(146e) Formation of Large Polysulfide Complex during Lithium-Sulfur Battery Discharge

Sulfur cathodes have much larger capacities than the transition-metal-oxide cathodes used in commercial lithium-ion batteries, but suffer from unsatisfactory capacity retention and long-term cyclability. Capacity degradation originates from soluble lithium polysulfides gradually diffusing into the electrolyte. Understanding the formation and dynamics of soluble polysulfides during the discharging process at the atomic level remains elusive, which limits further development of lithium-sulfur (Li/S) batteries. Here we report first-principles molecular dynamics simulations and density functional calculations, through which the discharging products of Li/S batteries were studied. We find that instead of simple Li₂S_n (1≤ n ≤8) clusters generated from single cyclooctasulfur (S₈) rings, large Li/S clusters form by collectively coupling several different rings to minimize the total energy. While some small clusters bind to the electrolyte molecules, such as DOL and DME, and diffuse out, the remainders conglomerate into large Li/S clusters. At high lithium concentration, a Li-S network forms at the sulfur surfaces. Our results explain the formation of insoluble Li-S complex at the end of the discharging process and the slow dynamics of phase transformation from Li₂S₂ to Li₂S. In addition, we show that oxygen impurities in graphene, particularly oxygen atoms bonded to vacancies and edges, may stabilize the lithium polysulfides that may otherwise diffuse into the electrolyte.

Support was provided by the Petroleum Institute, Abu Dhabi, and the McMinn Endowment at Vanderbilt University. DFT calculations were performed at the DoD AFRL.