Investigations on Tunnel‐Structure MnO₂ for Utilization as a High‐Voltage and Long‐Life Cathode Material in Aqueous Ammonium‐Ion and Hybrid‐Ion Batteries

Recently, nonmetal NH₄ ⁺ ions have attracted extensive attention for use as charge carries in the field of energy storage due to their abundant resources, environmental friendliness, and low cost. However, the development of aqueous ammonium-ion batteries (AAIBs) is constrained by the absence of high-voltage and long-life materials. Herein, different tunnel-structure MnO₂ materials (α-, β-, and γ-MnO₂) are utilized as cathodes for AAIBs and hybrid-ion batteries and compared, and α-MnO₂ is demonstrated to exhibit the most remarkable electrochemical performance. The α-MnO₂ cathode material delivers the highest discharge capacity of 219 mAh g^-1 at a current density of 0.1 A g^-1 and the best cyclability with a capacity retention of 95.4% after 10 000 cycles at 1.0 A g^-1. Moreover, aqueous ammonium-ion and hybrid-ion (ammonium/sodium ions) full batteries are successfully constructed using α-MnO₂ cathodes. This work provides a novel direction for the development of aqueous energy storage for practical applications.