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 Whakg^-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.