“Our team not only found an antifreeze electrolyte whose charging performance does not decline at -4°F, but we also discovered, at the atomic level, what makes it so effective.”Zhengcheng ‘John’ Zhang, Argonne National Laboratory
Many owners of electric vehicles worry about how effective their battery will be in very cold weather. Now, a new battery chemistry may have solved that problem.
In current lithium-ion batteries, cold weather mainly causes a problem for the liquid electrolyte. This key battery component transfers ions between the battery’s two electrodes, allowing the battery to charge and discharge. But being a liquid, the electrolyte can start to freeze at sub-zero temperatures, severely limiting the effectiveness of charging electric vehicles in cold regions and seasons.
To address that problem, a team of scientists from the US Department of Energy’s (DOE) Argonne and Lawrence Berkeley national laboratories developed a fluorine-containing electrolyte that performs well even in sub-zero temperatures. The scientists report their work in a paper in Advanced Energy Materials.
“Our team not only found an antifreeze electrolyte whose charging performance does not decline at -4°F, but we also discovered, at the atomic level, what makes it so effective,” said Zhengcheng ‘John’ Zhang, a senior chemist and group leader in Argonne’s Chemical Sciences and Engineering division. This low-temperature electrolyte shows promise for use in electric vehicle batteries, as well as in batteries for energy storage in electric grids and electronic devices like 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. The solvents dissolve the salt to form a liquid.
When a battery is charged, the liquid electrolyte shuttles lithium ions from the cathode (a lithium-containing oxide) to the anode (graphite). These ions migrate out of the cathode, then pass through the electrolyte on the way to the anode. While being transported through the electrolyte, they sit at the center of clusters of four or five solvent molecules.
During the initial few charges, these clusters strike the anode surface and form a protective layer called the solid-electrolyte interphase. Once formed, this layer acts like a filter, only allowing the lithium ions to pass through while blocking the solvent molecules. In this way, the graphite anode is able to store lithium atoms in its structure during charging. Upon discharge, electrochemical reactions release electrons from the lithium, thereby generating electricity to power vehicles.
The problem is that the liquid electrolyte begins to freeze in cold temperatures. As a result, it loses the ability to transport lithium ions into the anode during charging, because the lithium ions become tightly bound within the freezing solvent clusters. This means the ions require much higher energy to evacuate their clusters and penetrate the interface layer than at room temperature. For that reason, scientists have been searching for a better solvent for the electrolyte.
The team investigated several fluorine-containing solvents and was able to identify the composition with the lowest energy barrier for releasing lithium ions from the solvent clusters at sub-zero temperatures. They also determined at the atomic scale why that particular composition worked so well, finding that it depended on the position of the fluorine atoms within each solvent molecule and their number.
In testing with laboratory cells, the team’s fluorinated electrolyte retained stable energy storage capacity for 400 charge-discharge cycles at -4°F. Even at that sub-zero temperature, the capacity of the laboratory cell was equivalent to that of a cell with a conventional carbonate-based electrolyte at room temperature.
“Our research thus demonstrated how to tailor the atomic structure of electrolyte solvents to design new electrolytes for sub-zero temperatures,” Zhang said.
The antifreeze electrolyte also has a bonus property. It is much safer than the carbonate-based electrolytes that are currently used, since it will not catch fire.
“We are patenting our low-temperature and safer electrolyte and are now searching for an industrial partner to adapt it to one of their designs for lithium-ion batteries,” Zhang said.
This story is adapted from material from Argonne National Laboratory, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.