Elucidating the Electrochemical Activity of Electrolyte-Insoluble Polysulfide Species in Lithium-Sulfur Batteries

Abstract

The direct synthesis of Li 2 S 2 , a proposed solid intermediate in the discharge of lithium-sulfur (Li-S) batteries, was accomplished by treating elemental lithium with sulfur in liquid ammonia at −41 • C. The as-synthesized product was analyzed by X-ray photoelectron spectroscopy (XPS) as well as X-ray diffraction (XRD) and determined to be a mixture of crystalline Li 2 S, amorphous Li 2 S 2, and higher-order polysulfides (Li 2 S x , x > 2). Monitored filtration followed by a tailored electrochemical approach was used to successfully remove the higher-order polysulfides and yielded a powder, which was determined by XPS to be comprised of ∼9 mol% insoluble polysulfide species (mainly Li 2 S 2 ) and ∼91 mol% Li 2 S. This material was discharged galvanostatically in an electrochemical cell and, despite the lack of soluble polysulfide species, was shown to exhibit a discharge plateau at ∼2.1 V vs. Li/Li + . This result confirmed the electrochemical reducibility of electrolyte-insoluble polysulfides in Li-S batteries. Moreover, it was determined that the reduction of solid polysulfides was confined to areas where the sulfur-sulfur bonds were in intimate contact with the conductive current collector. Finally, it was observed that commercially available Li 2 S samples contain significant quantities of polysulfide-type impurities. Lithium-sulfur (Li-S) batteries have emerged in recent years as promising next-generation energy storage devices to meet the growing need for affordable, highly energy-dense systems for electrical vehicles and grid-scale applications.1,2 The high theoretical gravimetric energy density (1672 mAh g −1 ) of the Li-S system coupled with the low-cost and abundance of sulfur offers significant advantages over current lithium-ion batteries.2 However, the conversion nature of the Li-S system leads to a complicated charge/discharge mechanism that passes through multiple soluble and insoluble species on each halfcycle. 2,3 This opens up a multitude of mechanisms for active material loss during long-term cycling. Additionally, the insulating nature of sulfur and lithium sulfide, the final discharge product, limits the rate capability of the system and necessitates forming composite cathodes with a conductive material such as carbon. 1,2Of particular concern to the loss of capacity with cycling is the deposition of electrochemically-inaccessible solid sulfide species (Li 2 S x , x < 3).4,5 Multiple literature reports have reached different conclusions on the existence of the solid, electrolyte-insoluble species Li 2 S 2 and whether it forms as a solid intermediate during the discharge of Li-S batteries. [5][6][7][8][9][10][11][12] Recent studies have suggested there may be separate reaction pathways that are followed at different points during Li-S discharge: one where only Li 2 S is formed during reduction of soluble polysulfides (Li 2 S x , x > 2), and a second where both Li 2 S and Li 2 S 2 are formed simultaneously. 11 The question of whether any Li 2 S 2 that forms can then be reduced is debated in...

An Effective Lithium Sulfide Encapsulation Strategy for Stable Lithium–Sulfur Batteries

“…The same ratio for the unmodified Li 2 S electrode that was assembled into a cell, disassembled, and then washed with TEGDME was significantly less, corresponding to a lower S:Li ratio. Previous studies have noted that commercial Li 2 S powders contain soluble polysulfide impurities . We thus hypothesize that the soluble polysulfides are partially extracted in the uncycled Li 2 S cell, while they are protected by the MnS encapsulant layer in the Li 2 S/Mn cell.…”

Rational Design of Lithium–Sulfur Battery Cathodes Based on Experimentally Determined Maximum Active Material Thickness

“…Films with small quantities of soluble sulfur–sulfur species will form polysulfides in solution and the OCV will be in the range of 2.2 ± 0.2 V, as observed for the films in this work over 70 nm thick. The higher OCV (between 1.8 and 1.9 V) for the thin (<90 nm) films than the pure, crystalline Li 2 S reference likely reflect insoluble (Li 2 S 2 -type) solid polysulfides …”

“…Similarly, while the conductivity of Li 2 S 2 should be substantially larger than Li 2 S, only half of the theoretical capacity is realizable if the final discharge product of a Li–S battery is Li 2 S 2 . We have shown in the past that solid Li 2 S 2 reduces to Li 2 S only when in intimate contact with the current collector, so we expect that Li 2 S thickness should still be limiting to cell-level capacity.…”

Regulating the Stable Lithium and Polysulfide Deposition in Batteries by a Gold Nanoparticle Modified Vertical Graphene Host

“…The sulfur cathode in Li-S batteries also relies on a dissolution-precipitation process, and the nonuniform precipitation of insulating Li 2 S from the dissolved LiPSs leads to a high energy barrier for the charging process and blocks the pores in the electrode. [41][42][43] Therefore, we further explored the effects of such a vertical structure with homogeneous Au seeds on the regulation of Li 2 S deposition. The deposition of Li The full cells were also assembled with the Li metal as the anodes and the VG-GP or Au-VG-GP-5 loaded with sulfur as the cathodes (denoted S/VG-GP and S/Au-VG-GP-5).…”