Oxygen-containing functional groups were found to effectively boost the K+storage performance of carbonaceous materi-als,however,the mechanism behind the performance enhancement remains unclear.Herein,we report higher rate capability and bet-ter long-term cycle performance employing oxygen-doped graphite oxide(GO)as the anode material for potassium ion batteries(PIBs),compared to the raw graphite.The in situ Raman spectroscopy elu-cidates the adsorption-intercalation hybrid K+storage mechanism,assigning the capacity enhancement to be mainly correlated with reversible K+adsorption/desorption at the newly introduced oxygen sites.It is unraveled that the C=O and COOH rather than C-O-C and OH groups contribute to the capacity enhancement.Based on in situ Fourier transform infrared(FT-IR)spectra and in situ elec-trochemical impedance spectroscopy(EIS),it is found that the oxygen-containing functional groups regulate the components of solid electrolyte interphase(SEI),leading to the formation of highly conductive,intact and robust SEI.Through the systematic investiga-tions,we hereby uncover the K+storage mechanism of GO-based PIB,and establish a clear relationship between the types/contents of oxygen functional groups and the regulated composition of SEI.