New additives can help lithium-ion batteries perform over a wider range of temperatures, a potential boon for electric cars, a new study finds.
Electric cars struggle with extreme temperatures, which can degrade the electrolyte solutions that conduct ions between the negative electrodes, or anodes, and positive electrodes, or cathodes, within lithium-ion batteries.
A key additive to most of these electrolyte solutions is ethylene carbonate, which helps produce a protective layer that prevents further decomposition of electrolyte components when they interact with the anode. However, ethylene carbonate has a high melting point, which limits its performance at low temperatures.
Materials scientist Wu Xu at Pacific Northwest National Laboratory in Richland, Washington, and his colleagues previously showed they could extend the temperature range of lithium-ion batteries by partially replacing ethylene carbonate with propylene carbonate and adding cesium hexafluorophosphate. However, they wanted to improve the temperature range of lithium-ion batteries even further, so they could perform well from -40 to 60 degrees C.
In the new study, Xu and his colleagues tested the effects of five electrolyte additives on the performance of lithium-ion batteries within this temperature range. Through a combination of computational modeling, decades of experience with the chemical and electrochemical properties of liquid electrolytes and additives, and trial and error, they identified an optimized combination of three compounds that they added to their previous electrolyte solution.
This new mixture caused the formation of highly conductive, uniform and robust protective layers on both the anode and the cathode. At -40 degrees C, batteries containing this blend achieved 67 percent of the discharging performance they saw at room temperature. In comparison, regular lithium-ion batteries only experience about 20 percent discharge capacity, Xu says.
Normally, including a variety of additives within electrolytes results in thick layers on both positive and negative electrodes at low temperatures that are fairly resistant to ion transport, "leading to very poor low-temperature discharge performance," Xu says. "Our additive mixture still results in very thin surface layers on both electrodes, and their resistance is low, and does not change much with cycling. This is achieved by the synergistic effects of these additives."
The new batteries also displayed long-term cycling stability at 25 degrees C, retaining more than 85 percent of their original capacity after 1,000 cycles. In addition, at 60 degrees C, the new batteries maintained more than 60 percent of their original capacity after 300 cycles, whereas conventional lithium-ion batteries only kept about 10 percent of their original capacity, Xu says.
The scientists aim to validate these results in "commercial lithium ion batteries under practical testing conditions, and then hope that battery companies will use the electrolytes in their battery systems for electric vehicles," Xu says. They also hope to experiment with electrolyte additives to improve other aspects of battery performance, such as boosting their charging speed and reducing their flammability, Xu adds.
The scientists detailed their findings June 19 in the journal ACS Applied Materials & Interfaces.
Charles Q. Choi is a science reporter who contributes regularly to IEEE Spectrum. He has written for Scientific American, The New York Times, Wired, and Science, among others.