Cation Vacancies Activate Dual Sites of Magnesium Storage in Prussian Blue Analogs Cathode

Owing to their vast crustal abundance and inherently dendrite-free electrodeposition, rechargeable magnesium batteries (RMBs) have emerged as a compelling next-generation alternative to conventional lithium-ion technologies. In this context, Prussian Blue Analogs (PBAs) demonstrate significant potential as intercalation cathode candidates, as their open framework and continuous 3D diffusion pathways facilitate efficient divalent ion transport. However, their practical implementation is currently hindered by limited specific capacity and insufficient cycling stability, primarily due to lattice distortion and sluggish intercalation kinetics. This study presents a novel magnesium storage mechanism involving vacancy-mediated ion insertion (4b sites) and the activation of adsorption-active sites (32f sites), thereby significantly boosting the electrochemical performance of PBA materials. Notably, the cation-deficient PBA (PBA□Fe), synthesized via slow coprecipitation and acid treatment, delivered a high energy density of 240 Wh kg^-1, which is comparable to those of transition metal oxide materials, while maintaining 80.7% capacity retention over 1000 cycles at 200 mA g^-1. The dual role of Fe vacancies-providing Mg²⁺ storage sites and activating additional adsorption active sites (32f sites)-highlights a strategic innovation for the deployment of high-capacity and long-cycle cathodes for RMBs. This work underscores the potential of defect engineering in optimizing high-performance Mg-ion battery materials.