Unveiling the Self-Ion Exchange Mechanism in Na₂Mn₃O₇ for Enhanced Zinc-Ion Battery Performance

Manganese-based cathode materials have attracted significant interest in zinc-ion batteries (ZIBs) due to their high theoretical capacity, affordability, and environmentally friendly nature. Recently, Na₂Mn₃O₇ (NMO) has emerged as a promising electrode material for ZIBs owing to its unique triclinic crystal structure, which consists of infinite parallel [Mn₃O₇]²⁻ layers. This layered framework includes a vacancy at one of the seven Mn sites, which plays a crucial role in facilitating the insertion and accommodation of incoming Zn²⁺ ions. This structural feature allows for efficient zinc intercalation, transforming NMO into a Zn-Mn-based system electrochemically. In this study, the focus is on investigating the mechanism of self-ion exchange occurring in NMO, utilizing an electrolyte composed of 2 M ZnSO₄ and 0.1 M MnSO₄, exhibiting a reversible capacity of ∼240 mAh g^-1 at a C/10 rate with a Coulombic efficiency of ∼99%. The self-ion exchange mechanism and structural changes during the battery operation were investigated using ex-situ x-ray diffraction and x-ray photoelectron spectroscopy analysis. The reversible phase transformations between hydrated and partially dehydrated states suggest a robust mechanism for Zn²⁺ ion insertion and extraction, contributing to the stability and performance of the Zn-Mn electrode material in ZIB applications.