Urea decomposition enhancing the hydrothermal synthesis of lithium iron phosphate powders: Effect of the lithium precursor

The synthesis of LiFePO₄ (hereafter referred as LFP) using urea as a reducing agent under hydrothermal conditions was investigated. The synthesis was carried out by varying the temperature (150–200 °C), at different times from 6 to 24 h, and using different precursors of Li ions (LiCl, LiNO₃, LiOH, and Li₂SO₄), to determine the effect of these parameters on the crystallization process of the reaction products. These were characterized by XRD, FT-IR, and FE-SEM. The results showed that the differences in the lattice parameters calculated by Rietveld refinement are affected by the synthesis temperature. In addition, the crystallization of single-phase LiFePO₄ powders was achieved by the urea decomposition that occurred at reaction times longer than 12 h and at moderate temperatures (170–180 °C). The crystallization of LFP particles was promoted by a dissolution-precipitation mechanism, but it was not conducted in a single step, because intermediate secondary phases were produced at reaction intervals below 6 h. A crystal growth stage involving the dissolution-crystallization of intermediate secondary phases led to the formation of large particle agglomerates of LiFePO₄ exhibiting a flower-like morphology when the synthesis was carried out using Li₂SO₄. When LiCl, LiOH, and LiNO₃ were used, large bulky agglomerates were obtained.