UMD’s Long Chen (left) and Xiulin Fan (right) with their fluorine battery. Image: UMD.
UMD’s Long Chen (left) and Xiulin Fan (right) with their fluorine battery. Image: UMD.

The success of electric car batteries depends on the miles that can be driven on a single charge, but the current crop of lithium-ion batteries is reaching its natural limit for how much charge can be packed into any given space, keeping drivers on a short tether.

Now, researchers at the University of Maryland (UMD), the US Army Research Laboratory (ARL) and Argonne National Laboratory (ANL) have figured out how to increase a rechargeable battery's capacity by using aggressive electrodes and then stabilizing those potentially dangerous electrode materials with a highly-fluorinated electrolyte. They describe their research in a paper in Nature Nanotechnology.

"We have created a fluorine-based electrolyte to enable a lithium-metal anode, which is known to be notoriously unstable, and demonstrated a battery that lasts up to a thousand cycles with high capacity," said co-first authors Xiulin Fan and Long Chen, postdoctoral researchers at UMD's A. James Clark School of Engineering

The new batteries can thus charge and discharge many times over without losing the ability to provide a reliable and high-quality stream of energy. Even after 1000 charge cycles, the fluorine-enhanced electrolytes achieved 93% of battery capacity, which the authors call ‘unprecedented’. This means that a car running on this technology would reliably drive the same number of miles for many years.

"The cycle lives they achieved with the given electrode materials and operation voltage windows sound 'unprecedented’. This work is a great progress forward in the battery field in the direction of increasing the energy density, although further tuning might be needed to meet various standards for commercialization," said Jang Wook Choi, an associate professor in chemical and biological engineering at Seoul National University in South Korea, who was not involved with the research.

The team demonstrated the battery technology in a coin cell shaped like a watch battery and is now working with industry partners to use the electrolytes for a high voltage battery.

Aggressive electrode materials such as lithium-metal anode and nickel and high-voltage cathode materials react strongly with other materials. This means they can hold a lot of energy but also tend to ‘eat up’ any other elements they're partnered with, rendering them unusable.

Chunsheng Wang, professor in the Clark School's Department of Chemical and Biochemical Engineering, has collaborated with Kang Xu at ARL and Khalil Amine at ANL on these new electrolyte materials for batteries. Since each element on the periodic table has a different arrangement of electrons, Wang studies how each permutation of chemical structure can be an advantage or disadvantage in a battery. Wang and Xu also head up an industry-university-government collaborative effort called the Center for Research in Extreme Batteries, which aims to unite companies that need batteries for unusual uses with the researchers who can invent them.

"The aim of the research was to overcome the capacity limitation that lithium-ion batteries experience. We identified that fluorine is the key ingredient that ensures these aggressive chemistries behave reversibly to yield long battery life. An additional merit of fluorine is that it makes the usually combustible electrolytes completely unable to catch on fire," said Wang.

The team captured video of several battery cells catching fire in an instant, but the fluorine battery was impervious.

The high population of fluorine-containing species in the interphases is the key to making the material work, even though results have varied for different researchers in the past regarding fluorination.

"You can find evidences from literature that either support or disapprove fluorine as good ingredient in interphases," said Xu, laboratory fellow and team leader of the research at ARL. "What we learned in this work is that in most cases it is not just what chemical ingredients you have in the interphase, but how they are arranged and distributed."

This story is adapted from material from the University of Maryland, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.