LAUSR

Na 3 V 2 O 2 (PO 4 ) 2 F (NVOPF) as an attractive electrode material has received much attention based on the one-electron reaction of V 4+ /V 5+ . However, the electrochemical reactions involving lower vanadium valences were not investigated till now. Herein, we firstly synthesized a composite of graphene decorated nanosheet-assembled NVOPF microflowers (NVOPF/G) and conducted the multi-electron reaction of NVOPF/G by controlling the operation voltage windows. The reaction mechanism, structural changes and vanadium valences during the insertion/extraction of Li ions (from two to six) were elucidated clearly by in-situ X-ray diffraction and ex-situ X-ray photoelectron spectroscopy characterizations. Theoretical computations also reveal the Li ions locations in the structure of NaV 2 O 2 (PO 4 ) 2 F. Due to the additional redox couple of V 3+ /V 4+ , NVOPF/G displays a much higher initial capacity of 183.3 mAh g -1 in the wider voltage window of 1.0-4.8 V than that of 2.5-4.8 V (129.3 mAh g -1 ). Moreover, excellent Li-storage performance of NVOPF/G at a lower voltage (≤2.5 V) with the active reaction of V 2+ /V 3+ /V 4+ was obtained for the first time, demonstrating the high potential of NVOPF/G as an anode material for Li ion storage.