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.