The shuttling of polysulfide ions between the electrodes in a lithium-sulfur battery is a major technical issue limiting the self-discharge and cycle life of this high-energy rechargeable battery. Although there have been attempts to suppress the shuttling process, there has not been a direct measurement of the rate of shuttling. We report here a simple and direct measurement of the rate of the shuttling (that we term "shuttle current"), applicable to the study of any type of lithium-sulfur cell. We demonstrate the effectiveness of this measurement technique using cells with and without lithium nitrate (a widely-used shuttle suppressor additive). We present a phenomenological analysis of the shuttling process and simulate the shuttle currents as a function of the state-of-charge of a cell. We also demonstrate how the rate of decay of the shuttle current can be used to predict the capacity fade in a lithium-sulfur cell due to the shuttle process. We expect that this new ability to directly measure shuttle currents will provide greater insight into the performance differences observed with various additives and electrode modifications that are aimed at suppressing the rate of shuttling of polysulfide ions and increasing the cycle life of lithium-sulfur cells. The rechargeable lithium-sulfur battery is of great interest due to its high theoretical specific energy of 2600 Wh/kg and also the relatively low cost of sulfur. However, large-scale deployment of lithium-sulfur batteries has been limited by several performance issues relating to power density and cycle life. [1][2][3][4][5][6][7][8][9] With sulfur at the positive electrode and lithium metal at the negative electrode, the lithium-sulfur battery operates over a voltage range of 1.5 to 2.8 V. The reactions at the positive and negative electrode during charge and discharge are shown schematically in Figure 1. During discharge, sulfur at the positive electrode is reduced progressively to various polysulfides and eventually to the sulfide, while lithium metal at the negative electrode is oxidized to lithium ions. During charge, the lithium ions are reduced to lithium at the negative electrode, and the sulfide is re-oxidized at the positive electrode to the higher-order polysulfides. The organic electrolyte is a 0.5 M solution of lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) in a 1:1 mixture by volume of dioxolane (DOL) and dimethoxyethane (DME). A "solid electrolyte interphase" (SEI) protects the lithium anode from reacting freely with the organic electrolyte. The polysulfides produced during discharge are usually soluble in the battery electrolyte. Summary of technical challenges with lithium-sulfur batteries.-The principal challenge for the realization of a practical lithium-sulfur battery with long cycle life arises from the solubility of the higherorder polysulfides (S 8 2− , S 6 2− , S 4 2− ) in the electrolyte. These polysulfides, generated at the positive electrode during discharge, diffuse to the negative lithium electrode where they are reduced to...

The shuttling of polysulfide ions between the two electrodes of a lithium-sulfur battery is a major technical issue that limits the electrical performance and cycle life of this battery. This "polysulfide shuttle" causes self-discharge, low charging efficiencies, and irreversible capacity losses. Suppressing the polysulfide shuttle will bring us closer to realizing a rechargeable battery that has two to three times the energy density of today's lithium-ion batteries. We demonstrate a novel approach to the problem of the polysulfide shuttle by using a "mixed conduction membrane" (MCM). The MCM is a thin non-porous lithium-ion conducting barrier that simply restricts the soluble polysulfides to the positive electrode. Lithium-ion conduction occurs through the MCM by electrochemical intercalation or insertion reactions and concomitant solid-state diffusion, exactly as in the cathode of a lithium-ion battery. Because of the rapidity of lithium ion transport in the MCM, the internal resistance of the battery is not higher than that of a conventional lithium-sulfur battery. The MCM is as effective as the lithium nitrate additive in suppressing the polysulfide shuttle reactions. However, unlike lithium nitrate, the MCM is not used up during cycling and thus provides extended durability and cycle life. We establish the criteria for the selection of materials for MCMs and demonstrate the effectiveness of this novel MCM layer by proving the suppression of shuttling of polysulfides, demonstration of improved capacity retention during repeated cycling, and by the preservation of rate capability and impedance of the lithium-sulfur battery. Despite many advances in the area of lithium-ion batteries, the demand for more compact, lightweight, and long-life batteries for portable and automotive applications has continued to steadily increase. Thus, the search for battery solutions with increasingly higher specific energy remains a major quest. The rechargeable lithium-sulfur battery is particularly attractive for high-density electrical energy storage because of its high theoretical specific energy of 2600 Wh/kg and the relatively low cost of sulfur. Thus, lithium-sulfur batteries can provide two to three times the energy density of today's rechargeable lithium-ion batteries.1-3 However, the deployment of lithium-sulfur batteries has been limited by their relatively short cycle life.1-9 Practical cells have a cycle life of just 50-100 cycles. One of the major technical issues limiting the cycle life of the lithium-sulfur cell is the shuttling of soluble polysulfides between the two electrodes. In this article we demonstrate a new approach to suppressing the active shuttling of polysulfides in the lithium-sulfur battery by using a novel type of barrier layer. The novelty of this barrier layer lies in its ability to exclude the polysulfide ions while maintaining selectively the facile transport of lithium ions using a mixed conduction mechanism. We show that this type of barrier layer prevents capacity fade caused by the polysulfi...

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The development of a high cycle life rechargeable lithium-sulfur battery is of substantial interest due to its high theoretical specific capacity of 1675 mAh/g and energy density of 2600 Wh/kg. These properties combined with the low cost and abundance of sulfur are attractive for practical and high performance lithium-sulfur batteries.  However, the realization of these lithium-sulfur batteries have been limited by performance issues relating to their poor cycle life and low energy efficiency. The typical lithium-sulfur cell loses 20-50% of its initial capacity within 100 cycles due to a parasitic reaction known as the polysulfide shuttle. The polysulfide shuttle originates from the inherent solubility of the higher order polysulfides, S8 2-, S6 2-, and S4 2-, at the positive electrode. These polysulfides are capable of shuttling between the positive sulfur electrode and negative lithium electrode. When a polysulfide species contacts the negative electrode, it will be reduced to a lower order polysulfide species. If reduced to another soluble species such as S6 2- or S4 2-, it will shuttle back to the positive electrode where it must be reoxidized, resulting in charging inefficiency. If reduced to an insoluble species such as S2 2- or S2-, it will precipitate, resulting in irreversible capacity loss. Therefore, inhibiting the polysulfide shuttle is necessary to improving the energy efficiency and cycle life of lithium-sulfur batteries. We herein present a method of inhibiting the polysulfide shuttle through use of a thin barrier membrane. The thin barrier membrane is non-porous and capable of conducting lithium ions. The non-porosity of the membrane allows for physical exclusion of polysulfide species from the lithium electrode while the ionic conductivity allows for transport of lithium ions necessary for charge and discharge processes. In a cell configuration, the thin barrier membrane is sandwiched between two porous separators to maintain electronic isolation and inserted between the sulfur and lithium electrode. We have studied the effect of the thin barrier membrane in 2032-type coin cells consisting of sulfur-carbon composite positive electrodes, lithium foil negative electrodes, and 1 M LiTFSI electrolyte. The performance of the membrane was compared to cells of similar construction without the membrane and also to cells with 0.25 M lithium nitrate, the state-of-the-art polysulfide shuttle inhibiting additive. We have analyzed the performance of all cells using galvanostatic cycling, electrochemical impedance spectroscopy, and direct shuttle current measurements.1 We have found that the thin barrier membrane improves the energy efficiency and cycle life of lithium-sulfur cells by suppressing the polysulfide shuttle. References: 1. D. Moy, A. Manivannan, and S. R. Narayanan, J. Electrochem. Soc., 2015, 162, A1-A7 Figure 1

Lithium-sulfur chemistry has the potential to overtake Lithium-ion battery technology with its high theoretical specific capacity at 1675 mAh/g and energy density at 2600 Wh/kg. These properties coupled with the low cost and natural abundance of sulfur makes the lithium-sulfur battery attractive as a cost-effective and high energy density battery solution. However, the large scale deployment of these lithium-sulfur batteries have been severely limited by their poor cycle life. The typical lithium-sulfur cell loses over 20% of its initial capacity within its first 100 cycles. One of the major causes of these capacity losses is a parasitic reaction known as the polysulfide shuttle. The polysulfide shuttle is present in all lithium-sulfur cells due to the inherent solubility of the higher order polysulfide discharge products, S8 2-, S6 2-, and S4 2-. These soluble polysulfides constantly shuttle between the sulfur and lithium electrodes. When these species contact the negative lithium electrode, they are reduced to shorter chain polysulfides that must shuttle back towards to sulfur electrode to be re-oxidized. However, some polysulfides such as S4 2- are reduced to insoluble S2 2-on the surface of lithium electrode, resulting in an irreversible capacity loss. Inhibiting the polysulfide shuttle is necessary to protect the lithium electrode as well as to prevent capacity loss. We demonstrate the ability to inhibit the polysulfide shuttle using a lithium-ion selective membrane. The membrane is a thin, flexible, and non-porous barrier layer that restricts all soluble polysulfide species at the sulfur electrode and prevents their diffusion towards the lithium electrode. The membrane is ionically conducting to allow transport of lithium ions necessary for charge and discharge processes. The membrane remains inert to polysulfide species at all operating voltages and presents a barrier to growth of lithium dendrites and catastrophic failure. In a cell configuration, the membrane is electronically isolated from the two electrodes by using porous separator layers placed on either side of the membrane. We tested the lithium-ion selective membrane in 2032-type coin cells using sulfur-carbon composite positive electrodes, lithium foil negative electrodes, and 1 M LiTFSI electrolyte. The performance of cells containing the lithium-ion selective membrane was compared to cells without the membrane as well as to cells with 0.25 M lithium nitrate (the state-of-the-art polysulfide shuttle suppressing additive). We analyzed the performance of these cells using galvanostatic cycling, electrochemical impedance spectroscopy, and direct shuttle current measurements1. We found that the lithium-ion selective membrane improves the cycle life of lithium-sulfur cells through inhibition of the polysulfide shuttle. References 1. D. Moy, A. Manivannan, and S. R. Narayanan, J. Electrochem. Soc., 2015, 162, A1-A7 Figure 1