Controllable Synthesis And Electrochemical Property Of Porous Structure LiFePO₄

As a kind of high performance energy storage system,Li-ion batteries?LIBs?shows great promise as power sources for electric vehicle?EV?revolution,and is considered to be the most impressive success of modern electrochemistry in the last two decades.They power most of today's portable devices,and are developed as power supply of EVs.The research and development of new electrode materials,especially the cathode materials for LIBs are the focus of research in prominent groups in the field of materials science.Olivine-structured LiFePO₄ has been the focus of research in developing low cost,high performance cathode materials for LIBs.Its intrinsic structural and chemical stability lead to safe and long cycle life batteries.Besides,olivine LiFePO₄is composed of low cost and environmentally benign Fe and PO₄ moieties,which is an important merit for large scale applications.However,the low intrinsic electronic and ionic conductivity of LiFePO₄,which decrease the rate performance,limit its widespread applications.Combination of nanostructure with carbon coating is a widely adopted route to effectively resolve the aforementioned problem.Our previous studies have shown that nanostructured porous materials,benefiting from large surface areas,more active sites and shortened Li⁺diffusion paths,can largely improve their electrochemical properties.Based on the above considerations,in this thesis,we synthesize hierarchically porous/nano structured LiFePO₄ for LIBs cathode by microwave assisted solvothermal/hydrothermal method and normal solvothermal method.The effect of different experimental conditions on the morphology and performance were dicussed,the synthesis mechanism and electrochemical mechanism were studied and the relationship between structure and properties is explored.The main results were shown as follows:?1?Hierarchically 3D bicontinuous macro-mesoporous LiFePO₄/C nanocomposite constructed by nanoparticles in the range of 50¹00 nm has been synthesized via a microwave assisted solvothermal process followed by carbon coating.Theabundant 3D macropores allow for better penetration of electrolyte to promote Li⁺diffusion,the mesopores provide more electrochemical reaction sites and the carbon layers on the LiFePO₄ nanoparticles increase the electrical conductivity,thus ultimately facilitating reverse reaction of Fe³⁺to Fe²⁺and alleviating electrode polarization.In addition,the particle size in nanoscale can provide short diffusion lengths for the Li⁺intercalation-deintercalation.As a result,the LiFePO₄/C composite exhibits excellent electrochemical performance.?2?Porous LiFePO₄/C microspheres were successfully prepared by microwave assisted solvothermal/hydrothermal method and carbon coating procedure.The porous microspheres have good dispersion with the size of¹?m,and show excellent electrochemical performance as cathode materials.This can be ascribed to several factors.On the one hand,the porous structure offers more active sites,speeding up the Li⁺diffusion and e^-migration at the electrode/electrolyte interface.On the other hand,the carbon coating layer can improve conductivity of the materials.?3?Hierarchically LiFePO₄/C mesocrystal assembled by closely packed nanorods was successfully prepared by solvothermal method and the subsequent carbon coating procedure.The synthesized material shows excellent electrochemical properties.And the charge/discharge capacity increases as the cycling proceeds.This may be attributed to the unique hierarchically mesocrystal structure, offering strong electrochemical reaction kinetics,thus making more Li⁺ intercalation/deintercalation.Besides,the coated carbon layer can greatly improve the electronic conductivity.The synergy of the two aspects makes our material have excellent electrochemical performance.