Researchers in Drexel’s College of Engineering report the ability of stabilized titanium monoxide (TiO) nanoparticles in nanofibers to support high conductivity and to bind polysulfides in Li-S batteries. Their paper is published in the ACS journal Applied Materials & Interfaces.
The developed 3-D TiO/CNF architecture with the inherent inter-fiber macropores of nanofiber mats provides a much higher surface area (~427 m2 g-1) and overcomes the challenges associated with the use of highly dense powdered Ti-based suboxides/monoxide materials, thereby allowing for high active sulfur loading among other benefits.
Synthesis strategy for developing free-standing TiO/CNF nanofiber mats. Singh and Kalra.
The developed TiO/CNF-S cathodes exhibit high initial discharge capacities of ~1080 mAh g-1, ~975 mAh g-1, and ~791 mAh g-1 at 0.1C, 0.2C, and 0.5C rates, respectively with long term cycling.
Furthermore, the free-standing TiO/CNF-S cathodes developed with rapid sulfur melt infiltration (~5 sec) eradicate the need of inactive elements such as binders, additional current collectors (Al-foil) and additives.
We have created freestanding porous titanium monoxide nanofiber mat as a cathode host material in lithium-sulfur batteries. This is a significant development because we have found that our titanium monoxide-sulfur cathode is both highly conductive and able to bind polysulfides via strong chemical interactions, which means it can augment the battery’s specific capacity while preserving its impressive performance through hundreds of cycles. We can also demonstrate the complete elimination of binders and current collector on the cathode side that account for 30-50 percent of the electrode weight—and our method takes just seconds to create the sulfur cathode, when the current standard can take nearly half a day.—Vibha Kalra, PhD, an associate professor in the College of Engineering who led the research
The findings suggest that the nanofiber mat, which at the microscopic level resembles a bird’s nest, is an excellent platform for the sulfur cathode because it attracts and traps the polysulfides that arise when the battery is being used. Keeping the polysulfides in the cathode structure prevents “shuttling,” a performance-sapping phenomenon that occurs when they dissolve in the electrolyte solution that separates cathode from anode in a battery. This cathode design can not only help Li-S battery maintain its energy density, but also do it without additional materials that increase weight and cost of production, according to Kalra.
This research shows that the presence of a strong Lewis acid-base interaction between the titanium monoxide and sulfur in the cathode prevents polysulfides from making their way into the electrolyte, which is the primary cause of the battery’s diminished performance.—Arvinder Singh, PhD, a postdoctoral researcher in Kalra’s lab and co-author
Kalra’s previous work with nanofiber electrodes has shown that they provide a variety of advantages over current battery components. They have a greater surface area than current electrodes, which means they can accommodate expansion during charging, which can boost the storage capacity of the battery. By filling them with an electrolyte gel, they can eliminate flammable components from devices minimizing their susceptibility to leaks, fires and explosions. They are created through an electrospinning process; this means they have an advantage over the standard powder-based electrodes which require the use of insulating and performance deteriorating “binder” chemicals in their production.
In tandem with its work to produce binder-free, freestanding cathode platforms to improve the performance of batteries, Kalra’s lab developed a rapid sulfur deposition technique that takes just five seconds to get the sulfur into its substrate.
The procedure melts sulfur into the nanofiber mats in a slightly pressurized, 140-degree Celsius environment, eliminating the need for time-consuming processing that uses a mix of toxic chemicals, while improving the cathode’s ability to hold a charge after long periods of use.
Our Li-S electrodes provide the right architecture and chemistry to minimize capacity fade during battery cycling, a key impediment in commercialization of Li-S batteries. Our research shows that these electrodes exhibit a sustained effective capacity that is four-times higher than the current Li-ion batteries. And our novel, low-cost method for sulfurizing the cathode in just seconds removes a significant impediment for manufacturing.—Vibha Kalra
The group will continue to develop its Li-S cathodes with the goals of further improving cycle life, reducing the formation of polysulfides and decreasing cost.
This research was supported by Drexel Ventures Innovation Fund and National Science Foundation (CBET-1150528).
Arvinder Singh and Vibha Kalra (2018) “TiO Phase Stabilized into Free-Standing Nanofibers as Strong Polysulfide Immobilizer in Li-S Batteries: Evidence for Lewis Acid-Base Interactions” ACS Applied Materials & Interfaces doi: 10.1021/acsami.8b11029