Fluoride ion secondary batteries can have effects as shuttlecock secondary batteries. The effects are, for example, high stability, a high energy density, and a high output density. However, the fluoride ion secondary batteries are still in the process of research and development. If an electrolyte material having a high fluoride ion conductivity can be found, the performance of the fluoride ion secondary batteries can be improved. The present inventors have diligently studied and, as a result, have found a material of the present disclosure showing a high fluoride ion conductivity. According to the present disclosure, for example, a fluoride ion secondary battery having high performance is achieved.

Various Aspects of the Present Disclosure

A fluoride ion conductor according to a first aspect of the present disclosure contains rubidium, magnesium, and fluorine. In an average composition of the fluoride ion conductor, the ratio of the number M_Mg of moles of the magnesium to the total number M_Rb+M_Mg of moles of the rubidium and the magnesium, M_Mg/(M_Rb+M_Mg), is less than 0.4.

When the fluoride ion conductor has a single-phase structure of a compound phase, the term “average composition” refers to the composition of the compound. Alternatively, when the fluoride ion conductor has a multiple-phase structure of a compound phase and a mixture phase, the term “average composition” refers to the composition obtained by conducting elemental analysis without considering distinction of each phase of the fluoride ion conductor. The phrase “without considering distinction of each phase” typically means a composition obtained by conducting elemental analysis using a sample having a size larger than that of raw material powder of the fluoride ion conductor.