Self-Healing Cathode Material Could Make Lithium-Sulfur Batteries a Reality

The development of lithium-sulfur batteries has been plagued by issues, largely due to some unfavorable characteristics at play in their cathodes. But now, thanks to new research led by a team at the UC San Diego Sustainable Power and Energy Center, scientists are moving one step closer to making commercial lithium-sulfur batteries a reality.

Their new cathode material, as reported in Nature, can heal itself when it suffers damage during operation. It is also highly conductive and can remain stable within a battery for over 400 cycles.

The problem with lithium-sulfur batteries

Lithium-ion batteries are the most common battery type used in commercial products today. But that ubiquity doesn’t mean that they’re perfect; researchers are continuously investigating the potential of new battery chemistries to deliver improved capacities and performance.

Solid-state lithium-sulfur (Li-S) batteries are one of these alternative battery types. They are rechargeable, consisting of a solid electrolyte plus an anode made of lithium metal and a cathode made of sulfur.

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The theoretical potential of these batteries is huge, as they promise superior energy density and lower costs than lithium-ion batteries. Li-S batteries have the potential to store up to twice as much energy per kilogram as equivalent lithium-ion batteries – meaning they could potentially double the range of electric vehicles, with no additional weight.

So, why is the world not already running on these batteries?

Sulfur cathodes are tricky to work with. Sulfur is generally a poor electron conductor, which makes it a less-than-ideal material for battery design. But even more problematically, sulfur cathodes experience a significant amount of swelling and contraction during a battery’s charge and discharge cycles. This change in size can cause significant structural damage inside the battery and reduce the area of the cathode that is held in contact with the solid electrolyte. In turn, this can greatly affect the cathode’s ability to transfer charge, leading to large fall-offs in the battery’s performance and longevity.

Building a better cathode

In pursuit of making Li-S batteries a viable alternative to lithium-ion, the research team set out to create a new type of cathode material that would overcome these flaws. Their result is a crystalline sulfur cathode, with iodine molecules inserted into the crystal structure.

Performance testing found that the addition of these iodine molecules enhanced the cathode’s electrical conductivity by around 11 orders of magnitude, making it 100 billion times more conductive than pure sulfur crystals.

“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.”

The cathode’s conductivity isn’t the only significant advance. Melting point tests found that the sulfur iodide cathode has a relatively low melting point of just 65 degrees Celcius (149 degrees Fahrenheit), which is lower than a hot coffee mug.

“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,” explained study co-senior author Shyue Ping Ong, a professor of nanoengineering at the UC San Diego Jacobs School of Engineering.

“The low melting point of our new cathode material makes repairing the interfaces possible, a long sought-after solution for these batteries,” added study co-first author Jianbin Zhou, a former nanoengineering postdoctoral researcher from Liu’s research group. “This new material is an enabling solution for future high energy density solid-state batteries".

Could lithium-sulfur iodide batteries be the way forward?

The research team tested out the application of their new cathode material, incorporating it into a test battery that was subject to repeated charging and discharging.

With the sulfur iodide cathode in place, the test battery managed to last for more than 400 cycles while retaining around 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 upfront in acknowledging that there is still a significant amount of work to do before a battery like this is viable for commercial use. However, the creation of this vastly improved cathode material is a promising step forward.

Following on from their new cathode creation, the team says that their efforts will be focused on advancing solid-state lithium-sulfur battery technology by improving cell engineering designs and scaling up the cell format.

The researchers acknowledge funding from the U.S. Department of Energy (DOE) Advanced Research Projects Agency-Energy (DE-AR0000781) and the U.S. DOE Office of Science (DEAC02-05-CH11231).

Reference: Zhou J, Holekevi Chandrappa ML, Tan S, et al. Healable and conductive sulfur iodide for solid-state Li–S batteries. Nature. 2024:1-5. doi:10.1038/s41586-024-07101-z

This article is a rework of a press release issued by the University of California San Diego. Material has been edited for length and content.