Many electric car owners worry that their batteries won't last in very cold weather. Now a new approach to battery chemistry may have solved that problem. Scientists have developed a new, safer electrolyte for lithium-ion batteries that is as effective at temperatures below 0 degrees Fahrenheit (-17.8 degrees Celsius) as it is at room temperature.

In current lithium-ion batteries, the main problem is the liquid electrolyte. This key battery component transfers charge-carrying particles called ions between the two electrodes of the battery, causing the battery to charge and discharge. But the liquid starts to freeze at sub-zero temperatures. This situation severely limits the effectiveness of charging electric vehicles in cold regions and seasons.

To solve this problem, a team of scientists from the U.S. Department of Energy's (DOE) Argonne and Lawrence Berkeley National Laboratory has developed a fluorine-containing electrolyte that works well even at sub-zero temperatures. The scientists report their work in a paper in Advanced Energy Materials.

a) Solution design transition from carbonate to fluorinated ester. b) Atomic charge analysis of carbonyl groups in EA, EA-F, f-EA and F-EA-F.

(photo: www.onlinelibrary.wiley.com)

"Our team not only discovered an antifreeze electrolyte whose charging performance does not decline at -4 ° F (-20 ° C), but we also discovered what makes it so effective at the atomic level," said Zhengcheng Zhang, a senior chemist and group leader of Argonne's Chemical Science and Engineering Department.

This low-temperature electrolyte is expected to be used for batteries in electric vehicles, as well as for energy storage in consumer electronics such as computers and mobile phones.

In today's lithium-ion batteries, the electrolyte is a mixture of a widely used salt (lithium hexafluorophosphate) and a carbonate solvent (such as vinyl carbonate). The solvent dissolves the salt to form a liquid.

When the battery is charged, the liquid electrolyte shuttles lithium ions from the cathode (containing lithium oxide) to the anode (graphite). These ions migrate from the cathode and then through the electrolyte into the anode. When transported through the electrolyte, they are located at the center of clusters of four or five solvent molecules.

During the first few charges, these clusters hit the anode surface and form a protective layer called the solid electrolyte interface. Once formed, the layer acts like a filter. It allows only lithium ions to pass through the layer while blocking solvent molecules. In this way, the anode is able to store lithium atoms in an electrically charged graphite structure. An electrochemical reaction after discharge releases electrons from the lithium, producing electricity that can power a vehicle.

The problem is that at low temperatures, the electrolyte containing the carbonate solvent begins to freeze, causing it to lose its ability to transport lithium ions to the anode when charged. This is because lithium ions are tightly bound in solvent clusters, so these ions require higher energy than at room temperature to expel their clusters and penetrate the interface layer. For this reason scientists have been searching for better solvents.

The team studied several fluorine-containing solvents, and they were able to identify the components that had the lowest energy barrier when releasing lithium ions from clusters at sub-zero temperatures. They also determined, at the atomic scale, why this particular component is so effective. This depends on the position and number of fluorine atoms in each solvent molecule.

In tests with lab batteries, the team's fluorinated electrolyte maintained a stable energy storage capacity for 400 charge-discharge cycles at minus 4 degrees Fahrenheit (-20 degrees Celsius). Even at low temperatures, the capacity is equivalent to that of a battery using a conventional carbonate-based electrolyte at room temperature.

"Our study therefore shows how the atomic structure of the electrolyte solvent can be customized to design new electrolytes for sub-zero temperatures," says Zhang.

Cryogenic electrolytes have additional properties. It is much safer than the carbonate based electrolytes currently used because it does not catch fire.

"We are in the process of patenting our low temperature and safer electrolyte and are now looking for an industrial partner to adapt it to one of their lithium-ion battery designs," Zhang said.