Understanding the Role of Powder Protective Layers on the Chemical Reactivity of Sulfide-Based Solid Electrolytes in All-Solid-State Batteries

Sulfide-based solid-state electrolytes (SSE) are a promising class of materials to enable high-performance, all-solid-state Li-ion batteries due to 1) their favorable mechanical properties for processability at scale and 2) comparable Li-ion conductivities to conventional liquid electrolytes. However, their poor atmospheric stability remains a challenge toward commercial manufacturing processes as environmental moisture and oxygen content, even in dry room environments, are sufficient to degrade sulfide-based SSEs. To circumvent these limitations, we previously developed thin (~1 nm) Al₂O₃ coatings grown directly on Li₆PS₅Cl (LPSCl) <i>powders</i> via atomic layer deposition (ALD). Through this approach, we successfully demonstrated substantial suppression of the chemical reactivities of the coated LPSCl under aggressively oxidizing conditions as compared to the uncoated LPSCl. In this work, we provide a comprehensive investigation to understand the role of ALD alumina protective layers on the environmental reactivity of LPSCl utilizing pellets pressed from coated and uncoated powders. Thermogravimetric analysis indicates significantly suppressed weight gain from powders and pellets made from coated versus uncoated materials after exposure to humidified O₂, demonstrating the effectiveness of the ALD coating strategy in suppressing environmental reactivity regardless of material form factor. Surprisingly, X-ray photoelectron spectroscopy shows little to no changes in the surface chemistry on the uncoated LPSCl surface while the coated LPSCl exhibits a trace of the surface oxidation upon exposure to oxidizing conditions, suggesting that the surface oxidation products of LPSCl are volatile and evaporate under ultra-high vacuum conditions. Correlative X-ray and vibrational spectroscopic and diffraction analysis will be utilized to expand upon the mechanisms by which powder coatings stabilize LPSCl to environmental reactivity and the specific changes in the surface chemistry and the corresponding chemical reactivities. These results provide a clear pathway to enabling stable precursor powder materials with favorable properties for processing at scale in realistic manufacturing environments.