Flame‐Retardant Polyurethane‐Based Solid‐State Polymer Electrolytes Enabled by Covalent Bonding for Lithium Metal Batteries

Abstract Solid‐state lithium metal batteries (SSLMBs) have gained extensive attraction as one kind of next‐generation energy storage device. However, the drawbacks of flammability, low mechanical strength, and low ionic conductivity limit the further development of solid‐state polymer electrolytes (SPEs). In this work, a reactive flame‐retardant unit into the polymer framework via covalent bonding to create a nonflammable and stretchable polyurethane‐based SPEs is introduced. Meanwhile, the mechanical strength of the polymer backbone is increased by grafting a rigid benzene ring unit, which remarkably suppresses the lithium‐dendrite growth. As a result, these inflammable SPEs do not burn after contact with flames for 6 s. Furthermore, the obtained SPE expands the electrochemical stability window up to 5.1 V. Small monomers containing bromine decompose on the surface of lithium metal creating a LiBr riched solid electrolyte interface (SEI). Li|SPEs|Li symmetric battery offers a stable cycling life for more than 2100 h at 0.2 mA cm −2 and 0.2 mAh cm −2 . The LiNi 0.6 Co 0.2 Mn 0.2 O 2 |SPEs|Li cell equipped with the integrated cathode delivers a 142.1 mAh g −1 capacity after 330 cycles at 0.3 C with 85.2% capacity retention.