PolyPlus Lithium-Water Lithium metal is not directly compatible with water. However, the high gravimetric capacity of lithium metal, 3800 mA/g, and its highly negative standard electrode potential, Eo = -3.045 V, make it extremely attractive when combined as an electrochemical couple with oxygen or water. At a nominal potential of about 3 volts, the theoretical specific energy for a lithium/air battery is over 5000 Wh/kg for the reaction forming LiOH (Li + ¼ O2 + ½ H2O = LiOH) and 11,000 Wh/kg for the reaction forming Li2O2 (Li + O2 = Li2O2) or for the reaction of lithium with seawater, rivaling the energy density for hydrocarbon fuel cells and far exceeding Li-ion battery chemistry that has a theoretical specific energy of about 400 Wh/kg. With the invention of the protected lithium electrode (PLE), PolyPlus has introduced a unique technology that makes lithium metal electrodes compatible with aqueous and aggressive non-aqueous electrolytes, and enables the development of a new class of high energy density batteries. PolyPlus uses a solid electrolyte membrane to prevent direct electron transfer from the negative electrode to species in the aqueous electrolyte, therefore extending the voltage window from the oxidative limit of the aqueous electrolyte to the lithium electrode potential (~ 4.5 V). This technology allows the construction of practical aqueous lithium batteries with cell voltages similar to those of conventional Li-ion or lithium primary batteries, but with much higher energy density (for H2O or O2 cathodes). We have observed that the PLE is remarkably stable to aqueous electrolytes and does not appear to be susceptible to parasitic side reactions that can lead to self-discharge in batteries. The availability of a PLE enables the development of a new class of stable, high voltage (~ 3 V) aqueous batteries with exceptionally high energy density (> 1000 Wh/l & Wh/kg).