Journal of The Electrochemical Society, 161 (9) A1399-A1406 (2014) A1399 0013-4651/2014/161(9)/A1399/8/$31.00 © The Electrochemical Society On the Electrode Potentials in Lithium-Sulfur Batteries and Their Solvent-Dependence Holger Schneider, a, z Caroline Gollub, a Thomas Weiß, a Joern Kulisch, a Klaus Leitner, a Ruediger Schmidt, a Marina M. Safont-Sempere, a Yuriy Mikhaylik, b, * Tracy Kelley, b Chariclea Scordilis-Kelley, b, * Mike Laramie, b and Hui Du b a BASF SE, Ludwigshafen 67056, Germany b Sion Power Corp., Tucson, Arizona 85756, USA The influence of the electrolyte solvents on the cell voltage in lithium-sulfur (Li-S) batteries is investigated. It is found that changing the solvent does not only alter the reaction mechanisms taking place during charge and discharge, but also exerts a pronounced influence on the cell voltage. The changes monitored upon switching from standard ether-based electrolytes to more polar solvents are quite substantial. An increase in the open circuit voltage of up to ∼400 mV could be observed. Both experimental evidence and theoretical calculations are presented in order to elucidate and quantify these effects. It is demonstrated that both the observed trends and the order of magnitude of the measured values can be explained by the free solvation energies of the respective ionic species in the electrolyte systems. Among them, the lithium cation contributes most to the phenomena described. Given that the final reaction products are solid and precipitate from the solution, these effects cannot be exploited to increase the overall energy densities of standard Li-S batteries. However, they are still important both with respect to the fundamental understanding of the electrochemical processes involved as well as practical applications such as liquid, polysulfide-based redox flow batteries. © 2014 The Electrochemical Society. [DOI: 10.1149/2.0991409jes] All rights reserved. Manuscript submitted March 24, 2014; revised manuscript received May 30, 2014. Published June 20, 2014. Li-S cells are among the most promising next-generation post-Li- ion battery systems, due to their high specific charge and discharge capacities and energy densities (theoretically 1675 mAh/g and 3518 Wh/kg, respectively, based on the sulfur active material). 1–5 However, their practical breakthrough is hampered by several challenges such as loss of active material and growth of dendritic structures on the metallic lithium anode as well as continuous decomposition of the electrolyte. 2,6,7 In spite of many elegant experimental approaches and impressive progress which could be achieved with respect to cycling stability and cell performance ( 2,8–21 , to name just a few examples), none of the systems presented was able to address all of the problems mentioned above. Elucidating the electrochemical reactions taking place and their subtle interplay in this battery type is therefore of crucial importance for the development of tailor-made solutions for the above-mentioned issues and the improvement of the cell performance. The electrochem- ical charge and discharge reactions of the sulfur active material itself are key issues for these considerations. In addition to the numerous experimental challenges involved in the technical realization of this battery system, one principal disad- vantage is the substantially lower discharge voltage, which in average amounts to approximately 2.1 V and therefore only to roughly 50–60% of the value of current state-of-the-art Li-ion batteries. 2,22 Achieving higher voltages, however, is an advantage not only because of the simultaneous gain in energy density but also because in many cases technical applications require a certain overall voltage of a battery. Increasing the voltage of individual cells means that fewer cells are necessary to be incorporated into a battery pack and therefore less packaging and support material is necessary, which in turn means another gain in energy density for the overall system. Parameters critically important for achieving satisfactory cycle lives in Li-S batteries include the sulfur content, electrode designs as well as the electrolytes and contents used. 23–27 State-of-the art electrolytes for Li-S batteries are mostly based on mixtures of dioxolane with dimethoxyethane or other ethers, as they are both chemically stable against attack of the highly nucleophilic polysulfide intermediates formed in the course of discharging the bat- tery and reasonably stable against the metallic lithium usually used as anode in this battery system. 3,28–30 Moreover, they show a high solubility for the intermediate polysulfide species while maintaining a low enough viscosity to support the electrochemical reactions taking ∗ Electrochemical Society Active Member. z E-mail: holger.schneider@basf.com place at the electrodes. Additives such as nitrate salts provide a pro- tective layer on the metallic lithium anode suppressing the otherwise detrimental polysulfide shuttle. 31,32 This shuttle mechanism roots in the fact that the polysulfide intermediates formed electrochemically can diffuse in the electrolyte back and forth between the two elec- trodes and undergo electrochemical reactions. This leads to a very low coulombic efficiency of this battery type and a quick loss of active material. In ether-based electrolytes, typically two discharge plateaus are observed upon cycling a Li-S cell: One at ∼2.3–2.4 V, corresponding to the formation and reduction of long-chain polysulfides down to medium chain lengths (Li 2 S 4 ), contributing overall about 25% to the overall discharge capacity and another one at lower voltages around 2.1 V, accounting for the remaining 75%. 28,33,34 It is well-known that the sulfur chemistry depends heavily on the chemical environment, which in this battery system predominantly means the electrolyte. 7,35–41 Pioneering theoretical and experimental contributions based on different techniques helped to shed light on the complicated reac- tion chemistry and mechanisms taking place within an Li-S cell upon electrochemical cycling and the formation of the respective intermediates. 42–47 However, not only the reaction pathways at play might change together with the solvent and electrolyte system under consideration, but also the chemical state of the reaction products, namely solvated lithium ions and polysulfide species. This solution-based reaction chemistry is one important difference between Li-S and standard Li- ion battery systems, in which intercalation electrodes play the role of the hosts both at the anode and cathode. Any influence exerted on the chemical state and environment and therefore the chemical potential of the reactive intermediates and final products of an electrochemical reaction within a cell must be reflected in the cell potential during charge and discharge. Therefore, it can be expected that changes in the solvation environment of both the anionic and cationic species during charge and discharge and their free solvation enthalpy are observable in the external cell potential. In Li-ion batteries, the ions intermediately formed must be de-solvated again during the same charge or discharge process in order to enter the respective counter electrode and host material, thereby canceling any solvation effect and in turn any effect on the cell potential. Measuring the external cell voltage, the above-mentioned solvation effects can be clearly demonstrated in the Li-S battery system by a proper choice of the solvents and electrolyte systems. The differences in the open circuit voltage in lithium/lithium cells, the electrodes of which being immersed in different electrolyte systems illustrate the ) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 64.128.200.206 Downloaded on 2014-10-27 to IP