Unraveling the Origin of Grain Boundary Lithium Deficiency in Ceramic Solid Electrolytes

Realizing solid electrolytes with low grain-boundary (GB) resistance is critical for advancing all-solid-state batteries. High GB resistance in SEs is often attributed to deficiencies in mobile ions at these boundaries; yet, when and how these deficiencies form during synthesis remain unclear. Here, we use a unique in situ scanning transmission electron microscopy setup to guide solid electrolyte crystallization during annealing, enabling real-time observation of GB formation at the atomic scale, with Li0.33La0.56TiO3 as a model SE. We reveal an ultrathin, less than 1.5 nm thick, lithium-deficient layer that emerges at the crystallization front upon crystallization and persists as two adjacent crystals fuse to form a GB. We offer two hypotheses for the origin of the lithium-deficient layer, one based on thermodynamic stabilization and the other on kinetic constraints. Our results provide guidelines for designing synthesis strategies to reduce GB resistance in solid electrolytes.