Insights into Self-Discharge of Lithium- and Magnesium-Sulfur Batteries

Magnesium-sulfur (Mg-S) batteries represent a very promising emerging cell chemistry. However, developments in Mg-S batteries are in an early stage, and the system exhibits problems similar to those of early lithium-sulfur (Li-S) batteries. The significant challenges are the low Coulombic efficiency and short cycle life of Mg-S batteries, mainly associated with the well-known polysulfide shuttle. An obvious result of this phenomenon is the rapid self-discharge of Mg-S batteries. In this article, we present a multiscale simulation framework for metal-sulfur batteries. In our approach, we provide a continuum description of chemical and electrochemical processes at the positive and negative electrodes. In combination with a one-dimensional (1D) model for the transport of dissolved species in the electrolyte, this approach allows us to reproduce and interpret experimental data measured on Li-S and Mg-S batteries. We focus on the common properties of Li-S and Mg-S batteries as well as on the key differences causing the much more rapid self-discharge of the Mg system. We identify side reactions on the anode surface as a limiting process, while other factors, such as the mobility of dissolved species and solid-phase kinetics, play a minor role.