Phosphazene-Based Covalent Organic Framework Suppressing Shuttle Effect and Enabling Stable Cycling in Lithium–Sulfur Batteries

Lithium–sulfur batteries (LSBs) are considered one of the most attractive next-generation energy storage technologies, offering a high theoretical energy density of 2600 Wh/kg along with environmental compatibility. However, the “shuttle effect” of lithium polysulfides (LiPSs) and low electrical conductivity severely hinder their practical application. Herein, we report a two-dimensional phosphazene-based covalent organic framework (HM-COF) modified commercial polypropylene (PP) separators (HM-COF/AB/PP). The HM-COF features abundant N/P elements and a porous structure, enabling electrostatic repulsion of LiPSs anions and efficient Li+ transport. The modified separator exhibits enhanced electrolyte wettability (contact angle reduced to 24°) and a Li+ transference number of 0.28 (vs 0.14 for pristine PP). XPS analysis confirms strong chemical adsorption of LiPSs by HM-COF. LSBs with HM-COF/AB/PP separators deliver an initial discharge capacity of 872 mAh/g at 0.5 C, with a low capacity decay rate of 0.059% per cycle, outperforming cells with pristine PP separators. This strategy provides a facile approach for fabricating high-performance LSBs.