Characterization of Polysulfide Radicals Present in an Ether‐Based Electrolyte of a Lithium–Sulfur Battery During Initial Discharge Using In Situ X‐Ray Absorption Spectroscopy Experiments and First‐Principles Calculations

Self Cite

Lithium sulfur (Li-S) batteries are well known for their high theoretical specific capacities, but are plagued with scientific obstacles that make practical implementation of the technology impossible. The success of Li-S batteries will likely necessitate the use of thick sulfur cathodes that enable high specific energy densities. However, little is known about the fundamental reaction mechanisms and chemical processes that take place in thick cathodes, as most research has focused on studying thinner cathodes that enable high performance. In this work, in situ X-ray absorption spectroscopy at the sulfur K-edge is used to examine the back of a 115 μm thick Li-S cathode during discharge. Our results show that in such systems, where electrochemical reactions between sulfur and lithium are likely to proceed preferentially toward the front of the cathode, lithium polysulfide dianions formed in this region diffuse to the back of the cathode during discharge. We show that high conversion of elemental sulfur is achieved by chemical reactions between elemental sulfur and polysulfide dianions of intermediate chain length ( Lithium sulfur batteries have become a widely popular focus of energy storage research due to their high theoretical specific capacity of 1672 mA-h/g. 1,2 The overall reaction mechanism that governs the Li-S discharge processes is given by:The actual reaction mechanism involves a series of electrochemical reactions with lithium polysulfide reaction intermediates (Li 2 S x , 2 ≤ x ≤ 8, referred to as polysulfide dianions; or LiS x , 3 ≤ x ≤ 5, referred to as polysulfide radical anions). [3][4][5][6][7][8] One example of an electrochemical reaction pathway is:A decrease in the chain length of polysulfides is thus an unambiguous signature of electrochemical reactions. However, polysulfides are also known to undergo chemical reactions, for example:The evolution of polysulfide chain length in the presence of both electrochemical and chemical reactions is difficult to predict. Lithium polysulfides tend to dissolve into the electrolyte, resulting in loss of cell capacity. It is thus not surprising that a great deal of research focuses on solving the issues related to polysulfide dissolution. [10][11][12][13] Many researchers have recognized that in order for Li-S cells to have high specific energy density and be competitive in cost with current lithium ion battery technology, Li-S cell cathodes must have high area-specific sulfur loadings.14-16 These high sulfur loadings can only be achieved by making the Li-S cell cathodes * Electrochemical Society Member. Increasing cathode thickness amplifies concentration polarization effects and plating of Li 2 S (and possibly Li 2 S 2 ) at the front of the electrode.18 These problems are magnified if the battery is cycled at high rates of charge and discharge.Despite their apparent importance, little is known about the chemical and electrochemical reactions that take place in thick sulfur cathodes. The purpose of this paper is to shed light on these reactions. In partic...

Section: Resultsmentioning

confidence: 87%

Section: A19mentioning

confidence: 99%

In Situ X-ray Absorption Spectroscopy Studies of Discharge Reactions in a Thick Cathode of a Lithium Sulfur Battery

Wujcik

Wang

Pascal

et al. 2016

Self Cite

“…34,37 More specifically, the authors had hypothesized that the anodic cyclic voltammetry (CV) features of various polysulfides dissolved in tetrahydrofuran (THF) solvent could only be explained, if all the polysulfides were to chemically convert to the same intermediate, which would then be electrochemically oxidized at the electrode surface. 34,37 They based their reasoning on the fact that the value of the slope of the peak anodic current versus the scan rate plotted on a log scale had a value of significantly lower than 0.5, while the potential at which the sole anodic peak occurred was independent of the used polysulfide chain length and concentration. Because S 8 is the most stable oxidation product, it was concluded by the authors that S 8 and not a polysulfide was the most likely electrochemical product.…”

Section: Resultsmentioning

confidence: 99%

“…Because S 8 is the most stable oxidation product, it was concluded by the authors that S 8 and not a polysulfide was the most likely electrochemical product. 34,38 Due to similarities in the cyclic voltammetry features of Li-S batteries based on THF 19,34,37 and DOL-DME 17,35,39 solvents, it is likely that Li-S batteries based on these solvents share the same mechanism of oxidation.…”

Section: Resultsmentioning

confidence: 99%

Understanding the Charging Mechanism of Lithium-Sulfur Batteries Using Spatially Resolved Operando X-Ray Absorption Spectroscopy

Gorlin

Patel

Freiberg

et al. 2016

115

134

Replacement of conventional cars with battery electric vehicles (BEVs) offers an opportunity to significantly reduce future carbon dioxide emissions. One possible way to facilitate widespread acceptance of BEVs is to replace the lithium-ion batteries used in existing BEVs with a lithium-sulfur battery, which operates using a cheap and abundant raw material with a high specific energy density. These significant theoretical advantages of lithium-sulfur batteries over the lithium-ion technology have generated a lot of interest in the system, but the development of practical prototypes, which could be successfully incorporated into BEVs, remains slow. To accelerate the development of improved lithium-sulfur batteries, our work focuses on the mechanistic understanding of the processes occurring inside the battery. In particular, we study the mechanism of the charging process and obtain spatially resolved information about both solution and solid phase intermediates in two locations of an operating Li 2 S-Li battery: the cathode and the separator. These measurements were made possible through the combination of a spectro-electrochemical cell developed in our laboratory and synchrotron based operando X-ray absorption spectroscopy measurements. Using the generated data, we identify a charging mechanism in a standard DOL-DME based electrolyte, which is consistent with both the first and subsequent charging processes. Lithium-sulfur (Li-S) batteries are an emerging battery technology that has the potential to meet the energy density and cost requirements of electric vehicles. Recently, several studies have identified that the attainment of areal capacities as high as 4-8 mAh/cm 2 while minimizing the electrolyte content are the key factors in meeting these requirements.1-3 The only currently commercialized Li-S battery has a significantly lower areal capacity of 2.5 mAh/cm 2 and operates in the presence of excess electrolyte, 4 necessitating significant technological breakthroughs to facilitate the possible use of Li-S batteries in the transportation sector. One of the main barriers to achieving such breakthroughs is the lack of fundamental understanding of the mechanism behind the operation of Li-S batteries. 1,5,6 In particular, it is not yet clear how the mechanism of discharge differs from the charge mechanism, 5 and if these two processes might change upon an increase in active material loading or reduction in electrolyte volume. 1 Consequently, there is a pressing need for performing operando characterization of Li-S batteries under a variety of conditions to identify fundamental aspects of the charging and discharging processes.One attractive but insufficiently explored system for a mechanistic characterization of Li-S batteries is the charging process of a Li 2 S cathode, a possible alternative to the conventional S 8 cathode, with a potential to enable batteries with silicon or tin rather than lithium anodes. 7,8 Specifically, it has been recently reported that a Li-S battery, which is assembled in a discharged s...

“…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 the literature.…”

mentioning

confidence: 99%

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

Klein

Goossens

Bielawski

et al. 2016

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...