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The sulfide‐type solid electrolyte (SSE) is considered a promising candidate for solid‐state lithium metal batteries (SSLMBs) owing to its advantages of superior ionic conductivity. Nevertheless, the incompatibility of the sulfide and lithium metal can result in undesirable interface resistance and rapid Li dendrite growth, which seriously hinders its commercial applications. Herein, inspired by the moderation and long duration of sustained release drug carriers when combined with active pharmaceutical ingredients in the biomedical field, poly (propylene carbonate) (PPC) and lithium bis (trifluoromethanesulfonyl) imide (LiTFSI) gradually interact with a Li anode with constantly decreased Li/SSE interfacial resistance. In addition to intimate contact, the ultrastable LiF‐enriched solid electrolyte interphase (SEI) is in situ formed via a sustained release effect, which suppresses the Li dendrite effectively. As a result, the symmetric cells demonstrate stable cycling performance for 1200 h at a current density of 0.1 mA cm−2 and 300 h at 0.5 mA cm−2. Moreover, LiFePO4/ Li6PS5Cl /Li SSLMB delivers a high discharge capacity of over 132.8 mAh g−1 for 900 cycles at 1C with steady Coulombic efficiency. Therefore, this sustained release mechanism and its initially successful application in interfacial modification increase the potential for commercial applications of SSLMBs. [ABSTRACT FROM AUTHOR]