Understanding the Electrochemical Activity of Electrolyte-Insoluble Solid Polysulfide Species in the Lithium-Sulfur Battery System
Authors: Klein Michael , Manthiram Arumugam 
Published: February 2020

In the pursuit of electrochemical energy storage solutions for large-scale clean energy applications, including for electrical vehicles and at the grid-scale, current lithium-ion battery technologies are severely limited in terms of both their potential energy density and affordability. The lithium-sulfur (Li-S) battery represents a promising next-generation chemistry, due to the high theoretical gravimetric energy density of sulfur (1672 mAh/g), a material which is simultaneously earth-abundant and inexpensive. However, practical Li-S batteries remain difficult to implement due to a complex discharge mechanism that is bookended by solid, electrically insulating species—sulfur and lithium sulfide (Li2S)—and electrolyte-soluble polysulfide species formed at intermediate states of charge. Additionally, most Li-S batteries use metallic lithium as the anode, which requires meticulous surface treatment to avoid any unfavorable reactions and safety concerns. A promising alternative approach is to fabricate cells using Li2S as the starting cathode material, avoiding the need to use metallic lithium altogether. Li2S can be problematic to work with as it must be “activated” with a surface treatment or by nano-engineering.1 Otherwise, a large overpotential, outside the Li-S electrolyte stability window, must be applied on first charge.2

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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