Unveiling performance evolution mechanisms of MnO2 polymorphs for durable aqueous zinc-ion batteries

MnO2-based aqueous Zn-ion batteries (ZIBs) hold great promising for large-scale energy storage applications owing to their safe and sustainable nature. However, rapid capacity decay under high depth of discharge limits the applications of MnO2 cathodes. In the meantime, the reaction chemistry and degradation process of MnO2 cathodes cannot be fully understood, leading to improvement of their cycling stability lacks of robust methods. Herein, ZIB performances of MnO2 polymorphs are investigated to disclose their detailed reaction chemistry and degradation mechanisms. Ex situ characterizations at different cycles exhibit evolution of active materials (original MnO2→Mn2+→birnessite→ZnMn2O4/Mn3O4) and coexisted reactions from co-insertion, dissolution/deposition and chemical conversion mechanisms. Variational contributions from these intermediate products and different reaction mechanisms cause fluctuated performance during cycling. Initial performance activation is from enhanced activity of birnessite, while the degradation is caused by its conversion to electrochemically inactive ZnMn2O4 and Mn3O4. By optimizing tunnel structures, it is found that R-MnO2 shows low manganese dissolution with its reaction mainly achieved by intercalation/extraction of Zn2+/H+, and theoretical calculations verify its low Jahn-Teller distortion at discharged state. This specific property circumvents conversion of R-MnO2 to metastable birnessite, giving rise to a stable capacity under high depth of discharge.