Synthesis and characterization of nano-sized LiFePO4 by using consecutive combination of sol-gel and hydrothermal methods

Highlights: • Nano-sized LiFePO₄ was synthesized by merging sol-gel and hydrothermal methods. • Both samples have well olivine crystal structure and plate-like morphology. • The capacities of HY-SO-LiFePO₄ are close to those of HY-LiFePO₄ at low C. • HY-SO-LiFePO₄ displays higher capacities than those of HY-LiFePO₄ at high C. -- Abstract: Lithium iron phosphate (LiFePO₄) was synthesized by means of a new route which is based on the combination of sol-gel and hydrothermal methods (HY-SO-LiFePO₄). The results of HY-SO-LiFePO₄ were compared with those of LiFePO₄ which was synthesized by using only hydrothermal method (HY-LiFePO₄). The crystalline structure and morphology of LiFePO₄ nanoparticles were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). Based on XRD data, LiFePO₄ powders have a well olivine crystal structure with a space group of Pnma. The slight decrease of crystalline lattice parameters in HY-SO-LiFePO₄ was observed compared to that of HY-LiFePO₄. LiFePO₄ powders have homogeneous distribution of nanoparticles with a plate-like morphology. Also, the plate length decreases from 300-500 nm to 150–350 nm if sol-gel and hydrothermal methods are consecutively used together. The as-prepared LiFePO₄ coin cells were characterized via cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS), and their charge/discharge experiments were performed at different current rates in a range of 2.5-4.2V vs. Li/Li⁺. The discharge capacities of HY-SO-LiFePO₄ were found as 126 mAhg⁻¹ at 0.2C and 70 mAhg⁻¹ at 3C. Meanwhile, HY-SO-LiFePO₄ cathode exhibits a stable charge/discharge cycle ability (>97.5% capacity retention after 100 charge/discharge cycles compared with HY-LiFePO₄ cathode which is 77.7% at 0.5C). The overall experimental results revealed the idea that positioning the wet gel inside reactor may impede the growth of grains and lead to the formation of smaller LiFePO₄ nanoparticles with a narrow size distribution during reactive synthesis procedure. Hence, these results improve the electrochemical performance of cathode material.