Poly(ethylene oxide) (PEO)-based solid electrolytes are promising candidates for all-solid-state lithium metal batteries, yet high polymer crystallinity and uncontrollable lithium dendrite growth severely limit their practical application. In this work, researchers from the Technical University of Munich incorporated succinonitrile (SN) into a PEO composite electrolyte to enhance ion transport dynamics and interfacial compatibility. The addition of 10 wt.% SN reduces the crystallinity of the PEO matrix and more than doubles the ionic conductivity and enabling continuous stable cycling over 500 hours in lithium symmetric cells. To track interfacial reactions at the nanoscale, the team performed operando scanning nano-focus wide-angle X-ray scattering (nWAXS) with a sub-micron beam size. The real-time measurements directly revealed that lithium dendrites were successfully suppressed in the modified system. Furthermore, nWAXS captured the kinetic formation of the solid electrolyte interphase (SEI) layer during the first electrochemical cycle, discovering that key inorganic components LiF and Li₃N) initially deposit on the polymer electrolyte side before gradually extending toward the metallic lithium anode. This mechanism forms a robust protection layer that prevents parasitic electron transfer while facilitating uniform Li⁺ transport. This study provides critical fundamental insights into interfacial dynamics and establishes a versatile platform for real-time visualization of solid-state battery interfaces.

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