A team led by engineers at the University of California San Diego 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. The work was published in the journal Nature.

Solid-state Li–S batteries (SSLSBs) are made of low-cost and abundant materials free of supply chain concerns. Owing to their high theoretical energy densities, they are highly desirable for electric vehicles. However, the development of SSLSBs has been historically plagued by the insulating nature of sulfur and the poor interfacial contacts induced by its large volume change during cycling impeding charge transfer among different solid components.

Here we report an S_9.3I molecular crystal with I₂ inserted in the crystalline sulfur structure, which shows a semiconductor-level electrical conductivity (approximately 5.9 × 10⁻⁷ S cm⁻¹) at 25 °C; an 11-order-of-magnitude increase over sulfur itself.

Iodine introduces new states into the band gap of sulfur and promotes the formation of reactive polysulfides during electrochemical cycling. Further, the material features a low melting point of around 65 °C, which enables repairing of damaged interfaces due to cycling by periodical remelting of the cathode material. As a result, an Li–S_9.3I battery demonstrates 400 stable cycles with a specific capacity retention of 87%. The design of this conductive, low-melting-point sulfur iodide material represents a substantial advancement in the chemistry of sulfur materials, and opens the door to the practical realization of SSLSBs.

—Zhou et al.

(a) Schematic of a solid state battery with elemental sulfur as the active material. Poor solid/solid contact develops during cycling due to volume changes of the active material. (b) Schematic of a solid state battery with sulfur iodide as the active material. Ideal active material/electrolyte interface is achieved through periodical heating to melt the cathode, thus healing the interface. Zhou et al.

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.

To overcome these challenges, a team led by researchers at the UC San Diego Sustainable Power and Energy Center developed a new cathode material using a 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.

With its low melting point of 65 °C, 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. 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.

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

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. 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.

—study co-author Christopher Brooks, chief scientist at Honda Research Institute USA, Inc.

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.

This work was supported in part by the US Department of Energy (DOE) Advanced Research Projects Agency-Energy (DE-AR0000781), the US DOE Office of Science (DEAC02-05-CH11231).

Resources

• Zhou, J., Holekevi Chandrappa, M.L., Tan, S. et al. “Healable and conductive sulfur iodide for solid-state Li–S batteries.” Nature doi: 10.1038/s41586-024-07101-z