A secondary cell employs a nonaqueous electrolyte solution including a nonaqueous solvent and a salt, and a flame retardant material that is liquid at room temperature and pressure and substantially immiscible in the nonaqueous electrolyte solution. The nonaqueous electrolyte solution is formed by dissolving a salt, preferably an alkali metal salt, in a nonaqueous solvent. The nonaqueous solvent preferably includes a cyclic carbonate and/or a linear carbonate. The cyclic carbonate preferably contains an alkylene group with 2 to 5 carbon atoms, and the linear carbonate preferably contains a hydrocarbon group with 1 to 5 carbon atoms. Preferred salts include LiPF.sub.6 and LiBF.sub.4 at a concentration between about 0.1 and 3.0 moles/liter. The flame retardant material is preferably a halogen-containing compound in an amount by weight of nonaqueous solvent in a range of about 1 to about 99 wt %, and preferred halogen-containing compounds contain perfluoralkyl or perfluorether groups.