Membrane-less Aluminum Displacement Batteries Based on Transition Metal Chlorination in Molten Salts

Stationary electrochemical energy storage calls for low-cost and high-safety next-generation chemistries, among which the Na-NiCl₂ battery based on the displacement reaction stands out but is threatened by the fragile β″-Al₂O₃ membrane. Here we present a class of low-lost, medium-temperature (150 °C), membrane-less aluminum displacement batteries (ADBs) based on a reversible solid-to-solid displacement reaction between transition metals (TM; Fe, Co, Ni) and their chlorides (TMCs) in alkali chloroaluminate molten salts. Crucially, manipulation of the Lewis acidity of the chloroaluminate molten salt electrolyte enables membrane-less operation of the cell by restricting solubility of the TMC and its crosstalk with the Al negative electrode. The structured Ni cathode exhibits a high capacity of 380 mA h g^-1, low overpotential of ∼50 mV at the rate of 0.2 A g^-1, and high stability over 500 cycles. Furthermore, we demonstrate a membrane-less Al-Fe displacement battery that promises an ultralow-cost avenue for stationary energy storage.