Phase transformation in Na-Fe-S-O quaternary sulfate cathode materials

In last one decade, there have been a resurgence in interest followed by a rigorous research in sodium-ion intercalation chemistry for rechargeable battery application. Though lithium-ion chemistry has been a commercial success and is a lynchpin of the portable electronics era, sodium-ion chemistry can economically address the large-scale stationary rechargeable battery market and offers the exciting avenues of novel intercalation structures, some of which may not exist in their lithium equivalents. To realize the commercially viable large-scale Na-based batteries, the concept of earth abundance should consistently be applied throughout their design. Of significance, compounds constituting Na-Fe-S-O type elements show promising results. In this line, alluaudite structured Na₂Fe₂(SO₄)₃ has been reported benchmarking the highest Fe³⁺/Fe²⁺ redox potential at 3.8 V (vs. Na) with excellent rate capability and competent energy density. In pursuit of energy-savvy synthesis of this sulfate cathode, this work reports two aqueous based synthesis namely Pechini and spray dry routes. Further, various other Na-Fe-S-O quaternary cathodes along the Na₂SO₄ and FeSO₄ binary phase line and their possible phase transformation have been studied in detail.