Crystal Structure Engineering Enables Enhanced Ionic Conductivity in LAGP Solid‐State Electrolytes

Abstract Solid‐state lithium batteries (SSLBs) have emerged as promising energy storage systems, offering superior safety and energy density compared to conventional liquid electrolyte‐based lithium‐ion batteries. Among solid‐state electrolytes, NASICON‐type lithium aluminum germanium phosphate (LAGP) has attracted significant attention due to its exceptional air stability and wide electrochemical window. However, its practical ionic conductivity remains substantially below theoretical predictions. This study demonstrates a crystal structure engineering strategy using γ‐Al 2 O 3 instead of conventional α‐Al 2 O 3 as the aluminum source. The weaker Al─O bonds and higher reactivity of γ‐Al 2 O 3 facilitate increased Al 3 ⁺ incorporation into the LAGP framework, enhancing free Li⁺ concentration at M2 sites and boosting bulk ionic conductivity. Furthermore, γ‐Al 2 O 3 addition reduces AlPO 4 impurities and improves morphological uniformity, thereby optimizing grain boundary conductivity. The optimized γ‐Al 2 O 3 ‐derived LAGP achieves a threefold enhancement in lithium‐ion conductivity, reaching 6.04 × 10 −4 (total) and 2.77 × 10 −3 S cm −1 (intracrystalline), representing a significant advancement in solid electrolyte performance.