"Fluoride ion batteries" using fluorine are attracting increasing attention as a next-generation storage battery required for a decarbonized society. The performance of this battery to store electricity is improved to 6 to 7 times that of current lithium-ion batteries, enabling lighter and smaller batteries. A pure electric vehicle (EV) that travels 1,000 kilometers on a single charge will come into view. It is expected to become one of the strong competitors for batteries after 2030, and automakers such as Toyota have also started development.
Fluoride ion batteries are charged and discharged by the movement of fluoride ions between the positive and negative electrodes. Compared with lithium batteries, the material composition of the electrode can be simplified, and electrons can be efficiently generated by the reaction at the electrode. It is reported that the energy density (representing the performance of storing electricity) can reach 6 to 7 times that of lithium-ion batteries, which can make lighter and smaller batteries. In theory, a structure that does not use rare metals can also be realized, which is easy to avoid the risk of resource procurement.
Developed a fluoride ion battery using an electrolyte, and verified charge and discharge at room temperature (Photo courtesy of Ritsumeikan University Associate Professor Okazaki Kenichi)
Leading the development in Japan is Kyoto University. In 2017, using a liquid electrolyte (electrolyte), a fluoride ion battery that works at room temperature was successfully developed. Previous studies have mainly focused on batteries using solid electrolytes, and there is a problem that the conductivity of fluoride ions cannot be improved unless a high temperature of 150 degrees Celsius or more is reached. The research team overcame this problem by using materials that dissolve organic fluorides in special liquids called "ionic liquids" as electrolytes.
In 2020, Kyoto University and Toyota jointly produced a prototype for a fluoride ion battery using a solid electrolyte, which has been researched earlier, and confirmed the operation of the battery. Auto manufacturers are very concerned about this field, and Honda Research Institute, a subsidiary of Honda, also released research results on fluoride ion batteries in 2018 together with NASA and others.
However, there are many problems in practical application, for example, the optimal combination of electrode materials and electrolytes has not been found. Performance such as the repeatability of charge and discharge and electromotive force still lag behind lithium-ion batteries. The task is to improve the efficiency of finding materials by analyzing the working mechanism, etc.
In September, a team of associate professor Okazaki Kenichi at Ritsumeikan University (who was an associate professor at Kyoto University at the time of the paper) and others published results analyzing how the combination of electrode and electrolyte materials for fluoride ion batteries affects the electrode reaction. The bismuth used in the positive electrode reacts directly with fluoride ions, and the volume of the electrode changes. The lead used for the negative electrode is dissolved in the electrolyte and reacts with fluoride ions, causing many problems such as crystallization. This difference affects the repeatability of charge and discharge, among other things, and Okazaki said it "has shown a direction for material exploration."
Before 2025, the low-cost fluoride ion battery with low resource procurement risk will be trial-produced by using copper and aluminum as electrode materials. For the type using bismuth and lead, the electromotive force is only about 0.3 volts, and it is hoped that it will be improved to a practical level (2V or more) by changing electrode materials and the like. Okazaki said, "In order to move towards a decarbonized society, I hope to propose options other than lithium-ion batteries."
Japan hopes not to repeat the mistakes of lithium batteries
Lithium-ion batteries were put into practical use in the early 1990s, and have long occupied the leading role in storage batteries for personal computers and mobile phones. Even for pure electric vehicles, it is expected to become mainstream for some time to come.
