The multi-anion environment of Bi13S18I2 governs the mechanism of sodium storage.

Sodium-ion batteries (SIBs) are promising for large-scale energy storage. Alloy-type anodes such as Bi metal offer high capacity yet suffer from severe volume expansion and unstable cycling due to alloying reactions. Herein, we report a mixed-anion Bi13S18I2 nanorod anode synthesised via a mild solvothermal method, offering a unique platform to regulate the sodiation pathway. Compared with conventional Bi metal, Bi13S18I2 undergoes an initial conversion reaction that may convert the electrode into Bi embedded within conversion-derived Na2S composites. This conversion induced matrix modulates the subsequent alloying behaviour. Ex situ XRD analysis, together with the plateau capacity distribution, suggests that the sodiation process differs from the stepwise alloying behaviour of Bi metal. This different electrochemcial behaviour may be associated with the mixed-anion framework in Bi13S18I2, which results in heterogeneous local coordination environments for Bi and gives rise to a unique conversion-alloying pathway. This work highlights the important role of mixed-anion induced local environments in adjusting sodiation pathways, offering insights into the design of alloy-type anodes for SIBs.