Local Environments in Inorganic Oxide Solid Electrolytes

Abstract

All-solid-state batteries using inorganic solid electrolytes have been attracting great attention due to their enhanced stability and high volumetric capacity for the increased demand for high energy storage density and good operational safety for large-scale use[1]. However, crystalline electrolytes inevitably contain various kinds of non-periodic features (e.g. grain/domain boundaries, point defects) as a result of synthesis[2,3]. Since these defects have a significant effect on the overall conductivity, it is necessary to understand the local structure and ionic transport of crystalline electrolytes. For example, in lithium lanthanum titanate (Li3xLa2/3-xTiO3, denoted as LLTO), the random distribution of vacancies and La disorder creates a series of different local environments which affect the ionic mobility, following random walk theory[4,5]. While the movement of mobile ions is influenced by both the local chemical and structural environment, few methods exist that can directly observe the connection between the two[4].In this talk, we report the local chemical and structural environments of LLTO using aberration-corrected scanning transmission electron microscopy (STEM), and investigate their effect on ion mobility. Direct imaging is used to determine the dominant structural factors related to the ion migration. As shown in the STEM image in Fig. 1a, the simultaneously acquired ADF (annular dark field) and iDPC (integrated differential phase contrast) enable correlating the La/Li-site chemical distribution (Li/La/vacancy) and O bottleneck size. In addition, La/Li migration under the high energy electron beam induces a change of bottleneck size with a change in the La/Li-site chemical environment, revealing a direct relationship between the site chemistry and lattice distortion (Fig. 1b). Directly mapping the bottleneck size along with the local chemistry also enables direct comparisons with density functional theory (DFT) and molecular dynamics (MD). They show that the Li hopping mechanism is associated not only with Li itself but also with La, Ti, and O sublattices, suggesting that the structural changes along with the La/Li-site distribution can have a significant effect on Li transport and vice versa. This study of the local environment with high spatial and temporal resolution can serve as a cornerstone for a comprehensive and complete understanding of ion migration dynamics and establishing universal guidelines for further enhancement of ion conductivity.