Rich-oxygen-doped FeSe2 nanosheets with high pseudocapacitance capacity as a highly stable anode for sodium ion battery

Sodium ion battery (SIB) is considered as one of the most promising alternatives to Lithium ion battery (LIB) for large-scale energy storage, but wide usages of SIB are hindered by the lack of suitable anode materials with long cycling lifespan and high-rate capacity. With the merits of high theoretical specific capacity, preferable environmental benignity and wide resources availability, FeSe2-based materials have recently emerged as an attractive choice of SIB anode materials. In this work, a novel rich-oxygen-doped FeSe2 nanosheets (O-FeSe2 NSs) was constructed as a high-performance anode material for SIB. Substantially, the rich O on O-FeSe2 NSs effectively promotes the active material and its intermediate reaction products to form an amorphous phase during cycling and hence alleviates the electrode’s volume expansion which usually causes poor cycling stability. Besides, the rich O atoms enables the formation of stable Solid electrolyte interface (SEI) as well as fast diffusion rate of Na+, and therefore enhances the cycling life of the electrode. In addition, the unique sheet structure with rich O atoms endows O-FeSe2 NSs with super-high pseudocapacitance capacity which facilitates high-rate capabilities. Consequently, the O-FeSe2 NSs electrode displays prolonged cycling life and high-rate capacity with a large specific capacity of 268.4/268.6 mAh g−1 which is kept even after 700 cycles under the current density of 1.0 A g−1. Combined with detailed survey of electrochemical storage mechanism and Na+ diffusion kinetics, the present work is expected to provide a new research idea of FeSe2-based anode materials and motivates a deep understanding of the Na-storage in the O-FeSe2 NSs host for rechargeable SIB.