Background <p>Chemomechanically driven degradation at solid electrolyte-lithium metal interfaces is a critical barrier to the reliability of solid-state lithium-ion batteries. Although interphase formation at these interfaces is well recognized, how reaction-induced deformation and stress evolution depend on electrolyte microstructure remains poorly understood.</p> Objective <p>This study aims to directly quantify the coupled evolution of interphase growth, deformation, and stress, and to determine how electrolyte microstructure influences stress localization and fracture during interphase formation.</p> Methods <p>An integrated in situ experimental platform combining optical imaging, mechanical load measurement, and electrochemical impedance spectroscopy was developed to characterize interphase growth and stress evolution in a model sulfide electrolyte (Li₁₀SnP₂S₁₂). Reaction-induced volumetric strain and interphase roughness were quantified from operando imaging, and finite element simulations incorporating reaction-induced eigenstrain were performed to resolve stress localization mechanisms.</p> Results <p>Interphase growth kinetics were comparable across microstructures; however, dense electrolyte pellets exhibited larger reaction-induced volumetric strains, pronounced interphase roughness, sustained pressure buildup, and tensile microcracking despite globally compressive loading. Simulations revealed that rough interphase morphologies generate substantial localized tensile stress through geometric stress amplification, explaining the observed cracking behavior.</p> Conclusions <p>These results demonstrate that electrolyte microstructure critically governs how interphase evolution translates into stress localization and fracture, highlighting microstructural control as a key strategy for mitigating chemomechanical degradation in solid-state lithium-ion batteries.</p>