The Research On Synthesis And Modification Of LiFePO₄as The Cathode Materials Of Lithium-ion Secondary Battery

lithium iron phosphate (LiFePO4) has long been considered as one of the mostpromising cathode materials due to its advantages of low cost, high theoreticalcapacity, long cycle life, enviornmental friendliness and nontoxicity. However, thepoor electronic conductivity and low lithium-ion diffusion coefficient result in thepoor high-rate performance of LiFePO4, which limits its commercial applications. Inthis paper, LiFePO4was prepared by the traditional high temperature solid-statemethod and simple complexing sol-gel method respectively. Various modificationmethods were employed to enhance the its electrochemical performances. Theresearch aims at finding a method, which is beneficial for scale production ofLiFePO4and ensure its excellent electrochemical performance at the same time. Themain results are in the following:1. LiFePO4was synthesized via high temperature solid-state method by usingLi2CO3, FeC2O4·H2O and NH4H2PO4as main raw materials. Furthermore, themodified LiFe0.96Ti0.02PO4/C composite was prepared with PVA as carbon source andTiO2as dopant respectively. The results indicate that the doping and carbon coatingdo not destroy the crystal structure of active material, and LiFe0.96Ti0.02PO4/Cmaintains its excellent structure stability. Moreover, the low-temperature andhigh-rate performance of LiFe0.96Ti0.02PO4/C are much better than that of LiFePO4. Itsdischarge capacities at the rate of0.1C under0℃and5C under30℃are128.7mAh/g and97.4mAh/g respectively.2. The four process parameters, such as predecomposition temperature, calciningtemperature, sintering holding time and the pH of liquid system of LiFePO4synthesized via complexing sol-gel method were investigated. The results indicatethat all the process parameters have important effects on the morphologies andelectrochemical perfomances of samples. The best condition is as follow:predecomposition temperature is350℃，calcining temperature is700℃, sinteringholding time is15h and the pH of liquid system is3. Sample prepared with the bestprocess condiction possesses perfect crystallization, uniform and small particles,which have rule morphology and good monodispersity. The initial discharge capacityof at0.1C and charge-discharge efficiency are130.1mAh/g and93.5%. Its capacityretention ratio after20charge-discharge cycles is as high as98.2%..3. The nonionic surfactant PVP was added to the sol system to control the morphologies of the particles of samples. The effects of molecular mass and additiveamount of PVP on the electrochemial perfomances of LiFePO4/C was investigated.The results indicate that the molecular mass of PVP have important effect onmorphologies of the particles, and the additive amount plays a critical role in thecontent of coating carbon of the samples. When1g of PVP-k30was added to theliquid system for preparing0.02mol LiFePO4, sample prepared possesses not onlyuniform, small particles and rule morphology, but also a moderate content of coatingcarbon and perfect conductive network, as a consequence, sample prepared shows thebest electrochemical performance compared with samples synthesized with othersynthsis conditions: its discharge capacity of132.2mAh/g at the rate of1C.4. The research proposed a comparative study on the morphologies andelectrochemial perfomances of LiFe0.96Ti0.02PO4/C synthesized via high temperaturesolid-state method and complexing sol-gel method respectively. The results indicatethat the particles of LiFe0.96Ti0.02PO4/C prepared through complexing sol-gel methodare more regular, more homogenous and smaller. Moreover, the sample preparedpossesses better conductive network and doping uniformity. As a consequence, itpossesses better high-rate performance with a discharge capacity of91.3mAh/g at therate of8C.