Utilizing hydrolysis resistance of compressed Li3PS4 films to eradicate surface hydroxyls and form conformal coatings through atomic layer deposition

The susceptibility of sulfide solid electrolytes (SSEs) to rapid hydrolysis upon exposure to moist air significantly hinders their application, underscoring the necessity for effective protective strategies. This study countered initial expectations by demonstrating that Li3PS4 (LPS) films compressed at 400 MPa and subjected to an industrial-type dry room environment (4 % relative humidity at room temperature) for a specific period (e.g., 2 h) exhibit limited surface hydrolysis. This phenomenon was accompanied by a substantial accumulation of hydroxyls from hydration, predominantly affecting the topmost 100 nm of the film surface. Furthermore, upon prolonged exposure in a dry room, these surface hydroxyls were observed to progressively penetrate deeper into the films through inevitable surface cracks, thereby exacerbating hydrolysis and leading to the dislocation of the compressed LPS films. These findings indicate that these detrimental hydroxyls can be effectively harnessed as reactants in atomic layer deposition (ALD) with trimethylaluminum. This ALD technique successfully achieved two key objectives: the thorough removal of hydroxyl groups through the ALD coating process and the simultaneous creation of conformal Al2O3 layers on the surface and in cracks, which significantly enhanced the surface robustness of the compressed LPS films. Further experimental examination revealed that precise control over the thickness of the ALD-applied Al2O3 layer, particularly a 30-nm coating, substantially improved the air stability of the LPS films while concurrently maintaining their ionic conductivity.