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

Abstract

The influence of the electrolyte solvents on the cell voltage in lithium-sulfur (Li-S) batteries is investigated. It is found that changing the solvent does not only alter the reaction mechanisms taking place during charge and discharge, but also exerts a pronounced influence on the cell voltage. The changes monitored upon switching from standard ether-based electrolytes to more polar solvents are quite substantial. An increase in the open circuit voltage of up to ∼400 mV could be observed. Both experimental evidence and theoretical calculations are presented in order to elucidate and quantify these effects. It is demonstrated that both the observed trends and the order of magnitude of the measured values can be explained by the free solvation energies of the respective ionic species in the electrolyte systems. Among them, the lithium cation contributes most to the phenomena described. Given that the final reaction products are solid and precipitate from the solution, these effects cannot be exploited to increase the overall energy densities of standard Li-S batteries. However, they are still important both with respect to the fundamental understanding of the electrochemical processes involved as well as practical applications such as liquid, polysulfide-based redox flow batteries.