Electrochemical and Diffusional Investigation of Na<sub>2</sub>Fe<sup>II</sup>PO<sub>4</sub>F Fluorophosphate Sodium Insertion Material Obtained from Fe<sup>III</sup> Precursor

Sodium iron fluorophosphate (Na<sub>2</sub>Fe<sup>II</sup>PO<sub>4</sub>F) was synthesized by economic solvothermal combustion technique using Fe<sup>III</sup> precursors, developing one-step carbon-coated homogeneous product. Synchrotron diffraction and Mössbauer spectroscopy revealed the formation of single-phase product assuming an orthorhombic structure (s.g. <i>Pbcn</i>) with Fe<sup>II</sup> species. This Fe<sup>III</sup> precursor derived Na<sub>2</sub>Fe<sup>II</sup>PO<sub>4</sub>F exhibited reversible Na<sup>+</sup> (de)­intercalation with discharge capacity of 100 mAh/g at a rate of C/10 involving flat Fe<sup>III</sup>/Fe<sup>II</sup> redox plateaus located at 2.92 and 3.05 V (vs Na/Na<sup>+</sup>). It delivered good cycling stability and rate kinetics at room temperature. The stability of Na<sub>2</sub>FePO<sub>4</sub>F cathode was further verified by electrochemical impedance spectroscopy at different stages of galvanostatic analysis. Bond valence site energy (BVSE) calculations revealed the existence of 2-dimensional Na<sup>+</sup> percolation pathways in the <i>a–c</i> plane with a moderate migration barrier of 0.6 eV. Combustion synthesized Na<sub>2</sub>Fe<sup>II</sup>PO<sub>4</sub>F forms an economically viable sodium battery material. Although the capacity of this cathode is relatively low, this study continues systematic work, which attempts to broaden the scope of reversible sodium insertion materials.