Lithium-ion batteries serve as lightweight, rechargeable power sources for everyday technology such as portable electronics, power tools and electric vehicles. However, researchers are seeking a next-generation battery with high energy density and low cost that can surpass those made with lithium. One possibility is a fluoride-ion battery based on a fluoride shuttle, which could theoretically provide high energy densities.
Takami et al. report on an all-solid-state battery using ternary oxyfluoride as a cathode that demonstrates charge/discharge behavior via mobile fluoride ions. While most research has been devoted to binary metal fluorides as model electrodes, they have not met all desired conditions required for commercialization. Therefore, the researchers aimed to develop new materials to explore their potential as cathodes in fluoride-ion batteries.
The capabilities of ternary or higher order oxyfluorides have not been explored up to this point because synthesis is more difficult than for binary metal fluorides. In the current work, chemical fluorination with polyvinylidene difluoride resulted in a previously unrecognized quaternary Bi0.7Fe1.3O1.5F1.7 (BFOF) compound. The researchers investigated its crystal structure with X-ray diffraction and neutron powder diffraction.
Next, they performed electrochemical charging/discharging tests with BFOF as the cathode in a fluoride-ion battery. At 140 degrees Celsius, it had a discharge capacity of 360 mAh/g, and a charge capacity of 225 mAh/g. The results suggest that the novel quaternary BFOF compound has promise as a cathode of fluoride-ion batteries, and its performance could be improved based on engineered guidelines established for lithium-ion batteries.
In terms of future work, the authors wish to explore other multinary oxyfluorides as possibilities for cathodes to achieve higher capacities.
Source: “A new Bi0.7Fe1.3O1.5F1.7 phase: Crystal structure, magnetic properties, and cathode performance in fluoride-ion batteries,” by Tsuyoshi Takami, Takashi Saito, Takashi Kamiyama, Katsumi Kawahara, Toshiharu Fukunaga, and Takeshi Abe, APL Materials (2020). The article can be accessed at http://doi.org/10.1063/5.0005817.
