Temperature-dependent crystallinity and morphology of LiFePO4 prepared by hydrothermal synthesis

Lithium iron phosphate (LiFePO4) was synthesized by a hydrothermal method at 120, 140, 150, 160 and 175 degrees C from LiOH, FeSO4 and H3PO4 in an organic-free aqueous solution. The intermediates of the product synthesized at 175 degrees C were sampled at 120, 130 and 135 degrees C during the temperature rising period. The Rietveld refinement results of X-ray powder diffraction (XRD) show that the unit cell volume of LiFePO4 prepared at 120 degrees C is 0.2915 nm(3), which decreases to 0.2900 +/- 0.0001 nm(3) as the reaction temperature reaches above 140 degrees C. The results from scanning electron microscopy (SEM) show that the crystal morphology develops from a diamond plate to a polygonal plate and the thickness of the flakelet decreases from 130-150 nm to 80-90 nm with an increasing temperature from 120 to 175 degrees C. A critical temperature, ca. 135 degrees C, for the rapid nucleation is found from the XRD results of the intermediates. The variations in crystallinity, morphology and size between the samples prepared under and above 140 degrees C are due to the varied rates of nucleation and crystal growth of LFP particles, which are dependent on the concentration of Fe2+ and PO43- resulting from the dissociation of the intermediate Fe-3(PO4)(2)center dot 8H(2)O. The sample prepared at 160 degrees C exhibits specific discharge capacity of 161.2 mAh g(-1) at 25 degrees C and 121.6 mAh g(-1) at -20 degrees C measured at a current density of 17 mA g(-1).