While lithium-based batteries have become a standard for both electric vehicles and mobile devices, their performance is very limited by environmental conditions. The liquid electrolyte created by researchers at University of San Diego for a high energy density lithium sulfur battery allows it to offer high performance at extreme temperatures. The battery maintains its specifications in severe cold conditions and also at high temperatures.
Battery performance depends on keeping it within an optimal temperature range, preventing it from getting too hot when it is feeding the traction system or when it is recharging, especially at fast charging stations. They also affect environmental conditions that surround it, and that it is not limited to high temperatures, the cold also affects its performance, especially its recharging capacity. This limitation becomes a complication because it compromises its long-term operational viability.
The dilemma of critical temperatures
Engineers at the University of California at San Diego (UC San Diego) have developed a lithium-sulfur battery that works well at extreme temperatures on both sides of the spectrum, that is, hot or cold, in all cases offering a high energy density. According to the researchers, thanks to this work, it is possible to achieve greater autonomies for electric vehicles regardless of the weather in which they are driven. Another advantage is that manufacturers can reduce your manufacturing costs since it is possible to eliminate the need to implement cooling systems that prevent batteries from overheating in places where it is very hot. This cost saving could be directly affected in the final sale price of the vehicleswhich also favors consumers.
Because electric car battery packs are typically located under the vehicle floor, they are exposed to pavement heat, necessitating the need for “high-temperature operation in areas where ambient temperatures can reach triple digits.” “, explains in the press release Zheng Chen, professor of nanoengineering at UC San Diego Jacobs School of Engineering and director of the study. Chen further clarifies that the batteries heat up only with the passage of current during operation. “If the batteries cannot tolerate this high-temperature heating, their performance will quickly degrade.”
The key to design is a novel electrolyte developed by Chen’s team that can function optimally in a wide range of temperatures. It is also compatible with one anode and one high energy cathode which facilitates the development of a complete prototype battery. The electrolyte is made up of a liquid solution of dibutyl ether mixed with a lithium salt. “It is the liquid part of the material that gives it its ability to operate at extreme temperatures.”
The research has been published in an article in the journal Proceedings of the National Academy of Sciences (PNAS). Dibutyl ether has the peculiarity that its molecules are weakly bound to lithium ions. This means that the electrolyte molecules can easily release lithium ions while the battery is running. This weak molecular interaction, discovered in previous research, can improve battery performance in sub-zero temperatures. This electrolyte also works well at the higher end of the temperature spectrum because it remains liquid at high temperatures since it has a boiling point of 141ºC,
While its ability to operate at extreme temperatures is a very important feature, this electrolyte would not be useful if not compatible with a lithium-sulfur battery. In fact, the researchers say, it appears to prevent some of the key reactions within such a battery that have impeded its long-term viability for electric vehicles and other applications.
Advantages and disadvantages of lithium-sulfur batteries
Lithium-sulfur batteries are made up of a lithium metal anode and a sulfur-based cathode. Scientists believe they could be the chemical design of the dominant batteries in the future as they can store up to twice as much energy per kilogram than current lithium-ion-based designs. In the case of electric vehicles, this translates into double the autonomy without making the battery pack heavier.
This technology also promises lower manufacturing costs, especially because it uses silicon in its core, an abundant material on earth, instead of lithium, which is scarce and for which unethical procedures are sometimes used to obtain it.
On the other hand, lithium-sulfur batteries also have their drawbacks. Both the cathode and anode exhibit inherent performance issues that have prevented previous devices from lasting beyond tens of charge/discharge cycles. The sulfur cathodes tend to dissolve during battery operation, a scenario that worsens at high temperatures. Also, lithium metal anodes are prone to dendrite formation that can puncture other parts of the battery and cause short circuits that result in catastrophic failure.
This set of advantages and disadvantages makes it difficult to develop a viable lithium-sulfur battery that can operate at high energy density, a requirement for electric vehicles. a complex and daunting task. A task that becomes even more complicated when it has to work in a wide temperature range, Chen explains. “If you want a battery with high energy density, you generally need to use very hard and complicated chemistry: High energy means more reactions are happening, which means less stability and more degradation.”
the magic electrolyte
The dibutyl ether electrolyte developed by the UC San Diego team avoids these problems, even at high and low temperatures, the researchers describe. The team also designed the sulfur cathode by grafting it onto a polymer, to make it more stable, preventing more sulfur from dissolving into the electrolyte.
The result is a device that, in laboratory tests, can withstand more charge and discharge cycles than a conventional lithium-sulfur battery. Specifically, the prototype batteries retained 87.5% and 115.9% of its energy capacity at -40 ºC and 50 ºC respectively. They also showed high coulombic efficiency (ratio between the energy consumed during recharging and the actual energy contained in the battery that can be supplied to the electric vehicle) reaching at 98.2% and 98.7% at each temperature level. This means that the batteries can go through more charge and discharge cycles before they stop working.
“Our electrolyte helps improve both the cathode and anode sides while providing high conductivity and interfacial stability,” Chen concludes. The team will continue its work to achieve the battery scaling for use in real electric vehicles. His intention is to optimize it to work at even higher temperatures at both ends of the scale and further extend its life cycle.
