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[Display omitted] • PEO-based polymer-in-salt electrolytes are made by a facile coating process. • Rich hydrogen bonds are formed in polymer network for better structural stability. • High content of free Li+ and restricted anion movement obtain a high t Li + of 0.639. • Inhibition of Li dendrites renders Li/Li cells cycling over 3000 h at 0.2 mA cm−2. • The electrolytes boost the performances of LMBs using LFP or NCM88 significantly. Polyethylene oxide (PEO)-based solid polymer electrolyte (SPEs) is full of attraction due to its exceptional lithium ion dissolubility and strong resistance toward lithium metal reduction. Nevertheless, it still suffers from the unfulfilling room temperature ionic conductivity and poor mechanical properties. Herein, a flexible and robust polymer-in-salt electrolyte based on composite of polybenzimidazole (PBI) and PEO via a facile preparation approach has been developed for room-temperature lithium metal batteries (LMBs). The rigid framework structure of PBI and dense hydrogen bonds formed between PBI and PEO jointly reinforce the structural stability of SPEs at high lithium salt concentration, which makes the SPEs possess sufficient mechanical properties to resist the lithium dendrites growth. The thermal stability and fire resistance of SPEs have also been improved relying on the distinctive flame retardancy and incombustibility of PBI. Besides, PBI is prone to forming hydrogen bonds with lithium salt anions to limit their mobility, while the rich N atoms on PBI chains can promote the dissociation of lithium salt through electrostatic attraction interaction. Their synergistic effects make the PEO-based polymer-in-salt electrolytes achieve the high room temperature ionic conductivity of 5.7 × 10−4 S cm−1, wide electrochemical window of 4.45 V, and large lithium transference number of 0.639. When applied to LMBs with LiFePO 4 or LiNi 0.88 Co 0.06 Mn 0.06 O 2 cathode, both coin cells exhibit improved cycling performance and rate capability. Furthermore, the LiNi 0.88 Co 0.06 Mn 0.06 O 2 /SiO x -C pouch cells using the SPEs demonstrate remarkable flexibility and safety, clarifying that the PEO-based polymer-in-salt electrolytes possess a promising application prospect for safe and high-performance LMBs. [ABSTRACT FROM AUTHOR]