Like li-and Na-ion batteries, electrolyte reactivity (i.e., salt and solvent decomposition) is important in the electrochemical performance of K-ion batteries (KIBs). Indeed, X-ray photoelectron spectroscopy (XPS) analysis of a solid electrolyte interphase (SEI) in KIBs is still based on K-ion half-cells. This may lead to incorrect interpretations of the results considering the contamination of the studied electrode SEI due to the high K metal reactivity. Therefore, this study aims to provide a reliable XPS study without potassium metal, investigating the impact of various parameters, including open-circuit voltage (OCV) temperature, upper cutoff voltage (UCV), depth of discharge (DOD), and vanadium dissolution, on the electrochemical performance of KVPO 4 F 0.5 O 0.5 / graphite full cells. This work highlights the importance of SEI preformation, which is more pronounced at higher OCV temperatures. At specific UCV and DOD, the cell provides the best trade-off between the organic and inorganic passivation layer, leading to good electrochemical performance. The study also demonstrates that vanadium dissolution is a potential mechanism affecting capacity fading in K-ion batteries. Finally, understanding these SEI growth mechanisms should help researchers get reliable XPS analysis of the SEI in KIBs and move forward with the practical passivation layer characterizations.