The Importance of Chemical Reactions in the Charging Process of Lithium-Sulfur Batteries

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The underlying mechanism of lithium-sulfur batteries is still not fully established because it involves a series of both chemical and electrochemical reactions as well as the formation of soluble polysulfide intermediates. To improve the mechanistic understanding of lithium-sulfur batteries, this study investigates chemical reactions between the Li2S cathode and more oxidized sulfur species, such as S8 and polysulfides, during the electrochemical charge of the battery. By combining the electrochemistry with X-ray absorption spectroscopy, we show that chemical reactions and, in particular, the resulting accumulation of solution species in the electrolyte are essential to oxidize Li2S at a low overpotential. Additionally, by efficiently separating the anode and cathode compartments of a battery with a lithium ion-exchanged Nafion interlayer, we establish the adverse effect of the anode on the buildup of solution intermediates. In the absence of the interlayer, polysulfide intermediates can diffuse through the separator and react at the anode's surface, while the addition of the interlayer allows the intermediates to accumulate in the separator of the cathode compartment and facilitate the oxidation of Li2S.