Graphene-Loaded Bi₂Se₃: A Conversion-Alloying-type Anode Material for Ultrafast Gravimetric and Volumetric Na Storage

Sodium-ion battery (SIB) has been a promising alternative for sustainable electrochemical energy-storage devices. However, it still needs great efforts to develop electrode materials with ultrafast gravimetric and volumetric Na-storage performance, due to difficult balance between Na-ion diffusion kinetics and pressing density of materials. In this work, Bi₂Se₃/graphene composites, synthesized by a selenization reaction, are investigated as anode materials for SIBs. Na-ion storage mechanism of Bi₂Se₃ should be attributed to a combined conversion-alloying one by a series of ex situ measurements. In the composites, Bi₂Se₃ particles with an average diameter of 100 nm are uniformly dispersed onto graphene with strong interfacial interaction. Despite their nanoscale size, the pressing density of Bi₂Se₃/graphene composite could still reach a high value of 2.07 g/cm³. Therefore, the composites can deliver a high gravimetric specific capacity of 346 mAh/g and volumetric specific capacity of 716 mAh/cm³ at a current density of 0.1 A/g. Remarkably, the composites exhibit an ultrafast Na-storage capability and a negligible capacity fading with the increasing of current density from 0.2 to 5 A/g. Even at 10 A/g (≈30 C), the composites still possess a gravimetric capacity of 183 mAh/g and volumetric capacity of 379 mAh/cm³ with ultrastable cyclability up to 1000 cycles. This work introduces a valid route to design electrode materials with both excellent gravimetric and volumetric performance of Na-ion storage.