High-Performance MnO 2 /Al Battery with In Situ Electrochemically Reformed Al x MnO 2 Nanosphere Cathode.

Aqueous Al-ion battery (AAIB) is regarded as a promising candidate for large-scale energy storage systems due to its high capacity, high safety, and low cost, with MnO 2 proved to be a high-performance cathode. However, the potential commercial application of this type of battery is plagued by the frequent structural collapse of MnO 2 . Herein, an in situ, electrochemically reformed, urchin-like Al x MnO 2 cathode is developed for water-in-salt electrolyte-based AAIBs. Benefiting from its unique α-MnO 2 coated Mn 2 AlO 4 structure, a high Al ion storage capacity is achieved together with a high discharge voltage plateau of 1.9 V by reversible MnO 2 electrolysis. Consequently, the battery exhibits a high specific capacity of 285 mAh g -1 and a high energy density of 370 Wh kg -1 at a high current density of 500 mA g -1 . Improved stability with record capacity retention is also obtained at an ultrahigh current density of 5 A g -1 after 500 cycles. Such a high-capacity and high-stability Al x MnO 2 cathode would pave the way for in situ electrochemical transformation of cathode design and thus boost the practical application of AAIBs.