An Expansion-Mitigant Binder for Stable Cycling of High-Loading Lithium–Sulfur Batteries

Abstract

Lithium–sulfur batteries with high sulfur content and mass loading are promising energy storage technologies due to sulfur’s exceptional theoretical energy density. However, in practice, their actual capacity drastically decays when the sulfur cathode is loaded to the commercially required levels of 4 mg_sulfur cm^–2 and above, significantly reducing the energy density. This reduction is due to the excessive formation of polysulfides during sulfur lithiation, which not only deteriorates battery performance through their detrimental shuttling but also results in substantial stress buildup due to their significantly larger volume compared to sulfur. To address these challenges, we have developed an approach to suppress lithium polysulfide shuttling by limiting the space for sulfur expansion while improving the Li⁺ ion diffusion. This was achieved through a straightforward but effective method to cross-link the organic binder used in sulfur electrodes. Specifically, PVDF, one of the most common binder materials for battery electrodes, was studied. The chemical, mechanical, and structural properties of the cross-linked PVDF binder were thoroughly investigated, compared with standard PVDF, and correlated to the achieved electrochemical performance of sulfur electrodes. As a result, sulfur cathodes with cross-linked PVDF binder exhibited prolonged cycle life compared to their standard counterparts. Moreover, using this expansion-mitigant binder, cathodes with areal sulfur loading of 4 mg cm^–2 showed exceptional stability for more than 200 cycles and a Coulombic efficiency above 97%. This approach offers a promising avenue to alleviate the major roadblocks of lithium–sulfur battery commercialization while allowing the utilization of the commonly accessible and well-studied binder chemistries.