Chemically bonded Bi@C in porous carbon bundles for ultrafast and stable sodium storage

Metallic bismuth is one of the promising sodium ion anodes due to its exceptional conductivity, high theoretical capacity and suitable redox potential. However, severe volume changes (∼240%) during alloying reactions hinder its further development. In this study, carbon coated Bi nanoparticles embedded in micro-sized porous carbon bundles (Bi@C) are successfully fabricated. The dual-carbon confinement superstructure with inner covalent anchoring of C-O-Bi bonds significantly relieves the volume expansion of the Bi alloy anode and improves the electron transfer rate for the Bi@C anode. This unique structure promotes a pseudocapacitive sodium storage mechanism. As a result, Bi@C exhibits an excellent rate capability of 342.2 mAh g^-1 at 15 A g^-1 and long cyclability with 320.2 mAh g^-1 after 1000 cycles at 2 A g^-1 and 82.5% capacity retention. This strategy could be applied to other alloy-type anode materials for optimizing the rate performance and cycling stability.