Abstract: Precisely modulating the lithium vacancy concentration is critical for maximizing the ionic conductivity of garnet-type Li₇La₃Zr₂O₁₂ (LLZO) solid electrolytes. However, conventional high-temperature synthesis inevitably introduces parasitic impurities (e.g., Al³⁺ from alumina crucibles), which profoundly perturb the intrinsic defect chemistry. Consequently, the fundamental structure-property relationship between lithium vacancies and ionic transport remains obscured. To unveil the genuine ion-transport mechanism, we employ a strictly contamination-free sintering strategy. By integrating a tailored microstructural model to decouple morphological interferences such as grain size and boundary effects, we systematically investigate the crystallographic evolution and transport kinetics in a series of Nb⁵⁺-doped LLZO (0.2-0.7 per formula unit, pfu). Our findings unambiguously demonstrate that within a pristine, impurity-free lattice, the room-temperature ionic conductivity peaks at an optimal lithium vacancy concentration of exactly 0.45 pfu. This work not only identifies the intrinsic vacancy metric required for ultrafast ion transport in lithium garnets but also establishes a rigorous crystallographic foundation for the rational design of high-performance solid electrolytes.

Key words: garnet-type solid electrolyte, lithium vacancy concentration, Nb doping, transport kinetics