Low temperature hydrothermal synthesis of Na3Fe2(PO4)2F3 and its cathode electrochemistry in Na- and Li-ion batteries

Na3Fe2(PO4)2F3 has been synthesized under mild hydrothermal conditions. The structure of Na3Fe2(PO4)2F3 has been solved using single-crystal X-ray diffraction and the phase purity has been evaluated by Rietveld refinement from high resolution synchrotron powder X-ray diffraction data. Both methods establish the accuracy of P42/mnm space group and synthesis of so called beta2-polymorph. Room temperature 57Fe Mössbauer spectroscopic studies confirm 3+ oxidation state of the compound. The electrochemical properties have been investigated with respect to Li- and Na-ion batteries. Galvanostatic charge-discharge studies indicate that up to ~0.6 lithium ions and only ~0.4 sodium ions per Fe can be inserted at an average voltage of 2.82 and 2.42 ​V with respect to Li+/Li and Na+/Na in lithium and sodium ion batteries, respectively. The cycle-life studies indicate that Li-ion batteries show very stable capacity retention over long cycles while Na-ion batteries tend to lose capacity after ~35 cycles at C/5 rate. Na3Fe2(PO4)2F3 has been synthesized under mild hydrothermal conditions. The disordered sodium ions are located in the large cavity formed in the intersection of a- and b-axes of the three-dimensional structure of Na3Fe2(PO4)2F3. Reductive insertion of Na+ and Li+ ions have been carried out on this material of which Li-ion electrochemistry is reported here for the first time. [Display omitted] •Hydrothermal synthesis of Na3Fe2(PO4)2F3.•Cathode for Na-ion batteries.•Cathode for Lithium ion batteries.•Single-crystal structure of Na3Fe2(PO4)2F3.