Heterogeneous Doping via Nanoscale Coating Impacts Mechanics of Li Intrusion in Brittle Solid Electrolytes

Lithium metal electroplating and short-circuiting limit fast charging in solid-state batteries, yet the mechanisms and methods to regulate lithium intrusions are not well-understood. In this work, we discover that nanoscale heterogeneous Ag⁺ doping dramatically affects lithium intrusion in Li_6.6La₃Zr_1.6Ta_0.4O₁₂ (LLZO), a brittle solid electrolyte. We generate nanoscale Ag⁺ doping by thermally annealing a 3-nm-thick metallic film. The metallic Ag undergoes Ag-Li ion exchange, completely disappears, and diffuses into LLZO bulk and grain boundaries to a depth of 20-50 nm. Density functional theory calculations predict this Ag-Li ion exchange exhibits negligible impact on electronic properties. Mechanically, nanoindentation experiments (n = 69) show a fivefold increase in the force required to fracture Ag⁺ surface-doped LLZO (Ag⁺-LLZO), providing direct evidence that surface modification due to Ag⁺ incorporation prevents crack opening. Conducting 121 plating experiments via operando microprobe scanning electron microscopy, we further confirm that the Ag⁺-LLZO surface exhibits improved lithium plating even under a large local indentation stress of 3 GPa. Surprisingly, microprobe plating reveals that Ag⁺ increases the diameter of plated Li at failure by more than 4 times, demonstrating its role in enhancing the defect tolerance of LLZO. Our study reveals a chemo-mechanical mechanism via surface heterogeneous doping, complementing the present bulk design rules to prevent mechanical failures in solid-state batteries.