UC San Diego Engineers Develop Sulfur-Iodide Crystal for More Efficient Solid-State Batteries New Cathode Material Overcomes Limitations of Solid-State Lithium-Sulfur Batteries, Boosting Lifespan . Credit: scitechdaily.com

Scientists get closer to making lithium-sulfur solid-state batteries a viable option. A group of engineers from the University of California San Diego has developed a new cathode material for these batteries that overcomes the limitations of the current ones. The study was recently published in the journal Nature.

Solid-state lithium-sulfur batteries are rechargeable batteries that have a solid electrolyte, a lithium metal anode, and a sulfur cathode. They are believed to be a better alternative to current lithium-ion batteries due to their higher energy density and lower costs. These batteries have the potential to store twice the amount of energy per kilogram, giving electric vehicles a longer range without increasing the weight of the battery pack. Moreover, they use easily available materials, making them an economically feasible and environmentally-friendly option.

However, the development of solid-state lithium-sulfur batteries has faced challenges due to the characteristics of sulfur cathodes. Not only is sulfur a poor conductor of electricity, but it also expands and contracts during charging and discharging, causing structural damage and decreasing contact with the solid electrolyte. These issues reduce the cathode's ability to transfer charge, affecting the overall performance and longevity of the battery.

To combat these issues, a team of researchers from UC San Diego's Sustainable Power and Energy Center has developed a new cathode material made of a sulfur and iodine crystal. By inserting iodine molecules into the crystalline sulfur structure, the cathode's electrical conductivity has been drastically increased by 11 orders of magnitude, making it 100 billion times more conductive than pure sulfur crystals.

"We are thrilled about this new material," said Professor Ping Liu, co-senior author of the study. "The significant increase in electrical conductivity in sulfur is surprising and scientifically fascinating."

Furthermore, the new crystal material has a low melting point of 65 degrees Celsius, making it easy to re-melt the cathode after the battery is charged to repair any damage from cycling. This is a crucial feature as 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 solution to some of the main obstacles in the commercialization of lithium-sulfur batteries," explained co-senior author Professor Shyue Ping Ong. "Iodine breaks the bonds between sulfur molecules, lowering its melting point to the Goldilocks zone - above room temperature but low enough to re-heat the cathode periodically."

The team constructed a test battery using the new cathode material and subjected it to repeated charge and discharge cycles, where it remained stable for over 400 cycles while retaining 87% of its capacity. "This discovery has the potential to solve one of the biggest challenges in the introduction of solid-state lithium-sulfur batteries by significantly increasing their lifespan," said co-author Christopher Brooks from Honda Research Institute USA, Inc.

The team is now working on further improving the solid-state lithium-sulfur battery technology by enhancing cell engineering designs and scaling up the cell format. "While there is still a lot of work to be done to make solid-state batteries a viable option, our research is a significant step forward," said Professor Liu. "This achievement was made possible through excellent collaboration between our teams at UC San Diego and our research partners from national labs, academia, and industry."

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