Abstract
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.