A new Bi0.7Fe1.3O1.5F1.7 phase: Crystal structure, magnetic properties, and cathode performance in fluoride-ion batteries

We synthesize a Bi0.7Fe1.3O1.5F1.7 (BFOF) phase via a non-topochemical reaction with a fluorination agent. The crystal structure is refined by Rietveld refinement on the neutron diffraction patterns as a hexagonal lattice in the R3¯ space group, along with the defect structure. The sudden decrease in magnetic susceptibility below 250 K and the linear relationship between the magnetization and the magnetic field indicate that BFOF is an antiferromagnetic material. When BFOF is used as a cathode in fluoride-ion batteries (FIBs), a discharge (charge) capacity of 360 (225) mAh/g is achieved at 140 °C. Magnetization and x-ray diffraction measurements confirm that the F ions are transferred from the cathode to the Pb counter electrode during discharge and in the opposite direction during charge, in a manner analogous to the transfer of lithium (Li) ions in Li-ion batteries. These findings contribute to the development of quaternary oxyfluorides serving as FIB cathodes.