Abstract: In one aspect, an energy storage device component comprising a transition metal cathode comprising a transition metal selected from the group consisting of nickel, iron, cobalt, chromium, manganese, molybdenum, antimony and combinations thereof; a solid sodium halide phase; and an electrolyte phase is provided. The electrolyte phase comprises an electrolyte composition prepared from sodium chloride, lithium chloride and aluminum trichloride. The electrolyte composition is in contact with the cathode. The electrolyte composition comprises the reaction products obtained from an initial mixture of sodium chloride (NaCl), lithium chloride (LiCl) and aluminum trichloride (AlCl3). The initial mixture being characterized by an initial molar ratio of (NaCl+LiCl):AlCl3 in a range of from about 0.45:0.55 to about 0.55:0.45 and an initial molar ratio of NaCl:LiCl in a range of from about 0.1:1 to about 4:1. Also provided is an energy storage device and a method of operating the energy storage device.