Revealing the Multi‐Electron Reaction Mechanism of Na₃V₂O₂(PO₄)₂F Towards Improved Lithium Storage

Abstract

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 À 1 in the wider voltage window of 1.0-4.8 V than that of 2.5-4.8 V (129.3 mAh g À 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.

Lithium Polyacrylate as Lithium and Carbon Source in the Synthesis of Li3VO4 for High‐Rate and Long‐Life Li‐Ion Batteries

“…The O 1s peaks (Figure S1a) were fitted at about 531.8 and 530.1 eV, corresponding to C−O and V−O bonds, respectively [22] . The fitted Li 1s peak at 54.7 eV (Figure S1b) indicated the presence of LiOR [34,35] . A small amount of Na 2 S 2 O 8 used in the process caused the broad and weak Na 1s peak, fitted at 1071.8 eV (Figure S1c).…”

“…[22] The fitted Li 1s peak at 54.7 eV (Figure S1b) indicated the presence of LiOR. [34,35] A small amount of Na 2 S 2 O 8 used in the process caused the broad and weak Na 1s peak, fitted at 1071.8 eV (Figure S1c). The TG curve displays a mass loss of about 10 % between 200 and 800 °C, which is ascribed to the removal of C (Figure S2).…”

Crystallinity Tuning of Na3V2(PO4)3: Unlocking Sodium Storage Capacity and Inducing Pseudocapacitance Behavior

“…The two peaks at 524.3 and 517.2 eV match well with V 2p 1/2 and V 2p 3/2 of V 4+ ions, and the other two peaks are related to V 5+ 2p 1/2 (525.1 eV) and V 5+ 2p 3/2 (518.0 eV), indicating that three sodium ions per formula could be extracted from the sample. [ 22 ] As a comparison, we also did XPS test on the NVP‐S700 sample (Supporting Information, Figure S6b ). After charging to 4.5 V, only V 2p 1/2 (524.2 eV) and V 2p 3/2 (517.1 eV) XPS peaks related to V 4+ existed in the spectra, meaning that only two sodium ions per formula could be extracted in this voltage range.…”

Recent Advances and Perspectives on the Promising High‐Voltage Cathode Material of Na3(VO)2(PO4)2F

“…Mai et al conducted the multi-electron reaction of NVOPF(I4) for improved lithium storage. [123] The ultra-high energy density of 753.2 Wh kg −1 can be achieved with three Li ions insertion, and it deliv-ers a much higher capacity of 966.7 mAh g −1 when discharged to 0 V. The location of inserted Li ions and the final structure are simulated by density functional theory (DFT) and presented in Figure 14d. The first two Li ions insertion changed the Na ion locations and the third Li ion insertion resulted in a rearrangement of Na and Li ions.…”

“…d) Crystal structure illustration NVOPF(I4) for Li + storage at various states (with different Li ions insertion). Reproduced with permission [123]. Copyright 2021, Wiley-VCH.…”