Preparation, Structure And Electrochemical Properties Of LiFePO₄/C Nanosized Composite Cathode Materials

Olivine-structured LiFePO4as the cathode material has the advantages of high theoretical capacity(170mAh/g), good cycle performance, being environmentally benign and inexpensive, preferable safety, that LiFePO4 is considered as a promising cthode material for lithium-ion batteries. However, it suffers from low electronic conductivity and low lithium ion diffusivity, which prevents it from commercial use. In order to overcome these defaults, the researches focus the way of synthesizing small fine particles of LiFePO4, carbon coating and ion doping.LiFePO4/C composite cathode material was synthesized by liquid phase method using Fe(NO3)3·9H2O and LiH2PO4 as raw materials and using polyethylene glycol (PEG) as template and carbon source. The samples were characterized by XRD, SEM, TEM, CV, EIS and constant current charge-discharge experiments.At the preparation using single molecular weight PEG as carbon source, the results showed that with the increase of molecular weight of PEG, the particle size reduced, and the sample prepared with PEG10K(Mw=10000) revealed the best electrochemical properties and a discharge capacity of 165.3mAh/g under 0.1C was obtained with the capacity fading of 1.7% over 30 cycles. At the rate of 1C, the discharge capacity arrived at 128.5mAh/g. At the rate of 5C, the discharge capacity arrived at 105.4mAh/g.And we also researched the modification using both PEG with high molecular weight and PEG with low molecular weight as carbon source. The results showed that using both PEG with high molecular weight and PEG with low molecular weight as carbon source was better than using only PEG as carbon source in controlling the morphology and size of the particles. And the sample using both PEG200 and PEG35K(Mw=35000) as carbon source showed the best electrochemical performance with a fine particle size of 50nm below. Its initial discharge capacities under 0.1C and 1C rates were 168.3mAh/g and 133.5mAh/g, and the initial discharge capacities under 5C and 10C rates were 110.4mAh/g and 95.2mAh/g.