Crystal Structure and Electrochemical Behaviors of the New Sodium Cathode Material NaFe(SeO₃)₂

Sodium iron selenite NaFe(SeO₃)₂ (NFSeO) was synthesized through a solid-state reaction. The crystal structure of NFSeO was investigated by powder X-ray diffractions and density functional theory (DFT) calculations. The NFSeO crystallized in an orthorhombic unit cell with space group Pnma (no. 62) and cell parameters a = 12.5778(2) Å, b = 5.3620(3) Å, c = 7.9204(4) Å, and V = 534.17(2) ų from Rietveld refinements. The X-ray photoelectron spectroscopy (XPS) and X-ray absorption fine structure (XAFS) spectra of NFSeO were investigated with the behaviors agreeing with the crystal structure from Rietveld refinements. The XPS peaks of NFSeO are observed at 711.8 eV(Fe 2p_3/2), 725.7 eV(Fe 2p_1/2), 58.3 eV(Se 3d_5/2), 59.1 eV(Se 3d_3/2), 530.6 eV(O 1s), and 1071.1 eV(Na 1s). A pre-edge peak at 7112 eV and an absorption peak at 7133 eV are observed in the XAFS spectrum of NFSeO. The conductivity of NFSeO is 2.3 × 10^-8 S cm^-1 at 100 °C and increases to 8.9 × 10^-5 S cm^-1 at 360 °C. The NFSeO and carbon black composite (NFSeO/C) shows a Na⁺ capacity of 80 mAh g^-1 at 0.1C, making it a potential cathode material.