Electrochemical Redox Processes Involved in Carbon-Coated KVPO4F for High Voltage K-Ion Batteries Revealed by XPS Analysis

K-ion batteries need efficient positive electrode materials with open structural frameworks to accommodate the large ionic radius of K+. In that direction, polyanionic compounds are of great interest. Among them, KVPO4F is the most studied one. Its electrochemical redox processes still remain unclear, especially at high voltage. To tackle this issue, X-ray photoelectron spectroscopy was used to reveal the electrochemical redox processes of KxVPO4F. First, a carbon coating was performed and allowed increasing the overall electrochemical performance while mitigating electrolyte degradation at high voltage. Then, XPS analysis showed a high reversibility of the redox processes although the K+ extraction (and insertion) from x = 0.5 to 0 was hindered, possibly by structural constraints while electrolyte degradation occurred mostly above 4.5 V.