Mechanism for the Stable Performance of Sulfur-Copolymer Cathode in Lithium-Sulfur Battery Studied by Solid-State NMR Spectroscopy

Rechargeable lithium-sulfur (Li-S) batteries have drawn significant attention as next-generation energy storage systems. Sulfur-copolymers are promising alternative cathode materials to elemental sulfur in Li-S batteries as they provide high reversible capacity. However, the redox mechanisms of these materials are not well understood owing to the difficulty in characterizing amorphous structures and identifying individual ionic species. Here, we use solid-state NMR techniques together with electrochemistry experiments and quantum calculations to investigate the structural evolution of the prototype S-copolymer cathodes, sulfur-diisopropenylbenzene copolymers (poly(S-co-DIB)), during cycling. We demonstrate that polysulfides with different chain lengths can be distinguished by C-13 and Li-7 NMR spectroscopy, revealing that the structure of the copolymers can be tuned in terms of polysulfide chain lengths and resulting reaction pathways during electrochemical cycling. Our results show that the improved cyclability of these cathodes originates from the role of organic moieties acting as anchors that fixate polysulfides to the polymeric network during cycling, thus preventing their diffusion into the electrolyte. We provide a new methodological concept for the mechanistic studies to track the intermediate species and phase transition in Li-S batteries.