Theoretical Consideration of Side Reactions between the VS<sub>4</sub> Electrode and Carbonate Solvents in Lithium–metal Polysulfide Batteries

The lithium/vanadium tetra sulfide (Li|VS4) battery can be considered a promising next-generation battery because of its high theoretical energy density, and it is expected to overcome the problems inherent in Li-S batteries. However, the charge/discharge cycle degradation of the Li|VS4 battery strongly depends on the choice of the organic solvent. We investigated the equilibrium potentials of the decomposition reactions involving the VS4 electrode and organic solvent molecules using the density functional and classical solution theories. We first modelled the decomposition reactions between VS4 and different organic solvent molecules, such as ethylene, dimethyl, and propylene carbonates (EC, DMC, and PC). Next, we calculated the change in the Gibbs free energy of the decomposition reaction by assuming a thermodynamic cycle and estimated the equilibrium potential vs. Li/Li+. From the equilibrium potential, the overpotential of the DMC against the potential plateau of the Li|VS4 battery is negative and shows the lowest value in the considered solvents. This result suggests that the battery cycle with DMC deteriorates more quickly than that with EC and PC. This suggestion explains the experimental tendency of battery cycle degradation and will be a useful guide for improving the electrochemical performance of Li|VS4 batteries.