electric vehicle battery

Energy dense batteries developed that operate at extreme temperatures

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Energy-dense lithium-ion batteries that perform well at both hot and cold temperature extremes have been developed by engineers at the University of California San Diego.

The researchers say the key to their battery is a newly developed electrolyte that is more versatile and robust than traditional materials throughout a wide temperature range. It is also compatible with a high-energy anode and cathode.

Such batteries could allow electric vehicles in cold climates to travel farther on a single charge and they could also reduce the need for cooling systems to keep the vehicles’ battery packs from overheating in hot climates.

Zheng Chen, senior author of the study, said: “You need high temperature operation in areas where the ambient temperature can reach the triple digits [i.e. above 100°F] and the roads get even hotter.

“In electric vehicles, the battery packs are typically under the floor, close to these hot roads. 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 retained 87.5 per cent and 115.9 per cent of their energy capacity at -40°C and 50°C, respectively. They also had high Coulombic efficiencies of 98.2 and 98.7 per cent at these temperatures, respectively, which means the batteries can undergo more charge and discharge cycles before they stop working.

The batteries are both cold and heat tolerant thanks to their electrolyte. It is made of a liquid solution of dibutyl ether mixed with a lithium salt. A special feature about dibutyl ether is that its molecules bind weakly to lithium ions. In other words, the electrolyte molecules can easily let go of lithium ions as the battery runs.

This weak molecular interaction improves battery performance at sub-zero temperatures. Dibutyl ether can also easily operate at higher temperatures because it stays liquid up to 141°C.

The electrolyte is also compatible with lithium-sulphur batteries – a type of rechargeable battery that has an anode made of lithium metal and a cathode made of sulphur. Lithium-sulphur batteries could become a major 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 which could double 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.

Unfortunately, lithium-sulphur batteries have very reactive cathodes and anodes that can actually dissolve during battery operation. This issue gets worse at high temperatures. And lithium metal anodes are prone to forming needle-like structures called dendrites that can pierce parts of the battery, causing it to short-circuit. As a result, 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,” said Chen. “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 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.

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

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