Solid state lithium metal batteries incorporating ceramic solid electrolytes are visioned as a promising alternative for future energy storage. However much has to be investigated regarding the electro-chemo-mechanical stability of these electrolytes. Herein we investigate the role of microstructure of garnet type ceramic solid electrolyte in lithium dendrite propagation. Clear evidence of dendrite favoring the growth along the grain boundaries is observed which suggests the role both mechanical and electronic properties of solid electrolyte plays in dendrite propagation. Furthermore, in our present study, tantalum-doped LLZTO with a nominal composition of Li6.4La3Zr1.4Ta0.6O12 was fabricated using two approaches: one involving conventional powder processing followed by reactive sintering, and the other incorporating an intermediate-stage high-energy milling step. These fabrication routes resulted in distinct microstructures, influencing their electrochemical behavior during extended cycling. The relation between the two was then analyzed in detail using surface science and bulk techniques, as well as quantitative stereology. Notably, our findings demonstrate for the first time that compact grain size distribution critically affects short-circuit failure. Lithium metal dendrites propagate intergranularly along "weak-links" in the microstructure, characterized by percolating arrays of LLZTO grains significantly smaller than the average. Additionally, lithium metal accumulates inside the pores along the dendrite's path through the compact. These observations underscore the role of enhanced electrical conductivity—specifically, electron accumulation—in the dendrite-driven failure phenomenology. Complementing the experimental findings, mechanistic modeling was employed to provide a comprehensive understanding of the interrelations between microstructure, chemo-mechanical stress, and electrochemical stability in garnet-based solid-state electrolytes. Figure 1
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