Evaluating a Lithium-Seawater Battery on Gliders

Neutral-buoyancy vehicles demand high-density energy sources and lithium is light with high oxidation energy. PolyPlus Battery Company has developed a prototype lithium-seawater battery that is attractive for powering long-duration autonomous oceanographic vehicles (floats and underwater gliders). These batteries were tested in the laboratory and at sea. PolyPlus batteries use Protected Lithium Electrodes'' with proprietary windows'' protecting the volatile lithium anode from water while passing lithium ions. The cathode reduces oxygen dissolved in seawater, or hydrolyzes seawater to produce hydrogen. Not requiring additional electrolyte, fuel, or pressure cases, these cells have impressive weight advantages. Good electrode-seawater mass transfer is required but can increase drag and be impeded by biofouling. Tests assessing robustness of the PolyPlus batteries in oceanographic use, evaluating mass transfer issues, and observing biofouling impacts are reported. In sea trials, two cells were tested for 69 days mounted on a Spray glider. Findings are as follows: 1) the cells were robust over 900 dives, most to 400 m; 2) without antifouling measures, the cells became substantially biofouled, but their performance was undiminished; and 3) performance was complex, depending on current density, oxygen concentration, and flow conditions. For dissolved oxygen concentration above 1mL L-1, the cells delivered 9Wm(-2) of electrode surface at 3V. For low oxygen, the cell shifted to hydrolysis near 2.3 V, but mass transfer was less critical so current density could be increased and observed power reached 5Wm(-2). This could be increased using a lower resistance load.