Heterogeneous doping via nanoscale coating impacts the mechanics of Li intrusion in brittle solid electrolytes

Lithium dendrite intrusion in solid-state batteries limits fast charging and causes short-circuiting, yet the underlying regulating mechanisms are not well-understood. Here we discover that heterogeneous Ag⁺ doping dramatically affects lithium intrusion into Li_6.6La₃Zr_1.6Ta_0.4O₁₂ (LLZO), a brittle solid electrolyte. Nanoscale Ag⁺ doping is achieved by thermally annealing a 3-nm-thick metallic coating on LLZO, inducing Ag-Li ion exchange and Ag diffusion into grains and grain boundaries. Density functional theory calculations and experimental characterization show negligible impact on the electronic properties and surface wettability from Ag⁺ incorporation. Mechanically, nanoindentation experiments show a fivefold increase in the mechanical force required to fracture the surface Ag⁺-doped LLZO, indicating substantial doping-induced surface toughening. Operando microprobe scanning electron microscopy experiments show that the Ag⁺-doped LLZO surface exhibits improved lithium plating at >250 mA cm^-2 and an electroplating diameter that is expanded by over fourfold, even under an extreme indentation stress of 3 GPa. This demonstrates enhanced defect tolerance in LLZO, rather than electronic or adhesion effects. Our study reveals a chemo-mechanical mechanism via surface heterogeneous doping, complementing present bulk design rules to minimize mechanical failures in solid-state batteries.