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

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_0.6Co_0.2Mn_0.2O₂|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.