Electric vehicle (EV) range anxiety is what keeps EV owners from maximizing use of their cars. It’s also what makes would be owners nervous about purchasing a fully electric vehicle and tamps down demand. Resolving this issue would go a long way towards boosting the market and reducing carbon emissions from the transportation sector.
Electric car battery success depends on the miles that can be driven on a single charge, but the current crop of lithium-ion batteries are reaching their natural limit of how much charge can be packed into any given space, keeping drivers on a short tether. Now researchers at the University of Maryland, the Army Research Laboratory and Argonne National Laboratory have figured out how to increase a rechargeable battery’s capacity by using aggressive electrodes and then stabilizing these potentially dangerous electrode materials with a highly-fluorinated electrolyte.
“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 Dr. Xuilin Fan and Dr. Long Chen, post-doctoral researchers at the University of Maryland.
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 a thousand charge cycles, the fluorine enhanced electrolytes ensured 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 [sic] 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. Choi was not involved with the research.
The team demonstrated the batteries in coin-cell shape like a watch battery for testing, and is working with industry partners to use the electrolytes for a high voltage battery.
These aggressive materials, such as the Li-metal anode and high nickel and high-voltage cathode materials, are called such because they react strongly with other material, meaning that they can hold a lot of energy but also tend to “eat up” any other elements they’re partnered with, rendering them unusable.
Dr. Chunsheng Wang, a professor in the Chemical and Biochemical Engineering Department of the University of Maryland in College Park, has collaborated with Dr. Kang Xu at ARL and Dr. Khalil Amine at ANL on these new electrolyte materials for batteries. Since each element on the periodic table has a different arrangement of electrons, Dr Wang studies how each permutation of chemical structure can be an advantage or disadvantage in a battery. He 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 lithium-based battery cells catching on fire within instants, 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 the fluorination.
“You can find evidences from literature that either support or disapprove fluorine as good ingredient in interphases,” said Kang Xu, a laboratory fellow and team leader of the research at the Army Research Laboratory. “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.”
His team found that fluorine was smoothly distributed at the nanoscale, sought under high-powered microscopes –in the interphases covering the aggressive materials.
“We believe these fluorinated interphases serve as the key stabilization barriers to enable these aggressive electrode materials,” Xu said in an email.
Photos of the close structure of the battery were produced in the Advanced Imaging and Microscopy (AIM) Lab, part of the Maryland NanoCenter, which is headquartered in College Park.
The results were published July 16 in the journal Nature Nanotechnology. The research was funded by Nanostructures for Electrical Energy Storage, a DOE Energy Frontier Research Center, headquartered at the University of Maryland.