Ion Size Effects on the Thermodynamic, Kinetic, and Mechanical Properties during Ion Exchange in Solid-State Electrolytes

Ion exchange offers a pathway to impose residual compressive stresses to mitigate the electro-chemo-mechanical cracking of solid-state electrolytes such as lithium lanthanum zirconium oxide. This study uses a coupled multiscale framework (integrating density functional theory (DFT), molecular dynamics (MD), and continuum modeling) to examine how exchange ion size influences stress, diffusion, fracture toughness, and electronic properties. Larger isovalent ions (Na⁺, Ag⁺, K⁺) were exchanged with Li⁺, with DFT confirming their preference for octahedral 96h sites and a linear relationship between ion size and chemical free expansion coefficient. MD simulations reveal stress and concentration effects on exchange ion diffusivity at elevated temperatures, with Na⁺ and Ag⁺ maintaining favorable mobility while K⁺ showing concentration-dependent clustering. Continuum modeling predicts the range of fracture strength improvements and the required ion exchange concentration profile. It was shown that a 5% surface exchange concentration can induce ∼0.6 GPa of surface compressive stress using Na⁺ and ∼1.0 GPa of surface compressive stress using Ag⁺. On the other hand, larger ion exchange species may penalize Li⁺ diffusivity by increasing the activation volume and activation energy. Interestingly, Na⁺ has a negligible penalty on Li-ion diffusivity. The room temperature Li⁺ ion diffusivity is reduced by ∼40% with Ag⁺ ion exchange. Electronic band structure analysis shows no size-dependent change in the bandgap, though Ag⁺ introduces localized defect states near the valence band maximum. This study highlights ion size as a key factor in optimizing LLZO properties, offering a framework to improve the solid-state battery performance.