The lithium−sulfur (Li−S) system is an emerging, “beyond Li- ion” battery chemistry with high theoretical energy density and specific energy. Unfortunately, these benefits have not yet been realized in practical Li−S cells, largely due to two unmet challenges. These include polysulfide (PS) dissolution/shuttling and the insulating nature of the redox end members (REMs), α-S and Li2S. This presentation will describe recent atomic-scale computational studies aimed at characterizing and overcoming these challenges. First, the properties of the solid-phase redox end members, α-S, β-S, Li2S, and Li2S2, will influence the performance of Li-S batteries. Nevertheless, the fundamental thermodynamic and electronic properties of these phases remain poorly understood. Here we apply van der Waals augmented density functional theory, quasi-particle methods, and continuum solvation techniques to predict several structural, thermodynamic, spectroscopic, electronic, and surface characteristics of these phases. Second, the dissolution of PS intermediates during discharge is a well-known obstacle to achieving long cycle life in Li-S batteries. Prior work has shown that PS dissolution can be partially suppressed via\ encapsulation of S and PS within a porous cathode support. Metal−organic frameworks (MOFs) are promising cathode support materials because the encapsulation afforded by MOF pores can be augmented by chemical adsorption of PS onto coordinately unsaturated metal sites (CUS). Here we demonstrate that this additive approach-- restricting PS dissolution by combining encapsulation and adsorption within a MOF-- can be tuned to maximize PS anchoring via metal substitution on the CUS. Optimal MOF compositions are pinpointed by computationally screening metal-substituted variants of M2(dobdc) for their ability to chemically anchor prototypical species (S8, Li2S4, and Li2S) present during the cycling of Li−S batteries. Finally, the insulating nature of the REMs α-S and Li2S has the potential to limit capacity and efficiency. Nevertheless, the mechanisms responsible for ionic and electronic transport in these materials remain a matter of debate. These mechanisms are clarified -- in both the adiabatic and nonadiabatic charge transfer regimes-- by employing a combination of hybrid-functional-based and constrained density functional theory calculations. We conclude that low equilibrium carrier concentrations are responsible for sluggish charge transport in α-S and Li2S.

Atomic scale simulations of performance-limiting phenomena in lithium-sulfur battery cathodes

2019 Fall National Meeting

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