Probing Interfacial Reaction Pathways in Atomic Layer Deposition on Sulfide Superionic Conductors

Sulfide superionic conductors (e.g., argyrodite Li 6 PS 5 Cl, LPSCl) are extremely promising for all solid‐state batteries, but poor atmospheric stability and high interfacial reactivity limit widespread adoption. Coating LPSCl powders with ultrathin coatings using atomic layer deposition (ALD) mitigates these problems, protecting against atmospheric degradation and reducing reactivity with Li metal, yielding more stable cycling. Despite significant promise, the ALD mechanism is unknown, hampering the development of new coating chemistries. In this study, we elucidate the mechanism for Al 2 O 3 ALD on LPSCl using trimethyl aluminum (TMA) and H 2 O by combining in situ Fourier transform infrared spectroscopy, ex situ solid‐state magic angle spinning nuclear magnetic resonance, UV Raman spectroscopy, X‐ray photoelectron spectroscopy, and density functional theory calculations. We determine that ALD Al 2 O 3 nucleates promptly via TMA reaction with native OH, SH, and PS 3 ‐OH groups to form transient C–Al–O(S) species that are rapidly hydrolyzed during the subsequent H 2 O exposure. This reversible transformation maintains surface nucleophilicity and prevents sulfide decomposition. The resulting layer‐by‐layer growth leads to highly conformal Al 2 O 3 coatings on LPSCl that are readily scalable to ≥50 g quantities using a rotating drum fixture. This detailed understanding of ALD surface reactions provides critical insights guiding the selection of future ALD chemistries with improved performance.