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

Abstract 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 BF 3 ‐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 H 2 O 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 BF 3 , the CPEs endow Li/LiFePO 4 half batteries with excellent cycling stability. This work provides a paradigm for designing interface‐compatible electrolytes through spatially controlled orthogonal synthesis.