Researchers discovered that Li infiltration in solid oxide electrolytes for all-solid-state batteries is associated with local electronic structure at grain boundaries (GBs) rather than exclusively at Li-electrolyte interfaces as in Li-ion batteries. Results identify that tailoring GBs with sufficient electronic resistivity is a promising route to mitigate Li dendrite formation in polycrystalline solid electrolytes.
Solid electrolytes hold great promise for enabling the use of Li metal anodes. The major problem is that during cycling Li can infiltrate along grain boundaries (GBs) and cause short-circuits, resulting in potentially catastrophic battery failure. Presently this phenomenon is not well understood. Here, through electron microscopy measurements on a representative system Li7La3Zr2O12, we discover that Li infiltration in solid oxide electrolytes is associated to local electronic conductivity. About half of the Li7La3Zr2O12 GBs were found to have a reduced band gap, around 1-3eV, making them potential channels for leakage current. Instead of combining with electrons at the cathode, Li+ ions are hence prematurely reduced by electrons at GBs, forming local Li filaments. The eventual interconnection of these filaments results in the short circuit. Our discovery reveals that the grain-boundary electronic structure is a primary concern for optimization in future solid-state battery design.
In summary, we provide insight into the microscopic evidence for local electronic band structure affecting Li filament formation in a technologically important solid electrolyte, LLZO. The material is found to undergo a large band-gap reduction at some GBs and, in some cases, the magnitude of this reduction is larger than previously reported for other oxides. This phenomenon initiates the formation of lithium filaments within the solid electrolyte. The final connection of these filaments could result in the short circuit. These insights imply that tailoring GBs with sufficient electronic resistivity will be a promising route to mitigate the Li metal infiltration in polycrystalline solid electrolytes.
