Towards more sustainable negative electrodes in Na-ion batteries via nanostructured iron oxide

Na-ion technology could emerge as an alternative to Li-ion batteries due to limited costs and vast availability of sodium, as well as its similar chemistry. Several Na-rich compounds have been proposed as positive electrodes, whereas suitable negative counterparts have not been found yet. Nanostructured iron oxide is reported here for the first time as a potentially viable negative electrode for Na-ion cells based on conventional electrolytes and composite coatings with carboxymethyl cellulose. Electrochemical reactions of Na super(+) and Li super(+) ions with nanostructured Fe sub(2)O sub(3) are analysed and compared. Initial sodiation of Fe sub(2)O sub(3) yields a sloping profile in a voltage range characteristic for oxide conversion, which instead generates a typical plateau upon lithiation. Application of such earth-abundant, nontoxic material in upcoming Na-ion batteries is potentially groundbreaking, since it offers important advantages, namely: i. simple and cost-effective synthesis of Fe sub(2)O sub(3) nanostructures at low temperatures; ii. cheaper and more sustainable cell fabrication with higher energy densities, e.g., use of natural, water-soluble binders, as well as Al for both current collectors; iii. electrochemical performances with specific gravimetric capacities exceeding 400 mAh g super(-1) at 40 mA g super(-1), accompanied by decent specific volumetric energy densities, e.g., approximately 1.22 Wh cm super(-3), provided that cycle inefficiencies and long-term durability are addressed.