Extending Cycle Life of Lithium-Sulfur Cells Using a Flexible and Selective Lithium-Ion Conducting Membrane

Abstract

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.