Studies On Microstructures And Electrochemical Properties Of LiFePO₄/C Composites As Positive-electrode Materials Of Secondary Battery Prepared By Sol-gel Method

In this paper, the LiFePO4/C composites as positive-electrode materials of secondary battery are prepared by the following procedure:the phosphate iron lithium is obtained by sol-gel method, modified it by coating carbon with different carbon sources, such as ascorbic acid, D-fructose, after then it is doped with Zn+. The microstructures and morphologies of the LiFePO4/C composites have been characterized by XRD, TA-DTA, SEM and EDS. The electronic densities of these materials have been analyzed by the measurements of positron annihilation lifetime spectra. The electrochemical properties of these materials have been characterized by a. c. impedance spectroscopy, initial charge-discharge test and voltage-current curves. The experimental results are listed as follows:(1) The experimental results indicate that the electron density of LiFePO4/C composite with a lithium/iron ratio of1:1, a carbon/iron ratio of0.1:1, pre-sintering at400℃for3.5h, sintering at800℃for15h is relatively higher and the electrochemical properties of this material is the most optimized.(2) The influence of the coating carbons with2different carbon sources on the microstructure and electrochemical properties of the materials has been discussed. The results of XRD and SEM indicate that the shape and the size of the grain, the morphologies of LiFePO4coated with carbon by using D-fructose as a carbon source is better than that by using ascorbic acid as a carbon source. In analyzing the positron lifetime spectra of the samples, it has been found that the free electron density of LiFePO4coated with carbon by using D-fructose as a carbon source is higher than that by using ascorbic acid as a carbon source. The results of the measurements of electrochemical properties show that the initial charge and discharge capacity of LiFePO4/C coated with carbon by using D-fructose as a carbon source are120mAh/g and82mAh/g, respectively; while the initial charge and discharge capacity of LiFePO4/C coated with carbon by using ascorbic acid as a carbon source are92mAh/g and50mAh/g, respectively; the electrochemical property of the former is obviously better than that of the latter.(3) The effect of the addition of Zn into the LiFePO4/C coated with carbon on the electrochemical property of the material has been studied. It has been found that the crystal structure of the LiFePO4/C composite remain unchanged after doping with Zn. When the content of Zn is less than10%, with the content of Zn increasing, the electrochemical property of the LiFePO4/C composite is getting better; the electrochemical property is the most optimized improvement as the content of Zn in the sample is10%, the initial charge capacity and discharge capacity of this material are169.3mAh/g and121.1mAh/g, respectively; when the content of Zn is larger than10%, with the content of Zn increasing, the electrochemical property of the LiFePO4/C composite is getting worse.