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 mA h g⁻¹ in the Na⁺–Zn²⁺ dual salt electrolyte compared to 111 mA h 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 the 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, it does 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.