Abstract Serious safety risks caused by the high reactivity of lithium metal against electrolytes severely hamper the practicability of lithium metal batteries. By introducing unique polymerization site and more fluoride substitution, we built an in situ formed polymer‐rich solid electrolyte interphase upon lithium anode to improve battery safety. The fluorine‐rich and hydrogen‐free polymer exhibits high thermal stability, which effectively reduces the continuous exothermic reaction between electrolyte and anode/cathode. As a result, the critical temperature for thermal safety of 1.0 Ah lithium‐LiNi0.5Co0.2Mn0.3O2 pouch cell can be increased from 143.2 °C to 174.2 °C. The more dangerous “ignition” point of lithium metal batteries, the starting temperature of battery thermal runaway, has been dramatically raised from 240.0 °C to 338.0 °C. This work affords novel strategies upon electrolyte design, aiming to pave the way for high‐energy‐density and thermally safe lithium metal batteries.
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