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

Abstract

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.