General synthesis of metal selenides embedded in N-doped carbon nanofibers covered with in-situ grown carbon nanotubes as binder-free anodes for sodium-ion storage

Due to the high theoretical capacity, metal selenides are considered to be promising materials for energy storage. Unfortunately, their practical use is still hampered by their flat electronic conductivity, large volume change and poor ion transport kinetics. In this work, a general synthesis method is presented for the preparation of metal selenides embedded in N-doped carbon nanofibers with in-situ grown carbon nanotubes (CNTs) on the surface (MSe2@NCF/CNTs, M=Co, Ni, Fe) as binder-free anodes for sodium-ion batteries. Metal precursors can be used as catalysts and as active materials after selenization treatment. The metal selenides are dispersed in the carbon nanofibers and CNTs, which effectively prevents agglomeration of the particles. The in-situ grown CNTs are strongly coupled to the surface of nanofibers, which improves the electrical conductivity, allows for efficient electrolyte infiltration and buffers the volume expansion during the charge/discharge processes. As a result, CoSe2@NCF/CNTs achieves extremely high reversible capacity, stable cycling stability (441.2 mAh g−1 after 1000 cycles at 200 mA g−1), and excellent rate properties (375.4 mAh g−1 at 2 A g−1) when used as binder-free anodes for sodium-ion batteries. The fast charge transport and Na+ diffusion rates, as well as a pseudocapacitance Na+ storage mechanism, are revealed by kinetic analysis. This synthetic approach is extendable to assembling CNTs in-situ on other substrates for advanced energy storage systems.