Dendrite Growth-Microstructure-Stress-Interrelations in Garnet Solid-State Electrolyte

This study illustrates how the microstructure of garnet solid-state electrolytes (SSE) affects the stress-state and dendrite growth. Tantalum-doped lithium lanthanum zirconium oxide (LLZTO, Li6.4La3Zr1.4Ta0.6O12) is synthesized by powder processing and sintering (AS), or with the incorporation of intermediate-stage high-energy milling (M). The M compact displays higher density (91.5% vs 82.5% of theoretical), and per quantitative stereology, lower average grain size (5.4 +/- 2.6 vs 21.3 +/- 11.1 mu m) and lower AFM-derived RMS surface roughness contacting the Li metal (45 vs 161 nm). These differences enable symmetric M cells to electrochemically cycle at constant capacity (0.1 mAh cm-2) with enhanced critical current density (CCD) of 1.4 versus 0.3 mA cm-2. It is demonstrated that LLZTO grain size distribution and internal porosity critically affect electrical short-circuit failure, indicating the importance of electronic properties. Lithium dendrites propagate intergranularly through regions where LLZTO grains are smaller than the bulk average (7.4 +/- 3.8 mu m for AS in a symmetric cell, 3.1 +/- 1.4 mu m for M in a half-cell). Metal also accumulates in the otherwise empty pores of the sintered compact present along the dendrite path. Mechanistic modeling indicates that reaction and stress heterogeneities are interrelated, leading to current focusing and preferential plating at grain boundaries. Garnet-based LLZTO (Ta-doped Li7La3Zr2O12) solid-state electrolytes (SSEs) for all-solid-state batteries (ASSBs) display poor electrochemical stability. For the first time it is demonstrated that cycling-induced Li metal dendrites propagate preferentially through regions with smaller mean grain size versus the bulk. This occurs regardless of processing history or cell configuration. image