Study On LiFePO₄/C Composite Cathode Materials Prepared By Sol-gel Route

Olivine-structured LiFePO₄, due to its advantages such as inexpensive, nontoxic, good cycle electrochemical properties and environmentally friendly, is considered as a promising cathode material for lithium-ion batteries in place of LiCoO₂, which is expensive and toxic. However, it suffers from low conductivity and poor rate performance, which prevents it from commercial use. In this paper, the recent developments on LiFePO₄ were reviewed.In the aims of improving the electrochemical properties of LiFePO₄ as a cathode material.Carbon-coated LiFePO₄ composite (LiFePO₄/C) cathode materials were synthesized by sol-gel route. The micro-structures and morphologies of these composites were investigated by XRD, SEM, and TEM. The electrochemical properties of LiFePO₄/C composite cathode materials have been evaluated by galvanostatic charge-discharge, cyclic voltammetry(CV) and electrochemical impedance spectra (EIS). The effects of the synthesis parameters including sintering temperature,time and atmosphere,et al, on the physical-electrochemical properties of carbon-coated LiFePO₄ were systematically investigated.Different carbon source,such as glycol, citric acid and polypropylene were used for preparing LiFePCVC. LiFePCVC composite were synthesized by sol-gel route both on a so called one-step solid-state reaction by using Fe³+ compounds as iron source and glycol as conductive carbon source,and on a so called two-step solid-state reaction by using Fe²+ compounds as iron source and citric acid as conductive carbon source.The results show that in the one-step fabrication condition, increasing the sintering temperature led to higher crystallinity, but also to a larger particle size of LiFePCVC. In the sintering temperature range of 600⁸50℃ , 700℃ is the optimum synthetic temperature for the carbon-coated LiFePO₄ of which being with both small particle sizes and perfect crystal structure, which are the two key factors to enhance the electrochemical properties. LiFePCVC with low carbon content of 0.1 wt% only displayer a discharge capacity of 86.4mAh/g. However, the electrochemical properties of the material were improved when the carbon content of LiFePCVC was up to 2.6wt% and citric acid was used as carbon source instead of glycol.The LiFePCVC material display satisfiable electrochemical properties when prepared by two-step solid-state reaction by using Fe2+ compounds as iron source at a sintering temperature of 700℃. The effect of sintering times ranging 2.5h to 24h on the microstructure and electrochemical properties of LiFePCVC were also discussed.Results showed thatsintering time is an effective factor on the particle size and the homogeneity of LiFePCVC synthesized.Fine and homogeneous particle size favors the eletrochemical properties of LiFePCVC. The high rate discharge ability of LiFePCVC was improved by increasing sintering time, however,increasing sintering time also leads to particle growth, which decreases the discharge capacity. lOh is the optimum synthetic time for the carbon-coated LiFePC>4 with an all-around electrochemical performance according to the study of this thesis. The electronic conductivity of LiFePCVC was enhanced with increasing carbon content.The discharge capacity of the LiFePCVC cathode reached increased 138.3 mAh/g when carbon contant reaches 12.4wt%. Electrochemical impedance spectra indicated that overall impedance of LiFePCVC sintered for lOh is lower than that of either sintered for 2.5h or for 24h.The synthetic atmosphere, the fraction of H2 in N2+H2 mixture, affected effectively on the electrochemical properties of LiFePCVC.The discharge capacity of LiFePCVC lowered when the content of H2 was increased. Due to higher H2 content caused higher content of impurity in LiFePCVCthus lowered the discharge capacity and the high rate discharge ability when H2 content was 20%. 5%H2 is the optimum content for the synthesis atmosphere.LiFePCVC composites cathode materials prepared by carbon sources of glycol,citric acid, and polypropylene all showed long cycle life.However, LiFePCVC prepared from glycol and polypropylene displayed an unsatisfiable kinetics performance and poor high rate performance because of polarization. Citric acid is the optimum carbon source choice for the carbon-coated LiFePC<4 with an all-around electrochemical properties.