In situ Observation of Li Deposition-Induced Cracking in Garnet Solid Electrolytes

Lithium (Li) penetration through solid electrolytes (SEs) induces short circuits in Li solid-state batteries (SSBs), which is a critical issue that hinders the development of high energy density SSBs. While cracking in ceramic SEs has been often shown to accompany Li penetration, the interplay between Li deposition and cracking remains elusive. Here, we constructed a mesoscale SSB inside a focused ion beam-scanning electron microscope (FIB-SEM) for in situ observation of Li deposition-induced cracking in SEs at nanometer resolution. Our results revealed that Li propagated predominantly along transgranular cracks in a garnet Li_6.4La₃Zr_1.4Ta_0.6O₁₂ (LLZTO). Cracks appeared to initiate from the interior of LLZTO beneath the electrode surface and then propagated by curving toward the LLZTO surface. The resulting bowl-shaped cracks resemble those from hydraulic fracture caused by high fluid pressure on the surface of internal cracks, suggesting that the Li deposition-induced pressure is the major driving force of crack initiation and propagation. The high pressure generated by Li deposition is further supported by in situ observation of the flow of filled Li between the crack flanks, causing crack widening and propagation. This work unveils the dynamic interplay between Li deposition and cracking in SEs and provides insight into the mitigation of Li dendrite penetration in SSBs.