Synthesis Of LiFePO₄/C Composites By Microwave Heating Combined With Hydrothermal Method

LiFePO4has been considered as one of the most promising cathode materials for use in the electric and hybrid electric vehicles. However, when it comes to commercial application, there is a contradiction between its high electronic performance and its tap density. In this study, we are trying to figure out the problem above by the following work:(1) Microwave was used to heat the reaction system which contains the glycol-water as a solvent. Because microwave can heat the material on molecular level and make the system reach the desired temperature quickly, there are more crystal nuclei formed. Herein the impacts of different synthesis conditions on properties of the LiFePO4material were studied systematically to find out the optimal reaction condition.(2) A novel synthesis method has been developed to improve the particle morphology with the assistance of hydrothermal method. Following this line, we mainly considered two aspects to improve the properties of the LiFePO4material. One is to mitigate lithium diffusion limitations with a shorten diffusion lengh in LiFePO4by reducing the particle size and the other is to improve the electronic conductivity by carbon coating. To fulfill these targets, we investigated the reaction conditions during the hydrothermal progress. The result shows that pure spherical-like LiFePO4particles whose size is1-2μm can be prepared in the optimal condition and a uniform particle distribution can be obtained. Furthermore, sucrose coating delivers a better electrochemical performance than glucose. It exhibits initial discharge capacities of140,142and119mAh·g-1at rates of0.1C and1C respectively.The tap density turns out to be1.2g·cm-3.(3) Based on the studies above, the particle structure has been designed to solve the contradiction between the electrochemical performance and the tap density. In this process, several strategies have been employed to reach the aim. The result shows the LiFePO4/C sample consists of monodispersed spherical particles and the diameter of the microspheres is about1～2μm. It can be clearly seen that these microspheres are composed of many nano-particles with an average size about100～200nm. The LiFePO4/C composite which tap density is1.2g·cm-3exhibits a discharge capacity of158m·Ah g-1and150mAh·g-1at0.1C and1C charge/discharge rate respectively.