The presence of a metal sulfide has been discovered to be important for the effective deposit of sulfur on the iron anode. A battery with an iron anode seems to work better with sodium sulfide in the electrolyte, as the sulfide ends up in the iron anode as a performance enhancer after a few cycles. The sodium sulfide in essence is believed to increase the effective surface area of the iron, so one obtains more utilization of the iron. The capacity and power is therefore improved. In addition, the added sulfide is believed to form iron sulfides, two of the forms being FeS and Fe₂S₃, both of which are more electrically conductive than Fe(OH)₂ which normally forms on the iron surface. These conductive sites on the iron surface create a situation in which the oxidized layer that forms on the iron surface is thicker before true electrical passivation occurs allowing for more efficient use of the underlying iron active material. Various sulfide salts may be employed to obtain this desirable result. In one embodiment, the sulfide salt is sodium sulfide. Overall, it has been found that use of the present electrolyte improves specific power and power density of the battery. Improvements in the cycle life, the capacity (charge retention) and specific energy of a standard Ni—Fe battery are also improved.

While the use of metal sulfides such as sodium sulfide is described above, it will be understood that other sulfide compounds of suitable solubility may also be used. Examples of such sulfides include inorganic sulfides with sufficient solubility, but also organic sulfur compounds known to decompose in the electrolyte to inorganic sulfide.