Understanding the Tunnel-Structured Evolution in α-MnO2 Cathode Failure for Aqueous Zinc Ion Batteries.

Recently, extensive efforts have been devoted to aqueous Zn-Mn batteries owing to their inherent safety, high energy density, and affordable cost. Comprehensively unveiling the failure mechanism of hollandite cathode is conducive to developing efficient and stable cathodes that is of significant importance in improving their practical competitiveness. Herein, the tunnel-structured evolution of α-MnO2 during cation insertion and extraction processes has been monitored. Experimental results reveal that Ca- (Ca-MnO2) and Cr-doped α-MnO2 (Cr-MnO2) possess expanded tunnel dimension, delivering better charge transfer and ion diffusion capability than α-MnO2. Ex situ X-ray near-edge absorption spectroscopy measurements confirm the expanded tunnel structure and [MnO6] octahedra of the Ca-MnO2 feature breathable nature, enabling excellent ability in tolerating cation insertion/extraction, whereas Cr-MnO2 displayed a rigid expanded structure that fails to support repeated structural change. The expanded structure and breathable nature endow Ca-MnO2 with reduced polarization, boosted kinetics, suppressed Mn dissolution, and improved cycle stability, and the inherent or rigid structure enables α-MnO2 cathode structural collapse, showing rapid capacity fading. This work provides valuable insights into the origin of tunnel-structural evolution in α-MnO2 cathode failure and is promising in guiding the design of durable hollandite cathodes for rechargeable batteries.