Rhombohedral Zinc Hexacyanoferrate as a High-Voltage Cathode Material for Aqueous Mn-ion Batteries

Aqueous metal batteries have emerged as a promising alternative to lithium-ion batteries, offering enhanced safety through the use of aqueous electrolytes. Manganese-ion battery systems remain underexplored despite the low manganese redox potential of -1.19 V (vs the standard hydrogen electrode) as well as high operating voltage and capacity. In this study, a rhombohedral zinc Prussian blue analog (ZnHCF) is investigated for the first time as a cathode material for manganese-ion batteries, demonstrating the highest operating voltage reported in the field (0.55 V vs Ag/AgCl or 1.94 V vs Mn/Mn²⁺). ZnHCF exhibits a discharge capacity of 79.2 mAh g^-1 at 0.2 A g^-1 with excellent stability, retaining its original performance after 4000 cycles. By performing a comprehensive electrochemical characterization, advanced structural analysis, spectroscopic studies, and diffusion pathway and energy barrier calculations, the charge storage mechanism and structural behavior of ZnHCF are elucidated. This study underlines the application potential of ZnHCF as a high-performing cathode material for manganese-ion batteries and helps achieve a better understanding of Mn electrochemistry, offering valuable insights for advancing aqueous battery systems toward efficient and sustainable energy storage.