Parallel lithium-sulfur battery advances

Two research teams, one in the United States and the other in Australia – working on separate projects to improve lithium batteries – have reported significant breakthroughs recently that could make lithium-sulfur batteries a viable option.

As interest grows in harnessing green, emissions-free electricity to power everything from personal devices like smartphones and laptops to electric vehicles for consumers and industry, lithium batteries are becoming an important storage method for this new energy source.

With global sales of EVs more than doubling in 2021, prices of battery materials like lithium, nickel, manganese, and cobalt surged and supply chains for these raw materials, most of which are sourced from other countries, became bottlenecked due to the pandemic. The battery materials supply chain problem also focused attention on the primary providers of the raw materials – countries like Congo and China – and raised questions about the human and environmental impact of extracting them from the earth.

Including sulfur in batteries could expand their capacity, extend their lifespan, and provide a sustainable alternative to current cathode materials like cobalt, nickel, and manganese.

Well before the EV surge and battery material shortage, developing a commercially viable sulfur battery has been an industry goal because of sulfur's natural abundance and chemical structure, which enables it to store more energy.

Lithium-sulfur batteries offer the potential for significant energy density storage gains over the previous generation lithium-ion battery in concert with the use of sustainable electrode materials.

Li–S batteries, however, tend to lose their capacity very quickly because of a detrimental chemical reaction that occurs between cathode and electrolyte as batteries cycle. The reaction spawns polysulfides, compounds that interfere with a battery's anode and quickly cause the battery to shut down.

Researchers say the Li-S technology's cycling stability and slow-charging kinetics need significant improvement before it can be used in practical settings. Improving the charging and discharging functions within the battery cell is critical for promoting lithium transport while simultaneously retarding the movement of polysulfides, which limit a battery's capacity and stability.

To solve these problems, most researchers have focused on working with different electrolytes better able to cycle with sulfur or changes to the separator film keeping the two components apart.

Li-S battery breakthrough

A recent breakthrough by researchers in Drexel University's College of Engineering in Pennsylvania, published Feb. 10 in the journal "Communications Chemistry," could provide a way to sidestep the obstacles that have subdued Li-S batteries in the past, finally pulling the sought-after technology within commercial reach.

The U.S. team has developed a lithium-sulfur battery using a commercially available carbonate electrolyte, which retained more than 80% of its initial capacity after 4,000 cycles during testing. The group used a vapor deposition process which unexpectedly produced a form of sulfur that did not react with the electrolyte, overcoming one of the key challenges for this battery chemistry.

The scientists looked at modifying the sulfur-containing cathode to work better with commercially available carbonate electrolytes.

"Having a cathode that works with the carbonate electrolyte that they're already using is the path of least resistance for commercial manufacturers," said Vibha Kalra, the Drexel professor who led the research. "So rather than pushing for the industry adoption of a new electrolyte, our goal was to make a cathode that could work in the pre-existing Li-ion electrolyte system."

Initially, the researchers embedded sulfur in a carbon nanofiber mesh designed to mitigate the polysulfide reaction. This approach proved unsuccessful, but the team's cathode performed far better than expected in testing.

After further investigation, the team learned that during deposition, the sulfur in the experiment had crystallized in an unexpected way, forming a variation on the element known as "monoclinic gamma-phase sulfur."

This chemical phase of sulfur, which is not reactive with the carbonate electrolyte, had previously only been created at hot temperatures in labs and has only been observed in nature in the extreme environment of oil wells, the researchers said.

"At first, it was hard to believe that this is what we were detecting, because in all previous research monoclinic sulfur has been unstable under 95 degrees Celsius," said Rahul Pai, a doctoral student in Drexel's Department of Chemical and Biological Engineering and co-author of the research. "In the last century there have only been a handful of studies that produced monoclinic gamma sulfur, and it has only been stable for 20-30 minutes at most. But we had created it in a cathode that was undergoing thousands of charge-discharge cycles without diminished performance – and a year later, our examination of it shows that the chemical phase has remained the same."

After more than a year of testing, the sulfur cathode remains stable, and its performance has not degraded in 4,000 charge-discharge cycles, which is equivalent to 10 years of regular use. And, as predicted, the battery's capacity is more than three-fold that of a Li-ion battery, the researchers added.

Progress in Australia

In January, scientists working at Monash University in Melbourne, Australia, unveiled a new lithium-sulfur battery interlayer that improved the performance of the technology through 2,000 charging cycles while promoting fast lithium transfer.

Reported Jan. 14 in Issue 2, 2022 of the Journal of Materials Chemistry A, the breakthrough brings battery technology closer to sustainability.

The new interlayer sits in the middle of the battery and keeps the electrodes apart, helping lithium get from one side of the battery to the other faster, according to Australian Research Council Future Fellow Professor Matthew Hill.

"The new interlayer overcomes the slower charge and discharge rates of previous generation lithium-sulfur batteries," Hill told reporters.

Lithium batteries with the sulfur interlayer can store two to five times the energy by weight as the previous generation of batteries, delivering high capacity and longer life, according to the Monash team.

The new interlayer stops polysulfides, a detrimental chemical that forms inside this type of battery, from moving across the battery. "Polysulfides caused previous batteries to deteriorate rapidly and break down," said Ph. D. candidate Ehsan Ghasemiestabanati, who also worked on the project.

"It means the battery can be charged and discharged as many as 2,000 times without failing," Ghasemiestabanati added.

Moreover, lithium-sulfur batteries do not rely on metals like cobalt, nickel, and manganese – critical minerals which are used in lithium-ion batteries and are dwindling in supply globally.

Sulfur, an abundant element, is considered a waste or by-product at many mining operations around the world.

"These batteries are not dependent on minerals that are going to lack supply as the electrification revolution proceeds," Hill said. "This is another step towards cheaper, cleaner and higher-performing batteries that could be made within Australia."