Mechanical‐Stress‐Induced Lithiation and Structural Evolution Driven by Excess Lithium Predisposing Short Circuits at the Surface of Garnet Solid Electrolytes

Abstract Cubic‐garnet solid electrolyte has garnered significant attention in all‐solid‐state batteries (ASSBs) due to its ionic conductivity and chemical robustness against Li metal. However, the short‐circuit formation at low current density poses a significant obstacle with the main cause remaining ambiguous. Here, the lithium‐penetration mode originating from phase transformation is unveiled at the sintered pellet surface via mechanically induced lithiation. Mechanical stress applied during polishing under excess lithium content induces lithiation into the cubic‐garnet structure, leading to partial structural evolution into the tetragonal phase. This surface alteration induces current constriction, hindered by sluggish interfacial Li‐ion transport from the tetragonal phase, which exhibits low ionic conductivity, causing short circuits. By reducing mechanical stress, mitigating surface strain, and restoring the cubic phase, stable operation is ensured without short‐circuit formation in both Li symmetric and hybrid‐full cells. This insights illuminate the origin of lithium penetration related to phase transition at the surface of cubic‐garnet and pave the way for enhancements in ASSB development.