Decoupling the roles of grain boundary strength and grain size hidden in grain-level electro-chemo-mechanical failure of solid-state electrolyte

Garnet lithium lanthanum zirconium oxide (Li₇La₃Zr₂O₁₂, LLZO) is a benchmark solid-state electrolyte (SSE) material receiving considerable attention owing to its high conductivity and chemical stability against Li metal. Although its electro-chemo-mechanical failure mechanisms have been much investigated, the equivocal roles of grain boundary strength and grain size of LLZO remain under-explored, hindering further performance improvements. Here we decoupled the effects of grain size and grain boundary strength of polycrystalline LLZO via the combination of electrochemical kinetics and the cohesive zone model. We discovered that the disintegration of LLZO is initiated by the accumulation of local displacements, which strongly relates to the changes in both grain size and grain boundary strength. However, variations in grain boundary strength affect the diffusion and propagation pathways of damage, while the failure of LLZO is determined by the grain size. Large LLZO grains facilitate transgranular damage under low grain boundary strength, which can alter local chemo-mechanics within the bulk of LLZO, leading to more extensive damage propagation. The results showcase the structure optimization pathways by preferentially controlling the growth of lithium dendrites at grain boundaries and their penetration in garnet-type SSE.