Preparation of Lithium Hexafluorophosphate from LiF and P in Fluorine Atmosphere

Pure lithium hexafluorophosphate (LiPF6) has been successfully prepared by the reaction between the elemental fluorine and the equi-molar mixture of LiF and P (F2 direct method). The product was pure enough to be used as an electrolyte salt of lithium secondary battery. Especially, the stepwise introducing of fluorine gas into a reaction system was effective to prepare LiPF6 in a high yield. The results of Rietveld refinement of XRD data revealed that the structure of LiPF6 was trigonal (R3, Z = 3, a0 = 0.4932(2), c0 = 1.2641(5) nm, cell volume; 2.663(2) × 10^-28 m^3). The cell constants of LiPF6 prepared by the F2 direct method were almost the same as those of LiPF6 prepared by the reaction between LiF and PF5 in a liquid anhydrous hydrogen fluoride (L-AHF method).