Electrochemically Stable Rechargeable Lithium–Sulfur Batteries with a Microporous Carbon Nanofiber Filter for Polysulfide

As a primary component in lithium–sulfur (Li–S) batteries, the separator may require a custom design in order to facilitate electrochemical stability and reversibility. Here, a custom separator with an activated carbon nanofiber (ACNF)‐filter coated onto a polypropylene membrane is presented. The entire configuration is comprised of the ACNF filter arranged adjacent to the sulfur cathode so that it can filter out the freely migrating polysulfides and suppress the severe polysulfide diffusion. Four differently optimized ACNF‐filter‐coated separators have been developed with tunable micropores as an investigation into the electrochemical and engineering design parameters of functionalized separators. The optimized parameters that are verified by electrochemical and microstructural analyses require the coated ACNF filter to possess the following: (i) a porous architecture with abundant micropores, (ii) small micropore sizes, and (iii) high electrical conductivity and effective electrolyte immersion. It is found that the ACNF20‐filter‐coated separator demonstrates an overall superior boost in the electrochemical utilization (discharge capacity: 1270 mA h g −1 ) and polysulfide retention (capacity fade rate: 0.13% cycle −1 after 200 cycles). These results show that the modified thin‐film‐coating technique is a viable approach to designing ultratough ACNF‐filter‐coated separators with outstanding mechanical strength and flexibility as an advanced component in Li–S cells.