  Aluminum is the most earth-abundant metal, is trivalent, is inert in ambient
humid air, and has a density approximately four-times that of lithium at room
temperature. These attributes make it an attractive material for
cost-effective, long-duration storage of electrical energy in batteries.
Scientific discoveries in the past decade have established that secondary Al
batteries can be created by paring an Al anode with a graphite or transition
metal oxide cathode, in imidazolium-based, room-temperature
ionic-liquid-Aluminum chloride electrolytes. Here we report findings from a
systematic study that sheds light on the structural requirements,
physicochemical, and transport properties of the ionic liquid electrolytes
responsible for the high reversibility of Al battery anodes. We find that the
most important interfacial and transport properties of these electrolytes can
be achieved in other electrolytes, including ammonium-based molten salts that
are available at costs as much as twenty-times lower than the ionic
liquid-Aluminum chloride melt. High Al reversibility in ammonium- and
imidazolium-based electrolytes is specifically shown to require a critical
ratio of the solvation species, where Lewis acidity and beneficial interfacial
reactions continuously etch the alumina resistive interfacial layer and form
beneficial solid electrolyte interphase at the anode. We report further that
successful development of new electrolyte families that support high Al anode
reversibility also provides a good platform for detailed studies of the working
mechanisms of an intercalation graphite cathode using X-ray absorption
spectroscopy. Our findings therefore open new opportunities for developing
simple, cost-effective, room-temperature Al batteries that enable long-duration
electrical energy storage.
