The interfacial instability of the poly(ethylene oxide) (PEO)-based electrolytes impedes the long-term cycling and further application of all-solid-state lithium metal batteries. In this work, we have shown an effective additive 1-adamantanecarbonitrile, which contributes to the excellent performance of the poly(ethylene oxide)-based electrolytes. Owing to the strong interaction of the 1-Adamantanecarbonitrile to the polymer matrix and anions, the coordination of the Li+-EO is weakened, and the binding effect of anions is strengthened, thereby improving the Li+ conductivity and the electrochemical stability. The diamond building block on the surface of the lithium anode can suppress the growth of lithium dendrites. Importantly, the 1-Adamantanecarbonitrile also regulates the formation of LiF in the solid electrolyte interface and cathode electrolyte interface, which contributes to the interfacial stability (especially at high voltages) and protects the electrodes, enabling all-solid-state batteries to cycle at high voltages for long periods of time. Therefore, the Li/Li symmetric cell undergoes long-term lithium plating/stripping for more than 2000 h. 1-Adamantanecarbonitrile-poly(ethylene oxide)-based LFP/Li and 4.3 V Ni0.8Mn0.1Co0.1O2/Li all-solid-state batteries achieved stable cycles for 1000 times, with capacity retention rates reaching 85% and 80%, respectively.