A research team led by Professor Tsuyoshi Takami of Otemae University, in collaboration with Nagoya University and the High Energy Accelerator Research Organization, has used a compound with electrons present between lattices as a solid electrolyte capable of diffusing fluoride ions under an electric field for the first time, and demonstrated that fluoride ions are conducted through an exchange reaction between fluoride ions and anion electrons.

 An all-solid-state fluoride-ion battery is a storage battery in which fluoride ions travel between positive and negative electrodes through a solid electrolyte to charge and discharge. It has several times the capacity of a lithium-ion battery and is said to be highly stable and durable for long periods of use, making it a promising next-generation high-performance storage battery for the realization of a carbon-free society.

 Until now, compounds containing lanthanum, barium, and fluorine have been commonly used as solid electrolytes, and a mechanism has been used to create vacancies in which fluoride ions can move, thereby conducting the fluoride ions, but there have been limits to how much conductivity can be improved.

 A compound in which electrons are fixed at specific positions as anions (negative ions) is called an electride, and the electrons are called anion electrons. This time, the research group synthesized an electride made of barium, nitrogen, and fluorine. In order to suppress Ba impurities, they devised the selection of Ba3N2, which contains nitrogen, as the raw material powder. The addition of sodium or potassium showed a tendency for electrical conductivity to increase.

 Furthermore, they came up with the idea that the anion electrons between the lattice hinder the smooth conduction of fluoride ions, and attempted an exchange reaction between the anion electrons and fluoride ions in xenon fluoride. This demonstrated the exchange reaction and also demonstrated that fluoride ions were conducted.

 This is the first time that fluoride ion conductivity has been achieved using a material derived from an electride, and it is expected that this will lead to the exploratory development of new solid electrolytes for the development of all-solid-state fluoride ion batteries.

Paper information:[Chemistry of Materials] Topochemical fluoride exchange reaction with anionic electrons toward fluoride-ion conduction in layered Ba2-xAxNF1-x (A = Na, K)

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