On the Electrode Potentials in Lithium-Sulfur Batteries and Their Solvent-Dependence

The influence of the electrolyte solvents on the cell voltage in lithium-sulfur (Li-S) batteries is investigated. It is found that changingthe solvent does not only alter the reaction mechanisms taking place during charge and discharge, but also exerts a pronouncedinfluence on the cell voltage. The changes monitored upon switching from standard ether-based electrolytes to more polar solventsare quite substantial. An increase in the open circuit voltage of up to ∼400 mV could be observed. Both experimental evidence andtheoreticalcalculationsarepresentedinordertoelucidateandquantifytheseeffects. Itisdemonstratedthatboththeobservedtrendsand the order of magnitude of the measured values can be explained by the free solvation energies of the respective ionic species inthe electrolyte systems. Among them, the lithium cation contributes most to the phenomena described. Given that the final reactionproducts are solid and precipitate from the solution, these effects cannot be exploited to increase the overall energy densities ofstandard Li-S batteries. However, they are still important both with respect to the fundamental understanding of the electrochemicalprocesses involved as well as practical applications such as liquid, polysulfide-based redox flow batteries.© 2014 The Electrochemical Society. [DOI: 10.1149/2.0991409jes] All rights reserved.Manuscript submitted March 24, 2014; revised manuscript received May 30, 2014. Published June 20, 2014.