An improved electrolyte for a cell having an anode comprising magnesium or magnesium alloy. The cell's cathode may desirably include iron disulfide (FeS2) as cathode active material. The improved electrolyte comprises a magnesium salt, preferably magnesium perchlorate dissolved in an organic solvent which preferably includes acetonitrile or mixture of tetrahydrofuran and propylene carbonate. The electrolyte includes an additive to retard the buildup of deleterious passivation coating on the magnesium anode surface, thereby enhancing cell performance. Such additive may preferably include 1-butyl-3-methylimidazolium tetrafluoroborate (BMIMBF4), 1-butyl-3-methylimidazolium hexafluorophosphate (BMIMPF6), lithium hexafluorophosphate (LiPF6), or aluminum chloride (AlCl3)