Synthesis,Characterization And Electrochemical Performance Of Nitrogen-Doped Nanocarbon Modified LiFePO₄ Positive Electrode Materials

Along with the rapid development of science and technology,energy and environmental issues have become increasingly prominent,which bring forward higher requirements for the development and utilization of new energies.Lithium-ion batteries?LIBs?,with the advantages of high voltage,small self-discharge,non-memory effect and environmental friendly,have become the most promising energy storage devices.The olivine-type lithium iron phosphate?LiFePO₄?is considered as one of the most promising positive electrode materials of lithium-ion batteries,due to its high theoretical capacity,low cost,long cycle ability,high safety and environmental friendliness,etc.However,low electrical conductivity and lithium-ion diffusion coefficient lead to the poor electrochemical performance of LiFePO₄ and limit its broader application in high-rate power type batteries.Therefore,how to improve the electrical conductivity and lithium-ion diffusion coefficient of LiFePO₄ has become the research hotspot.Highly conductive nanocarbon materials,such as carbon nanotubes and graphene,have been demonstrated as a kind of excellent conducting additives for its unique properties.The nitrogen-doped nanocarbon materials can further improve the physicochemical properties.In order to improve the performance of LiFePO₄,in this thesis,the nitrogen-doped nanocarbon modified porous LiFePO₄ composite has been synthesized by a freeze-drying method,and the nitrogen-doped nanocarbon modified spherical LiFePO₄ composite has been synthesized via a two-step hydrothermal process,while the nitrogen-doped graphene aerogel modified LiFePO₄ composite has been synthesized via a solvothermal method.X-ray diffraction,scanning electron microscopy,transmission electron microscopy,X-ray photoelectron spectroscopy,nitrogen adsorption and thermogravimetry have been used to analyze the morphology and structure of the obtained materials,and galvanostatic charge/discharge tests,cyclic voltammograms and electrochemical impedance spectroscopy have been further used to investigate the electrochemical performances of the LiFePO₄ composites.The main contents and conclusions have been summarized as following:?1?The LiFePO₄/N-CNTs composites synthesized by the freeze-drying method present a three-dimensional porous structure,and the crosslinked N-CNTs conductivity networks and pores can provide abundant transport paths for electrons and lithium-ion and remarkably increase the lithium-ion insertion/de-insertion kinetics.Therefore,the composites exhibit enhanced specific capacity,rate characteristic and cyclic stability.?2?The LiFePO₄/N-CNTs microspheres obtained by the hydrothermal and chemical lithiation processes possess high tap density.The three-dimensional N-CNTs conductive networks incorporated in the composite microspheres not only improve the conductivity of LiFePO₄,but also create abundant transport paths for lithium-ion and electrons,shorten the ionic diffusion path and further improve the utilization rate of the material.The LiFePO₄/N-CNTs microsphere composites display a reversible discharge capacity of 153 and 140 m Ah g^-1 at 0.1 and 1 C,and the capacity retention is 97.6% after 500 cycles at 1 C.?3?The LiFePO₄/NGA composites prepared by a solvothermal method show good electronic conductivity,high porosity and large surface area,such unique structure can provide the three-dimensional transmission channals for lithium-ion and electrons.Nitrogen doping is an effective method to enhance the electronic conductivity and the wettability of the graphitic materials,to further improve the transfer rate of electrons and diffusion rate of lithium-ion.Therefore,the LiFePO₄/NGA composites exhibit enhanced rate characteristic and cyclic stability.