Atomic-Scale Probing of Reversible Li Migration in 1T-V_{1+ x}Se₂ and the Interactions between Interstitial v and Li

Ionic doping and migration in solids underpins a wide range of applications including lithium ion batteries, fuel cells, resistive memories, and catalysis. Here, by in situ transmission electron microscopy technique we directly track the structural evolution during Li ions insertion and extraction in transition metal dichalcogenide 1T-V_1+xSe₂ nanostructures which feature spontaneous localized superstructures due to the periodical interstitial V atoms within the van der Waals interlayers. We find that lithium ion migration destroys the cationic orderings and leads to a phase transition from superstructure to nonsuperstructure. This phase transition is reversible, that is, the superstructure returns back after extraction of lithium ion from Li_yV_1+xSe₂. These findings provide valuable insights into understanding and controlling the structure and properties of 2D materials by general ionic and electric doping.