Understanding the Electrochemical Activity of Electrolyte-Insoluble Solid Polysulfide Species in the Lithium-Sulfur Battery System

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 (Li₂S_x, 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 Li₂S_x product, corresponding to the higher concentration of insoluble polysulfide species. Additional analysis of as-prepared and discharged Li₂S_x 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.

References

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4. P. Dubois, J.P. Lelieur, G. Lepoutre, Inorg. Chem., 27, 73 (1988)