Room temperature fluoride ion conductivity in defective β-KSb1-δF4-3δ polycrystals

Abstract

For a room temperature operation of fluoride ion secondary batteries, it is strongly demanded for a development of fast fluoride ion conductors. Potassium tetrafluoroantimonate (KSbF4) is an air stable crystalline F− ion conductor that can be synthesized by a liquid phase method. Taking account for the effects of lattice defects in ion conductive crystals, we elucidate and propose that a heat treatment just below the melting temperature (550 K) introduces significant amounts of lattice defects in β-KSb1-δF4-3δ (δ = 0.54), which drastically increases the F− ion conductivity. The F− ion conductivity of the material is now reached into the order of 10−4 S cm−1 at room temperature and 10−2 S cm−1 at 420 K, which are two orders of magnitude higher than those in the sintering at 490 K. Spectroscopic analyses revealed that, during the heat treatment, SbF3 evaporates from the bulk with retaining the lattice framework, suggesting that a number of vacancies at Sb (54%) and F (41%) atomic sites remarkably promote the F− ion diffusivity in β-KSb1-δF4-3δ polycrystals. The present heat treatment method just below the melting point should be applicable to improve the F− ion conductivity of complex salt fluorides.