Morphological Heterogeneity Impact of Film Solid-State Cathode on Utilization and Fracture Dynamics.

Structural heterogeneity in solid-state batteries can impact the material utilization and fracture mechanisms. Crystallographically oriented LiCoO film cathodes serve as a model electrode system for exploring how void distribution contributes to stress relief and buildup during cycling. Real- and reciprocal-space and synchrotron-based experiments are utilized to understand structural changes across multiple length scales that contribute to stress generation and fracture. Nanotomography uncovers a depth-dependent porosity variation in the pristine electrode and highlights the preferential fracture in regions of lower porosity during delithiation. Energy-dispersive X-ray diffraction and three-dimensional (3D) X-ray absorption near-edge spectroscopy (XANES) reveal the underutilization of cathode material in these regions. 3D XANES also confirms preferential delithiation near the subgrain boundaries. Chemo-mechanical modeling coupled with site-specific mechanical characterization demonstrates how stress accumulation in dense regions of the electrode leads to fracture and underutilization of active material. Our findings reveal the importance of material design to alleviate stress in small-volume changing cathodes.