Concentration-Controlled and Phytic Acid-Assisted Synthesis of Self-Assembled LiFePO4 as Cathode Materials for Lithium-Ion Battery

Abstract

The olivine LiFePO4 with various morphologies and different growth lattice planes was prepared by a controllable hydrothermal method with changing precursor concentration and using phytic acid as phosphorus source. The microstructure, crystal orientation and electrochemical performance of the prepared samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HR-TEM) and charge–discharge tests. The results show that the morphologies of all samples change from spindle-like to hierarchical plate-like and then to long plate-like shape, and the main exposed facets transform from (100) to (001). This indicates that the precursor concentration and phytic acid play important roles in exposing facets and controlling the morphology of LiFePO4. In order to illustrate these phenomena, a reasonable assembly process is provided and the formation is explained. Li ion diffusion coefficient along [100] and [001] directions was calculated by using electrochemical impedance spectroscopy (EIS). The results show that the diffusion coefficient of (100) facet is higher than that of (001) facet, indicating a good electrochemical performance for (100) facet. In addition, the capacity test is carried out, which also confirms the above results. With the precursor concentration of 0.5[Formula: see text]M, the obtained LiFePO4 with self-assembled hierarchical structure, smaller size and (100) facet shows the best electrochemical performance: 162.1[Formula: see text]mAh/g at 0.1[Formula: see text]C and 112.4[Formula: see text]mAh/g at 10[Formula: see text]C. Using phytic acid as phosphorus source and controlling precursor concentration to prepare high performance LiFePO4 open up a new prospect for the production of cathode materials for lithium ion batteries.