The cells can be used for high altitude unmanned aircraft, polar operation and space missions where heaters are needed to warm up traditional batteries. But the technique that allows the cold operation can also stop thermal runaway that end up with batteries catching fire and gives a higher energy density for electric vehicles.
The team took a very different approach to battery design, looking at liquified gas as the electrolyte for batteries and capacitors rather than more viscous liquids or solid polymers. This allows the battery to operate at lower temperatures, but also as the temperature rises above room temperature the electrolyte stops conducting, Interestingly this effect is reversible, so as the battery cools down the battery starts working again.
The new lithium battery electrolyte was made using liquefied fluoromethane gas and provides an energy density of 400Wh/kg with charging and discharging at -60°C, compared to 250Wh/kg at 0 °C for today’s lithium cells. The electrochemical capacitor electrolyte was made using liquefied difluoromethane gas with 7Wh/kg and operating from -80 °C to +65 °C.
“Better batteries are needed to make electric cars with improved performance-to-cost ratios. And once the temperature range for batteries, ultra-capacitors and their hybrids is widened, these electrochemical energy storage technologies can be adopted in many more emerging markets. This work shows a promising pathway and I think the success of this unconventional approach can inspire more scientists and researchers to explore the unknown territories in this research area,” said Prof Shirley Meng, nanoengineering professor at the UC San Diego Jacobs School of Engineering who leads the Laboratory for Energy Storage and Conversion.
“Mars rovers have a low temperature specification that most existing batteries cannot meet. Our new battery technology can meet these specs without adding expensive and heavy heating elements,” said researcher Cyrus Rustomji. “The low viscosity leads to high ion mobility, which means high conductivity for the battery or capacitor, even in the extreme cold,” he said.
The thermal shutdown is also important. “This is a natural shutdown mechanism that prevents the battery from overheating,” said Rustomji. “As soon as the battery gets too hot, it shuts down. But as it cools back down, it starts working again. That’s uncommon in conventional batteries.”
This also helps with safety, for example in a car accident when the battery is crushed and shorted, the electrolyte gas may vent away from the cell and so prevent the thermal runaway that has caused fires in electric vehicles.
Using a low viscosity electrolyte needed changes to the electrodes and a fluorinated electrolyte additive to protect the lithium metal electrode from dendrites. The design shows a highly uniform and dendrite free surface allowing for a high Coulombic efficiency of over 97% and improved battery conductivity for both lithium metal and classical cathode materials.
The team is commercialising the technology through a startup called South 8 Technologies Moving forward, researchers aim to improve the energy density and cyclability of both batteries and electrochemical capacitors and to run at even lower temperatures — down to -100 °C for new technology to power spacecraft sent to explore the outer planets such as Jupiter and Saturn.
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