Recently developed in the US, new lithium-ion batteries are able to perform well at freezing cold and scorching hot temperatures while still packing a lot of energy.
Different from previous research focusing on the anode, the devices that were created by engineers at the University of California San Diego are able to endure extreme temperatures thanks to an electrolyte made of a liquid solution of dibutyl ether mixed with a lithium salt.
In a paper published in the journal Proceedings of the National Academy of Sciences, the researchers explain that dibutyl ether improves battery performance at sub-zero temperatures because its molecules bind weakly to lithium ions. In other words, the electrolyte molecules can easily let go of lithium ions as the battery runs. In addition, dibutyl ether can easily take the heat because it stays liquid at high temperatures as has a boiling point of 141 degrees Celsius.
In tests, the proof-of-concept batteries retained 87.5% and 115.9% of their energy capacity at -40 and 50 degrees Celsius, respectively. They also had high Coulombic efficiencies of 98.2% and 98.7% at these temperatures, respectively, which means the batteries can undergo more charge and discharge cycles before they stop working.
According to Zheng Chen, senior author of the study, what is also special about this electrolyte is that it is compatible with lithium-sulphur batteries, which are a type of rechargeable battery that has an anode made of lithium metal and a cathode made of sulphur.
Chen explained that 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—this could double the range of electric vehicles without any increase in the weight of the battery pack. Also, sulphur is more abundant and less problematic to source than the cobalt used in traditional lithium-ion battery cathodes.
However, there are problems with lithium-sulphur batteries. Both the cathode and anode are super reactive. Sulphur cathodes are so reactive that they dissolve during battery operation. This issue gets worse at high temperatures. And lithium metal anodes are prone to forming dendrites that can pierce parts of the battery, causing it to short-circuit. As a result, lithium-sulphur batteries only last up to ten cycles.
“If you want a battery with high energy density, you typically need to use a very harsh, complicated chemistry,” Chen said. “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 scientists noted that the dibutyl ether electrolyte prevents these issues, even at high and low temperatures. In fact, the batteries Chen and his team 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,” he said.
The team also engineered the sulphur cathode to be more stable by grafting it to a polymer. This prevents more sulphur from dissolving into the electrolyte.
Given the positive results they’ve obtained, Chen and his group now want to scale up the battery chemistry, optimizing it to work at even higher temperatures and further extending cycle life.