Fingerprinting Lithium-Sulfur Battery Reaction Products by X-ray Absorption Spectroscopy

Wang

Pascal

et al. 2016

J. Electrochem. Soc.

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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: Resultssupporting

confidence: 59%

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

Wang

Pascal

et al. 2016

J. Electrochem. Soc.

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“…[ 2,3,5,6,[18][19][20][21][22] The attractiveness of XAS is that it is element-specifi c while also being sensitive to the local bonding chemistry and solvent environment. When properly interpreted, XAS can provide powerful insights into molecular structure and electronic charge state, and, for in situ studies, changes in chemistry due to an external bias.…”

Section: Introductionmentioning

confidence: 99%

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

Advanced Energy Materials

Pascal

Pemmaraju

et al. 2015

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115

118

The presence and role of polysulfide radicals in the electrochemical processes of lithium sulfur (Li–S) batteries is currently being debated. Here, first‐principles interpretations of measured X‐ray absorption spectra (XAS) of Li–S cells are leveraged with an ether‐based electrolyte. Unambiguous evidence is found for significant quantities of polysulfide radical species (LiS3, LiS4, and LiS5), including the trisulfur radical anion S3 −, present after initial discharge to the first discharge plateau, as evidenced by a low energy shoulder in the S K‐edge XAS below 2469 eV. This feature is not present in the XAS of cells at increased depth of discharge, which, by our analysis, exhibit increasing concentrations of progressively shorter polysulfide dianions. Through a combination of first‐principles molecular dynamics and associated interpretation of in situ XAS of Li–S cells, atomic level insights into the chemistries are provided that underlie the operation and stability of these batteries.

“…In recent studies, X-ray absorption spectroscopy (XAS) at the sulfur K edge has been used to detect the presence of lithium polysulfide intermediates in Li-S battery electrolytes (16,17). The benefit of XAS is that it is an element-specific spectroscopic probe of both the electronic structure and the local environment surrounding sulfur atoms.…”

mentioning

confidence: 99%

“…The polymer electrolyte was a polystyrene-bpoly(ethylene oxide) (SEO) copolymer, and solubility of Li 2 S 8 in this electrolyte was studied by Wujcik et al (16). The two solids were contacted by hand-pressing at 75°C for 3 d. A small piece of the hybrid pellet was taken, and XAS was performed on the side of the pellet that was in direct contact with the SEO/Li 2 S 8 membrane.…”

mentioning

confidence: 99%

Compliant glass–polymer hybrid single ion-conducting electrolytes for lithium batteries

Villaluenga

Proc. Natl. Acad. Sci. U.S.A.

Tong

et al. 2015

Self Cite

105

Despite high ionic conductivities, current inorganic solid electrolytes cannot be used in lithium batteries because of a lack of compliance and adhesion to active particles in battery electrodes as they are discharged and charged. We have successfully developed a compliant, nonflammable, hybrid single ion-conducting electrolyte comprising inorganic sulfide glass particles covalently bonded to a perfluoropolyether polymer. The hybrid with 23 wt% perfluoropolyether exhibits low shear modulus relative to neat glass electrolytes, ionic conductivity of 10 −4 S/cm at room temperature, a cation transference number close to unity, and an electrochemical stability window up to 5 V relative to Li + /Li. X-ray absorption spectroscopy indicates that the hybrid electrolyte limits lithium polysulfide dissolution and is, thus, ideally suited for Li-S cells. Our work opens a previously unidentified route for developing compliant solid electrolytes that will address the challenges of lithium batteries.hybrid electrolytes | inorganic sulfide glasses | fluorinated polymers | lithium batteries | lithium-sulfur batteries E lectrolytes used in lithium ion batteries that power personal electronic devices and electric vehicles comprise lithium salts dissolved in flammable organic liquids. Catastrophic battery failure often begins with the electrolyte decomposition and combustion. In addition, side reactions between the electrolyte and anode particles result in steady capacity fade. Some of the byproducts of side reactions can dissolve in the electrolyte and migrate from one electrode to the other. This effect is minimized in the case of solid electrolytes because of limited solubility and slow diffusion (1). Mixtures of liquids and salts have additional limitations. The passage of current results in an accumulation of salt in the vicinity of one electrode and depletion close to the other electrode, because only the cation participates in the electrochemical reactions. Both overconcentrated and depleted electrolytes have lower conductivity, which accentuates cell polarization and reduces power capability. Concentration polarization is absent in single-ion conductors, wherein the anions are immobilized (2). Nonflammable, single ionconducting solid electrolytes have the potential to dramatically improve safety and performance of lithium batteries (3-6).Solid electrolytes, such as inorganic sulfide glasses (Li 2 S-P 2 S 5 ), are single-ion conductors with high shear moduli (18-25 GPa) and high ionic conductivity (over 10 −4 S/cm) at room temperature (7,8). However, these materials, on their own, cannot serve as efficient electrolytes, because they cannot adhere to moving boundaries of the active particles in the battery electrode as they are charged and discharged. Hayashi et al. (9) prepared hybrid electrolytes by mixing sulfide glasses and poly(ethylene oxide) (PEO) polymers. Although the addition of PEO improves mechanical flexibility, there is a dramatic decrease in ionic conductivity because of the insulative nature of PEO. For exampl...