Formation and Propagation of Fluorine-Deficient Phases in Large LaF3 Single Crystals during Electrochemical Defluorination

Abstract

LaF3 is expected to be useful as an electrolyte and an active material of fluoride shuttle batteries (FSBs), which are considered as next-generation batteries with high energy densities. Diffusivity of fluoride ions in solid electrolytes and active materials is a key property for achieving high battery performance of an FSB. In the present work, the formation and propagation of phases with dilute fluorine vacancies at surfaces of single crystals of LaF3 (2.2 × 3.2 × 4.2 mm3 in size) were studied during electrochemical defluorination by using in situ wide-view scanning Raman spectroscopy. The area of the peak at 387 cm–1,which was assigned by first-principles calculations to vibrations of F atoms between La planes, sensitively decreased during defluorination, indicating desorption of F atoms between La planes. During defluorination of pure LaF3, the fluorine-deficient phase extended over 250 μm from the (001) plane where defluorination occurred. During defluorination of Eu(0.9 mol %)-doped LaF3, segregation of the fluorine-deficient phase was observed in the vicinity of the (00–1) plane on the opposite to the (001) plane where defluorination occurred. The diffusivities of fluorine-deficient phases in pure and Eu-doped LaF3 were estimated to be 3 × 10–9 cm2/s to 7 × 10–9 cm2/s and above 4 × 10–7 cm2/s, respectively, which are larger than previously reported diffusion coefficients of F–. The unique method to observe phases with dilute fluorine vacancies and their large scale migration will be useful for the development of superior electrolytes and active materials for FSBs and also other materials.