Accelerated Li-ion transport through a zwitterion-anchored separator for high-performance Li–S batteries

Although considerable studies have focused on suppressing polysulfide shuttling in lithium–sulfur (Li–S) batteries by modifying separator surfaces, simultaneously achieving fast Li-ion transport and full active-material use remains a technological limitation. Herein, we present an effective method for anchoring zwitterionic sulfobetaine (SB) moieties on a separator surface that concurrently facilitates both selective Li-ion transport and polysulfide conversion. The platform that anchors the zwitterions was sequentially constructed by first introducing a polydopamine (PDA) coating to promote surface wettability and activity for further covalent SB binding using the aza-Michael addition reaction, which enabled the formation of a robust PDA/SB-coated separator. The zwitterion-functionalized separator exhibited a significant enhancement in ionic conductivity from 0.837 to 1.827 mS cm−1 and the Li-ion transference number from 0.186 to 0.511 due to the facile dissociation of lithium salts and selectively promoted the interactions between Li cations and the anionic sulfonate SB end groups. Along with the redox promoting effect, the polysulfide shuttling was further inhibited through strong dipole–dipole interactions. Furthermore, as Li2S precipitation was effectively mediated by the SB zwitterions and improved sulfur-conversion kinetics was achieved, outstanding Li–S battery performance with an initial discharge capacity of 1365.9 mA h g−1 was obtained, while delivering an ultra-low capacity decay rate of 0.034% during long-term cycling (up to 1200 cycles) at 3.0C, even at a high load. Therefore, we believe that the proposed study harnessing zwitterionic separator functionalization would enable the rational design of functional separators for the promoted kinetics and suppressed diffusion of unfavorable reaction intermediates for high performance batteries.