High-Resolution Tracking Asymmetric Lithium Insertion and Extraction and Local Structure Ordering in SnS₂

In the rechargeable lithium ion batteries, the rate capability and energy efficiency are largely governed by the lithium ion transport dynamics and phase transition pathways in electrodes. Real-time and atomic-scale tracking of fully reversible lithium insertion and extraction processes in electrodes, which would ultimately lead to mechanistic understanding of how the electrodes function and why they fail, is highly desirable but very challenging. Here, we track lithium insertion and extraction in the van der Waals interactions dominated SnS₂ by in situ high-resolution TEM method. We find that the lithium insertion occurs via a fast two-phase reaction to form expanded and defective LiSnS₂, while the lithium extraction initially involves heterogeneous nucleation of intermediate superstructure Li_0.5SnS₂ domains with a 1-4 nm size. Density functional theory calculations indicate that the Li_0.5SnS₂ is kinetically favored and structurally stable. The asymmetric reaction pathways may supply enlightening insights into the mechanistic understanding of the underlying electrochemistry in the layered electrode materials and also suggest possible alternatives to the accepted explanation of the origins of voltage hysteresis in the intercalation electrode materials.