Synthesis And Propetries Of The Polyanion-type LiMPO₄（M=Fe,Mn） And Li₃V₂（PO₄）₃Cathode Materials For Li-ion Batteries

Lithium-ion batteries have attracted extensive attention due to the superiorenergy density, environmental benignity, outstanding cycling performance and nomemory effect, etc. At present, it has been widely used in every aspect of our lives,such as cell phones, laptop computers, digital cameras, and energy storage devices forsolar and wind electricity generation. Usually, the used anode materials of lithium ionbatteries are graphite, and cathode materials have become the important factor for theproperties and price of Li-ion batteries. In1997, Goodenough et al. proposed LiFePO₄as cathode material due to its good structure stability, high safety and excellentcycling performance. Since then, some polyanion materials have been investigated toimprove the electrochemical performance, such as LiMnPO₄, Li₃V₂（PO₄）₃, Li₂MnSiO₄,Li₂FeSiO₄and so on. Among them, LiFePO₄, LiMnPO₄and Li₃V₂（PO₄）₃have highenergy density, and both of them are considered ideal cathode materials of powerbatteries. However, the electrochemical performance is greatly limited by the poorelectronic conductivity and low lithium ion diffusivity. Therefore, this thesis isdevoted to improve the electrochemical performances of the LiFePO₄, LiMnPO₄andLi₃V₂（PO₄）₃materials.Firstly, the LiFePO₄/C composite was prepared by an in situ polymerizationrestriction method using resorcinol-formaldehyde （RF） gel as carbon source andhexamethylenetetramine as potential precipitant, and we also discussed the effect ofvarious calcination temperatures for the electrochemical performance of LiFePO₄/C.It is known that hexamethylenetetramine can be cleaved to give formaldehyde andammonium ions under acidic conditions. On the one hand, the pH value of thesolution increases and facilitates the formation of FePO4precipitation; on the other hand, formaldehyde, formed in situ, as one reactant for the raw material of theresorcinol-formaldehyde gel. The residual carbon from the pyrolysis of RF gel caneffectively increase the electron conduction. We studied the structure properties of thematerial using XRD and Raman, and we also discussed the effect of variouscalcination temperatures for the structure of LiFePO₄/C. The carbon content of theLiFePO₄/C was determined by thermogravimetric analysis, and the morphology andparticle size were observed through SEM and TEM. Then we studied the structurestability and electrochemical properties of the material by galvanostaticcharge/discharge tests and cyclic voltammetry （CV）. Electrochemical test indicatedthat the obtained LiFePO₄/C at750°C delivered better electrochemical properties.Even at the high rates of10C,20C and50C, the initial discharge capacities of theelectrodes exhibited115.6mAh/g,84.5mAh/g and67.8mAh/g, and the electrodesdelivered capacity retention of89.5%,90.9%and85.7%after1000cycles,respectively.Then, we developed a more convenient method for the synthesis of LiFePO₄/Ccomposite. Poly（furfuryl alcohol）（PFA） can be easily obtained from FA by heating oracidic catalysis. The mixture of poly（furfuryl alcohol） and FePO4was preparedthrough the polymerization of furfuryl alcohol under heating and acidic conditions.The LiFePO₄/C composite was synthesized by an in situ polymerization restrictionmethod, and we also discussed the effect of various calcination temperatures for theelectrochemical performance. We studied the structure properties of the material usingvaries techniques including XRD, Raman, SEM and TEM. Then we studied theelectrochemical properties of the material by galvanostatic charge/discharge tests andcyclic voltammetry （CV）. The material delivered a high discharge capacity of156.1mAh/g in the first cycle at0.5C, and it remained145.2mAh/g after50cycles. Thedischarge capacity retention of sample after500cycles was94.4%,90.6%,87.8%and90.9%at rates of5C,10C,20C and50C, respectively.Thirdly, the LiFe_0.4Mn_0.6PO₄/C composite was prepared via a carbon gel process.The carbon gel process between resorcinol and formaldehyde can ensure themolecular-level homogeneity of the chemical product, and the doping of Fe²⁺can improve the structure stability of the material. The produced carbon can effectivelyimprove the electronic conductivity. The initial discharge capacity of the electrodewas64.6mAh/g, and it remained53.8mAh/g after1000cycles.Finally, The Li₃V₂（PO₄）₃/C composites were prepared by a combustion method.The structure properties of the material were studied by XRD, Raman, SEM and TEM.It was discussed that the effects of the content of carbon and synthesis temperature forthe electrochemical performances of the Li₃V₂（PO₄）₃/C composites by galvanostaticcharge/discharge tests. The results showed that a suitable carbon content andcalcination temperature had an important influence for the electrochemical propertiesof the material.In short, these works give us the understanding in depth on the structure of LiFePO₄,LiMnPO₄and Li₃V₂（PO₄）₃materials, as well as their existing problems andelectrochemical properties, and the theoretical and technical guidance for the practicalapplication of the materials.