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.
Competing Interests: There are no conflicts to declare.
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