Temperature-resilient batteries: Pioneering the functionality of EVs

A research team from the University of California, San Diego, have developed temperature-resilient batteries that perform well at both cold and hot temperatures, whilst storing abundant amounts of energy.

Engineers at the University of California, San Diego, have developed lithium-ion batteries that perform well at both freezing cold and scorching hot temperatures, whilst still storing a lot of energy. This was accomplished by developing an electrolyte that is not only versatile and robust throughout a wide temperature range, but also compatible with a high-energy anode and cathode.

This research was recently published in the Proceedings of the National Academy of Sciences.

The possibilities of temperature-resilient batteries

Temperature-resilient batteries have the potential to allow electric vehicles (EVs) in cold climates to travel farther on a single charge. Additionally, they could also reduce the requirement for cooling systems to keep the Ev’s battery packs from overheating in hot climates, noted Zheng Chen, senior author of the study, and a Professor of Nanoengineering at the UC San Diego Jacobs School of Engineering.

“You need a high-temperature operation in areas where the ambient temperature can reach the triple digits and the roads get even hotter. In EVs, the battery packs are typically under the floor, close to these hot roads,” explained Chen, also a faculty member of the UC San Diego Sustainable Power and Energy Centre. “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.”

Electrolytes: Making batteries tolerant to both heat and cold

Testing revealed that the batteries retained 87.5% and 115.9% of their energy capacity at -40°C and 50°C respectively. They also had high coulombic efficiencies of 98.2% and 98.7% at these temperatures, which means the batteries can be exposed to more charge and discharge cycles before they stop working.

The batteries developed by Chen’s research team are both tolerant to the cold and heat, due to their electrolytes, which are made of a liquid solution of dibutyl ether mixed with a lithium salt. A unique feature of dibutyl ether is that its molecules bind weakly to lithium ions. This means that the electrolyte molecules can effortlessly release lithium ions as the battery runs.

In a previous study, the research team discovered that this weak molecular interaction improves battery performance at sub-zero temperatures. Additionally, dibutyl ether can easily take the heat because it stays liquid at high temperatures (it has a boiling point of 141°C or 286°F). 

Lithium-sulphur battery compatibility

Furthermore, this electrolyte is compatible with a lithium-sulphur battery, which is a type of rechargeable battery that has an anode composed of lithium metal and a cathode made of sulphur. Lithium-sulphur batteries are an indispensable part of next-generation battery technologies because they provide higher energy densities and lower costs.

Additionally, they can store up to two times more energy per kilogram than current lithium-ion batteries, which could double the range of EVs without any increase in the weight of the battery pack. Sulphur is also more abundant and less problematic to source than the cobalt utilised in traditional lithium-ion battery cathodes.

However, scientists have observed potential issues with lithium-sulphur batteries. Both the cathode and anode are super reactive, and sulphur cathodes are so reactive that they dissolve during battery operation. This becomes more of an issue at high temperatures, as lithium metal anodes are prone to forming needle-like structures called dendrites that can pierce parts of the battery, which causes it to short-circuit. Thus, this results in lithium-sulphur batteries only lasting 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 developed by scientists 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,” concluded 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, the team intents to scale up the battery chemistry, optimising it to work at even higher temperatures and further extending cycle life.

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