High fluoride-ion conductivity and fluoride-ion conductor−insulator transition in fluorinated hexagonal boron nitride

Abstract Several two-dimensional (2D) materials exhibit a rich variety of transport properties from insulating to metallic conduction, and further to superconductivity. However, almost all charge carriers for ionic conduction have been so far limited to the cations such as sodium ions and protons. Here, we report a superhigh in-plane fluoride-ion conductivity of 200 mS cm−1 at room temperature for 2D fluorinated boron nitride (F-BN). F-BN also exhibits unique fluoride-ion transport at higher temperatures. An ionic conductor–insulator transition (iCIT) is responsible for the temperature variation of fluoride-ion conductivity at higher temperatures. The most drastic iCIT shows a change of approximately six orders of magnitude in conductivity. Neutron diffraction and X-ray photoemission spectroscopy measurements reveal that the mobile fluoride ions in the 2D void space are trapped by the formation of covalent N–F bonds, which is the primary factor influencing the iCIT. Tuning the bonding state yields exciting guidelines for designing fluoride-ion conductors, with broad implications for fluoride-ion batteries, chemical sensors, and heat-responsive on/off-switching electric devices.