Synthesis and performance evaluation of nanostructured NaFe(x)Cr(1−X)(SO(4))(2) cathode materials in sodium ion batteries (SIBs)

This research work focuses on the synthesis and performance evaluation of NaFe(x)Cr(1−X)(SO(4))(2) (X = 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 NaFe0(.8)Cr(0.2)(SO(4))(2) when compared to NaFe(SO(4))(2) and NaCr(SO(4))(2). Galvanostatic charge/discharge analysis indicates that the intercalation/de-intercalation of a sodium ion (Na(+)) into/from NaFe(SO(4))(2) ensues at about 3.2 V due to the Fe(2+)/Fe(3+) active redox couple. Moreover, ex situ 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 NaFe(0.8)Cr(0.2)(SO(4))(2)which has not been reported earlier. The galvanostatic measurements and X-ray photoelectron spectroscopy (XPS) analysis confirm that the Cr(2+)/Cr(3+) redox couple is inactive in NaFe(x)Cr(1−X)(SO(4))(2) (X = 0, 0.8) and thus does not contribute to capacity augmentation. However, suitable carbon coating may lead to activation of the Cr(2+)/Cr(3+) redox couple in these inactive materials.