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Solid polymer electrolytes (SPEs) make contact with highly reductive lithium (Li) metal anodes, forming the interphase that largely determines the battery performance. In this work, trace iodine doping in a poly(ethylene oxide) (PEO) electrolyte to achieve a stable interphase on Li metal surface for long battery cycling, is proposed. The triiodide ion (I3−) stemming from iodine additives can coordinate with the COC bond of PEO to enable the increased ionic conductivity of the SPE. The I‐doped interphase contains I3− and IO3−, which spontaneously react with the dead Li and Li2O at the initial interphase to smooth the Li metal surface, eventually leading to significant improvements in interfacial resistance and dendrite suppression. When matching with the LiFePO4 cathode, the full cell exhibits higher capacity (150 mAh g−1) and excellent cycling stability after 300 cycles (capacity retention of 96.5%) at 0.5 C and 50 °C. This work opens up a promising avenue for using halogen to design solid‐state Li metal batteries.

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