A High-Capacity K-Rich Fe-Based Polyanion Cathode Material for K-Ion Batteries: Structure, Phase Evolution, and Electrochemical Behavior

K-rich Fe-fluorophosphate, having only earth-abundant Fe as the transition metal, has been developed in this work and structural, as well as electronic, features have been reported; with the material also been tested as a low-cost, high-performance cathode material for the sustainable K-ion battery chemistry. The preparation involves a room-temperature potassiation process of K-deficient KFePO4F, which is, otherwise, incapable of acting as a true cathode material, viz., as a K-reservoir in K-ion cells. Comprehensive sets of structural, spectroscopic, and electrochemical analyses reveal that just 15 min of potassiation leads to an optimal K-content of close to 2 p.f.u. in K1+xFePO4F, with Fe in +2 oxidation state (as per XPS, ESR, Mössbauer spectroscopy), which also corresponds to the lowest-energy structures, as per DFT-based simulations. DFT also reveals the associated diffraction patterns of this compound, as hitherto not available. However, further potassiation results only in the formation of KF-based surface species (as per NMR, XPS), which are detrimental toward the electrochemical performance. The as-developed, optimized 15 min potassiated K1+xFePO4F delivers a reversible K-storage capacity of ∼122 mAh/g (based on Fe2+/3+), along with ∼100% Coulombic efficiency, which are among the best reported for K-ion battery cathodes. Overall, while unraveling the intricacies and phase/structural evolutions during a room-temperature potassiation process toward developing structurally stable Fe-based polyanion materials, this work paves the way for developing high-performance cathodes for the K-ion battery chemistry and beyond.