Lithium-Sulfur (LiS) batteries are a front runner for next-generation secondary battery systems. This is partially due to the low cost, relative abundance, and environmentally benign nature of their active materials, but arguably most important is the order of magnitude increase in theoretical specific capacity of LiS cells compared to current state of the art lithium-metal oxide systems (~1,600 mAhg^-1 vs ~160mAhg^-1, respectively). Despite these inherent benefits, the commercialization of LiS systems has been impeded by a rapid loss of capacity upon repeated cycling due to the complexities of lithiation and delithiation at the carbon/sulfur composite cathode.

During discharge, elemental sulfur is sequentially reduced to lithium sulfide following the reaction S₈ + 16Li⁺ + 16e^-→ 8Li₂S_, where intermediates Li₂S_(8-2) are formed, their length depending on depth of discharge. Long-chain lithium polysulfides (LiPS), Li₂S_(8-4) are highly soluble in organic electrolytes and, once solvated, will migrate to the lithium metal anode. This leads to permanent capacity loss and internal shorting via lithium polysulfide redox shuttling. To combat this limitation, extensive efforts have been focused on trapping long-chain LiPS by both: 1. physically confining them in mesoporous carbons, and 2. chemically inducing electrostatic attractions with electron rich heteroatoms on the cathode.

In this work we not only aim to find synergy between these two approaches, but also investigate the potential for doubling the binding energy of electrostatic interactions by covalent tethering of long chain LiPS to the cathode surface via reversible di-sulfide bonding. Through a highly tunable one-step reaction, we functionalize mesoporous carbon surfaces with aromatic small molecules using in-situ generation of diazonium radicals. The flexibility of our approach allows for the introduction of a wide variety of surface functionality, and thus a platform to broaden our understanding of cathode/electrolyte interactions.

Our current progress focuses on the spectroscopic, physical, and electrochemical characterization of a thiophenol terminated carbon. Through this work we have demonstrated not only successful modification of carbon particles, but an ability to control the density of modifier groups on the surface. In conjunction with finding an optimal concentration of surface modifiers, we investigate the role of modifiers and carbon pore size in modulating the kinetics and capacity retention of LiS cells. By optimizing pore-size and modifier concentration, we have drastically increased the cycle life of our devices, which maintain >900 mAhg^-1 over 150+ cycles at 0.1C. We hypothesize that the mechanism responsible for this improvement is a reversible covalent interaction based on electrochemical measurements, which indicate a concentration-dependent shift toward solid-phase reaction pathways.