Facile synthesis of hierarchical (NiCo)Se/(NiCo)Se2@C nanostructure via the synergistic effect of carbonization and selenization

Transition-metal selenides (TMSs) possess excellent redox reversibility and Se-rich TMSs have high theoretical specific capacity for sodium storage. Although carbon coating is a useful strategy to construct robust protective layer with additional high electronic conductivity, the formation of Se-poor TMSs inevitable results in low ca-pacity. Herein, by simply adopting the synergistic strategy of carbonization and selenization, a novel kind of hierarchical (NiCo)Se/(NiCo)Se2@C nanostructure can be fabricated. The formation and redistribution of mul-tiple phases can create porous structure and facilitate to expose maximum active sites, finally endowing (NiCo) Se/(NiCo)Se2@C multifunctional properties such as stable, large, fast and reversible sodium storage. The sodium -ion battery using this kind of electrode material exhibits a high initial specific capacity of 658.7 mA h g-1 at 1 A g-1, excellent rate capability of 324.9 mA h g-1 at 30 A g-1, superior cycle stability with a low capacity decay rate of 0.0046% per cycle for 1100 cycles at 20 A g-1. This work provides a facile strategy for designing unique hierarchical nanostructure for sodium-ion batteries.