Crystal Structure\nand Electrochemical\nBehaviors of the New Sodium Cathode\nMaterial NaFe(SeO<sub>3</sub>)<sub>2</sub>

Sodium iron selenite NaFe(SeO<sub>3</sub>)<sub>2</sub> (NFSeO)\nwas synthesized through a solid-state reaction. The crystal structure\nof NFSeO was investigated by powder X-ray diffractions and density\nfunctional theory (DFT) calculations. The NFSeO crystallized in an\northorhombic unit cell with space group <i>Pnma</i> (no.\n62) and cell parameters <i>a</i> = 12.5778(2) Å, <i>b</i> = 5.3620(3) Å, <i>c</i> = 7.9204(4) Å,\nand <i>V</i> = 534.17(2) Å<sup>3</sup> from Rietveld\nrefinements. The X-ray photoelectron spectroscopy (XPS) and X-ray\nabsorption fine structure (XAFS) spectra of NFSeO were investigated\nwith the behaviors agreeing with the crystal structure from Rietveld\nrefinements. The XPS peaks of NFSeO are observed at 711.8 eV(Fe 2p<sub>3/2</sub>), 725.7 eV(Fe 2p<sub>1/2</sub>), 58.3 eV(Se 3d<sub>5/2</sub>), 59.1 eV(Se 3d<sub>3/2</sub>), 530.6 eV(O 1s), and 1071.1 eV(Na\n1s). A pre-edge peak at 7112 eV and an absorption peak at 7133 eV\nare observed in the XAFS spectrum of NFSeO. The conductivity of NFSeO\nis 2.3 × 10<sup>–8</sup> S cm<sup>–1</sup> at 100\n°C and increases to 8.9 × 10<sup>–5</sup> S cm<sup>–1</sup> at 360 °C. The NFSeO and carbon black composite\n(NFSeO/C) shows a Na<sup>+</sup> capacity of 80 mAh g<sup>–1</sup> at 0.1C, making it a potential cathode material.