Synthesis and performance evaluation of nanostructured NaFe Cr (SO) cathode materials in sodium ion batteries (SIBs).

This research work focuses on the synthesis and performance evaluation of NaFe Cr (SO) ( = 0, 0.8 and 1.0) cathode materials in sodium ion batteries (SIBs). The novel materials having a primary particle size of around 100-200 nm were synthesized through a sol-gel process by reacting stoichiometric amounts of the precursor materials. The structural analysis confirms the formation of crystalline, phase pure materials that adopt a monoclinic crystal structure. Thermal analysis indicates the superior thermal stability of NaFe0Cr(SO) when compared to NaFe(SO) and NaCr(SO). Galvanostatic charge/discharge analysis indicates that the intercalation/de-intercalation of a sodium ion (Na) into/from NaFe(SO) ensues at about 3.2 V due to the Fe/Fe active redox couple. Moreover, XRD analysis confirms that the insertion/de-insertion of sodium into/from the host structure during charging/discharging is accompanied by a reversible single-phase reaction rather than a biphasic reaction. A similar sodium intercalation/de-intercalation mechanism has been noticed in NaFeCr(SO)which has not been reported earlier. The galvanostatic measurements and X-ray photoelectron spectroscopy (XPS) analysis confirm that the Cr/Cr redox couple is inactive in NaFe Cr (SO) ( = 0, 0.8) and thus does not contribute to capacity augmentation. However, suitable carbon coating may lead to activation of the Cr/Cr redox couple in these inactive materials.