Thermally rearranged covalent organic framework with flame-retardancy as a high safety Li-ion solid electrolyte

Solid polymer electrolytes have demonstrated high promise to solve the safety problems caused by conventional liquid electrolytes in lithium ion batteries. However, the inherent flammability of most polymer electrolyte materials remains unresolved, hence hindering their further industrial application. Addressing this challenge, we designed and constructed a thermal-responsive imide-linked covalent organic framework (COF) bearing orthopositioned hydroxy groups as precursors, which can conduct a thermal rearrangement to transform into a highly crystalline and robust benzoxazole-linked COF upon heating. Benefiting from the release of carbon dioxide through thermal rearrangement reaction, this COF platform exhibited excellent flame retardant properties. By contrast, classic COFs (e.g., boronate ester, imine, olefin, imide linked) were all flammable. Moreover, incorporating polyethylene glycol and Li salt into the COF channels can produce solid polymer electrolytes with outstanding flame retardancy, high ionic conductivity (6.42 x 10-4 S cm-1) and a high lithium-ion transference number of 0.95. This thermal rearrangement strategy not only opens a new route for the fabrication of ultrastable COFs, but also provides promising perspectives to designing flame-retardant materials for energy-related applications.