Phase Evolution and Crystallization Mechanism of Glass Ceramic Solid-State Electrolyte from In Situ Synchrotron X-ray Diffraction

Lithium aluminum germanium phosphate (LAGP) is a promising solid state electrolyte system for all solid state lithium ion batteries. A common route for the synthesis of LAGP is through the crystallization of the glass precursors hence detailed understanding of crystallization behavior is critical for the synthesis of pure phase and highly conductive LAGP based solid state electrolyte. In this work, in situ synchrotron X-ray diffraction was used to investigate the nucleation and crystallization processes of LAGP Li1.5Al0.5Ge1.5(PO4)3 based solid-state electrolytes together with scanning electron microscopy, atomic force microscopy, and atomistic computer simulations. Structural models from molecular dynamics simulations were used to interpret the diffraction patterns acquired from synchrotron diffraction. The strain and average size of crystal grains were calculated by using the Williamson–Hall equation, and the results suggest a compressive stress on the grains in the early stage of nucleation. This stress increases the solubility of Al in the nuclei that was explained by the Gibbs–Thomson effect. It was also found that aliovalent ion-substituted pure phase LAGP can be obtained by heat treatment at a temperature significantly lower than previously reported through the two-step heat treatment process.