In Situ Orthogonal Chemistry for Facile Preparation of Uniform COF-Based Composite Polymer Electrolytes

Covalent organic framework (COF)-based composite polymer electrolytes (CPEs) represent a promising platform for advanced lithium-metal batteries (LMBs) with high energy density. However, conventional physical fabrication methods may suffer from cumbersome processing, filler agglomeration, and high interfacial resistance. Here we report the in-situ preparation of COF-based CPEs via a BF₃-mediated concurrent Schiff base condensation and cationic ring-opening polymerization (ROP) directly from the monomer mixture of 4,4',4″-(1,3,5-triazine-2,4,6-triyl)trianiline (TAPT), 2,3,5,6-tetrafluoroterephthalaldehyde (TFTA), and tetrahydrofuran (THF) within LMBs. Notably, the H₂O generated from the polycondensation reaction effectively serves as the chain transfer agent for THF ROP, reducing the molecular weight of polytetrahydrofuran (PTHF) to facilitate Li⁺ transference, instead of corroding the lithium anode. This orthogonal reaction protocol enables uniform dispersion of COF fillers within the polymer matrix, establishes long-range ordered Li⁺ transport pathways, and weakens the Li⁺-ether oxygen coordination. Consequently, the ionic conductivity increases 80-fold (2.3 × 10^-3 S cm^-1) with a Li⁺ transference number of 0.81 at room temperature. Coupled with a fluorine- and boron-rich solid-electrolyte interphase (SEI) derived from BF₃, the CPEs endow Li/LiFePO₄ half batteries with excellent cycling stability. This work provides a paradigm for designing interface-compatible electrolytes through spatially controlled orthogonal synthesis.