Unveiling the Unique Phase Transformation Behavior and Sodiation Kinetics of 1D van der Waals Sb₂S₃ Anodes for Sodium Ion Batteries

Carbon-coated van der Waals stacked Sb₂S₃ nanorods (SSNR/C) are synthesized by facile hydrothermal growth as anodes for sodium ion batteries (SIBs). The sodiation kinetics and phase evolution behavior of the SSNR/C anode during the first and subsequent cycles are unraveled by coupling in situ transmission electron microscopy analysis with first-principles calculations. During the first sodiation process, Na⁺ ions intercalate into the Sb₂S₃ crystals with an ultrafast speed of 146 nm s⁻¹. The resulting amorphous Na_xSb₂S₃ intermediate phases undergo sequential conversion and alloying reactions to form crystalline Na₂S, Na₃Sb, and minor metallic Sb. Upon desodiation, Na⁺ ions extract from the nanocrystalline phases to leave behind the fully desodiated Sb₂S₃ in an amorphous state. Such unique phase evolution behavior gives rise to superb electrochemical performance and leads to an unexpectedly small volume expansion of ≈54%. The first-principles calculations reveal distinctive phase evolution arising from the synergy between the extremely low Na⁺ ion diffusion barrier of 190 meV and the sharply increased electronic conductivity upon the formation of amorphous Na_xSb₂S₃ intermediate phases. These findings highlight an anomalous Na⁺ ion storage mechanism and shed new light on the development of high performance SIB anodes based on van der Waals crystals.