Synthesis, Structure And Electrochemical Performance Of Olivine LiFePO₄ As Cathodic Materials For Lithium-ion Batteries

Lithium ion batteries have become the primary choice of the green secondary batteries in the 21th century due to their high voltage,high energy density,no memory effect and non-pollution.At present,lithium cobaltate(LiCoO₂) is a major cathode material used in commercial lithium ion batteries.However,because of its expensive cost,toxicity and safety, it couldn't meet lithium ion batteries development.This work aims to develop the olivine LiFePO₄ cathodic material with low cost,safety and excellent electrochemical performances. The spray-drying process and solid state reaction for synthesizing LiFePO₄/C materials were investigated in detail.The structure and morphology of these products were investigated by means of XRD and SEM.The electrochemical performances were evaluated by galvanostatic charge-discharge cycling and cyclic voltammetry tests.The effects of F ion doping on the electrochemical performances were also investigated.In Chapter 3,olive LiFePO₄/C powder was prepared by a spray-drying method,in which the corresponding acetate salts as the reagents.The product showed high purity,good crystallinity,small particle sizes and homogenous size distribution.It was found that the as-prepared precursors were pre-sintered at 400℃for 1 h and re-sintered at 700℃for 6 h, and the resulting LiFePO₄/C powder exhibited good electrochemical performance.The initial discharge capacity of the cathode at 0.1 C rate is 161.5 mAh·g^-1 and 130.3 mAh·g^₁ at 1 C rate, respectively.And also it exhibited excellent cycle stability.The particle size of LiFePO₄/C is about 0.5-1μm,and it is distributed homogeneously.The homogeneous distribution of nano-sized carbon webs is effective in enhancing the electronic conductivity of LiFePO₄ and hindering the particle growth,consequently improving its electrochemical performance.In Chapter 4,a simple method for synthesizing carbon-coated LiFePO₄ material has been developed,using inexpensive Fe³⁺ compounds as the iron source,citric acid as the reductive agent and the conductive carbon source.The carbon coating and the formation of LiFePO₄ crystal were carried out synchronously.The advantages of this method are the direct synthesis of carbon-coated LiFePO₄ powders withiout any post-treating,low cost,simple procedure and easy to be scalled up.It was found that hydrogen and carbon generated from pyrolysis of polypropylene play the key roles in the synthesis process:(1) hydrogen and carbon as the reductive agents for the reduction of Fe⁺³ to Fe⁺³;(2) carbon as the electronic conductor for the enhancement of the LiFePO₄ conductivity;(3) carbon as the obstructer film for the hinderment of LiFePO₄ particle growth.The first discharge capacity of the powder synthesized at 700℃is 143.3 mAh·g^-1 at 0.1 C,110.3 mAh·g^-1 at 0.5 C,97.8 mAh·g^-1 at 1 C rate.After cycling 50 cycles at 0.1 C rate,the capacity was over 90%of primary capacity.