Encapsulating Trogtalite CoSe2 Nanobuds into BCN Nanotubes as High Storage Capacity Sodium Ion Battery Anodes

Trogtalite CoSe₂ nanobuds encapsulated into boron and nitrogen codoped graphene (BCN) nanotubes (CoSe₂@BCN-750) are synthesized via a concurrent thermal decomposition and selenization processes. The CoSe₂@BCN-750 nanotubes deliver an excellent storage capacity of 580 mA h g⁻¹ at current density of 100 mA g^-1 at 100th cycle, as the anode of a sodium ion battery. The CoSe₂@BCN-750 nanotubes exhibit a significant rate capability (100-2000 mA g^-1 current density) and high stability (almost 98% storage retention after 4000 cycles at large current density of 8000 mA g^-1). The reasons for these excellent storage properties are illuminated by theoretical calculations of the relevant models, and various possible Na⁺ ion storage sites are identified through first-principles calculations. These results demonstrate that the insertion of heteroatoms, B-C, N-C as well as CoSe₂, into BCN tubes, enables the observed excellent adsorption energy of Na⁺ ions in high energy storage devices, which supports the experimental results.