Unveiling the Energy Storage Mechanism of MnO<sub>2</sub> Polymorphs for Zinc‐Manganese Dioxide Batteries
Qingyong Zhang, Jun Zhao, Xiuyuan Chen, Ruijie Yang, Ting Ying, Chong Cheng, Bilu Liu, Jun Fan, Shuang Li, Zhiyuan Zeng
Abstract
Abstract The energy storage mechanism of MnO 2 in aqueous zinc ion batteries (ZIBs) is investigated using four types of MnO 2 with crystal phases corresponding to α‐, β‐, γ‐, and δ‐MnO 2 . Experimental and theoretical calculation results reveal that all MnO 2 follow the H + and Zn 2+ co‐intercalation mechanism during discharge, with ZnMn 2 O 4 , MnOOH, and Zn 4 (SO 4 )(OH) 6 ·4H 2 O being the main products. ZnMn 2 O 4 is formed from Zn 2+ intercalation, while MnOOH and Zn 4 (SO 4 )(OH) 6 ·4H 2 O are formed from H + intercalation. Charging generates MnO 2 through the extraction of Zn 2+ and H + from ZnMn 2 O 4 and MnOOH, indicating reversible reactions. The study also reveals that H + intercalation exhibits better kinetics than Zn 2+ intercalation due to its higher diffusion ability and easier charge transfer. The crystal structures and diffusion energy barriers differences are responsible for the differences of H + and Zn 2+ diffusion in MnO 2 , with the layered δ phase exhibiting the lowest diffusion energy barrier and highest apparent diffusion coefficient, while the α phase exhibits the highest diffusion energy barrier and lowest apparent diffusion coefficient. These findings highlight the importance of crystal phases in determining the diffusion ability and energy storage of MnO 2 , which can inform strategies for design of multiphase MnO 2 cathodes for high‐performance ZIBs.