Abstract：Solid-state lithium batteries are widely regarded as a key technology for achieving both high energy density and enhanced safety in next-generation energy storage systems. However, the structural stability of solid electrolytes remains a fundamental bottleneck limiting their long-term cycling performance. In this Review, we systematically revisit the failure mechanisms of solid electrolytes from a mechanical fatigue perspective. The paper summarizes the origins of the internal and external forces experienced during battery operation, including electrode volume changes, electrochemically induced stress, current-density inhomogeneity, as well as stack pressure and structural constraints. Collectively, these factors constitute a cyclic loading environment intrinsically characterized by low-cycle fatigue driven by the coupling of internal and external mechanical forces. Distinct from conventional analyses based solely on static strength criteria or isolated dendrite/crack mechanisms, we propose that the failure of solid electrolytes can be more comprehensively understood as a quasi-brittle fatigue process. In this framework, local current-density and stress concentrations, often triggered by intrinsic material defects and interfacial voids, act as initiation sites. Subsequent degradation is accelerated through the mutually reinforcing interaction between crack propagation and lithium dendrite growth. By comparatively analyzing soft and stiff material systems with different mechanical response characteristics, we demonstrate that low-modulus electrolytes tend to exhibit dendrite-dominated failure initiation pathways, whereas high-modulus electrolytes are more prone to crack-dominated fatigue damage. Despite these distinct initiation mechanisms, both systems ultimately converge toward coupled crack–dendrite penetration and catastrophic failure under cyclic mechanical loading. From the perspective of cyclic mechanical loading, we establish a unified framework for understanding the long-term degradation of solid electrolytes. Based on this framework, we further summarize and critically evaluate existing mechanical optimization strategies targeting material defects, interfacial contact stability, and the evolution of cracks and dendrites. Finally, we outline future research directions for solid electrolytes, emphasizing a paradigm shift from a solely high-modulus design philosophy toward “stiff–compliant” architectures that synergistically balance mechanical rigidity and stress accommodation. We advocate for the development of fatigue-lifetime-oriented evaluation metrics and multi-scale predictive models tailored to realistic solid-state battery operating conditions, thereby enabling a transition from empirical parameter optimization to fatigue-informed, life-predictive design spanning from material properties to battery lifetime.
2026, Vol. 47 
