Iodine creates the Goldilocks zone for lithium-sulphur batteries

25 March 2024 | Muriel Cozier

‘Iodine disrupts the intermolecular bonds holding sulphur molecules together by just the right amount to lower its melting point to the “Goldilocks zone”’

A new cathode material developed by researchers at the University of California San Diego, US, could make solid state lithium-sulphur batteries a viable alternative to lithium-ion batteries.

Lithium-sulphur batteries hold promise as an alternative to lithium-ion batteries due to their increased energy density and lower costs. With the potential to store up to twice as much energy per kilogram as conventional lithium-ion batteries, the range of an electric vehicle could be doubled.

However, their development has been hampered by sulphur’s poor electron conductivity and the fact that sulphur cathodes expand and contract during charging and discharging, leading to structural damage.

Publishing their work in Nature, the research team at the UC San Diego Sustainable Power and Energy Centre have developed a new cathode material which is a crystal composed of sulphur and iodine. The researchers said that by inserting iodine molecules into the crystalline sulphur structure the cathode material’s electrical conductivity increased by 11 orders of magnitude, making it ‘100 billion times more conductive’ than crystals made of sulphur alone.

In addition, the new material has a melting point of 65°C, which means that it can easily be re-melted after the battery is charged to repair the damaged interfaces that occur from cycling. The researchers said that this is an important feature to address the cumulative damage that occurs at the solid-solid interface between the cathode and the electrolyte during repeated charging and discharging.

‘Iodine disrupts the intermolecular bonds holding sulphur 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 be re-healed via melting,’ said Shyue Ping Ong, a professor of nanoengineering at the UC San Diego Jacobs School of Engineering and study co-senior author.

A test battery made up of the new cathode material was subjected to repeated charge and discharge cycles. It remained stable for over 400 cycles while retaining 87% of its capacity.

Christopher Brooks, chief scientist at the Honda Research Institute, US and co-author added ‘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 applications of solid-state batteries.’

The research team is working on improving the cell engineering designs and scaling up the cell format.

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