| Lithium iron phosphate?LiFePO4?is a promising candidate cathode material for lithium ion batteries due to its low material cost?safety and excellent cycling stability in comparision with other positive materials.Because of its low electronic conductivity and low diffusion coefficient,it is highly desirous to improve the properties of LiFePO4 in order to realize the commercialization.In this thesis,in-situ carbon coated LiFePO4/C composites were synthesized by one-step solid-state reaction method.The crystal structure,morphologies and physical properities of the composites was investigated by XRD,SEM,etc.The electrochemical performance of as-synthesized cathode materials were investigated by galvanostatic charge-discharge performance test.The influences of synthetic conditions such as reaction temperature and time have been investigated in detail.The synthesis temperature is the key influence factor on the microstructures and performances of LiFePO4.The crystalline in the sample synthesized at 700? possesse perfect crystal structure,high crystalline degree and few defects.The grain sizes is uniform in distribution.The sample has the best electrochemical performance.Its initial discharge capacity is 142.6mAh/g.Its initial charge and discharge efficiency is 97.8%.It displays good rate and cycling capability.To improve the electrochemical performance and the stability of the LiFePO4/C material,small amuounts of vanadium compound?NH4VO3?was used to dope the LiFePO4/C through the body phase in order to form Li1-xVxFePO4/C composite.The vanadium content?expressed as x=0.01,0.03,0.05?in the composite can influence the electrochemical properties of the materials.The electrochemical test result showed that when x is 0.03 the composite has best charge-discharge abilities.The discharge capacity can reach 152.8mAh/g at 0.1 C rate condition.This result shows that the V doping is effective to improve the eletrochemical properties of the LiFePO4/C positive materials. |
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