Understanding Li-S Chemistry with First-Principles Analysis

Fulfilling the promise of Li-S batteries depends on the implementation of strict controls on the degradation processes that are involved in a strongly correlated system. A highly reactive Li anode requires of a special electrolyte formulation able to control dendrite formation and its subsequent effects on the cell performance. In addition, the presences of soluble polysulfides (PS) that travel from cathode to anode substantially change the anode interfacial chemistry. On the other hand, intimately mixed sulfur-carbon cathode formulations are usually employed to enhance the cathode electronic conductivity. As a consequence, sulfur reduction reactions at the cathode interface are affected by the presence of porous carbon structures.  Thus, a complex chemistry requires of numerous tools to identify processes and mechanisms with the goal of guiding improvements in materials aimed to extended battery life. Among these tools, first-principles density functional theory and ab initio molecular dynamics simulations yield information regarding molecular mechanisms that is not readily available or difficult to obtain from experimentation. Here we focus on two aspects: interfacial electrolyte decomposition reactions on the Li anode surface leading to the initial stages of the solid-electrolyte interphase (SEI), and sulfur reduction at the cathode.  We study the reduction mechanisms of a series of electrolyte solutions including  1M solution of Lithium bis(trifluoromethane sulfonyl) imide (LiTFSI) in pure solvents such as Dioxolan (DOL), 1,1,2,2-Tetrafluoroethyl 2,2,3,3-tetrafluoropropyl ether (D2), dimethoxyethane (DME), and ethylene carbonate (EC) and mixtures  of DOL/DME, and DOL/D2. The reduction mechanisms of soluble PS species such as S₈Li₂, S₆Li₂ and S₄Li₂ at the anode surface are also investigated, along with their effect on the electrolyte decomposition reactions.  At the cathode side we characterize discharge reactions and their voltage profiles for sulfur reduction in contact with the electrolyte, versus sulfur reduction in contact with carbon and the electrolyte. Also we analyze effects of carbon structure on diffusion and dissolution of sulfur anions and PS species as well as on Li₂S precipitation.