Combining Simulations and Spectroscopy to Design Protective Coatings on Sulfide Solid Electrolytes
Sulfide solid electrolytes are attractive for all-solid-state batteries because of their high Li-ion conductivity and favorable processability, but their practical use is limited by electrochemical instability and moisture-induced degradation. Molecular surface coatings offer a scalable strategy to improve humidity tolerance and interfacial stability, yet there is limited understanding of the molecular-level factors governing surface interactions. By combining large-scale atomistic simulations enabled by a machine-learning potential with targeted experiments, we present a comprehensive framework for designing molecular coatings on Li$_6$PS$_5$Cl (LPSCl). Using a PET-MAD interatomic potential fine-tuned on an r$^2$SCAN reference dataset covering LPSCl surfaces, molecular adsorbates, and their interactions, we model surface adsorption of carboxylic (CA), sulfonic (SA), and phosphonic (PA) acids with alkyl chains of varying lengths and the interaction of these coated surfaces with water at finite temperature. The simulations show that lithiated head groups generally exhibit more favorable adsorption energies. Impedance spectroscopy shows that the lithiated coatings maintain 40--55\% of the initial ionic conductivity after powder processing. While adsorption strength is largely controlled by head-group chemistry and lithiation state, coating morphology depends primarily on alkyl-chain length and surface coverage. In particular, long-chain lithiated carboxylates form dense, brush-like hydrophobic layers that reduce water access to reactive Li surface sites, with protection setting in at a coating thickness of approximately 0.8 nm. Consistent with these predictions, longer carboxylate chains suppress H$_2$S evolution more effectively. Because thicker coatings also hinder Li-ion transport, intermediate chain lengths offer the best balance between moisture protection and ionic conductivity.