Bismuth nano-spheres encapsulated in porous carbon network for robust and fast sodium storage

•The Bi-NS@C composite has been synthesized via a molten salt method.•Bi nanoparticles are well-dispersed and incorporated into the carbon matrix.•The composite boosts the electric conductivity and mass transport of Na+.•The composite delivers excellent cycle performance for sodium ion battery.

Sodium ion batteries (SIBs) have been considered as a promising cost-effective alternative for grid energy storage for renewable energy sources such as wind- and solar power. In this work, a bismuth nano-spheres and porous carbon composite (Bi-NS@C) is developed via an oleate-oriented dual-phase interfacial reaction and a molten salt calcination process. Materials characterizations suggest that the Bi-NS with a size of 20-30 nm are uniformly distributed in the sponge-like porous carbon network. Such a structure could enable a conductive network, prevent particle aggregation, shorten the ions transportation pathways, accommodate volume change and prevent the collapse of the electrode. As a result, this anode delivers a reversible discharge capacity of 106 mAh g−1 after even 1000 cycles at 0.2 A g−1. Even at 2 A g−1, the specific capacity of the electrode can still retain at ∼110 mAh g−1. The remarkable electrochemical performance of the Bi-NS@C composite suggests that the as-prepared nanocomposite can simultaneously enhance the Na+ ion conductivity and electronic conductivity in the charge/discharge processes, which offer guidance in anode materials design and synthesis in SIBs.