A novel Li/H2O primary battery is reported. The battery consists of a thin film lithium metal anode evaporated on hybrid inorganic/organic lithium ion conductive solid electrolyte, H2O as the electroactive cathode material, and a Pt mesh current collector. The hybrid electrolyte consists of a two-phase ceramic, lithium aluminum titanium phosphate/aluminum phosphate (LATP/AlPO4), sealed with polyvinylidene fluoride. These cells display an open cell potential of 0.90V, a current output range between 0.78 and 0.50μA under constant 1MΩ load discharge, and specific energy of 2660Wh/kg. Battery life is shown to be proportional to the mass of the Li metal anode employed and has been demonstrated for up to 37.5h.