Study On Synthesis And Modification Of LiFePO₄Used As Cathode Material For Li-ion Battery

The increasing concerns of energy, environmental and the rapid development ofmodern science and technology bring on more demands on lithium-ion batteries. Theolivine structured LiFePO₄has been extensively studied due to its low cost,environmental compatibility, good safety, relatively high specific capacity and longcycle life. However, the electronic conductivity and lithium-ion diffusion of LiFePO₄are poor due to the restriction of its crystal structure, so these are the two main obstaclesfor the use of LiFePO₄as a commercial cathode material. Therefore, synthesisconditions and modifications are of great significance to improve the electronicconductivity and lithium-ion diffusion of LiFePO₄.In this paper, olivine LiFePO₄were chosen as cathode material. The LiFePO₄、LiFePO₄/C and Li1-xNdxFePO₄/C cathode materials were synthesized by solid-statereaction without using inert gas. The effects of thermal processing conditions, carboncoating and ion-doping on the properties of the materials are investigated by TG-DTA,XRD, EDS, SEM, TEM, charge-discharge tests, EIS and CV.The results indicated that the sintering temperature and time have a great effect onthe phase-purity, morphology and the electrochemical performance of the materials, andthe best thermal processing conditions are as following: the sintering temperature is750℃and the sintering time is12h. The samples prepared on this conditions havewell-ordered olivine phase, small particle sizes and best electrochemical performance.In this experiment, carbon coating was adopted to improve the electrochemicalproperties of LiFePO₄. The carbon can not only increase the conductivity of LiFePO₄but also prevent the oxidation of Fe2+. Therefore, the as prepared LiFePO₄/C is smaller and distributed uniformly. The experiments explore the influence of carbon coating onthe properties of LiFePO₄with different coating content, when the carbon content is8.6wt.%, the obtained sample has the best electrochemical properties with the firstdischarge capacity of143.6mAh/g at0.1C rate.Nd-doped LiFePO₄/C samples were synthesized based on carbon coated LiFePO₄.The XRD analysis shows that the perfect olivine structured samples can be prepared andthe Nd3+doping modification does not affect the structure of the samples. After carbonand Nd3+modification, the Li1-xNdxFePO₄/C samples have higher specific capacity andbetter cycle performance. When the doping content is1.0%mol, the obtained samplehas the best electrochemical performance with the first discharge capacity of161.5mAh/g at0.1C rate, and even at2.0C, the capacity remained98.7%after20cycles.