All-solid-state batteries based on sheet-type sulfide-based solid electrolytes (SEs) represent a groundbreaking advance in energy storage technology, offering high-energy density and enhanced safety. Sheet-type sulfide-based SEs are particularly advantageous owing to their thin feature, enabling both high energy density and scalable manufacturing. Despite their promise, the unavoidable formation of interior cavities during fabrication poses a significant challenge, as it disrupts continuous ion-transport pathways and compromises interfacial stability. To address this issue, an innovative solution is presented: an ion-conducting cavity filler (ICCF) formulated from ambient-temperature molten salts (ATMSs) specifically tailored for SEs. Through comprehensive electrochemical investigations and advanced simulations, we demonstrate that even a drop of ICCF can significantly eliminate the internal cavities, effectively preventing ionic conductivity loss. Furthermore, the ICCF enhances interfacial stability, as evidenced by improved cycling performance. Reactivity experiments conducted using spectroscopy and chemical affinity testing confirm that compatibility between sulfide-based SEs and ATMSs is vital for stability. We propose an ICCF-integrated sheet-type SE as a new concept for a functional solid electrolyte, demonstrating that even a minimal amount of ICCF can enhance performance throughout the entire cell. The “functional solid electrolyte” provides a crucial cornerstone for practical strategies in developing all-solid-state batteries.