New Insights into Phase‐Mechanism Relationship of MgxMnO2 Nanowires in Aqueous Zinc‐Ion Batteries

In response to the call for safer energy storage systems, rechargeable aqueous manganese-based zinc-ion (Zn-ion) batteries using mild electrolyte have attracted extensive attention. However, the charge-storage mechanism and structure change of manganese-based cathode remain controversial topics. Herein, a systematic study to understand the electrochemical behavior and charge storage mechanism based on a 3 × 3 tunnel-structured Mgx MnO2 as well as the correspondence between different tunnel structures and reaction mechanisms are reported. The energy storage mechanism of the different tunnel structure is surface faradaic dissolution/deposition coupled with an intercalation mechanism of cations in aqueous electrolyte, which is confirmed by in situ X-ray diffraction, in situ Raman and ex situ extended X-ray absorption fine structure. The deposition process at the cathode is partially reversible due to the accumulation of a birnessite layer on the surface. Compared to smaller tunnels, the 3 × 3 tunnel structure is more conducive to deposit new active materials from the electrolyte. Therefore, pristine Mgx MnO2 nanowires with large tunnels display an excellent cycling performance. This work sheds light on the relationship between the tunnel structure and Mn2+ deposition and provides a promising cathode material design for aqueous Zn-ion batteries.