This illustration shows the hot-press procedure, developed at Drexel, for producing the novel cathode for Li-S batteries, which involves melting sulfur into the nanofiber mats in a slightly pressurized, 140°C environment. Image: Drexel University.
This illustration shows the hot-press procedure, developed at Drexel, for producing the novel cathode for Li-S batteries, which involves melting sulfur into the nanofiber mats in a slightly pressurized, 140°C environment. Image: Drexel University.

In late July 2008, a British solar plane set an unofficial flight-endurance record by remaining aloft for more than three days straight. Lithium-sulfur (Li-S) batteries emerged as one of the great technological advances that permitted the flight – powering the plane overnight with efficiency unmatched by the top batteries of the day.

Ten years later, the world is still awaiting the commercial arrival of Li-S batteries. But a breakthrough by researchers at Drexel University has just removed a significant barrier that has been blocking their viability.

Technology companies have known for some time that the evolution of their products, whether laptops, cell phones or electric cars, depends on the steady improvement of batteries. Technology is only ‘mobile’ for as long as the battery allows it to be, and lithium-ion batteries (Li-ion) – considered the best on the market – are reaching their limit for improvement.

With battery performance approaching a plateau, companies are trying to squeeze every last volt into, and out of, the storage devices by reducing the size of some of their internal components that do not contribute to energy storage. Some unfortunate side-effects of these structural changes are the malfunctions and meltdowns that occurred in a number of Samsung tablets in 2016.

Researchers and the technology industry are looking at Li-S batteries to eventually replace Li-ion batteries, because this new chemistry theoretically allows more energy to be packed into a single battery – a measure called ‘energy density’ in battery research and development. This improved capacity – between five to 10 times that of Li-ion batteries – equates to a longer run time for batteries between charges.

The problem is that Li-S batteries haven't been able to maintain their superior capacity after the first few recharges. It turns out that the sulfur, which is the key ingredient for improved energy density, migrates away from the electrode in the form of intermediate products called polysulfides, leading to the loss of this key ingredient and performance fade during recharges.

For years, scientists have been trying to stabilize the reaction inside an Li-S battery to physically contain these polysulfides, but most attempts have created other complications, such as adding weight or expensive materials to the battery or requiring several complicated processing steps.

But a new approach, reported by researchers in Drexel's College of Engineering in a paper in Applied Materials and Interfaces, is able to hold polysulfides in place, maintaining the Li-S battery's impressive stamina, while reducing its overall weight and the time required to produce it.

"We have created freestanding porous titanium monoxide nanofiber mat as a cathode host material in lithium-sulfur batteries," said Vibha Kalra, an assistant professor in Drexel’s College of Engineering and lead author of the research. "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% 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."

Their 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 the polysulfides dissolve in the electrolyte solution that separates the cathode from the anode. This cathode design can not only help a Li-S battery to maintain its energy density, but also do it without additional materials that increase weight and cost of production, says Kalra.

To achieve these dual goals, the group closely studied the formation and reaction mechanisms of polysulfides to better understand how an electrode host material could help contain them.

"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," explained Arvinder Singh, a postdoctoral researcher in Kalra's lab who was an author of the paper.

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 and so can accommodate expansion during charging, which can boost the storage capacity of the battery. They can be filled with an electrolyte gel, which eliminates flammable components from devices and minimizes their susceptibility to leaks, fires and explosions. They are also created through an electrospinning process, similar to making cotton candy. This gives them an advantage over 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 procedure that takes just five seconds to get the sulfur into its substrate. The procedure works by melting sulfur into the nanofiber mats in a slightly pressurized, 140°C environment. This eliminates 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," Kalra said. "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."

Since Zephyr-6's record-setting flight in 2008, many companies have invested in the development of Li-S batteries in hopes of increasing the range of electric cars, making mobile devices last longer between charges, and even helping the energy grid accommodate wind and solar power sources. Kalra's work now provides a path for this battery technology to move past a number of impediments that have slowed its progress.

The group will continue to develop its Li-S cathodes with the goals of further improving cycle life, further reducing the formation of polysulfides and decreasing cost.

This story is adapted from material from Drexel University, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.