The lithium-sulfur battery, long thought to be better at energy storage capacity than its more popular lithium-ion counterpart, was hampered by its short cycle life. Currently the lithium-sulfur battery can be recharged 50 to 100 times — impractical as an alternative energy source compared to 1,000 times for many rechargeable batteries on the market today.
The solution devised by Narayan and lead author and research assistant Moy is something they call the “Mixed Conduction Membrane,” or MCM, a small piece of non-porous, fabricated material sandwiched between two layers of porous separators, soaked in electrolytes and placed between the two electrodes.
The membrane works as a barrier in reducing the shuttling of dissolved polysulfides between anode and cathode, a process that increases the kind of cycle strain that has made the use of lithium-sulfur batteries for energy storage a challenge. The MCM still allows for the necessary movement of lithium ions, mimicking the process as it occurs in lithium-ion batteries. This novel membrane solution preserves the high-discharge rate capability and energy density without losing capacity over time.
At various rates of discharge, the researchers found that the lithium-sulfur batteries that made use of MCM led to 100 percent 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,” said Narayan, senior author and professor of chemistry at the USC Dornsife College of Letters, Arts and Sciences. “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.”
The lithium-sulfur battery with a mixed conduction membrane barrier to stop polysulfide shuttling (Illustration/Courtesy of Sri Narayan)
Cheap and abundant building blocks
Lithium-sulfur batteries have a host of advantages over lithium-ion batteries: They are made with abundant and cheap sulfur, and are two to three times denser, which makes them both smaller and better at storing charge.
A lithium-sulfur battery would be ideal for saving space in mobile phones and computers, as well as allowing for weight reduction in future electric vehicles, including cars and even planes, further reducing reliance on fossil fuels, researchers said.
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.
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.
SOURCES – USC, Journal of the Electrochemical society