Reversible Intercalation of Multivalent Al³⁺ Ions into Potassium‐Rich Cryptomelane Nanowires for Aqueous Rechargeable Al‐Ion Batteries

Abstract The development of new battery technology that utilizes abundant electrode materials that are environmentally benign is an important area of research. To alleviate the reliance on Li‐ion batteries new energy storage mechanisms are urgently needed. To address these issues, MnO 2 nanowires were investigated as a possible electrode material for use in rechargeable Al ion batteries that can operate in aqueous conditions. The use of this type of material and an aqueous electrolyte ensures safe operation as well as easy recycling of spent batteries. A potassium‐rich cryptomelane structure was presented, and a new mechanism of electrochemical energy storage was elucidated based on the intercalation and deintercalation of small‐radius Al 3+ ions interchanging with larger K + ions in the cryptomelane MnO 2 nanowires, which was supported by DFT calculations. This first‐time use of a cryptomelane MnO 2 cathode for an aqueous Al ion system yielded a discharge capacity of 109 mAh g −1 , which indicates the potential commercial viability of rechargeable aqueous Al‐ion batteries.

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