Atomically Determining the Interstitial Lithium in the Three-Dimensional Lattice of a Lithium-Ion-Conducting Solid Electrolyte

Identifying all the possible Li sites in a crystal structure is the first step toward understanding the Li-ion migration in solid electrolytes for Li batteries. Here, we discover that the perovskite structure, a widely studied prototypic structure for solid electrolytes, may host Li ions in three different sites instead of only two sites as commonly believed. Thanks to integrated differential phase contrast (iDPC) scanning transmission electron microscopy (STEM), the Li ions that are too light to be detected in the conventional high-angle annular dark-field (HAADF) STEM are successfully visualized in the lattice. Combined with first-principles computations, this microscopy technique discloses the existence of Li ions in an interstitial site that has not been reported to host Li ions before in the perovskite structure and different Li-ion transport behaviors. This discovery enables a more comprehensive understanding of Li-ion transport in the perovskite structure.