β-FeOOH: An Earth-Abundant High-Capacity Negative Electrode Material for Sodium-Ion Batteries

Thanks to the great earth abundance and excellent energy density of sodium, sodium-ion batteries are promising alternative energy storage devices for large-scale applications. Developing cheap, safe, and high-capacity sodium-ion battery anode materials is one of the critical challenges in this field. Here, we show that β-FeOOH is a very promising low-cost anode material, with a high reversible capacity (>500 mAh g^-1 during initial cycles). The fundamental characteristics associated with the discharge/charge processes, in terms of the redox reactions, formation/deformation of the solid electrolyte interface (SEI) layers, and structural and morphological changes, are comprehensively investigated. In addition, a comparison study shows that the smaller-sized FeOOH has more serious kinetic restrictions, and thus lower capacities, while it shows better cyclability than the bigger one. Origins of the large overpotential are discussed, and it is suggested that the overpotential should be mainly due to the features of the surface-concentration-dependent potential and the slow diffusion of Na⁺; in addition, the presence of the SEI layers may also contribute to the overpotential.