In situ XRD and solid state NMR characterization of Na₃V₂(PO₄)₂F₃ as cathode material for lithium-ion batteries

In this work, Na3V2(PO4)2F3/C composite fluorophosphate cathode material is synthesized by a sol-gel process. The as prepared material shows excellent electrochemical performance. A capacity of 125 mAh/g of the material is obtained at the current density of 150 mA/g. When cycling at the current density of 3000 mA/g, it still delivers a capacity of 110 mAh/g (88% of the initial capacity at 150 mA/g). The charging and discharging mechanism of this material is investigated by synchrotron-based in situ XRD and ex situ MAS SS-NMR. In situ XRD results indicate a single phase electrochemical delithiation/lithiation process and the material shows excellent structure stability during charge/discharge process. The ex situ MAS SS-NMR results show that Li-Na exchange happens when Na3V2(PO4)2F3 contacts with electrolyte, and the charge/discharge mechanism is: Li+ and Na+ in the Na1 site are deintercalated from the structure in the first charge process; Li+ and Na+ are co-intercalated in the structure in the following discharge process.