Many electric vehicle owners worry about how effective their battery will be in very cold weather. Now a new battery chemistry may have solved that problem.
In today’s lithium-ion batteries, the main problem is the liquid electrolyte. This main component of the battery transfers charge-carrying molecules called ions between the two electrodes of the battery, causing the battery to charge and discharge. But the liquid begins to freeze at sub-zero temperatures. This requirement severely limits the effectiveness of electric vehicle charging in cold regions and seasons.
To address this problem, a team of scientists from the US Department of Energy (DOE) National Argonne and Lawrence Berkeley Laboratories developed an electrolyte that contains fluorine and works well even at sub-zero temperatures.
said Zhengcheng “John” Zhang, a senior chemist and group lead in the Department of Chemical Science and Engineering at Argonne.
This low-temperature electrolyte shows promise in working for batteries in electric vehicles, as well as in energy storage for electrical grids and consumer electronics such as computers and phones.
In current lithium-ion batteries, the electrolyte is a mixture of a widely available salt (lithium hexafluorophosphate) and a carbonate solvent such as ethylene carbonate. Solvents dissolve the salt to form a liquid.
When the battery is charged, the liquid electrolyte transfers the lithium ions from the cathode (lithium-containing oxide) to the anode (graphite). These ions migrate from the cathode, then pass through the electrolyte on their way to the anode. As it is transported through the electrolyte, it lies at the center of groups of four or five solvent molecules.
During the initial few charges, these groups strike the anode surface and form a protective layer called the solid-electrolyte interphase. Once formed, this layer acts as a filter. Only lithium ions are allowed to pass through the layer while the solvent molecules are prevented. In this way, the anode is able to store lithium atoms in the graphite structure when charging. Upon discharge, electrochemical reactions release electrons from the lithium to generate electricity that can power vehicles.
The problem is that at cold temperatures, the electrolyte with the carbonate solvents starts to solidify. As a result, it loses the ability to transfer lithium ions to the charged anode. This is because lithium ions are tightly bound within the solvent groups. Thus, these ions require much higher energy to evacuate their groups and penetrate the interface layer compared to room temperature. For this reason, scientists have been looking for a better solvent.
The team investigated several fluorine-containing solvents. They were able to identify the structure that had the lowest energy barrier to releasing lithium ions from the clusters at sub-zero temperatures. They’ve also identified on an atomic level why this particular combination works. Depends on the position and number of fluorine atoms within each solvent molecule.
In testing with laboratory cells, the team’s fluorinated electrolyte held a constant energy storage capacity for 400 charge-discharge cycles at minus 4 F. Even at sub-zero temperatures, the capacity was equivalent to a cell with a conventional carbonate-based electrolyte at room temperature.
“Our research thus shows how to adapt the atomic structure of electrolyte solvents to design new electrolytes for sub-zero temperatures,” Zhang said.
The antifreeze electrolyte has an additional property. It is safer than the carbonate-based electrolytes currently in use, as it will not catch fire.
“We are patenting a lower-temperature, safer electrolyte, and are now looking for an industrial partner to adapt it to one of their designs for lithium-ion batteries,” Zhang said.
This search appears in advanced energy materials. In addition to Jun Zhang, the composers for Argon are Dong Joo Yoo, Kian Liu, and Mingyu Kim. Berkeley Lab authors are Orion Cohen and Kristin Pearson.
This work was funded by the Department of Energy’s Office of Energy Efficiency and Renewable Energy, Office of Vehicle Technologies.