Structure–Conductivity Correlation and Phase Evolution Mechanism in LLZO‐Based Solid Electrolytes

Garnet‐type Li 7 La 3 Zr 2 O 12 (LLZO) solid electrolytes are among the most promising candidates for all‐solid‐state lithium batteries owing to their high intrinsic Li + conductivity. However, poor sinterability and unstable Li interfaces hinder practical application. Although Ta/Sr codoping has been employed to stabilize the cubic phase and enhance conductivity, the high cost and sluggish sintering kinetics of Ta limit scalable fabrication. Here, a cost‐effective Nb/Sr co‐doping strategy is developed to simultaneously promote densification and Li + transport. Sr substitution at La‐sites induces transient glassy phases that accelerate sintering and stabilize Li interfaces, achieving an ionic conductivity of 8.8 × 10 −4 S cm −1 and a critical current density (CCD) of 0.65 mA cm −2 . When paired with a LiFePO 4 cathode, the optimized LLSZNO‐0.08 (Nb = 0.5, Sr = 0.08) electrolyte delivers 164.8 mAh g −1 with 88% coulombic efficiency at 0.2 C over 150 cycles and retains 140.6 mAh g −1 at 0.5 C with 89% capacity retention. The Nb/Sr co‐doped LLZO avoids intentional Al incorporation and achieves stable Li interfacial contact without surface modification. This work correlates structural evolution with ionic transport and interfacial stability, providing mechanistic insight and a scalable route for high‐performance LLZO‐based electrolytes.