Unlocking the Secrets of Divergent Sodium Storage Performances in Sb and Bi Anodes

Alloy-type anode materials have garnered significant interest for sodium-ion batteries (SIBs) due to their high theoretical capacity. However, the underlying mechanisms affecting their rate capability and cycling stability remain inadequately elucidated. In this study, we systematically compare the electrochemical performance of Sb and Bi anodes, which belong to group VA and exhibit analogous sodium alloy composition. The results reveal that the marked performance disparity arises from a "blockade effect" of Na⁺ within the Sb alloy intermediate phase, which impedes the phase transformation kinetics. This not only deteriorates rate capability but also induces structural degradation during cycling. In contrast, the Bi anode undergoes a sequential phase transition that enables rapid Na⁺ diffusion and promotes the formation of a thin and uniform solid-electrolyte interphase (SEI). As a result, the Bi anode delivers a high capacity of 325.1 mAh g^-1 even at an ultrahigh current rate of 155.8C (1C = 385 mA g^-1), and maintains a specific capacity of 323 mAh g^-1 (88% retention) after 2500 cycles at 2 A g^-1, significantly outperforming the Sb anode. The phase transition mechanism uncovered in this work provides valuable insights for the rational design of advanced alloy-based anodes for alkali metal ion batteries.