Chemical bonding in multiple encapsulation geometry of Bi₂Se₃-based conversion-alloying anode materials for superior sodium-ion storage

Conversion-alloying-based materials have been regarded as potential anode electrodes for low-cost sodium-ion batteries (SIBs), but their applications are limited owing to the large volume variation and poor electrochemical kinetics. In this study, Bi₂Se₃ nanoflowers assembled by ultra-thin na-nosheets, vertically anchored on reduced graphene oxide (rGO) via strong chemical bonding of C-O-Bi, and encapsulated in the N-doped C nanolayer (Bi₂Se₃@rGO@NC), are constructed as anodes for Na-ion storage. The physico-chemical encapsulation geometry of graphene and N-doped C is conductive to acquiring excellent electrode integrity by accommodating large lattice strain, as well as boosting fast electrochemical kinetics process by dispelling the band gap and decreasing Na-ion diffusion barrier. Bi-ion is used as a redox site for Na-ion insertion/extraction via conversion-alloying dual-mechanism with 12-electron transport per formula (Bi₂Se₃ + 12Na⁺ + 12e⁻ ⟷ 2Na₃Bi + 3Na₂Se). Thus, a high initial charge capacity of 288.4 mA h g⁻¹ at 50 mA g⁻¹, excellent cycling stability with an ultra-long lifespan of over 1000 cycles, and good rate property (119.9 mA h g⁻¹ at 5.0 A g⁻¹) can be achieved for Bi₂Se₃@rGO@NC. This study may open up systematic research on conversion-alloying anodes and shed insights into the illumination of the electrochemical reaction mechanism for SIBs. [Figure not available: see fulltext.].