On the irreversible sodiation of tin disulfide

Highlights: • In situ TEM and DFT results clarified the asymmetric reaction pathways of SnS₂ during sodiation and desodiation. • The real sodiation pathway of 2D SnS₂ is intercalation → disordering → conversion → alloying. • Only the rock-salt phase (Na_ySnS₂) is finally formed thru desodiation reaction, limiting the reversible capacity. Tin disulfide is considered as a promising electrode material for sodium-ion batteries because of its two-dimensional layered structural characteristics allowing the intercalation of Na ions. Understanding the underlying reaction mechanisms and the decisive step of the reaction reversibility is critical for its applications. Herein, we investigate the sodiation and desodiation processes of SnS₂ by employing in situ transmission electron microscopy (TEM). After the initial intercalation reaction, a rock-salt Na_ySnS₂ phase with disordering Na and Sn cations is observed, followed with a conversion reaction and an alloying reaction. The disordering reaction occurs along < 1–10 > direction of pristine SnS₂ phase which is correlated with local bonding rearrangements induced by the exchange of Sn and Na cations. In-situ TEM studies and first-principles calculations indicate that the original 2D SnS₂ structure could not be recovered during desodiation. Instead, the disordered Na_ySnS₂ phase is finally formed, which indicates that the irreversible disordering transition is the determining step of irreversible cycling. This work probes the structural evolution of sodiation, providing a fundamental understanding of the electrochemical properties of metal sulfides and inspiring rational designs of high performance electrodes for sodium-ion batteries.