Ultrathin CuSe nanosheets as the anode for sodium ion battery with high rate performance and long cycle life

Sodium-ion batteries (SIBs) have quickly developed to be important alternative energy storage devices other than lithium-ion batteries in the explosively growing market of large-scale grid storage and low-speed electric vehicles, thanks to their low cost and the abundance of sodium resources. However, the sluggish sodiation kinetics remains major concerns in design of competent anode materials for SIBs. Herein, by using a one-step, facile hydrothermal synthesis, well-defined ultrathin CuSe nanosheets (thickness of ∼5 nm) were fabricated, leading to a narrow bandgap, flexible Cu–Se bonding and abundant electrochemically active sites. As a result, the product presents high rate performance: a high capacity of 404 mAh g^–1 is achieved at a current density of 0.1 A g^–1 after 100 operation cycles, and the capacity can be maintained to 276 mAh g^–1 even at a high current density of 20 A g^–1. According to kinetics analysis, surface capacitance contributes dominatly in the electrochemical process, facilitating fast sodiation. Moreover, coordinated the ultrathin thickness and two-dimensional morphology of the nanosheets with their three-dimensional open framework, the volume expansion-related issues during charge/discharge processes have been well addressed, resulting in ultrahigh cycling stability (with 100% capacity maintenance after 500 cycles at 0.5 A g^–1) together with ultralong cycle life (up to 10,000 working cycles) at 20 A g^–1.