FeSe₂ clusters with excellent cyclability and rate capability for sodium-ion batteries

Sodium-ion batteries (SIBs) have great promise for sustainable and economical energy-storage applications. Nevertheless, it is a major challenge to develop anode materials with high capacity, high rate capability, and excellent cycling stability for them. In this study, FeSe₂ clusters consisting of nanorods were synthesized by a facile hydrothermal method, and their sodium-storage properties were investigated with different electrolytes. The FeSe₂ clusters delivered high electrochemical performance with an ether-based electrolyte in a voltage range of 0.5–2.9 V. A high discharge capacity of 515 mAh·g^–1 was obtained after 400 cycles at 1 A·g^–1, with a high initial columbic efficiency of 97.4%. Even at an ultrahigh rate of 35 A·g^–1, a specific capacity of 128 mAh·g^–1 was achieved. Using calculations, we revealed that the pseudocapacitance significantly contributed to the sodium-ion storage, especially at high current rates, leading to a high rate capability. The high comprehensive performance of the FeSe₂ clusters makes them a promising anode material for SIBs.