Understanding the Electrochemical Activity of Electrolyte-Insoluble Solid Polysulfide Species in the Lithium-Sulfur Battery System
All these problems demand a better understanding of the fundamental electrochemical processes that occur at the solid cathode-electrolyte interface. This includes both when solid species are formed by the reduction of soluble polysulfides during discharge and when Li2S is oxidized to soluble species on charge. In particular, literature reports are inconclusive about the formation of a solid Li2S2 polysulfide species on discharge.3 We have sought to identify solid, electrolyte-insoluble polysulfide species and to verify and characterize their electrochemical activity.
Past reports4 supposedly synthesizing Li2S2 by direct chemical reaction of lithium and Li2S were duplicated and found to produce a powder which was determined by x-ray photoelectron spectroscopy (XPS) to be predominantly Li2S with significant fractions (> 8 mol%) of Li2S2 and higher-order polysulfides (Li2Sx, x>2). Subsequent filtration in the Li-S battery electrolyte effectively removed the higher-order polysulfides (to < 0.5 mol%, as determined by XPS), while a significant fraction (~ 8 mol%) of polysulfide species ascribed to Li2S2 remained. Non-negligible amounts (~ 5 mol%) of polysulfide-type species were additionally identified in nominally pure, reference Li2S powders.
These powders were sandwiched between carbon nanotube paper current collectors and assembled into cells to evaluate their electrochemical discharge behavior. Discharge capacities in excess of 25 mAh/g were achieved by reference Li2S, on the order of the discharge capacity predicted for the polysulfide species identified by XPS (58 mAh/g). Likewise, even higher discharge capacities were identified in the synthesized Li2Sx product, corresponding to the higher concentration of insoluble polysulfide species. Additional analysis of as-prepared and discharged Li2Sx powders by XPS and electrochemical impedance spectroscopy provide further evidence of the electrochemical activity of these solid polysulfide species. This work demonstrates that electrolyte-insoluble solid polysulfide species can be identified chemically and are electroactive such that they can be reduced to Li2S in a Li-S battery.
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