The Synthesis Of LiFePO₄ As Cathode Material And The Investigation Of Its Electrochemical Properties In Lithium Ion Battery

At present,although olivine-structured lithium iron phosphate has occupied half of the battery industry market in China,it still has some disadvantages,for example,The lithium ion battery production cost is too high because of the high price of the raw materials;the overcharge and the overdischarge phenomenon are easy to happen in the practical application,therefore,the circuit board protection is needed in the production process;now there is mainly solid-state method with high temperature in the industry,this method has high energy consumption,long cycle and large partical size,and these problems lead to the poor electrochemical performance.And these shortcomings will hinder the promotion of LiFePO₄ battery,in order to get through the bottleneck,some measures must be taken to offset its own shortcomings.First of all,we must start with the synthesis method and technology.In this paper,a green environmental method was designed to prepare LiFePO₄ cathode material.Firstly,LiFePO₄ was synthesized by a two step liquid-phase method,and then the LiFePO₄ was sintered at high temperature in inert gas.Firstly,the pH values of the solution for the preparation of the FePO₄·xH₂O precusor were studied.The structure and the morphology of FePO₄·xH₂O and LiFePO₄ were analyzed by X-ray diffraction?XRD??scanning electron microscopy?SEM?and so on.The effect of the precusor FePO₄·xH₂O prepared under different pH values on the properties of LiFePO₄ was investigated.The results show that no impurities exist in the FePO₄·xH₂O synthesized under different pH values from 1 to 5.But LiFePO₄ prepared from FePO₄·xH₂O synthesized with the pH value of 2 shows excellent electrochemical performance.The first discharge specific capacity is 132mAh·g-1 at 0.1 C,and capacity retention remains 98.9% after 10 cycles.The selection of iron source and calcination temperature and time for the synthesis of LiFePO₄ was further studied.FePO₄·xH₂O precursor synthesized with the pH value of 2 and the FePO₄·xH₂O purchased were regarded as the iron source to prepare LiFePO₄ respectively,and then study their performances.The results show that the LiFePO₄ prepared with the former precursor FePO₄·xH₂O has the better performances.And the effect of calcination condition on the properties of LiFePO₄ cathode materials is great.LiFePO₄ sintered at 700 °C for 2 h has uniform particlesizes and better crystallinity.At 0.1 C,its discharge specific capacity is 137.3 mAh·g-1,and the capacity is 136.2 mAh·g-1 after 10 cycles.In order to increase the electrochemical performances of LiFePO₄,the modification of carbon coating and doping metal ions?Mn2+,Mg2+,Ni2+,Zn2+?were studied.The results show that when the amount of glucose as carbon source was 10%,LiFePO₄/C has the best electrochemical performance: Its discharge specific capacity is 158.6 mAh·g-1 at 0.1 C,and capacity retention remains 98.4% after 10 cycles.And when LiFePO₄ was doped with Mg2+,the material has the best electrochemical performance,at 0.1 C,after 10 cycles the discharge capacity decreased from 146.7mAh·g-1 to 141.7 mAh·g-1,and capacity retention remains 96.6%.