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Poly(ethylene oxide) (PEO)-based solid composite electrolytes suffer from poor conductivity and lithium dendrite growth, especially toward the metallic lithium metal anode. In this study, succinonitrile (SN) is incorporated into a PEO composite electrolyte to fabricate an electrode-compatible electrolyte with good electrochemical performance. The SN-doped electrolyte successfully inhibits the lithium dendrite growth and facilitates the SEI layer formation, as determined by the operando nanofocus wide-angle X-ray scattering (nWAXS), meanwhile, stably cycled over 500 h in Li/SN-PEO/Li cell. Apart from the observation of lithium dendrite, the robust SEI layer formation mechanism in the first cycle is investigated in the SN-enhanced composite electrolyte by nWAXS. The inorganic electrochemical reaction products, LiF and Li 3 N, are found to initially deposit on the electrolyte side, progressively extending toward the lithium metal anode. This growth process effectively protected the metallic lithium, inhibited electron transfer, and facilitated Li⁺ transport. The study not only demonstrates a high-performance interfacial-stable lithium metal battery with composite electrolyte but also introduces a novel strategy for real-time visualizing dendrite formation and SEI growth directing at the interface area of electrolyte and metallic lithium., (© 2025 The Author(s). Advanced Science published by Wiley‐VCH GmbH.)