Introduction: Many electric car owners are concerned that their batteries may not be durable in very 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 below 0 degrees Fahrenheit (-17.8 degrees Celsius) as at room temperature.
The main problem in current lithium-ion batteries is the liquid electrolyte. This key battery component transfers charge carrying particles called ions between the two electrodes of the battery, leading to battery charging and discharging. But the liquid begins to freeze at temperatures below zero. This situation severely limits the effectiveness of charging electric vehicles in cold regions and seasons.
To address this issue, a group of scientists from the US Department of Energy (DOE) Argonne and Lawrence Berkeley National Laboratories have developed a fluorinated electrolyte that works well even at temperatures below zero. Scientists reported their work in a paper published in Advanced Energy Materials.
a) The transition scheme of solvent design from carbonate to fluorinated ester. b) Atomic charge analysis of carbonyl groups in EA, EA-f, f-EA, and f-EA-f.
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Our team not only discovered an antifreeze electrolyte whose charging performance does not decrease at -4 ℉ (-20 ℃), but we also found the reasons why it is so effective at the atomic level, "said Zhengcheng Zhang, a senior chemist and leader of the Science and Engineering Department of Argonne Chemical.
This low-temperature electrolyte is expected to be used in batteries for electric vehicles and energy storage for consumer electronics products such as computers and mobile phones.
In today's lithium-ion batteries, the electrolyte is a mixture of widely used salts (lithium hexafluorophosphate) and carbonate solvents (such as vinyl carbonate). Solvent dissolves 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 out of the cathode and then enter the anode through the electrolyte. When transported through electrolytes, they are located at the center of four or five solvent molecule clusters.
In the first few charges, these clusters collide with the anode surface and form a protective layer called the solid electrolyte interface. Once formed, the layer acts like a filter. It only allows lithium ions to pass through the layer while blocking solvent molecules. In this way, the anode can store lithium atoms in a charged graphite structure. After discharge, the electrochemical reaction releases electrons from lithium, generating electricity that can provide power for vehicles.
The problem is that at low temperatures, electrolytes containing carbonate solvents begin to freeze, causing them to lose their ability to transport lithium ions to the anode when charged. This is because lithium ions are tightly bound in solvent clusters, so compared to room temperature, these ions require higher energy to expel their clusters and penetrate the interfacial layer. For this reason, scientists have been searching for better solvents.
The team studied several fluorinated solvents and was able to determine the component with the lowest energy barrier for 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 quantity of fluorine atoms in each solvent molecule.
In laboratory battery testing, the team's fluorinated electrolyte maintained a stable energy storage capacity of 400 charge discharge cycles at minus 4 degrees Fahrenheit (-20 degrees Celsius). Even at low temperatures, its capacity is equivalent to that of a battery using traditional carbonate based electrolytes at room temperature.
Therefore, our research demonstrates how to customize the atomic structure of electrolyte solvents to design new electrolytes for sub zero temperatures, "said Zhang Zhengcheng.
Low temperature resistant electrolytes have additional characteristics. It is much safer than the carbonate based electrolytes currently used because it does not ignite.
We are applying for a patent for our low-temperature and safer electrolyte, and are currently seeking an industrial partner to adapt it to one of their lithium-ion battery designs, "said Zhang Zhengcheng.
