Understanding Dual-Polar Group Functionalized COFs for Accelerating Li-Ion Transport and Dendrite-Free Deposition in Lithium Metal Anodes

Lithium metal batteries (LMBs) have attracted wide attentions because of their high theoretical specific capacity and low electrochemical potential. However, the growth of lithium dendrites seriously affects the practical application of LMBs. Thus, the lithium-philic carbonyl and carboxy dual-group-modified covalent organic framework (COF-COOH) is designed to coat the polypropylene (PP) separator (COF-COOH@PP separator), realizing the regulation of ion transport and uniform lithium deposition. The plentiful and negative charge sites in the COF-COOH can suppress the diffusion of the freely movable lithium salt anion by the electrostatic interaction. Density functional theory (DFT) calculations demonstrate that the COF-COOH possesses the function of anchoring anion and desolvation. Consequently, the Li⁺ transference number (0.7), ion conductivity (0.64 mS cm⁻¹), and desolvating of Li⁺ are obviously improved by using the COF-COOH@PP separator. The modified Li-Li symmetric battery delivers stable cycle for more than 1000 h and lower voltage hysteresis (0.02 V). This dendrite-free deposition strategy holds great promise for practical application of Li metal anodes.