Rechargeable aluminum batteries attracted growing attentions in recent years as one candidate beyond lithium ion battery system. This is due to the great potential of aluminum metal anode owing to its high capacity (2.98 Ah/g and 8.05 Ah/cc), its negative electrochemical potential (-1.67 V vs. NHE), and its highest abundancy among metals in earth crust. Moreover, aluminum has significantly lower reactivity towards moisture and air compared to Li, Na and Mg. Reversible Al deposition/stripping in room temperature is extremely challenging due to the tendency of Al to form real passivation on its surface with both poor ionic and electronic conductivity. Nevertheless, use of aluminum anode was shown to be possible by in ionic liquid based electrolyte (RTIL-AlCl3) with up to 100% coulombic efficiency for aluminum deposition/stripping.

As a high capacity cathode material (1,675 mAh/g), sulfur has attracted intense interest in Li/S, Mg/S and Na/S systems. A rechargeable Al/S battery is also of great interest because the full cell capacity could achieve 1072 mAh/g-(total electrode mass) and voltage could achieve ~1.25 V, estimated from the Gibbs formation energy of Aluminum sulfide( -724 kJ/mol). The gravimetric energy density is hence 1340 Wh/kg, over three times of a commercial LiCoO₂/graphite cell and close to that of a Li₂S/silicon cell. The high energy density and low cost of both Al and S makes Al/S system a very promising battery chemistry for large scale applications like electric vehicle and grid storage.  Motivated by the potential, Cohn et al. recently demonstrated a proof-of-concept non-aqueous Al/S battery. Though demonstrating close to theoretical capacity with operating voltage of 1.2 V, the reported Al/S cell cannot be recharged, resulting in the loss of all capacity within three cycles. Up to now, there was no demonstration of reversible Al/S battery, probably due to the inability to recharge AlSx.

In this work, we demonstrate a reversible Al/S cell with specific cathode capacity of ~1200 mAh/g_s for over 20 cycles. Kinetics analysis point out the system is limited by slow diffusion of Al containing anion due to the high viscosity of the ionic liquid electrolyte. Raising temperature is proved to be an effective approach to mitigate the kinetic limitation thus decrease the hysteresis during charge and discharge. Spectroscopic and microscopic observations explore the mechanism of sulfur redox in the RTIL-AlCl₃ electrolyte and enabled us to monitor the interfacial processes on both the cathode and anode. We believe that the new scientific insights obtained in this work will demonstrate inspiring progress on the way to realize a rechargeable Al/S battery.