Framework Electronegativity Governs Interfacial Transport Kinetics in Lithium-Metal Batteries

Modulating interfacial ion transport via separator engineering is pivotal for overcoming the rate capability and cycle life limitations of lithium metal batteries (LMBs). Although polarity-tuning strategies have been widely explored, a clear physical linkage between molecular-level design and transport kinetics, particularly as quantified by Sand's time, remains unresolved, making separator optimization largely empirical. Herein, we report a systematic series of isoreticular covalent organic frameworks (COFs) that decouple pore topology from electronic effects and introduce framework electronegativity (χ) as a quantitative descriptor for ion-selective transport. By modulating χ, exemplified through progressive fluorination, a push-pull electrostatic microenvironment is established that promotes Li⁺ transport while repelling PF₆^-, thereby suppressing concentration polarization and revealing an effective scaling relationship between χ and Sand's time. Guided by this descriptor, an optimized TFCOF@PP separator increases the Li⁺ transference number by 73% and markedly suppresses concentration polarization, enabling stable cycling of LiFePO₄ full cells at 5 C with 83.1% capacity retention after 4000 cycles. The system further demonstrates robust performance under demanding conditions, including high-voltage NCM811 cells, elevated temperatures (60 °C), and mechanical abuse. This work elucidates the physical origin linking framework electronegativity to interfacial transport kinetics, providing a rational, descriptor-based strategy for separator design to mitigate kinetic failure in metal batteries.