Synthesis Of LiFePO₄/C Cathode Materials Via Sol-Gel Method And The Key Factors Influencing The Structure And Electrochemical Performance Of LiFePO₄/C

Olivine structured LiFePO₄ is a promising cathode material for the next generation of lithium ion batteries for its low cost,environmental benign,good cycling performance and safety,etc.However,its intrinsic poor electrical conductivity and diffusion capability of lithium ion greatly hinder its application in the high power rechargeable battery.The improvement of its high-rate performance is key important to realize the application in high power rechargeable batteries.In this thesis,a sol-gel method was used to synthesize carbon coated LiFePO₄(LiFePO₄/C) materials.The effects of the synthesis technique and parameters, including the valence of the iron source(Fe³⁺ and Fe²⁺),chelating agents and carbon sources (citric acid and ethylene glycol),the pH value of the sol,pre-treatment of the dried gel,the sintering temperature,time and atmosphere(N₂ with different content of H₂) on the structure and electrochemical properties of the LiFePO₄/C were systematically investigated by means of XRD/Rietveld,SEM,TEM,elemental analysis,etc.and galvanostatic charge-discharge, EIS,CV etc.The relationships between the structure and electrochemical properties of LiFePO₄/C materials,and the key factors that influence the structure and the rate capability, especially the high-rate capability of LiFePO₄/C,have been revealed.The results of the investigation show that LiFePO₄/C materials can be successfully prepared by the present synthesis parameters,and the structure and morphology of the materials have been optimized by modifying the synthesis parameters.LiFePO₄/C materials with low amount of carbon coating(2 wt%),small particle size and suitable amount of high electrical conductive phases of iron phosphides have been obtained,showing capacities of 157,124,92 and 70 mAh/g at 0.1 C,1 C,5 C and 10 C,respectively.The LiFePO₄/C materials in the present work all display good cycling performance.The rate capacities are affected by the content,the crystallization,the particle size and its distribution of LiFePO₄,and the content of iron phosphides.However,the above factors act differently on the LiFePO₄/C materials at various discharge rates.At the low discharge rate, the content,crystallization and the particle size of LiFePO₄ play key effect on the capacity. High content,well crystallization and small particle size of LiFePO₄ lead to the high capacity. But with the discharge rate increasing,certain amount of high electronic-conductive iron phosphides becomes key important on the improvement of the high rate capability.Iron phosphides can greatly enhance the utilization of LiFePO₄ at high discharge rate.Combing with the small size of LiFePO₄/C,low amount of Fe₂P of 2 wt%can obviously increase the rate capability.Even when the particle size of LiFePO₄ is a bit larger,certain content of Fe₂P can also improve the high rate performance pronouncedly.The high rate capability of LiFePO₄/C material increases with the increase of the Fe₂P content within a certain range. However,too much Fe₂P decreases the capacity due to the decrease of the fraction of the active material of intercalation/deintercalation of lithium ion.EIS and CV studies confirm that the kinetic property of LiFePO₄/C is obviously improved by the reduction of the particle size and the proper amount of iron phosphides,which is consistent with the high rate performance of LiFePO₄/C.