Unveiling the microscopic origin of asymmetric phase transformations in (de)sodiated Sb₂Se₃ with in situ transmission electron microscopy

Sodium-ion batteries (SIBs) have huge application potential in large-scale energy storage systems due to high abundance and low cost of sodium resource. Metal chalcogenides, which store Na ions by multiple reactions of intercalation, conversion, and alloying, are considered promising anode materials with high theoretical capacity but often suffer from fast capacity fading and underlying reason remains elusive due to the lack of a precise understanding of microscopic behaviors. Here, combined with in situ high-resolution transmission electron microscopy and consecutive electron diffraction, we tracked phase transformations during (de)sodiation of Sb₂Se₃ nanowires in real time and revealed multi-step reaction mechanisms. During sodiation, Sb₂Se₃ NWs firstly underwent the intercalation of Na ions and forming Na_xSb₂Se₃ phase, following by the conversion reaction into Na₂Se and Sb phases; afterwards, metallic Sb could further alloy with Na to form Na₃Sb phase via sequential intermediate Na_xSb phases. While during desodiation, despite the reversible dealloying of Na₃Sb phase, the following deconversion of Na₂Se and Sb phases back to Sb₂Se₃ phase was incomplete, which is deemed to be the key factor causing rapid capacity decay. Furthermore, we reveal that contact interfaces between NWs greatly affected the degree of deconversion reaction due to possibly changed electrode reaction kinetics. This work not only reports the first experimental visualization proof showing the effect of contact interfaces on phase transformations of electrode materials, but also affords new insights into capacity decay mechanisms and rational design of high-capacity SIBs.