Cation Symmetry Breaking in Molten-Salt Electrolytes Enables Reversible, Long-Duration Storage in Al Batteries

Aluminum is the most earth-abundant metal, is trivalent, and offers a volumetric energy density more than twice that of lithium. It has emerged in recent years as among the most promising candidate materials for cost-effective, long-duration storage of electrical energy in batteries. Scientific discoveries in the past decade have established that rechargeable Al batteries can be created by pairing an Al metal foil with a graphitic carbon sheet in imidazolium-based ionic-AlCl₃ electrolytes. Although such cells are of increasing scientific interest, their utility remains limited by the high cost and environmental sensitivity of the ionic liquid, as well as by the surprisingly poor reversibility of Al electrodes in other electrolyte chemistries. We report that application of <i>Carnelley’s rule</i> to short-chain alkylammonium chlorides enables design of new families of low-cost alkylammonium chloride-AlCl₃ molten-salt electrolytes that support highly reversible redox reactions at an Al electrode. Through straightforward manipulation of the composition of [AlCl₄]^- and [Al₂Cl₇]^- species in electrochemical cells, we achieve near-unity efficiency for thousands of charge/discharge cycles and conformal Al electrodeposit morphology at a planar substrate. As a first illustration of the practical relevance of our findings, Al||graphite full cell batteries are created and reported to support stable charge/discharge cycling for more than 1000 cycles. Further, the successful development of this new group of electrolytes creates a robust platform to study in detail the working mechanisms of the electrodes using advanced characterization tools. Our findings open up new opportunities for developing simple, cost-effective, room-temperature Al batteries that enable long-duration electrical energy storage.