Lithium growth involves coupled structural and mechanical evolution that remains difficult to probe non-destructively during cycling, especially at buried interfaces. Here, we establish an operando platform by integrating guided-wave ultrasound with an optically accessible lithium symmetric glass cell, enabling continuous tracking of lithium evolution during plating and stripping. Two distinct ultrasonic observables are identified with different physical meanings: the velocity change primarily tracks the thickness evolution of dense deposited lithium, whereas the attenuation change rate is more sensitive to the development of porous and dendritic morphologies.
By combining guided-wave measurements with simulation and cryo-FIB-SEM characterization, we further resolve the microstructural origin of these acoustic responses. Simulation shows that, when lithium remains dense, thickness is the dominant factor controlling the velocity response, while the contribution of growth stress is negligible under the present conditions. Cryo-FIB-SEM reveals progressive porosity development, a two-layer morphology after stripping, and through-thickness structural anisotropy in degraded lithium, providing the structural basis for the continued wave-velocity reduction and attenuation increase. These results show that ultrasound does not merely detect lithium growth qualitatively, but can decouple thickness evolution from dendrite-related structural degradation through mechanistically interpretable signal features.
This work establishes a non-destructive operando framework for linking lithium plating behaviour, dendrite evolution, and buried microstructural heterogeneity with measurable acoustic signatures. For the battery field, it provides a new route to study how mechanical degradation evolves together with electrochemical processes in lithium metal anodes, offering insight that is difficult to obtain from optical observation alone. More broadly within battery research, this approach opens opportunities for tracking hidden structural evolution in buried interfaces and for developing mechanistically informed diagnostics of interfacial degradation, failure initiation, and cycling instability.