Synthesis and performance evaluation of nanostructured NaFe_xCr_1−X(SO₄)₂ cathode materials in sodium ion batteries (SIBs)

This research work focuses on the synthesis and performance evaluation of NaFe_xCr₁-_X(SO₄)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 NaFe_0.8Cr_0.2(SO₄)₂ when compared to NaFe(SO₄)₂ and NaCr(SO₄)₂. Galvanostatic charge/ discharge analysis indicates that the intercalation/de-intercalation of a sodium ion (Na⁺) into/from NaFe(SO₄)₂ensues at about 3.2 V due to the Fe²⁺/Fe³⁺ 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_0.8Cr_0.2(SO₄)₂which has not been reported earlier. The galvanostatic measurements and X-ray photoelectron spectroscopy (XPS) analysis confirm that the Cr²⁺/Cr³⁺ redox couple is inactive in NaFe_xCr₁-_X(SO₄)₂ (X = 0, 0.8) and thus does not contribute to capacity augmentation. However, suitable carbon coating may lead to activation of the Cr²⁺/Cr³⁺ redox couple in these inactive materials

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Published in: RSC Advances
License: https://creativecommons.org/licenses/by-nc-sa/3.0/  
See article on publisher's website: https://doi.org/10.1039/c8ra06583g