Bromination of carbon nanohorns to improve sodium-ion storage performance

Carbon nanohorns (CNHs) have low layering, internal cavities, a large number of defects and these structural features are important for the effective accumulation of sodium ions. Since nanohorns can be easily modified, this opens up possibilities for controlling their properties. In this work, initial CNHs and CNHs opened by thermal oxidation in the air were successfully brominated to bromine concentrations of 7 and 5 at%, respectively. Cyclic voltammetry curves revealed two peaks at potentials of 1.4 and 2.1 V vs Na/Na+ and the related processes contributed to the capacity of the brominated samples. Density functional theory calculations showed that the peaks correspond to reversible adsorption/desorption of Na+ ions on the surface of CNHs near the brominated tips and the adsorbed bromine molecules. As a result, brominated open CNHs with a large amount of Br2 had the highest specific capacity of 129-62 mAh g-1 at 0.1-2 A g-1 as compared to the corresponding values of 117-41 mAh g-1 for the initial CNHs. Our results show that the bromination of carbon materials, pretreated for controlling the introduction of certain forms of bromine, is a way to improve their performance in sodium-ion batteries.