Local Structural Environments in Perovskite Oxide Solid Electrolytes

Solid-state lithium batteries using inorganic solid electrolytes have become increasingly important due to their potential to offer enhanced safety and higher energy density compared to the conventional liquid electrolyte-based lithium-ion batteries [1]. However, various structural defects, such as varying degrees of structural order or grain/domain/antiphase boundaries, are directly related to the overall ionic conductivity. [3, 4] For example, lithium-lanthanum titanate Li3xLa2/3-xTiO3 (LLTO) exhibits high ionic conductivity at room temperature (∼ 10-3 S cm-1) [2], but the synthesis conditions (e.g. La/Li concentration, post-treatment, cooling rate, etc.) can vary the degree of La/Li ordering and/or give rise to other structural defects that alter the conductivity. Because Li+ ion mobility/conductivity has been suggested to largely depend on the area between O atoms comprising the O-Ti octahedra, also known as the bottleneck size [5], the study on local structural features and environments can provide essential insights related to ionic transport and design structures with high ionic conductivity. In this talk, we report local structural characteristics of LLTO, which are related to the Li+ ion mobility, using aberration-corrected scanning transmission electron microscopy (STEM). First, the local distribution of La/Li is identified using atomic resolution ADF (annular dark field) intensities. Depending on the processing conditions, as shown in Figure 1, two types of structures are observed: one is a partially ordered structure in which the La-rich (La1) and La-poor (La2) layers are separated along the c-axis, and the other is a disordered structure with a nearly homogeneous intensity distribution. The simultaneously acquired iDPC (integrated differential phase contrast) images enable determining the position of the O atoms and the bottleneck size. A combination of ADF and iDPC images shows the decrease in bottleneck size with the increased La concentration, suggesting that La atoms act as migration barriers of Li+ ions. Additionally, the large bottleneck size in the La2 layers, which contain less La, indicates that the La2 layers are the major pathway of Li+ ions, and supported by previous reports [6]. With these direct measurements, we then examine the local bottleneck sizes at and near extended defect structures, including twins (Figure 2) and antiphase boundaries, to determine the effect of each type on the bottleneck size and thus ionic conductivity [7]. ADF (left column) and iDPC (right column) STEM images of (a) partially ordered and (b) disordered LLTO with overlaid atomic structure of LLTO. (c,d) Statistic analysis of bottleneck area according to the A-site ADF intensity. ADF STEM image of LLTO with a domain boundary (DB) (a) and atomic columns with a smaller than averaged bottleneck size of 7.7 Å2 overlapped by red circles (b).