Engineers at the University of California San Diego have developed lithium-ion batteries that perform well at freezing cold and scorching hot temperatures, while packing a lot of energy. The researchers accomplished this feat 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.
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, Professor of nanoengineering at the UC San Diego Jacobs School of Engineering.
“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,” explained Chen. “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 percent and 115.9 percent of their energy capacity at -40 °C and 50 °C (-40 °F and 122 °F), respectively. They also had high Coulombic efficiencies of 98.2 percent and 98.7 percent at these temperatures, respectively, which means the batteries can undergo more charge and discharge cycles before they stop working.
The batteries that Chen and colleagues developed 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.
What’s also special about this electrolyte is that it is compatible with a lithium-sulfur battery, which is a type of rechargeable battery that has an anode made of lithium metal and a cathode made of sulfur.
The batteries the team tested had much longer cycling lives than a typical lithium-sulfur 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 sulfur cathode to be more stable by grafting it to a polymer. This prevents more sulfur from dissolving into the electrolyte.
Next steps include scaling up the battery chemistry, optimizing it to work at even higher temperatures, and further extending cycle life.