Reversible Superdense Ordering of Tetragonal Lithium in a Layered Material

Understanding lithium (Li) ordering and dynamics is foundational in energy storage. X-ray based experimental methods do not simultaneously provide atomic structure information together with chemical composition and local bonding information for lithium in solids. Here we employ scanning transmission electron microscopy (STEM) and combine imaging, spectroscopy, and diffraction within a single experiment, to observe, in situ, an all-solid-state electrochemical cell. By integrating multimodal STEM with other complementary techniques, we report a complete mapping of lithium intercalation in a layered system, LaTe3. We identify three ordered phases of LixLaTe3 with x ranging from 1/3 to 3 with in-plane strain of up to 5%. At a very high lithium concentration of Li3LaTe3, we discover an unexpected three-layer, superdense lithium phase with tetragonal symmetry occupying the van der Waals gap. This represents a new Li phase that is reversible. Our multimodal approach thus enables complete tracking of lithium ordering and dynamics, important for next-generation energy storage applications.