Mechanistic insights into operational lithium–sulfur batteries by in situ X-ray diffraction and absorption spectroscopy

100

156

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...

mentioning

confidence: 99%

Operando Characterization of Intermediates Produced in a Lithium-Sulfur Battery

Gorlin

Siebel

Piana

et al. 2015

100

156

“…Different groups [13][14][15] showed that operando XRD can be used as an excellent tool to characterize the formation and consumption of a crystalline active species for instance Li 2 S and different allotropes of sulfur. Early studies were not concentrated on the detection of polysulfides, since they were considered as amorphous species detected in the background of the XRD spectra.…”

mentioning

confidence: 99%

Polysulfides Formation in Different Electrolytes from the Perspective of X-ray Absorption Spectroscopy

Dominko

Vižintin

Aquilanti

et al. 2017

Li-S batteries are promising energy storage technology for the future, however there two major problems remained which need to be solved before successful commercialization. Capacity fading due to polysulfide shuttle and corrosion of lithium metal are directly connected with the type and quantity of electrolyte used in the cells. Several recent works show dependence of the electrochemical behavior of Li-S batteries on type of the electrolyte. In this work we compare and discuss a discharge mechanism of sulfur conversion in three different electrolytes based on measurements with sulfur K-edge XAS. The sulfur conversion mechanism in the ether based electrolytes, the most studied type of solvents in the Li-S batteries, which are enabling high solubility of polysulfides are compared with the fluorinated ether based electrolytes with a reduced polysulfide solubility and in carbonate based electrolytes with the sulfur confined into a ultramicroporous carbon. In all three cases the sulfur reduction proceeds through polysulfide intermediate phases with a difference on the type polysulfides detected at different steps of discharge. Electrification of road transport has increased the pressure on materials' scientists to improve performance of the current Li-ion batteries and to develop new advanced high energy battery technologies. 1Among several post lithium-ion technologies, the lithium sulfur (Li-S) batteries are recognized as the most promising for commercialization in the near future. A combination of lithium and sulfur in the electrochemical cell corresponds to the theoretical energy density of 2600 Wh/kg, while the maximum practically accessible energy density is predicted to be close to 600 Wh/kg.2,3 This is much higher compared to Li-ion batteries and besides that, sulfur is inexpensive and naturally abundant. Nevertheless, problems related to the solubility of polysulfides in the electrolyte and the related redox shuttle phenomena cause short cycle life, potential safety problems, poor cycling efficiency, and relative fast self-discharge. Additional problems of Li-S batteries are very low electronic conductivity of the both end members in the discharge and charge process (i.e. sulfur and Li 2 S) and extensive corrosion of the metal lithium anode.Different directions how to improve Li-S battery cycle life have been explored, like synthesis of the optimized porous host matrices with active sites for polysulfide anchoring, 4,5 design and optimization of separators which can effectively suppress polysulfide cross communication between electrodes 6,7 and protection of lithium by artificial SEI 8 or by additives. 9 While most of the research was performed in the binary mixture of solvents using alkyl ethers (glymes) and heterocyclic acetyl (dioxolane), less attention has been paid to the development of new electrolytes for Li-S batteries. 10 Reasons for that are nested in the requirements which should be fulfilled in the development of new formulations. First of all, electrolytes used in the electrochemical cells must...

“…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.…”

mentioning

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...