Mechanical Stress Induced Current Focusing and Fracture in Grain Boundaries
Solid state electrolytes (SSEs) with garnet structures and chemical composition Li7La3Zr2O12 (LLZO) are being actively considered as the electrolyte for next-generation lithium ion batteries. These LLZO-based ceramic SSEs are polycrystalline and possess distinct grain-interior/grain-boundary (GI/GB) microstructure. Lithium dendrite growth through these GI and GB domains is considered to be the largest bottleneck preventing their successful implementation. Dendrite growth has been observed to occur predominantly within the GB regions. While this would be expected if the GB regions were more ionically conductive than the GI regions, there is significant disagreement over the relative conductivities of these two regions. The present work demonstrates that dendrite growth can occur preferentially in the GB regions even when the GI regions have higher ionic conductivity. Electrochemical potential changes induced by mechanical stress within the GI and GB domains reveals that the magnitude of effective stress induced current density within the elastically softer GB is larger than in the mechanically stiffer GI, which leads to enhanced lithium deposition in GB and subsequent dendrite growth. The present work additionally examines the impact of applied current density and grain size on current focusing in the GB regions, and also finds that SSE fractures initiate from the GB regions.