Eldfellite-structured NaCr(SO₄)₂: a potential anode for rechargeable Na-ion and Li-ion batteries

Recent global concerns over continuously increasing air pollution and the related health risks due to automobile exhaust have shifted our attention towards green transportation. Recent decades have witnessed a revolution in portable energy-storage systems, mainly lithium-based energy-storage devices. However, the uneven distribution of global lithium reserves and its scarcity lead to huge price differences and geopolitical imbalances, and hence the research in energy-storage materials has shifted towards the development of cost-effective, abundant electrode materials. Here, NaCr(SO₄)₂, a transition metal-based polyanionic layered material with low cost and high stability during the charge/discharge process vs. Na, operating on the basis of the Cr³⁺/²⁺ redox couple, is presented. The test materials were characterized by techniques like XRD, FTIR, SEM, UV, XPS, TGA-DTA, and a detailed electrochemical analysis of the charge/discharge capacity of the materials is presented here. Here, the findings provide insights towards achieving a Cr³⁺/Cr²⁺ redox-couple-based sodium-ion battery with a specific capacity of 75 mA h g^-1 and 150 mA h g^-1 at operating voltages of 0.95 V vs. Na and 1.05 V vs. Li, respectively, with 100% coulombic efficiency. Cr²⁺ is a very special oxidation of Cr that cannot be obtained easily and CrTa₂O₆ is the only known oxide where Cr exists in the 2+ state. Here, a shift in the redox energy of the Cr^3+/2+ couple was obtained due to its bonding with (SO₄)^2- polyanions in eldfellite that made the accessibility of Cr^3+/2+ possible, resulting in the superior intercalation/deintercalation of Na and Li and the superior energy-storage capacity of the NaCr(SO₄)₂vs. Na/Li cell.