Hydrophobic Polymer Coatings for Enhanced Air Stability of Inorganic Solid Electrolytes for All-Solid-State Batteries

Achieving improved safety in energy storage devices has been intensively pursued as the demand for electrification increases. All-solid-state Li batteries (ASLBs) employing inorganic solid electrolytes (SEs) are considered the Holy Grail because of the possibility to achieve not only enhanced safety but high energy/power density. Sulfide and halide-based SEs are considered promising because of their superionic conductivity and deformability. However, they suffer from poor air stability, which hinders the commercialization of ASLBs. Sulfide-based SEs react with humid air and release H₂S gas with rapid structural degradation. Halide-based SEs also suffer from irreversible degradation to form MCl₃-H₂O and LiCl-H₂O. The poor chemical stability of these inorganic SEs causes not only the degradation of the electrochemical performance of ASLBs but drastically increased processing costs because the strict exclusion of moisture is required. Thus far, several strategies to enhance air stability of inorganic SEs have been proposed: i.e., elemental substitution, functional additives, and surface treatment. However, those strategies have limitations, such as limited applicability, severe decrease in ionic conductivity, and insufficient air stability.<br/>In this study, we report on our universal hydrophobic polymer coating strategies for enhanced air stability of inorganic SEs. The hydrophobic coating layer impedes the diffusion of moisture into the SEs and enhance the air stability. Furthermore, the structural degradation mechanism in humid air is discussed. Finally, to investigate practical application, dry room compatibility of the protected SEs will be presented.