Engineering Side-Chain Steric Effects to Build Selective COF Channels for Polysulfide Suppression in Li-S Batteries.

Lithium-sulfur (Li-S) batteries suffer from severe capacity fading due to the dissolution and shuttle diffusion of lithium polysulfide (LiPS). Although separator regulation that relies on pore confinement can effectively suppress polysulfide migration, its coupled influence on lithium-ion (Li+) transport kinetics remains poorly understood. Herein, covalent organic frameworks (COFs) with continuously tunable pore are used as a model system by introducing oligo(ethylene glycol) (OEG) chains of varying lengths. A series of OEGnCOFs (n = 0 - 3) which share identical backbone topology while differing only in side-chain length, provide a well-defined platform for precisely modulating the pore. Confinement induced by side-chains monotonically suppresses polysulfide migration, whereas Li+ transport shows a pronounced nonmonotonic dependence on side-chain length. Excessively long side-chains impose highly fluctuating steric constraints that disrupt continuous lithium-ion transport, while OEG2COF achieves an optimal balance, delivering the lowest migration barrier and improved rate capability and cycling stability. This work elucidates, from a model perspective, the competitive interplay among pore confinement, steric effects, and ion transport kinetics, offering insights for the rational design of high-performance separators for Li-S batteries.