An Electric Vehicle Battery for All Seasons

Many owners of electric vehicles are concerned about how effective their batteries will be in extreme cold weather. Now a new battery chemistry may have solved that problem.

In current lithium-ion batteries, the main problem lies in the liquid electrolyte. This key battery component transports charge-carrying particles called ions between the battery’s two electrodes, allowing the battery to charge and discharge. But at temperatures below zero, the liquid begins to freeze. This condition severely limits the effectiveness of charging electric vehicles in cold regions and climates.

To address that problem, a team of scientists at the US Department of Energy’s (DOE) Argonne and Lawrence Berkeley National Laboratories has developed a fluorine-rich electrolyte that performs well even in sub-zero temperatures.

appears in research advanced energy materials,

“Our team has not only found an antifreeze electrolyte whose charging performance does not degrade at minus 4 degrees Fahrenheit, but we have also discovered, at the atomic level, what makes it so effective,” said Zhengcheng “John” Zhang, a senior chemist in the group. makes it effective.” Leader in Argonne’s Chemical Sciences and Engineering Division.

This low-temperature electrolyte shows promise for work in batteries in electric vehicles, as well as in energy storage for electric grids and consumer electronics such as computers and phones.

In today’s lithium-ion batteries, the electrolyte is a mixture of a widely available salt (lithium hexafluorophosphate) and carbonate solvents such as ethylene carbonate. Solvents dissolve the salt to make it a liquid.

When a battery is charged, the liquid electrolyte shuttles lithium ions from the cathode (lithium-containing oxide) to the anode (graphite). These ions exit the cathode, then travel through the electrolyte to the anode. While being carried through the electrolyte, they sit at the center of clusters of four or five solvent molecules.

During the first few charges, these clusters hit the surface of the anode and form a protective layer called the solid-electrolyte interface. Once formed, this layer acts like a filter. This allows only lithium ions to pass through the layer while blocking out solvent molecules. In this way, the anode is able to store lithium atoms in the graphite’s structure on charge. When discharged, electrochemical reactions release electrons from the lithium to generate electricity that can power vehicles.

The problem is that in colder temperatures the electrolyte tends to freeze with carbonate solvents. As a result, it loses its ability to transport lithium ions to the anode when charged. This is because the lithium ions are so tightly bound within the solvent clusters. Therefore, these ions require much more energy to free their groups and enter the interface layer than at room temperature. That’s why scientists are searching for a better solvent.

The team investigated several fluorinated solvents. They were able to identify the structure that had the lowest energy barrier to release lithium ions from the clusters at sub-zero temperatures. They also determined, on an atomic scale, why that particular composition worked so well. It depended on the position and number of fluorine atoms within each solvent molecule.

In testing laboratory cells, the team’s fluorinated electrolyte maintained stable energy storage capacity for 400 charge-discharge cycles at minus 4 F. Even at that sub-zero temperature, the capacity was comparable to that of a cell with a conventional carbonate-based electrolyte. at room temperature.

“Thus our research demonstrated how to tailor the atomic structure of electrolyte solvents to design new electrolytes for sub-zero temperatures,” said Zhang.

Antifreeze electrolyte has a bonus property. It is much safer than currently used carbonate-based electrolytes, as it will not catch fire.

“We are patenting our low-temperature and safe electrolyte and are now looking for an industrial partner to adapt one of our designs for lithium-ion batteries,” Zhang said.

In addition to John Zhang, Argonne authors are Dong-Ju Yu, Qian Liu and Minkyu Kim. The authors are Orion Cohen and Kristin Persson from Berkeley Lab.