One of the major issues hobbling the commercialization of high energy-density lithium-sulfur batteries is the “polysulfide shuttle”—the shuttling of polysulfide ions between the cathode and anode. The polysulfide shuttle is a major technical issue that limits the electrical performance and cycle life of this type of battery. Addressing this polysulfide shuttle—which causes self-discharge, low charging efficiencies, and irreversible capacity losses—has been a major focus of research and development.
Now, in an open-access paper published in the January issue of the Journal of the Electrochemical Society, Sri Narayan and Derek Moy of the USC Loker Hydrocarbon Research Institute report a novel approach to the problem. The USC team developed a “mixed conduction membrane” (MCM)—a thin non-porous lithium-ion conducting barrier that simply restricts the soluble polysulfides to the positive electrode.
The MCM is small piece of non-porous, fabricated material sandwiched between two layers of porous separators, soaked in electrolytes and placed between the two electrodes.
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.
At various rates of discharge, the researchers found that the lithium-sulfur batteries that made use of MCM led to 100% capacity retention and had up to four times longer life compared to batteries without the membrane.
This advance removes one of the major technical barriers to the commercialization of the lithium-sulfur battery, allowing us to realize better options for energy efficiency. We can now focus our efforts on improving other parts of lithium-sulfur battery discharge and recharge that hurt the overall life cycle of the battery.—Sri Narayan
The actual MCM layer that Narayan and Moy devised is a thin film of lithiated cobalt oxide, though future alternative materials could produce even better results. According to Narayan and Moy, any substitute material used as an MCM must satisfy some fundamental criteria: The material must be non-porous, it should have mixed conduction properties and it must be electrochemically inert.
|Lithium-sulfur battery with Mixed Conduction Membrane barrier to stop polysulfide shuttling. Credit: Sri Narayan and Derek Moy. Click to enlarge.|
The study was funded by the University of Southern California and the Loker Hydrocarbon Research Institute. The authors presented this paper earlier at the Honolulu, Hawaii, Meeting of the Electrochemical Society in October 2016.
Derek Moy and S. R. Narayanan (2017) “Mixed Conduction Membranes Suppress the Polysulfide Shuttle in Lithium-Sulfur Batteries” J. Electrochem. Soc. volume 164, issue 4, A560-A566 doi: 10.1149/2.0181704jes