Investigating the Calcination and Sintering of Li7La3Zr2O12 (LLZO) Solid Electrolytes Using Operando Synchrotron X-ray Characterization and Mesoscale Modeling

The crystallographic structure and microstructure of solid electrolytes, such as Li7La3Zr2O12 (LLZO), have a profound impact on their reactivity, conductivity, and stability toward dendrites in solid-state batteries. Controlling the material's structure and morphology requires fine control during the synthesis process, where multiple conditions (precursor particle size/distribution, calcination/sintering temperature, ramp rate, etc.) influence performance. This paper describes, for the first time, the operando characterization of the calcination process using synchrotron X-ray diffraction combined with a mesoscale model of grain growth during the calcination and densification of LLZO. The model is then used to guide synthesis conditions to enhance the densification process. The X-ray data reveal significant coarsening of the initial nanophase lanthanum zirconate precursors during conversion to LLZO. The mesoscale model shows that the activation energy for diffusion during calcination is lower than that during sintering, indicating the inherent coupling between the chemical reaction and grain growth processes. Simulations suggest that particles with small and bimodal size distribution experience better densification, as does precise grading (smaller particles near the surface and larger particles at the center) of different-sized particles. The approach described here can be adapted to understand and guide the synthesis of other materials that undergo calcination and sintering (e.g., transition metal oxide cathodes).