The electrolyte solution for a primary nonaqueous lithium metal air battery is optimized. A novel oxygen selective membrane consisting of Teflon coated fiberglass cloth is used to prevent corrosion of the lithium metal negative electrode by water vapor during discharge in ambient air. The air electrode capacity is increased by increasing the amount of dimethyl carbonate and methoxybenzene in the electrolyte solvent, increasing the lithium tetrafluoroborate concentration to 1.5M, and adding 4 mass % pentafluorophenylboron oxalate. Cells using either the starting or optimized electrolyte were discharged at a current density of 0.1mAcm−2 in air at 23°C and 20% relative humidity. The discharge capacity (averaged for five discharges using each electrolyte solution) was increased by a factor of 2.2 for the cells using the optimized electrolyte solution of 1.5M lithium tetrafluoroborate in propylene carbonate:dimethyl carbonate:methoxybenzene (1:5:1 by volume)+4 mass % pentafluorophenylboron oxalate.