Electrolyte promises battery operation in freezing and searing temperatures

Developed by engineers at the University of California San Diego, the temperature-resilient batteries are described in a paper published in Proceedings of the National Academy of Sciences (PNAS).

Such batteries could allow electric vehicles in cold climates to travel farther on a single charge; they could also reduce the need for cooling systems to keep the vehicles’ battery packs from overheating in hot climates, said Zheng Chen, a professor of nanoengineering at the UC San Diego Jacobs School of Engineering and senior author of the study.

“You need high temperature operation in areas where the ambient temperature can reach the triple digits and the roads get even hotter. In electric vehicles, the battery packs are typically under the floor, close to these hot roads,” said Chen, a faculty member of the UC San Diego Sustainable Power and Energy Center. “Also, batteries warm up just from having a current run through during operation. If the batteries cannot tolerate this warmup at high temperature, their performance will quickly degrade.”

In tests, the proof-of-concept batteries are said to have retained 87.5 per cent and 115.9 per cent of their energy capacity at -40 and 50oC, respectively. They also had high Coulombic efficiencies of 98.2 per cent and 98.7 per cent at these temperatures, respectively, so the batteries can undergo more charge and discharge cycles before they stop working.

The team’s temperature-resilient batteries contain an electrolyte made of a liquid solution of dibutyl ether mixed with a lithium salt. According to UCSD, a special feature about dibutyl ether is that its molecules bind weakly to lithium ions. In practise, the electrolyte molecules can easily release lithium ions as the battery runs. This weak molecular interaction, the researchers had discovered in a previous study, improves battery performance at sub-zero temperatures. Dibutyl ether can take the heat because with a boiling point of 141oC it stays liquid at high temperatures.

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The electrolyte is compatible with a lithium-sulphur battery, a rechargeable cell that has an anode made of lithium metal and a cathode made of sulphur. The UCSD team said lithium-sulphur batteries are an essential part of next-generation battery technologies because they promise higher energy densities and lower costs. They can store up to two times more energy per kilogram than today’s lithium-ion batteries, potentially doubling the range of electric vehicles without any increase in the weight of the battery pack. Sulphur is also more abundant and less problematic to source than the cobalt used in traditional lithium-ion battery cathodes.

Sulphur cathodes, however, are so reactive that they dissolve during battery operation, an issue that worsens at high temperatures. Lithium metal anodes are prone to forming dendrites that can pierce parts of the battery, causing it to short-circuit. Consequently, lithium-sulphur batteries only last up to tens of cycles.

“If you want a battery with high energy density, you typically need to use very harsh, complicated chemistry,” Chen said in a statement. “High energy means more reactions are happening, which means less stability, more degradation. Making a high-energy battery that is stable is a difficult task itself—trying to do this through a wide temperature range is even more challenging.”

The dibutyl ether electrolyte developed by the UC San Diego team prevents these issues, even at high and low temperatures. The batteries they tested had much longer cycling lives than a typical lithium-sulphur battery. “Our electrolyte helps improve both the cathode side and anode side while providing high conductivity and interfacial stability,” said Chen.

The team also engineered the sulphur cathode to be more stable by grafting it to a polymer, which prevents more sulphur from dissolving into the electrolyte.

Next steps include scaling up the battery chemistry, optimising it to work at even higher temperatures and further extending cycle life.