TPU-DMBA@KB-Modified Separator with Multifunctional Anionic Polymer/KB Composite: Enhancing Electrochemical Performance and Suppressing Dendrite Penetration in Lithium–Sulfur Batteries

Lithium–sulfur (Li–S) batteries hold great promise for next-generation energy storage owing to their ultrahigh theoretical energy density; however, their practical application is severely hampered by the polysulfide shuttle effect and the penetration of lithium dendrites through the separator. In this work, a carboxyl-containing anionic polymer (TPU-DMBA) is synthesized and composited with Ketjen Black (KB), and the resulting mixture is coated onto a commercial polypropylene separator via a simple doctor-blade method. In this design, the porous KB network provides physical adsorption to capture polysulfides, while the dissociated carboxylate groups (–COO−) generate strong electrostatic repulsion against negatively charged polysulfide anions (Sn2−). This dual-mechanism strategy—adding electrostatic repulsion on the basis of physical adsorption—effectively suppresses the shuttle effect. In addition, the flexible polymer backbone increases the tensile strength of the separator by approximately 30%, enhancing its resistance against dendrite penetration. The carbon material also significantly improves electrolyte wettability (the contact angle decreases from 41.6° to 11.7°) and ionic conductivity (from 0.48 × 10−3 to 0.88 × 10−3 S cm−1). The polymer itself acts as a binder, eliminating the need for additional binder addition. Benefiting from the synergy of electrostatic repulsion, physical adsorption, and mechanical reinforcement, the prepared modified separator endows the Li–S battery with an initial specific discharge capacity of 1373.15 mAh g−1 at 0.1 C and an initial discharge capacity of 714.46 mAh g−1 at a high rate of 2 C. After 200 cycles at 2 C, the capacity remains 577.93 mAh g−1, with a capacity retention of 80.89%. This work provides a low-cost, scalable, and binder-free separator modification strategy that simultaneously suppresses the polysulfide shuttle and resists dendrite growth, opening a new and effective pathway toward practical high-performance Li–S batteries.