Morphology Controlled Synthesis And Electrochemical Properties Investigation Of LiFePO₄/C For Lithium Ion Batteries

Olivine-type lithium iron phosphate(LiFePO4)has been highlighted as one of the most promising cathode materials because of its inherent merits including environment compatibility,low cost,and high safety.However,the low intrinsic electronic conductivity,ionic conductivity,voltage window and tap density of the original LiFePO4 significantly constrains its development for high-power lithium-on rechargeable battery.In this thesis,we have synthesized LiFePO4/C nanostructures with different morphologies,LiFexln1-xPO4 with high voltage window,and porous spherical LiFePO4/C with high tap density by hydrothermal/solvothermal method combined with high-temperature calcinations.The influences of experimental conditions,such as reaction time and reaction temperature,on the morphologies and electrochemical properties of the resulting products have been investigated.The main results are concluded as follows:(1)Olivine LiFePO4 single-crystalline nanoplates have been synthesized by a facile solvothermal method.The thin side of LiFePO4 nanoplates with large exposure of bc-plane is along the a-axis,differing from that along b-axis or c-axis reported previously.Although lithium ion diffusion in the olivine crystal structures occurs preferably through one-dimensional channels(b-axis),the synthesis of LiFePO4 single-crystalline nanoplates dominated with bc-planes are important for understanding the growth habit of LiFePO4 crystal.LiFePO4/C composite nanoplates exhibit good electrochemical performances with discharge capacities of 165(0.1C),154(0.5C),146(1C),130(2C),and 96 mAh/g(5C).LiFePO4 nanostructures with different morphologies have been synthesized by using different solvents,water/DMSO volume ratio,and P/Fe molar rate.When the molar rate of P/Fe is 1:1,LiFePO4/C nanorods is synthesized,which exhibit better electrochemical properties than LiFePO4/C nanoplates.(2)LiFePO4has low energy density because of its lower voltage window(3.45 V,Fe2+/Fe3+ vs.Li/Li+)as compared with the 4 V class LiMO2(M=Ni,Co,and Mn)or LiMn2O4.To solve this problems,olivine LiFexMni-xO4(x=0,0.25,0.5,0.75)nanoparticles with high voltage window have been synthesized by a template-free solvothermal process.Among them,LiFe0.5Mn0.5PO4/C nanorods show high voltage window and good rate capabilities with discharge capacities of 157(0.1C),151(0.5C),and 143 mAh/g(1C).The mass energy density is improved by 7.2%and 5.9%in comparison with LiFePO4/C nanoplates and LiFePO4/C nanorods at 0.1C,respectively.(3)Although nanoscale LiFePO4 particles with surface carbon coating shows excellent rate performances,its volumetric energy density is not satisfactory for high-power/energy lithium-on rechargeable battery because of the low tap density.In order to get an ideal electrode material with high tap density,porous spherical LiFePO4/C microscale secondary particles have been synthesized by low-temperature hydrothermal method combined with high-temperature calcinations.The spherical microscale secondary particles(5-6 ?m)with porous structures are composed of carbon-coated primary nanoparticles(40-80 nm),which posses high tap density of approximately 1.4 g/cm3,desirable electrochemical performances with a high rate capability of 95.2 mAh/g at 5 C and stable cycle life.The sizes and shapes of LiFePO4 microparticles can be controlled by adjusting the synthetic parameters such as temperature,and types of lithium precursor in this reaction system.The spherical LiFePO4 microparticles composed of sheet-like nanostructures have been synthesized by precipitation method,which exhibit good electrochemical properties after coating with carbon.