by the U.S. Department of Energy. The Government has certain rights in the invention.