FeOF Nanocomposites With Tunable Graphitic Carbon Shells as High-Performance Conversion-Type Cathode for Potassium...

Potassium-ion batteries (KIBs) have gained increasing interest for large-scale energy storage because potassium is abundant, inexpensive, and offers a relatively high energy density. While most reported KIB cathodes rely on intercalation-type mechanisms, this work focuses on a less-explored conversion-type system. Among conversion-type candidates, iron oxyfluoride (FeOF) is a promising conversion-type candidate due to its high theoretical capacity (∼855 mAh g^-1), low cost, and environmental friendliness. However, its practical use is limited by poor ionic and electronic conductivity, as well as structural instability during cycling. In this work, FeOF nanocomposites with tunable graphitic carbon (GC) shells were synthesized via a chelation-assisted polymerization method, where citric acid (CA) chelates Fe species and polymerizes with ethylene glycol. The carboxylic and hydroxy functional groups of CA promote uniform dispersion of Fe ions within the polymeric matrix, enabling in situ reduction to Fe(0), which catalyzes graphitic carbon formation during pyrolysis and facilitates homogeneous GC shell encapsulation around FeOF particles. The FeOF@GC-1 electrode exhibited superior electrochemical performance, delivering a reversible discharge capacity of 132.7 mAh g^-1 after 200 cycles at 0.1 A g^-1 in the voltage range of 1.2-4.0 V. Additionally, a full cell assembled with hard carbon as the anode demonstrated a high energy density of 181 Wh kg^-1, confirming its practical potential for KIB applications. Only a few reports have explored conversion-type cathodes for KIBs, this study provides an early investigation of FeOF as a conversion-type KIB cathode and highlights a controllable synthesis strategy for optimizing graphitic carbon encapsulation and understanding the K⁺ storage mechanism in FeOF.