Morphology evolution and impurity analysis of LiFePO4 nanoparticles via a solvothermal synthesis process

Abstract

A solvothermal method is applied for synthesizing LiFePO₄ nanoparticles using ethylene glycol as solvent. Crystals are obtained with quite different morphologies at solutions of various acidity prepared via changing the primary LiOH/H₃PO₄ mole ratios. SEM, TEM, and HRTEM are used to analyze the samples. Element distribution in solid LiFePO₄ particles, mother solutions and washing solutions are tracked by ICP-OES and pH tests. Morphological test results show that the main exposed faces of samples transform from (100) as a rectangular shape to (010) as a spindle shape with the pH of the mother solutions increasing. Samples with predominant (010) faces are formed at less acidic solvothermal solutions. At the intermediate pH from 3.11 to 3.73, powders like long hexagon nanorods are synthesized with (100) and (010) faces exposed. XRD results show that the long hexagon nanorods have better crystal structures when synthesized at LiOH/H₃PO₄ = 2.7–3.0. Impurities like Fe₃O₄, Li₃PO₄, etc. are detected in the spindle shape LiFePO₄ powders. The amount of impurities is related to the synthesis process and increases with the pH of solvothermal solution increasing. High temperature treatment is useful for impurities transforming to LiFePO₄ and thus reduces the impurities. The long hexagon nanorods show better electrochemical performances: 169.9 mA h g⁻¹ at 0.1 C, and 129.8 mA h g⁻¹ at 10 C.