Cobalt selenide decorated carbon spheres for excellent cycling performance of sodium ion batteries

Development of electrode materials with high capacity, superior rate performance, long cycle life and low cost is critical to commercialize sodium ion batteries (SIBs). Herein, a unique composite consisted of cobalt selenide (Co0.89Se) nanoparticles (3–10 nm) embedded in carbon spheres (CoSe@CSs) is presented as anode for SIBs. The exclusive structure of CoSe@CSs enables the pseudocapacitive charge storage, improves the electronic properties and provides buffer for volumetric changes during the repeated charge-discharge processes. The composite with an average diameter of ~100 nm, denoted as CoSe@100CSs, presented the best electrochemical performance (with a reversible charge storage capacity of 554 mA h g-1 after 50 cycles at a rate of 100 mA g-1) when compared with composites of 300 nm and 500 nm diameters and also pure CoSe. Additional to high storage capacity, the CoSe@100CSs also showed excellent rate capability and ultra-long cycling life when examined at higher discharge rates. Storage capacities are as high as 350 mA h g-1 at the rate of 1000 mA g-1 and 260 mA h g-1 at the rate of 4000 mA g-1 after 4000 and even 10,000 cycles, respectively. Hence, due to this synergetic effect of uniformly distributed cobalt selenide nanoparticles and carbon matrix, the CoSe@CSs composite exhibits exceptional electrochemical performance for SIBs. This strategy is expected to open up new methodologies for the development of advanced electrode materials of high performance SIBs.