Self-Assembled FeSe2 Microspheres with High-Rate Capability and Long-Term Stability as Anode Material for Sodium- and Potassium-Ion Batteries.

Sodium- and potassium-ion batteries have attracted intensive attention recently as low-cost alternatives to lithium-ion batteries with naturally abundant resources. However, the large ionic radii of Na+ and K+ render their slow mobility, leading to sluggish diffusion in host materials. Herein, hierarchical FeSe2 microspheres assembled by closely packed nano/microrods are rationally designed and synthesized through a facile solvothermal method. Without carbonaceous material incorporation, the electrode delivers a reversible Na+ storage capacity of 559 mA h g-1 at a current rate of 0.1 A g-1 and a remarkable rate performance with a capacity of 525 mA h g-1 at 20 A g-1 . As for K+ storage, the FeSe2 anode delivers a high reversible capacity of 393 mA h g-1 at 0.4 A g-1 . Even at a high current rate of 5 A g-1 , a discharge capacity of 322 mA h g-1 can be achieved, which is among the best high-rate anodes for K+ storage. The excellent electrochemical performance can be attributed to the favorable morphological structure and the use of an ether-based electrolyte during cycling. Moreover, quantitative study suggests a strong pseudocapacitive contribution, which boosts fast kinetics and interfacial storage.