Transforming Mn ₃ O ₄ into MnOOH via Electrochemical Reconstruction for Aqueous Ammonium‐Ion Storage

Aqueous ammonium‐ion batteries (AAIBs) have gained considerable attention as a promising solution for sustainable energy storage owing to the natural abundance, cost‐effectiveness, and biocompatibility of ammonium‐ion (NH 4 + ) carriers, along with the intrinsic safety of aqueous electrolytes. Despite the considerable potential of manganese‐based oxides for NH 4 + storage derived from their multiple valence states and diverse structures, existing research remains focused on Mn 4+ ‐based oxides, with low‐valent manganese oxides largely unexplored. Herein, it is demonstrate that Mn 3 O 4 nanoparticles can undergo in situ electrochemical reconstruction during cycling in aqueous (NH 4 ) 2 SO 4 electrolyte, transforming into MnOOH (e‐MnOOH) nanorods with an open channel structure and abundant defects. The resulting e‐MnOOH exhibits significantly enhanced NH 4 + storage performance, providing a high specific capacity of 170.7 mAh g −1 at 0.3 A g −1 , good rate capability. Besides, it also maintains 98.9% capacity retention after 300 cycles at 0.5 A g −1 . Ex situ characterizations reveal that the charge storage process is regulated by surface pseudocapacitance, which involves reversible Mn 3+ /Mn 4+ redox and NH 4 + adsorption/desorption via hydrogen bonding. Furthermore, full AAIB is assembled by combining with an e‐MnOOH cathode and PTCDA anode, which delivers a specific capacity of 91.7 mAh g −1 and good cycling durability. This work not only unveils the electrochemical reconstruction behavior of low‐valent Mn 3 O 4 for NH 4 + storage but also provides insights into the pseudocapacitive NH 4 + storage in e‐MnOOH beyond conventional MnO 2 .