Molecular skeleton programming of premediators in sulfur electrochemistry

Molecular mediators have demonstrated broad applicability in electrolyte chemistry of lithium-sulfur batteries, transforming sulfur conversion from traditional multiphase reactions to highly reactive pathways^1-6. Despite tremendous efforts to elucidate the mechanistic roles of molecular mediators^7-9, the influence of molecular skeleton regulation on their mediating effects remains barely understood. Here we propose 2-chloropyrimidine as a potential 'premediator' and a model material for molecular skeleton design, which can be in situ activated into a molecular mediator during sulfur reaction progression by means of aromatic nucleophilic substitution, homogeneously inducing a rapid redox loop over the electrode. Integrating quantum chemistry and machine learning, we develop a molecular skeleton programming strategy that illuminates the structure-property relationship between electronic, geometric and site features of side-chain groups and mediating performance, offering control over the activation rate and mediating activity of premediators. The strategy identifies 2-chloro-4-(trifluoromethyl)pyrimidine as a favourable premediator from 196 candidates, enabling lithium-sulfur batteries to achieve an average capacity retention of 81.7% over 800 cycles together with an energy density of 549 Wh kg^-1 in a 14.2-Ah-level pouch cell. We expect that our work on molecular skeleton programming may find application in designing functional molecules in broader organic chemical spaces.