of an extensive range of materials forming a bilayer separator using first-principles calculations. Notably, several bilayer separators are found to be thermodynamically stable, amongst them, the stability of the Li 3 PO 4 /Li 3 InCl 6 pairing is further verified experimentally using a combination of X-ray diffraction, solid-state nuclear magnetic resonance, and X-ray photoelectron spectroscopy.This study underscores the importance of understanding the chemical compatibility of bilayer separators when engineering high-energy density all-solid-state batteries.