An Inverse Aluminum Battery: Putting the Aluminum as the Cathode

Aluminum has long been regarded as a promising anode for energy storage because of its high energy density and low cost, but its application is hindered by the inability of cathodes to provide reversible Al3+ insertion. In contrast, we show how the use of Al as cathode enables a rechargeable high-energy battery. The battery comprises a molten sodium anolyte and a molten NaAl2Cl7 catholyte, separated by a NaSICON solid Na+ electrolyte. It is operated at 200 °C to overcome the ceramic separator kinetics and to keep sodium and NaAl2Cl7 in the molten state. Because of the simple composition and trivalence of Al, the sodium anolyte and NaAl2Cl7 catholyte together show a high energy density of 366 Wh kg–1, although its voltage is only about 1.55 V and only 60% of the capacity can be realized. The high energy density, low-cost, and internal safety make this new cell chemistry applicable to the large-scale energy storage market.