Unmasking the Intrinsic Mechanistic Limits of Manganese Prussian Blue Analogues in Aqueous Zn-Ion Batteries

Abstract

Manganese Prussian blue analogues (Mn PBAs) are widely investigated as cathodes for aqueous zinc ion batteries (AZIBs) owing to their open framework and potential two-electron redox, yet their true electrochemical stability and charge storage mechanism remain contentious. Here, we systematically disentangle the roles of electrolyte composition and compositional variations in Mn-PBAs using a combination of electrochemical analysis, operando X ray diffraction, solution phase analysis, microscopy, soft X ray absorption spectroscopy, and density functional theory. It is revealed that, independent of composition and electrolyte formulation, Mn PBAs do not operate as stable intercalation hosts with aqueous zinc-ion electrolytes. Initial Na+ deintercalation triggers a monoclinic-to-cubic transition, accompanied by Mn3+ driven disproportionation and manganese dissolution, followed by electrochemical deposition of manganese oxide that contributes capacity beyond one electron, reaching 136 mAh g-1 in the Na+-Zn2+ dual salt electrolyte compared to 111 mAh g-1 in the Zn2+ based single salt system after activation. Subsequent discharge induces irreversible conversion of the Mn PBA framework to a rhombohedral Zn PBA phase, with later cycling dominated by solid solution behavior of Zn PBA alongside repeated manganese dissolution–redeposition. As a result, the capacity of Mn-PBA converges toward that of Zn-PBA and decays rapidly, with only 60% (dual-salt) and 43% (single-salt) retention after 100 cycles. While the dual salt electrolyte delays manganese dissolution and enables partial (de)intercalation within Zn PBA, they do not alter the fundamental reaction pathway. Across the compositions investigated, differences are reflected primarily in activation behavior and electrochemical kinetics. These findings reconcile reports of high-rate cycling stability at modest capacities and establish intrinsic limitations of Mn-PBAs in AZIBs, highlighting the need for interfacial and electrolyte strategies to suppress Mn3+ disproportionation.