Biomimetic Ultrathin Cellulose Composite Separators for High‐Performance Lithium–Sulfur Batteries

Abstract Lithium–sulfur (Li–S) batteries are considered promising candidates for next‐generation energy storage systems due to their high energy density and low cost. However, their commercial production is impeded by the lithium polysulfides (LiPSs) shuttle effect and lithium dendrite growth, which degrade capacity and cycling stability. Inspired by the selectivity of biological ion channels, an ultrathin (8 µm) cellulose composite separator with high ion selectivity and permeability is developed by leveraging the compatibility of chitosan (CS) to cellulose fiber (CF) with covalent organic frameworks (COFs), promoting the self‐assembly of COFs with a preferred (100) orientation. The resulting biomimetic bilayer CF@ICOF exhibits a high lithium‐ion flux (2.186 mS cm ⁻¹ ) and exceptional Li ⁺ /S ₆ ²⁻ selectivity (275.97), effectively suppressing both the LiPSs shuttle and lithium dendrite formation. As a result, the Li–S battery utilizing the CF@ICOF separator delivers ultra‐stable cycling performance, with a capacity decay of only 0.047% per cycle over 800 cycles at 0.5C. Even with a high sulfur‐loaded cathode (11.12 mg cm ⁻² ) and a lean electrolyte (3.5 µL mg ⁻¹ ), the battery retains over 92.73% capacity after 200 cycles. Moreover, a flexible pouch cell is integrated into a wearable display system, demonstrating both high energy density (440.98 Wh kg ⁻¹ ) and excellent stability.