In operando study of orthorhombic V2O5 as positive electrode materials for K-ion batteries

Herein, the electrochemical performance and the mechanism of potassium insertion/deinsertion in orthorhombic V2O5 nanoparticles are studied. The V2O5 electrode displays an initial potassiation/depotassiation capacity of 200 mAh g$^{-1}$/217 mAh g$^{-1}$ in the voltage range 1.5–4.0 V vs. K+/K at C/12 rate, suggesting fast kinetics for potassium insertion/deinsertion. However, the capacity quickly fades during cycling, reaching 54 mAh g-1 at the 31st cycle. Afterwards, the capacity slowly increases up to 80 mAh g$^{-1}$ at the 200th cycle. The storage mechanism upon K ions insertion into V$_2$O$_5$ is elucidated. In operando synchrotron diffraction reveals that V$_2$O$_5$ first undergoes a solid solution to form K$_{0.6}$V$_2$O$_5$ phase and then, upon further K ions insertion, it reveals coexistence of a solid solution and a two phase reaction. During K ions deinsertion, the coexistence of solid solution and the two-phase reaction is identified together with an irreversible process. In operando XAS confirms the reduction/oxidation of vanadium during the K insertion/extraction with some irreversible contributions. This is consistent with the results obtained from synchrotron diffraction, ex situ Raman, X-ray photoelectron spectroscopy (XPS), and transmission electron microscopy (TEM). Moreover, ex situ XPS confirms the ‘‘cathode electrolyte interphase” (CEI) formation on the electrode and the decomposition of CEI film during cycling