Revealing the Multi‐Electron Reaction Mechanism of Na 3 V 2 O 2 (PO 4 ) 2 F Towards Improved Lithium Storage

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, 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 NaV 2 O 2 (PO 4 ) 2 F. Due to the additional redox couple of V 3+ /V 4+ , 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 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.