Local electronic structure variation resulting in Li 'filament' formation within solid electrolytes

Solid electrolytes hold great promise for enabling the use of Li metal anodes. The main problem is that during cycling, Li can infiltrate along grain boundaries and cause short circuits, resulting in potentially catastrophic battery failure. At present, 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 strongly associated with local electronic band structure. About half of the Li₇La₃Zr₂O₁₂ grain boundaries were found to have a reduced bandgap, around 1-3 eV, making them potential channels for leakage current. Instead of combining with electrons at the cathode, Li⁺ ions are hence prematurely reduced by electrons at grain boundaries, forming local Li filaments. The eventual interconnection of these filaments results in a short circuit. Our discovery reveals that the grain-boundary electronic conductivity must be a primary concern for optimization in future solid-state battery design.