Introduction: Many owners of electric vehicles are concerned about the durability of their batteries in extremely cold weather. Now, a new battery chemistry method may have solved this problem. Scientists have developed a new and safer electrolyte for lithium-ion batteries, which is as effective at temperatures below 0°F (-17.8°C) as it is at room temperature.
In current lithium-ion batteries, the main issue lies in the liquid electrolyte. This crucial battery component transfers charge-carrying particles called ions between the two electrodes of the battery, enabling the battery to charge and discharge. However, the liquid begins to freeze at sub-zero temperatures. This severely limits the effectiveness of charging electric vehicles in cold regions and seasons.
To address this issue, a team of scientists from the U.S. Department of Energy (DOE) Argonne and Lawrence Berkeley National Laboratories developed a fluorine-containing electrolyte that functions effectively even at sub-zero temperatures. The scientists reported their work in a paper published in Advanced Energy Materials.

a) Transition plan for solvent design from carbonate to fluorinated ester. b) Analysis of atomic charges of carbonyl groups in EA, EA-f, f-EA, and f-EA-f.
"Our team not only discovered an antifreeze electrolyte whose charging performance does not degrade at -4℉ (-20℃), but we also uncovered the reasons for its effectiveness at the atomic level," said Zhengcheng Zhang, a senior chemist and a Chinese-American scientist who leads the Chemical Sciences and Engineering Division at Argonne.
This low-temperature electrolyte is expected to be used in batteries for electric vehicles, as well as 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 ethylene carbonate). The solvent dissolves the salt to form a liquid.
When the battery is charging, the liquid electrolyte shuttles lithium ions from the cathode (containing lithium oxide) to the anode (graphite). These ions migrate from the cathode and then enter the anode through the electrolyte. When transported through the electrolyte, they are located at the center of clusters of four or five solvent molecules.
During the initial few charges, these clusters collide with the anode surface and form a protective layer known as the solid electrolyte interface. Once formed, this 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 the charged graphite structure. Upon discharge, the electrochemical reaction releases electrons from lithium, generating electricity that can power vehicles.
The issue arises at low temperatures, where the electrolyte containing a carbonate solvent begins to freeze, rendering it incapable of transporting lithium ions to the anode when charged. This is due to the tight binding of lithium ions within the solvent clusters, necessitating higher energy for these ions to break out of their clusters and penetrate the interfacial layer compared to room temperature. Consequently, scientists have been searching for improved solvents.
The team studied several fluorinated solvents, and they were able to determine the composition that has the lowest energy barrier for releasing lithium ions from clusters at subzero temperatures. They also identified on an atomic scale why this particular composition is so effective. This depends on the position and quantity of fluorine atoms in each solvent molecule.
In tests on laboratory batteries, the team's fluorinated electrolyte maintained a stable energy storage capacity over 400 charge-discharge cycles at minus 4 degrees Fahrenheit (-20 degrees Celsius). Even at such low temperatures, its capacity was comparable to that of batteries using traditional carbonate-based electrolytes at room temperature.
"Therefore, our research demonstrates how to tailor the atomic structure of electrolyte solvents to design new electrolytes for subzero temperatures," said Zhang Zhengcheng.
The low-temperature resistant electrolyte possesses additional characteristics. It is significantly safer than the currently used carbonate-based electrolytes, as it does not catch fire.
"We are applying for a patent for 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," said Zhang Zhengcheng.