Beyond Electrochemistry: Tailoring Mechanical Properties of Halogen-Substituted Argyrodites for Conformal Dry Coating...

Dry coating of solid electrolytes (SEs) onto cathode materials is a promising strategy for promoting homogeneous interfacial reactions in all-solid-state batteries (ASSBs). While electrochemical properties are considered a key criterion, the coating process fundamentally depends on the mechanics of SE particles. Herein, we demonstrate that the mechanical properties of sulfide SEs govern coating quality and, consequently, the performance of ASSB cathodes. Argyrodites with controlled Cl/Br compositions are synthesized to tune their electrochemical and mechanical properties. Atomic force microscopy-based force spectroscopy and nanoindentation measurements reveal that halogen substitution systematically governs the Young's modulus, yield strength, and adhesion energy in accordance with the nature of the halogen substituent and compositional ratio. Quantitative analysis of unbound SE fragments shows a strong correlation between the mechanical parameters and coating quality. Finite element method simulations indicate that particle fracture occurs readily, suggesting that fracture is unlikely to be a limiting factor, while density functional theory calculations identify interfacial adhesion as the key factor governing coating quality. Electrochemical analysis combined with ultraviolet photoelectron spectroscopy suggests that coating quality, beyond ionic conductivity and oxidative stability, critically governs interfacial kinetics. Accordingly, Li5.4PS4.4Cl0.8Br0.8-coated LiNi0.8Co0.1Mn0.1O2 (NCM) exhibits superior coating quality and reduced interfacial resistance, resulting in enhanced capacity and cycle stability compared to Li6PS5Cl0.5Br0.5-coated NCM. This study offers a design guideline for high-performance ASSBs through mechanical tuning of coating materials.