KIT researchers develop fluoride-ion batteries; potential for much higher energy densities than conventional Li-ion batteries

Setup of the fluoride-ion battery: A fluoride-containing electrolyte separates the metal anode from the metal fluoride cathode. Click to enlarge.

Researchers from the Karlsruher Institut für Technologie (KIT) have developed and demonstrated secondary (i.e., rechargeable) battery cells based on a fluoride shuttle. A paper on their work is published in the RSC Journal of Materials Chemistry. The team also presented the work at the recent 220^th meeting of the Electrochemical Society (ECS) in Boston.

Metal fluorides may be applied as conversion materials in lithium-ion batteries. They also can enable lithium-free batteries comprising a fluoride-containing electrolyte; a metal anode; and metal fluoride cathode. Instead of the lithium cation (as in Li-ion batteries), the fluoride anion takes over charge transfer. The resulting fluoride-ion batteries can reach a much better storage capacity and possess improved safety properties, according to the team.

In the search for new concepts to build batteries with high energy densities it may be interesting to transfer the well-established, so-called rocking-chair principle of Li-ion batteries also to other ions so that electrochemical cells can be built which do not necessarily contain lithium but have the potential to reach theoretical energy densities which are considerably beyond the state-of-the art.

Fluoride ion batteries are capable of meeting this high energy density demands. The reaction of highly electronegative fluorine with metal leads to the formation of metal fluorides which are accompanied by large change in free energy and thus the high voltage. By choosing appropriate metal/metal fluorides, high voltage electrochemical cells can be built. Though few reports are known towards this end, such battery chemistry was largely overlooked in the past years. The recent report by Potanin et al. revised the interest in this field.

—Reddy et al., ECS 220

The fluoride anion acts as a charge transfer ion between a metal/metal fluoride pair where it will react with metal or evolve from metal fluoride depending on the flow of current—i.e. at the cathode and anode, a metal fluoride is formed or reduced, depending on whether the battery is charging or discharging. The fluoride-ion batteries could surpass the storage capacity reached by lithium-ion batteries, according to Drs. Maximilian Fichtner and Munnangi Anji Reddy.

As several electrons per metal atom can be transferred, this concept allows to reach extraordinarily high energy densities—up to ten times as high as those of conventional lithium-ion batteries.

—Dr. Maximilian Fichtner

Composition of the electrodes and architecture of the samples used in the study. Credit: RSC, Reddy et al. Click to enlarge.

In the Journal of Materials Chemistry study, they prepared five different cathode composites comprising a metal fluoride, carbon, and La_0.9Ba_0.1F_2.9 electrolyte, and paired those with the La_0.9Ba_0.1F_2.9 electrolyte and a cesium metal (Ce) anode.

The KIT researchers are now working on the further development of material design and battery architecture in order to improve the initial capacity and cyclic stability of the fluoride-ion battery.

Another challenge lies in the further development of the electrolyte. The solid electrolyte applied so far is suited for applications at elevated temperatures only; the researchers are looking for a liquid electrolyte that is suited for use at room temperature.

Source: Green Car Congress