Enhanced Performance of KVPO₄F_0.5O_0.5 in Potassium Batteries by Carbon Coating Interfaces

Potassium vanadium oxyfluoride phosphate of composition KVPO₄F_0.5O_0.5 was modified by a carbon coating to enhance its electrochemical performance. Two distinct methods were used, first, chemical vapor deposition (CVD) using acetylene gas as a carbon precursor and second, an aqueous route using an abundant, cheap, and green precursor (chitosan) followed by a pyrolysis step. The formation of a 5 to 7 nm-thick carbon coating was confirmed by transmission electron microscopy and it was found to be more homogeneous in the case of CVD using acetylene gas. Indeed, an increase of the specific surface area of one order of magnitude, low content of C sp², and residual oxygen surface functionalities were observed when the coating was obtained using chitosan. Pristine and carbon-coated materials were compared as positive electrode materials in potassium half-cells cycled at a C/5 (C = 26.5 mA g^-1) rate within a potential window of 3 to 5 V vs K⁺/K. The formation by CVD of a uniform carbon coating with the limited presence of surface functions was shown to improve the initial coulombic efficiency up to 87% for KVPFO₄F_0.5O_0.5-C₂H₂ and to mitigate electrolyte decomposition. Thus, performance at high C-rates such as 10 C was significantly improved, with ∼50% of the initial capacity maintained after 10 cycles, whereas a fast capacity loss is observed for the pristine material.