Mechanistic Origins of Chemo‐Mechanical Instability and Failure in Solid‐State Lithium‐Sulfur Electrodes

ABSTRACT Solid‐state lithium‐sulfur (SSLS) batteries offer a promising route toward high‐energy‐density storage, yet their practical implementation is hindered by mechanical degradation associated with large conversion‐induced volume expansion. Although these volumetric changes can promote particle fracture and capacity fade, the links between cathode microstructure, strain‐energy accumulation, and damage evolution remain poorly understood. Here, we develop a microstructure‐resolved electro‐chemo‐mechanical framework to identify the fundamental drivers of fracture in SSLS cathodes. By explicitly coupling conversion‐induced expansion with mechanical constraints imposed by neighboring particle arrangements, we show that crack initiation and propagation are governed by stress localization arising from local microstructural heterogeneity. Our results reveal that constrained expansion produces localized strain‐energy hotspots that dictate crack nucleation and lead to distinct fracture morphologies, including annular, radial, and through‐particle cracking. We further identify sulfur particle size and cathode packing density as key design parameters controlling mechanical degradation. Larger sulfur particles intensify stress localization and accelerate fracture onset, whereas increased porosity relaxes mechanical constraint, delays crack initiation, and reduces overall damage. These findings reveal a fundamental trade‐off between energy density and mechanical stability, providing a mechanistic basis for designing SSLS cathodes with improved structural integrity and cycling‐relevant durability.