In late July 2008, a British solar-powered aircraft set an unofficial flight endurance record by flying at high altitude for three consecutive days. Perhaps little known, the lithium-sulfur (Li-S) battery became one of the greatest technological advances in its components, powering the aircraft at night with an efficiency unmatched by even the top batteries of the time.
Ten years later, the world still seems to be waiting for Li-S batteries to arrive commercially. A breakthrough by Drexel University researchers has just removed a huge obstacle to its survival.
Technology companies have learned the hard way that the development of both laptops, cell phones and electric cars depends on steady improvements in battery performance. Technology can only move forward as far as the battery will allow, and lithium-ion batteries - currently considered the best on the market - are reaching the limits of improvement.
As battery performance approaches stability, some companies are trying to cram the last volt into storage devices by squeezing and reducing the size of some internal components that don't contribute to energy storage. These structural changes can have some unfortunate side effects, such as the series of explosions that occurred in Samsung phones in 2016.
A Samsung phone after a battery explosion (image via web)
Researchers and technology companies are studying how lithium-sulfur (Li-S) batteries could eventually replace lithium-ion batteries, as this new chemistry could theoretically pack more energy into a single cell - a measure known as energy density in battery development. This improvement brings about 5-10 times the capacity of a lithium-ion battery, which equates to a longer run time between this charge and the next.
The problem is that after completing the first few charges, Li-S batteries cannot continue to maintain their superior capacity. It turns out that sulfur, a key component in increasing energy density, migrates from the electrodes as a "polysulfide" intermediate, resulting in the loss of this key component and degradation of performance during recharging.
Scientists have been trying for years to stabilize the reactions inside Li-S cells that produce these polysulfides, but most attempts have resulted in other complications, such as adding weight to the cell, requiring expensive materials or adding several complex processing steps.
But now a new approach has been born, as researchers at the Drexels School of Engineering published in a recent issue of the American Chemical Society journal Applied Materials and Interfaces, entitled "Titanium monoxide (TiO) nanofibers as polysulfide immobilizers in Li-S cells ": Evidence of Lewis acid-base interactions suggests that it can immobilize polysulfides in place, maintaining the impressive endurance of such cells while reducing overall weight and the time required to produce them.
We have created freestanding porous titanium monoxide nanofiber mats as the cathode body material in lithium-sulfur batteries, said Dr. Vibha Kalra. He is an assistant professor in the School of Engineering and the lead research author. "This is a significant development because we found that our titanium monoxide - sulfur cathode has high electrical conductivity and is able to bind polysulfides through strong chemical interactions, which means it can enhance the specific capacity of the battery while maintaining its impressive performance. We can also demonstrate that this can completely eliminate the binder and collector on the cathode side, which make up 30-50% of the electrode weight - our method takes only a few seconds to form the sulfur cathode. Current standards can take nearly half a day."
Their findings suggest that the nanofiber mat, which resembles a bird's nest at the microscopic level, is an excellent platform for a sulfur cathode because it attracts and traps the polysulfides produced during battery use. Keeping the polysulfides in the cathode structure prevents shuttling, a degradation of performance that occurs when they dissolve in the electrolyte solution that separates the cathode from the anode in a battery.
According to Kalra, this cathode design not only helps Li-S batteries maintain their energy density, but also achieves this goal without adding additional materials that would lead to higher weight and production costs.
To achieve these dual goals, the team has studied this in depth. This includes the reaction mechanism and the formation of polysulfides to better understand how the electrode body materials can help them.
The study showed that strong Lewis acid-base interactions between titanium monoxide and sulfur in the cathode prevent polysulfide production from entering the electrolyte, a major cause of degraded battery performance, said Dr. Arvinder Singh, one of the paper's authors and a postdoctoral fellow in the Kalras lab.
This means their cathode design can help Li-S batteries maintain their energy density - and do so without the added weight and production costs of additional materials, Kalra.Kalras' previous research on nanofiber electrodes has shown them to be overcurrent battery components with multiple advantages. They have a larger surface area than current electrodes, which means they can accommodate expansion during charging, which can increase the battery's storage capacity. By filling them with electrolyte gel, they can eliminate flammable components from the device, minimizing their susceptibility to leaks, fires and explosions.
They are created through an electrostatic spinning process that looks like making marshmallows, which means they have an advantage over standard powder-based electrodes, which require the use of insulation and binder chemicals that can deteriorate performance during the manufacturing process.
The electrostatic spinning process creates what looks like making marshmallows. (Image from the web)
To produce a binder-free, freestanding cathode platform for improved cell performance, the Kalras lab has developed a rapid sulfur deposition technique that adds sulfur to its substrate in just five seconds.
"The procedure melts sulfur into nanofiber mats in a mildly pressurized environment at 140 degrees Celsius - eliminating the need for time-consuming processing or the use of mixed toxic chemicals - while improving the cathode's ability to remain effective after extended use. Our Li-S electrodes provide the right structure," Kalra said, "to minimize capacity degradation during battery cycling, which is one of the most significant barriers to commercialization of Li-S batteries."
"Our research shows that these electrodes have four times the sustained effective capacity of current Li-ion batteries. And our novel, low-cost method of building sulfide cathodes in seconds removes a major barrier to manufacturing."
Whether it's laptops, cell phones or electric cars, development depends on steady improvements in battery performance. Technology can only move forward as far as the battery will allow, and lithium-ion batteries - currently considered the best on the market - are reaching the limits of improvement. (Image from the web)
Since the record-breaking flight of the Zephyr-6s solar-powered aircraft in 2008, many companies have invested in developing Li-S batteries in hopes of increasing the range of various electric vehicles, making mobile devices last longer between charges, and even helping entire energy networks to adapt to the intermittent nature of wind and solar power. kalras' work now offers a path for this battery technology to break through a series of obstacles to development.
The team will continue to develop its Li-S cathode with the goal of further improving cycle life, reducing polysulfide formation and reducing costs.
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