Tuning the electrolyte network structure to invoke quasi-solid state sulfur conversion and suppress lithium dendrite formation in Li–S batteries

The lithium–sulfur battery is promising as an alternative to conventional lithium-ion technology due to the high energy density of both sulfur and lithium metal electrodes. An extended lifetime has been demonstrated, but two notable challenges still exist to realize its full potential: to overcome the undesired high electrolyte/sulfur ratio required for the catholyte-type mechanism that governs most cell configurations, and to inhibit Li dendrite growth and its parasitic reaction with the electrolyte that results in cell degradation. Here, we demonstrate that by tuning the electrolyte structure, the challenges at both electrodes can be tackled simultaneously. Specifically, the sulfur speciation pathway transforms from a dissolution–precipitation route to a quasi-solid state conversion in the presence of a lowered solvent activity and an extended electrolyte network, curtailing the need for high electrolyte volumes. Ab initio calculations reveal the nature of the network structure. With such an optimized structure, the electrolyte allows dendrite-free Li plating and shows a 20-fold reduction in parasitic reactions with Li, which avoids electrolyte consumption and greatly extends the life time of a low electrolyte/sulfur (5 µl mg–1) sulfur cell. Realization of the full potential of Li–S batteries requires effective methods to treat problems associated with both sulfur cathodes and lithium anodes. Here, the authors report suppression of electrolyte depletion and dendrite formation in Li–S batteries by tuning the solvent/salt molar ratio in a diglyme electrolyte to favour quasi-solid state conversion.