A supramolecular ionic liquid, a solid-state electrolyte membrane, a solid-state lithium metal battery, and an apparatus are provided. The supramolecular ionic liquid of this disclosure has a benzophenanthrene structure represented by formula (I), where R1 is selected from ether groups, polyether groups, halogenated ether groups, or halogenated polyether groups having 1 to 16 carbon atoms; and R2 is selected from ether groups or polyether groups having 1 to 16 carbon atoms. The supramolecular ionic liquid of this disclosure has an ionic conductivity as high as 10−3 S/cm at room temperature. A solid-state electrolyte membrane made therefrom also has an ionic conductivity as high as 10−3 S/cm at room temperature. This is close to performance of a liquid electrolyte, proposing a solution to the problem of low ionic conductivity in existing solid-state electrolytes.