This new cathode material for lithium-sulfur materials is structurally healable and highly conductive. (Image: David Baillot/UC San Diego Jacobs School of Engineering)

A team led by engineers at the University of California San Diego has developed a new cathode material for solid-state lithium-sulfur batteries that is electrically conductive and structurally healable — features that overcome the limitations of these batteries’ current cathodes.

Solid-state lithium-sulfur batteries are a type of rechargeable battery consisting of a solid electrolyte, an anode made of lithium metal, and a cathode made of sulfur. These batteries hold promise as a superior alternative to current Li-ion batteries as they offer increased energy density and lower costs. They have the potential to store up to twice as much energy per kilogram as conventional Li-ion batteries. Additionally, the use of abundant, easily sourced materials makes them an economically viable and environmentally friendlier choice.

However, the development of lithium-sulfur solid-state batteries has been historically 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, compromising the overall performance and longevity of the solid-state battery.

(Image: David Baillot/UC San Diego Jacobs School of Engineering)

To overcome these challenges, the team developed a new cathode material: a crystal composed of sulfur and iodine. By inserting iodine molecules into the crystalline sulfur structure, the researchers drastically 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 Professor and Co-Senior Author Ping Liu. “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 °C (149 °F), which is lower than the temperature of a hot mug of coffee. This means that the cathode can be easily re-melted after the battery is charged to repair the damaged interfaces from cycling. This is an important feature to address 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 Professor and Co-Senior Author Shyue Ping Ong. “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.”

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 percent of its capacity.

Jianbin Zhou prepares a test battery. (Image: David Baillot/UC San Diego Jacobs School of Engineering)

“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 Co-Author Christopher Brooks. “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.”

Here is an exclusive Tech Briefs interview — edited for length and clarity — with Liu.

Tech Briefs: I’m sure there were too many to count, but what was the biggest technical challenge you faced while developing this new cathode material?

Ping: There are two main difficulties. One is to resolve its structure. It is not a traditional inorganic crystalline material. An x-ray diffraction (XRD) examination did not reveal anything other than sulfur. However, iodine signals were nowhere to be found. With the guidance from the theorist, Professor Ong, we discovered a diffraction peak at a very low angle, not where one would normally look. This points to a super lattice type of structure. The lack of long-range order for iodine was also later corroborated by X-ray pair density function analysis by Dr. Enyuan Hu and Sha Tan at Brookhaven.

The second difficulty is to understand its operating mechanism in a solid-state battery. We use a sulfide-based electrolyte to work with a sulfur-based cathode material. While that promotes material compatibility, it makes it more difficult to analyze the reaction of the silicon-iodine material. We had to conduct chemical reactions with lithium (outside a battery and without the electrolyte). That helped us to confirm the role of polysulfide, something previously thought not possible.

Tech Briefs: Can you explain in simple terms the process by which you developed it?

Ping: It is well known sulfur is a highly insulating material. The battery community has focused on mixing it with carbon to make sulfur very small or bind it into a polymer. We decided that we need to give sulfur "intrinsic" electronic conductivity, which led us down the path of iodine doping.

Tech Briefs: What are its pros and cons?

Ping: The pros of the material are mainly its appreciable electronic conductivity and its low melting point. The conductivity, on the order of 1x10-6 to 1x10-7 S/cm, makes it on par with common semiconductors. In contrast, sulfur itself is 11 orders of magnitude lower. The melting point of 65 °C allows us to remelt the material after the battery shows signs of degradation. Sulfur goes through a large volume change (~ 80 percent) during battery operation. In a solid-state battery, this creates cracks and delamination. The ability to remelt it is crucial to reverse this aging process.

Tech Briefs: The article I read says, “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.” How is that coming along? Are there any updates you can share?

Ping: This is a work in progress. We have made cells with electrolyte layers <100 um thick. We have also lowered the pressure to <10 MPa. These two steps are crucial toward realizing a practical solid-state cell. We still have a way to go to further drive down both numbers.

Tech Briefs: Ping, you’re quoted in the article as saying, “While much remains to be done to deliver a viable solid-state battery, our work is a significant step.” With regards to much needing to still be done, what are your next steps? Do you have any plans for further research/work/etc.?

Ping: In addition to the above, we are working on scaling up the fabrication of electrolyte sheets to >100 cm2. We are also working on improving the stability of the electrolyte in the presence of water so that the solid-state battery can be manufactured in a common dry room currently used for Li-ion. This is a shared concern for all sulfide solid state battery technology.

Tech Briefs: Do you have any advice for engineers/researchers aiming to bring their ideas to fruition?

Ping: The most important is to "not to follow the crowd." Li-S is not a new topic; many ideas have been tried. When a topic is progressing slowly, it is important to intentionally depart from the "beaten path" and look for something very different.