Structural Transformation by Crystal Engineering Endows Aqueous Zinc‐Ion Batteries with Ultra‐long Cyclability

Manganese oxide is a promising cathode material for aqueous zinc batteries. However, its weak structural stability, low electrical conductivity, and sluggish reaction kinetics lead to rapid capacity fading. Herein, a crystal engineering strategy is proposed to construct a novel MnO2 cathode material. Both experimental results and theoretical calculations demonstrate that Al-doping plays a crucial role in phase transition and doping-superlattice structure construction, which stabilizes the structure of MnO2 cathode materials, improves conductivity, and accelerates ion diffusion dynamics. As a result, 1.98% Al-doping MnO2 (AlMO) cathode shows an incredible 15 000 cycle stability with a low capacity decay rate of 0.0014% per cycle at 4 A g-1 . Additionally, it provides superior specific capacity of 311.2 mAh g-1 at 0.1 A g-1 and excellent rate performance (145.2 mAh g-1 at 5.0 A g-1 ). To illustrate the potential of 1.98%AlMO to be applied in actual practice, flexible energy storage devices are fabricated and measured. These discoveries provide a new insight for structural transformation via crystal engineering, as well as a new avenue for the rational design of electrode material in other battery systems.