Synchrotron radiation investigation of reaction mechanism of manganese Prussian blue analogue in aqueous Zn–Mn electrolyte

Objective: The performance of manganese hexacyanoferrate (MnHCF) in aqueous zinc in batteries has been extensively studied. However, severe dissolution of MnHCF in an aqueous electrolyte leads to significant composition changes and phase transitions, hindering its further development. Electrolyte engineering is one of the most effective approaches to address this issue. Approach: In this study, Mn2+ was added to the ZnSO4 electrolyte. A significant difference in specific capacity was observed between the 3 M ZnSO4 and 3 M ZnSO4 + 0.1 M MnSO4 electrolytes after approximately 50 cycles. The intercalation mechanism of the MnHCF electrode in the two electrolytes was comprehensively studied in terms of compositional distribution, local coordination environment, and long-range crystal structure evolution using different synchrotron x-ray techniques: x-ray fluorescence, x-ray absorption spectroscopy and x-ray diffraction. Main results and significance: The results demonstrate that the Mn2+ additive alleviates the dissolution of Mn in the Zn–Mn electrolyte, and reduces Zn incorporation into the framework. This helps to preserve the MnHCF structural framework and suppresses the phase transformation to the ZnHCF during the initial reaction stage. The formation of new rhombohedral and cubic MnHCF intermediate phases in the early cycles accounts for the high specific capacity observed in the Zn–Mn electrolyte. After long-term cycling, significant changes in the Mn coordination were detected, and the same crystal structure data were observed in the two different electrolytes, which were also reflected in the electrochemical performance changes, as the capacities in both systems declined. The results not only provide a deeper understanding of the working mechanism of MnHCF-A in the two electrolyte systems, but also clearly illustrate the structure-composition-performance relationship.