Operando monitoring of mechanical failure in all-solid-state batteries via embedded optical sensing

The chemo-mechanical behavior within the electrodes and at the interfaces of all-solid-state batteries (ASSBs) remains poorly understood, primarily due to the lack of direct and in-operando characterization techniques. In this study, multi-grating Fiber Bragg Grating (FBG) sensors are embedded within the polyethylene oxide (PEO)-Li_6.5La₃Zr_1.5Ta_0.5O₁₂ (LLZTO) solid-state electrolyte (SSE) and on the surface of the cathode. This configuration enables real-time and spatial monitoring of the strain inside the SSE and at the cathode-electrolyte interface. The results reveal a significant strain inhomogeneity within the electrode during cycling, which is identified as a critical factor of the electrode cracking in ASSBs. Furthermore, we find a pronounced strain mismatch between the cathode and SSE, which leads directly to the interfacial delamination and contact loss of ASSBs. Remarkably, the decay of differential strain amplitude (DSA) serves as a critical precursor of internal damage, as it precedes other electrical indicators such as terminal voltage, coulombic efficiency, and capacity. The accumulated mechanical damage including cracks and voids provides pathways for soft short circuit that trigger the electrical failure. The developed in-operando methodology provides a powerful approach for the design of ASSBs with both enhanced mechanical stability and prolonged cycle life.