Bismuth Embedded in Holey Expanded Graphite as a Sodium Ion Battery Anode with Superior Rate Capability

As a promising anode for sodium-ion batteries (SIBs), bismuth (Bi) with a high theoretical volumetric capacity of 3750 mAh cm^-3 and optimal operation voltage plateau suffers from severe volume expansion and the formation of an unstable solid-electrolyte interphase. Although expanded graphite (EG) mitigates volume changes of Bi in the Bi@EG composite, the sluggish transportation kinetics of Na⁺ between graphitic structures remains a bottleneck. Herein, we report a series of Bi_x@EG_1-x (x = 0.2, 0.4, 0.6, and 0.8) and Bi_x@hEG_1-x (x = 0.6 and 0.8) composites as anodes for SIBs. Benefiting from the synergistic effect between the high capacity of Bi nanoparticles and the layered/holey structures of holey expanded graphite (hEG), Bi_0.6@hEG_0.4 exhibited exceptional sodium storage properties including excellent rate capabilities (e.g., 232.0 and 206.3 mAh g^-1 at 10 and 20 A g^-1, respectively) and long cycling stability (e.g., a high reversible capacity of 234.9 mAh g^-1 after 4000 cycles at 5 A g^-1 with an initial capacity retention of 93.5%). When it was paired with Na₃V₂(PO₄)₃ as the cathode (NVP//Bi_0.6@hEG_0.4), NVP//Bi_0.6@hEG_0.4 achieved a high initial capacity of 102.1 mAh g^-1 at 1 C (1 C = 117 mA g^-1) and maintained a high reversible capacity of 86.7 mAh g^-1 after 900 cycles at 10 C with an 84.9% initial capacity retention.