Recent studies pay off with major improvements in charging times and conductivity in solid-state lithium-sulfur battery technology.

A flurry of scientific advances from Australia to California in recent months could soon propel lithium-sulfur (Li-S) batteries into direct competition with their lithium-ion cousins.

Researchers at the University of Adelaide, Tianjin University in China, and the Australian Synchrotron recently reported results of a study that suggests the next generation of lithium-sulfur batteries may be capable of being charged in less than five minutes instead of several hours, as is currently the case.

Scientists in the United States, meanwhile, report developing a new material for Li-S batteries that is both healable and highly conductive.

High-power rechargeable lithium-sulfur batteries are considered a promising alternative to lithium-ion batteries for powering various devices such as smartphones, laptops, and electric vehicles.

One of the reasons for this is sulfur is abundant on Earth, which could lead to cheaper and more environmentally friendly batteries with potentially higher energy densities than their lithium-ion battery counterparts.

Solid-state lithium-sulfur batteries – consisting of a solid electrolyte, an anode made of lithium metal, and a cathode made of sulfur – have the potential to store up to twice as much energy per kilogram as conventional lithium-ion batteries.

Researchers say they have the potential to double the range of EVs without increasing the battery pack's weight. Additionally, the use of abundant, easily sourced materials makes them an economically viable and environmentally friendlier choice.

Despite the potential advantages, deployment of Li-S batteries has been limited because many of them also have a low cycle life and a high self-discharge rate. Also, the predicted high energy density of Li-S batteries often becomes far lower when in real-world applications due to the high rates at which the devices charge and discharge.

A chemical reaction that plays a vital role in ensuring the high capacity of Li-S batteries is the so-called "sulfur reduction reaction," or SRR.

Though widely studied, the sulfur reduction reaction and its kinetic tendencies at high current rates remain poorly understood.

New research holds promise

Based on the kinetic trend, a team of researchers led by University of Adelaide Professor Shizhang Qiao examined the sulfur reduction reaction that is pivotal to the charge-discharge rate of lithium-sulfur batteries.

University of Adelaide

Professor Shizhang Qiao, director of the Centre for Materials in Energy and Catalysis at the University of Adelaide.

The study is the first comprehensive approach to tackling the problem of slow charge/discharge rates in lithium-sulfur batteries, the scientists said.

Results published in the Feb. 16 edition of "Nature Nanotechnology" introduced a nanocomposite carbon electrocatalyst that was found to boost the performance of Li-S batteries, attaining a discharge capacity retention of about 75%.

"The activity of electrocatalysts for the sulfur reduction reaction can be represented using volcano plots, which describe specific thermodynamic trends," the team wrote in their research paper. "However, a kinetic trend that describes the SRR at high current rates is not yet available, limiting our understanding of kinetic variations and hindering the development of high-power Li-S batteries. Using Le Chatelier's principle as a guideline, we establish an SRR kinetic trend that correlates polysulfide concentrations with kinetic currents."

The team also collected synchrotron X-ray adsorption spectroscopy measurements and ran various molecular orbital computations. Overall, their results suggest that orbital occupancy in catalysts based on transition metals is linked to a concentration of polysulfide in batteries and, consequently, SRR kinetic predictions.

"Reaction rates increased with higher polysulfide concentrations, as polysulfide serves as the reactive intermediates during SRR," Qiao observed.

The team investigated various carbon-based transition metal electrocatalysts, including iron, cobalt, nickel, copper, and zinc during the SRR.

The scientists designed a new nanocomposite electrocatalyst comprised of a carbon-based material and cobalt-zinc clusters. They then integrated the catalyst into a Li-S battery cell and tested its performance, focusing on its charge-discharge rates.

"Our research shows a significant advancement, enabling lithium-sulfur batteries to achieve full charge/discharge in less than five minutes," said Qiao.

With another electrocatalyst found to enhance the capacity retention and cyclic stability of Li-S batteries, the researchers believe their work could inspire the design of other promising catalysts, potentially contributing to the development of new high-power Li-S battery technologies.

New cathode material holds promise

Meanwhile, a team of U. S. researchers, led by engineers at the University of California San Diego's Sustainable Power and Energy Center, have developed a new cathode material for solid-state lithium-sulfur batteries that is electrically conductive and structurally healable, features they say overcome the limitations of current cathodes.

The researchers reported their work in the March 6 edition of Nature in a research paper titled "Healable and Conductive Sulfur Iodide for Solid-State Li-S Batteries."

Historically, the development of lithium-sulfur solid-state batteries has been plagued by the inherent characteristics of sulfur cathodes.

Not only is sulfur a poor electron conductor, but sulfur cathodes also experience significant expansion and contraction during charging and discharging, leading to structural damage and decreased contact with the solid electrolyte.

These issues collectively diminish the cathode's ability to transfer charge, which compromises the overall performance and longevity of solid-state batteries.

To overcome these challenges, the researchers developed a new cathode crystal composed of sulfur and iodine. By inserting iodine molecules into the crystalline sulfur structure, the researchers increased the cathode material's electrical conductivity by 11 orders of magnitude, making it 100 billion times more conductive than crystals made of sulfur alone.

"We are very excited about the discovery of this new material," said study co-senior author Ping Liu, a professor of nanoengineering and director of the Sustainable Power and Energy Center at UC San Diego. "The drastic increase in electrical conductivity in sulfur is a surprise and scientifically very interesting."

Moreover, the new crystal material possesses a low melting point of 65 degrees Celsius (149 degrees Fahrenheit), which is lower than the temperature of a hot mug of coffee.

This means that the cathode can easily be re-melted after the battery is charged to repair damaged interfaces from cycling. This is an important feature because it addresses the cumulative damage that occurs at the solid-solid interface between the cathode and electrolyte during repeated charging and discharging.

"This sulfur-iodide cathode presents a unique concept for managing some of the main impediments to commercialization of Li-S batteries," said study co-senior author Shyue Ping Ong, a professor of nanoengineering at the UC San Diego Jacobs School of Engineering.

"Iodine disrupts the intermolecular bonds holding sulfur molecules together by just the right amount to lower its melting point to the Goldilocks zone – above room temperature yet low enough for the cathode to be periodically re-healed via melting, he explained."

Study co-first author Jianbin Zhou, a former nanoengineering postdoctoral researcher from Liu's research group, said the low melting point of the new cathode material makes repairing the interfaces possible, which has been a long sought-after solution for the batteries.

"This new material is an enabling solution for future high-energy density solid-state batteries," Zhou added.

David Baillot; UC San Diego Jacobs School of Engineering

This cathode material heals by melting from a brown powder to a deep purple-red liquid at only 149 degrees Fahrenheit.

The cathode material heals by melting from a brown powder to a deep purple-red liquid. To validate the effectiveness of the new cathode material, the researchers constructed a test battery and subjected it to repeated charge and discharge cycles. The battery remained stable for over 400 cycles while retaining 87% of its capacity.

"This discovery has the potential to solve one of the biggest challenges to the introduction of solid-state lithium-sulfur batteries by dramatically increasing the useful life of a battery," said study co-author Christopher Brooks, chief scientist at Honda Research Institute USA, Inc. "The ability for a battery to self-heal simply by raising the temperature could significantly extend the total battery life cycle, creating a potential pathway toward real-world application of solid-state batteries."

The team is working to further advance the solid-state lithium-sulfur battery technology by improving cell engineering designs and scaling up the cell format.

"While much remains to be done to deliver a viable solid-state battery, our work is a significant step," added Liu.