Structural Stability of NMC and LLZO during Thermal Annealing Studied with In Situ Bragg Coherent Diffractive Imaging

Abstract

Composite electrodes combine cathode active materials with solid electrolytes and are essential for all-solid-state batteries. Nevertheless, co-sintering these components introduces mechanical stresses that can lead to microstructural degradation. Understanding how defects evolve at the grain level during thermal processing is critical for designing mechanically robust composite electrodes. Here, we use in situ Bragg coherent diffractive imaging (BCDI) to investigate the microstructural stability of two key materials: the cathode LiNi0.8Mn0.1Co0.1O2 (NMC811) and the solid electrolyte Al-doped cubic Li7La3Zr2O12 (c-LLZO). We track individual crystalline grains during heating and image three-dimensional displacement fields to visualize embedded dislocations. NMC811 exhibits remarkable stability, with stationary dislocation networks and minimal changes in diffraction patterns up to 400 °C. In contrast, c-LLZO undergoes significant microstructural changes beginning at 200 °C, including strain relaxation and dislocation migration. These findings reveal that c-LLZO exhibits dislocation mobility at moderate temperatures that may enable it to accommodate thermal stresses when co-sintered with more rigid oxide cathodes, while the stability of NMC811 ensures structural integrity of the active material. This work demonstrates the value of single-grain characterization for understanding mechanical compatibility in composite electrode design.