Sulfide solid electrolytes suffer from unstable interfaces with the cathode active materials, leading to fatal side reactions and performance degradation. Although various surface-coating strategies are being explored, conventional methods typically require complex processing and high-temperature treatments, which limit their scalability and result in nonuniform, island-like coatings. For the first time, a high-ionic-conductivity phase-convertible ion conductor (PCI) is introduced as a coating material capable of forming a uniform and thin interfacial layer. A novel self-liquefying coating method is developed in which internal heat during mild mechanical mixing triggers the phase transformation of PCI into a liquid state, enabling the spontaneous formation of a conformal nanoscale coating. Upon natural cooling, the liquefied PCI resolidifies while maintaining the coating integrity. This approach significantly enhances the ion transport across the interface and effectively suppresses interfacial side reactions, thereby improving the interfacial stability and electrochemical performance of all-solid-state batteries.