In‐Situ Transmission Electron Microscope Investigation of the Calcination Behavior and Mechanism of Solid‐State Electrolyte Li ₁ . ₃ Al ₀ . ₃ Ti ₁ . ₇ (PO ₄ ) ₃ (

ABSTRACT Solid‐state electrolytes are key materials for all‐solid‐state lithium batteries (ASSLBs) due to their high safety, thermal stability, and ability to suppress lithium dendrite growth. Among them, Li 1 . 3 Al 0 . 3 Ti 1 . 7 (PO 4 ) 3 (LATP), a NASICON‐type oxide electrolyte, is attractive for its high ionic conductivity and stability, but its formation mechanism during calcination remains poorly understood. Here, we combine ex situ XRD, Raman spectroscopy, and atomic‐scale in situ TEM to elucidate the structural evolution of LATP upon calcination. The results reveal a transformation from amorphous precursors to intermediate phases (Li 4 P 2 O 7 and AlPO 4 ), and finally to crystalline LATP at 800°C. In situ TEM directly visualized the structural evolution associated with Al incorporation into the LTP* lattice and the subsequent Ti‐site substitution, which reconstructs the phosphate framework and stabilizes the NASICON structure. High‐resolution TEM/FFT, supported by XPS and XAS analyses, confirms that Al substitution perturbs local Ti–O coordination while preserving global lattice stability. This study establishes, for the first time, the complete thermal‐induced formation mechanism of LATP at the atomic scale, providing new insights for optimizing synthesis and improving the performance of solid‐state electrolytes.