Direct Observation of Li Migration into VS: Order to Antisite Disorder Intercalation Followed by the Topotactic-Based Conversion Reaction.

Two-dimensional transition-metal dichalcogenides hold great potential in rechargeable lithium-ion batteries. Their electrochemical properties are closely related to the structural evolutions during lithium-ion migration. Understanding these migration/reaction mechanisms is important to help improve battery performance. Herein, we report the real-time and atomic-scale observation of phase transitions during the lithiation and delithiation for VS via in situ electron diffraction and high-resolution transmission electron microscopy techniques. We find that the phase transformation proceeds via a sequence of order to antisite disorder intercalation and topotactic-based conversion reaction. During the intercalation reaction, the lithium ion destroys the orderings of the interstitial V with the formation of Li/V antisite. Such a reaction is found to be reversible, i.e., the extraction of lithium from LiVS leads to the recovery of V orderings. The conversion reaction involves heterogeneous nucleation of LiS with 3-20 nm nanodomains, which maintain the crystallographic integrity with LiVS. These findings elucidate the complex interactions between the lithium ion and host VS during ionic migration in solids, which should be helpful in understanding the relationship between phase transformation kinetics and battery performance.