Na3 V2 O2 (PO4 )2 F (NVOPF) as an attractive electrode material has received much attention based on the one-electron reaction of V4+ /V5+ . However, the electrochemical reactions involving lower vanadium valences were not investigated till now. Herein, a composite of graphene decorated nanosheet-assembled NVOPF microflowers (NVOPF/G) was synthesized and the multi-electron reaction of NVOPF/G was conducted by controlling the operation voltage windows. The reaction mechanism, structural changes, and vanadium valences during the insertion/extraction of Li ions (from 2 to 6) were elucidated clearly by in-situ X-ray diffraction and ex-situ X-ray photoelectron spectroscopy. Theoretical computations also revealed the Li-ion locations in the structure of NaV2 O2 (PO4 )2 F. Due to the additional redox couple of V3+ /V4+ , NVOPF/G displayed 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 V2+ /V3+ /V4+ was obtained for the first time, demonstrating the high potential of NVOPF/G as an anode material for Li ion storage.
Keywords: Li-ion storage; batteries; electrode materials; structural change; vanadium.
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