Mechanical origins of lithium-metal intrusions into garnet solid electrolytes revealed by failure statistics

Solid-state electrolytes are an attractive option to increase the chemical stability and energy density of rechargeable lithium-metal batteries. However, they are susceptible to lithium-metal short-circuiting during charging, especially under high current densities. In this work, using a force-controlled electrical microprobe inside a focused-ion beam scanning electron microscope, we performed 48 experiments of electroplating lithium metal onto Li6.6La3 Ta0.4Zr1.6O12 (LLZO) and observed the initiation of lithium penetration (termed intrusions) into the solid electrolyte, with and without externally applied stress. Employing a statistical approach, we find that the cumulative probability of intrusion as a function of lithium metal diameter follows a two-parameter Weibull distribution, implying defect-governed fracture behavior for the intrusion initiation. Upon applying a contact force of 5 mN with a 5-10 µm microprobe, the characteristic failure diameter of lithium metal decreases by a factor of 2.6 due to the generation of nanoscale cracks as suggested by nanoindentation and finite element simulations. Furthermore, we introduce in-plane compressive strain to LLZO through a cantilever bending experiment and demonstrate that strain as small as 0.067% strongly influences the direction of intrusion propagation immediately following initiation. Overall, our results suggest that both mechanically-generated and pre-existing defects as well as mechanical strain dominate the intrusion behavior in LLZO, a phenomenon that could extend to other solid electrolytes.