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.