Aluminum ion battery (AIB) technology is an exciting alternative for post-lithium energy storage. AIBs based on ionic liquids have enabled advances in both cathode material development and fundamental understanding on mechanisms. Recently, unlocking chemistry in rechargeable aqueous aluminum ion battery (AAIB) provides impressive prospects in terms of kinetics, cost, safety considerations, and ease of operation. To review the progress on AAIB, we discuss the critical issues on aluminum electrochemistry in aqueous system, cathode material design to overcome the drawbacks by multivalent aluminum ions, and challenges on electrolyte design, aluminum stripping/plating, solid-electrolyte interface (SEI) formation, and design of cathode materials. This review aims to stimulate exploration of high-performance AAIB and rationalize feasibility grounded on underlying reaction mechanisms.

Considering the high safety, low-cost and high capacity, aqueous zinc ion batteries have been a potential candidate for energy storage ensuring smooth electricity supply. Herein, we have synthesized inverse opal manganese dioxide constructed by few-layered ultrathin nanosheets by a solution template method at mild temperature. The ultrathin nanosheets with the thickness as small as 1 nm are well separated without obvious aggregation. Used as cathode material for aqueous zinc ion batteries, the few-layered ultrathin nanosheets combined with the inverse opal structure guarantee excellent performance. A high specific discharge capacity of 262.9 mAh·g^-1 is retained for the 100th cycle at a current density of 300 mA·g^-1 with a high capacity retention of 95.6%. A high specific discharge capacity of 121 mAh·g^-1 at a high current density of 2, 000 mA·g^-1 is achieved even after 5, 000 long-term cycles. The ex-situ X-ray diffraction (XRD) patterns, selected-area electron diffraction (SAED) patterns and high-resolution transmission electron microscopy (HRTEM) results demonstrate that the discharge/charge processes involve the reversible formation of zinc sulfate hydroxide hydrate on the cathode while in-plane crystal structure of the layered birnessite MnO₂ could be maintained. This unique structured MnO₂ is a promising candidate as cathode material for high capacity, high rate capability and long-term aqueous zinc-ion batteries.