A Self‐Constructed Mg²⁺/K⁺ Co‐Doped Prussian Blue with Superior Cycling Stability Enabled by Enhanced Coulombic Attraction

Abstract Prussian blue (PB) is regarded as a promising cathode for sodium‐ion batteries because of its sustainable precursor elements (e.g., Mn, Fe), easy preparation, and unique framework structure. However, the unstable structure and inherent crystal H₂O restrain its practical application. For this purpose, a self‐constructed trace Mg²⁺/K⁺ co‐doped PB prepared via a sea‐water‐mediated method is proposed to address this problem. The Mg²⁺/K⁺ co‐doping in the Na sites of PB is permitted by both thermodynamics and kinetics factors when synthesized in sea water. The results reveal that the introduced Mg²⁺ and K⁺ are immovable in the PB lattices and can form stronger K‒N and Mg‒N Coulombic attraction to relieve phase transition and element dissolution. Besides, the Mg²⁺/K⁺ co‐doping can reduce defect and H₂O contents. As a result, the PB prepared in sea water exhibits an extremely long cycle life (80.1% retention after 2400 cycles) and superior rate capability (90.4% capacity retention at 20 C relative to that at 0.1 C). To address its practical applications, a sodium salts recycling strategy is proposed to greatly reduce the PB production cost. This work provides a self‐constructed Mg²⁺/K⁺ co‐doped high‐performance PB at a low preparation cost for sustainable, large‐scale energy storage.