(83e) Ab-Initio Investigation of Dimethyl Disulfide As an Additive for Lithium-Sulfur Batteries

Energy storage technology is an important field of study with strong economic and environmental driving forces. Batteries are ubiquitous and critical for much of modern technology like cell phones and electric vehicles. The current leader in rechargeable battery technology is the lithium ion (Li) battery. However, the Li battery is close to the theoretical limit of its energy storage but our technology requires more capacity. One promising battery system is the Lithium-Sulfur (LiS) battery which can surpass the Li battery by about 3 times the energy density.

Unfortunately, the LiS battery faces several challenging problems before it can be utilized commercially. One problem is known as the polysulfide shuttle reaction. This reaction is the consequence of the solubility of sulfur reaction products (polysulfides) in the electrolyte. The polysulfide shuttle causes active material loss from the cathode and passivates both electrodes. Donghai Wang’s group at Penn State University recently published a new strategy to counteract this effect: use a different electrolyte. Their new electrolyte consisted of 50% DME and 50% dimethyl disulfide (DMDS) which showed increased battery performance. Wang et. al’s experiment postulated several potential reasons for why they saw increased performance but were not able to determine exactly why.

We used density functional theory (DFT) to explore the theory and fundamentals behind DMDS’s effect on battery performance to complement Wang et al’s experimental work. Computational chemistry calculations were completed with Gaussian09 to determine electron affinities and how the reduction mechanism is modified from reducing polysulfides to reducing dimethyl polysulfides. . We also investigated the passivation and solubility of new reduction products on the electrodes with the DMDS modified reduction pathway by solid state calculations with VASP. Based on the DOS and the mobility of charge carriers like hole polarons and charged defects we show that even with a modified reaction, the electrode will still be passivated. Regarding the reversibility of the reactions, the new electrode precipitate was determined to be more soluble than Li₂S which indicates that the precipitate is more reversible. Together these studies provide insight into how the LiS battery performance can be improved with the addition of DMDS.