Driving Redox Reversibility in Aqueous Magnesium-Ion Batteries: The Synergy of Fe-Doping and Mn 2+ Electrolyte Additives in Mn-Based Prussian Blue Analogues

Manganese-based Prussian Blue Analogues (Mn-PBAs) are one of the promising cathodes for aqueous Mg 2+ -ion batteries. However, the Jahn-Teller-induced dissolution of Mn 3+ ions leads to severe framework degradation. We demonstrate that while 35% Fe-doping in Mn-PBA (KFMHCF 3565) enhances redox activity, it is insufficient to prevent significant metal (Mn and Fe) leaching and capacity fade in pure 2.4 m MgCl 2 electrolyte. In contrast, the addition of 0.5 m MnCl 2 enables an impressive ∼95% capacity retention over 100 cycles. Ex-situ XRD reveals that this electrolyte-engineering strategy drives a reversible monoclinic-to-cubic phase transition, thereby mitigating structural strain during Mg 2+ insertion. XPS analysis confirms that the Mn 2+ additive stabilizes the Mn 2+/3+ redox couple by facilitating a comproportionation mechanism that suppresses net Mn loss. Furthermore, ICP-OES and SEM-EDS analyses reveal a dynamic surface reconstruction, where Mn redeposition from the electrolyte offsets Fe dissolution, thereby maintaining the structural integrity of the host. This synergistic approach, combining lattice doping and electrolyte modification, provides a robust pathway for high-stability Mg 2+ -ion storage.