Observations of stress accumulation and relaxation in solid-state lithiation and delithiation of suspended Si microcantilevers

Motion of microfabricated cantilevers is demonstrated as an in situ technique for mechanical characterization during the solid-state electrochemical lithiation and delithiation of silicon. The composite cantilevers consist of suspended single-crystal silicon cantilevers, onto which LiAlF₄ electrolyte, Li₂WO₄ lithium reservoir, and Ag electrode layers are deposited. Using white light interferometry, the cantilevers are observed to deflect downward as the silicon is charged with lithium (lithiation), but the cantilevers experience little motion as the lithium is discharged (delithiation). An analytical cantilever-bending model featuring a moving phase boundary is developed to describe motion of the beams and explore stress profiles within the lithiated layer. Cantilever deflection during lithiation allows comparison to several models for stress within the LiSi_x layer. Specifically, a purely elastic model overestimates cantilever motion, as does a plastic bending model. The observed cantilever deflection distances are appropriate for a model of gradual accumulation of stress during charging, best fit with an exponential function indicating compressive stress of more than 1 GPa at the Si-LiSi_x phase boundary. After both charge and discharge cycles, the cantilevers relax upwards, indicating that this material system experiences time-dependent stress relaxation and continues to restructure itself after both lithiation and delithiation.

During lithiation (black lines), microcantilevers are observed to deflect downward with increasing charge, while similar behavior is not observed during delithiation (gray lines). This experimental technique was demonstrated using a model solid electrolyte system supported on Si microcantilevers, and resulted in observation of stress relaxation without current flow.