Operando Characterization of Intermediates Produced in a Lithium-Sulfur Battery

Abstract

One of the technological barriers to electrification of transport is the insufficient storage capacity of the Li-ion batteries on which the current electric cars are based. The lithium-sulfur (Li-S) battery is an advanced technology whose successful commercialization can lead to significant gains in the storage capacity of batteries and promote wide-spread adoption of electric vehicles. Recently, important Li-S intermediates, including polysulfides, S 3•-, and Li 2 S, have been shown to present unique X-ray absorption near edge structure (XANES) features at the sulfur K-edge. As a result, a combination of XANES characterization with electrochemistry has the potential to contribute to the understanding of Li-S chemistry. In this study, we present an operando XANES cell design, benchmark its electrochemical and spectroscopic performance, and use it to track reaction intermediates during the discharge of the battery. In particular, by employing electrolyte solvents with either a high or a low dielectric constant, we investigate the influence of the solvent on the conversion of polysulfide species to Li 2 S. Our results reveal that Li 2 S is already formed after ∼25-30% discharge in both types of electrolyte solvents, but that further conversion of polysulfides to Li 2 S proceeds more rapidly in a solvent with a low dielectric constant. Electric vehicles represent a promising alternative to conventional transport based on an internal combustion engine and, when combined with renewable energy sources, have the potential to decrease worldwide CO 2 emissions by 20-30%.1,2 In recent years, they have been gaining popularity in the form of passenger cars, with the number of sold units surpassing the 100,000 mark in 2012.2 One of the main technological barriers to an even wider adoption of electric cars and a complete displacement of the CO 2 -emitting vehicles is the insufficient storage capacity of the current battery technology, which leads to a limited vehicle driving range.2,3 The useable energy density of state-of-the-art lithium-ion batteries employed in recent electric vehicles amounts to only ≈120 Wh/kg system , 4 which corresponds to a practical driving range of 150-200 km. If one succeeds in the development of sufficiently durable high-voltage or high-energy cathode materials and combines them with silicon anodes, an energy density of ≈250 Wh/kg system would be in reach. 4 An alternative promising battery concept is the lithium-sulfur (Li-S) battery.3,5-7 Current development in Li-S battery research includes successful commercialization of Li-S cells with 260 Wh/kg system energy density by Sion Power corporation.8 This value already exceeds the DOE target for 2022 (250 Wh/kg system ) 9 and has the potential to also comply with the system cost target of 125 USD/kWh 9 because of the abundance and low cost of sulfur. To achieve even greater gains in the performance and, most importantly, to improve the cycle life of Li-S batteries, it will be necessary to gain a deeper mechanistic understanding of the 16 e -/S 8 in...

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

“…Electrochemical measurements.-Electrochemical measurements were performed in an operando cell developed in our laboratory 21 and in a standard T-cell made from Swagelok components. In all studies, the same electrolyte was used.…”

“…21 Li 2 S standard was prepared from a commercially available powder (99.98% trace metal basis, Sigma-Aldrich). To minimize the self-absorption experienced by the standard, the sample was both ball-milled to reduce the particle size of Li 2 S and diluted with boron nitride to 0.5 wt%.…”

“…X-ray absorption spectroscopy (XAS) is an alternative operando characterization technique that is especially suited for the characterization of Li-S batteries, because it is capable of detecting solid S 8 and Li 2 S as well as polysulfides dissolved in the electrolyte. 21,22 To date, operando XAS has only been applied to the study of Li-S batteries assembled in a charged state using S 8 cathodes, 21-25 and has not been used to identify intermediates during the initial charge of Li 2 S. Since the initial charging process differs significantly from all the subsequent charges, XAS characterization of both the starting species and the intermediates associated with these charging processes has an opportunity to facilitate a significantly improved mechanistic understanding of the operation of Li-S batteries.Motivated by advancing the mechanistic understanding of Li-S chemistry, we have performed XAS characterization of Li 2 S-Li battery using a spectro-electrochemical cell designed in our laboratory, 21 which is capable of spatially resolved measurements. In the experiments, we obtain, for the first time, information about both solid and solution phase intermediates produced in two locations of the cell, the Li 2 S cathode and the separator, and thus generate a unique set of spatially resolved data that is able to discriminate between the changes in the concentration of the species in the separator and the changes in the composition of the species in both locations of the cell.…”

“…X-ray absorption spectroscopy (XAS) is an alternative operando characterization technique that is especially suited for the characterization of Li-S batteries, because it is capable of detecting solid S 8 and Li 2 S as well as polysulfides dissolved in the electrolyte. 21,22 To date, operando XAS has only been applied to the study of Li-S batteries assembled in a charged state using S 8 cathodes, [21][22][23][24][25] and has not been used to identify intermediates during the initial charge of Li 2 S. Since the initial charging process differs significantly from all the subsequent charges, XAS characterization of both the starting species and the intermediates associated with these charging processes has an opportunity to facilitate a significantly improved mechanistic understanding of the operation of Li-S batteries.…”

“…Motivated by advancing the mechanistic understanding of Li-S chemistry, we have performed XAS characterization of Li 2 S-Li battery using a spectro-electrochemical cell designed in our laboratory, 21 which is capable of spatially resolved measurements. In the experiments, we obtain, for the first time, information about both solid and solution phase intermediates produced in two locations of the cell, the Li 2 S cathode and the separator, and thus generate a unique set of spatially resolved data that is able to discriminate between the changes in the concentration of the species in the separator and the changes in the composition of the species in both locations of the cell.…”