Study On Fe Based MOF Derivatives As Negative Electrode Materials For Lithium-ion Batteries

Since the beginning of the 21st century,human’s demand for energy has been increasing.With the depletion of fossil resources and its serious pollution to the environment,various forms of green energy have attracted more and more attention.Due to clean and efficient characteristics,electric energy has gradually become the main form of energy in many fields.As an important energy storage system,lithium-ion batteries,especially lithium-ion secondary batteries,have become one of the most mature and widely used battery types due to their high energy density,long cycle life and reusability.In the current mainstream commercial lithium-ion battery system,graphite is used as the active material for anode.Commercial graphite usually has a theoretical specific capacity of about 372 mAh g^-1,which enables Li-ion batteries to relieve people’s energy anxiety for a long time.However,with the rapid development of portable electronic devices and electric vehicles in recent years,the development of a novel lithium-ion battery system with higher capacity density,higher safety and stronger rate performance has been put on the agenda,and the development of anode materials is also key link.The low theoretical specific capacity of graphite anode material has been difficult to meet the current needs,meanwhile poor rate performance,weak structural stability and low discharge platform severely limit its charge and discharge performance,cycle life and safety.Transition metal oxides（TMOs）usually have much higher theoretical specific capacity than graphite,excellent rate performance,higher safety,abundant reserves,and low price,becoming potential substitutes for graphite anodes.However,TMOs are insulating in nature,which limits the transport of electrons.At the same time,TMOs particles experience a severe"volume effect"during the charge and discharge process,which makes the particles easy to pulverize and fall off from the electrode,resulting in capacity loss.Therefore,the preparation of TMOs composites by carbon coating and other methods is a common modification method,which can effectively improve the performance of TMOs.Metal-organic frameworks（MOFs）are emerging coordination polymers lately.Metal ions form a three-dimensional porous structure through self-assembly with organic ligands,so MOFs usually have a large specific surface area.More importantly,the structures of MOFs are well controllable,and their surfaces are easy to be functionalized.After proper modification,it can be directly used as anode material for lithium-ion batteries or converted into TMOs/carbon composite anode material by calcination.In order to explore a new-generation high-performance anode materials,MOFs were used as precursors to prepare TMOs composite materials to modify the properties of TMOs materials in this thesis proving the unique advantages of MOFs’derivatives as anode materials for lithium-ion batteries.The main research contents are as follows:（1）Ferric oxide（Fe₂O₃）is an important member of TMOs anode materials,and its high theoretical specific capacity(about 1007 m Ah g^-1)and low cost make it very competitive.However,it also suffers from low conductivity and volume expansion during cycling.Therefore,in order to improve the electronic conductivity of Fe₂O₃ and suppress its volume expansion,Fe-based MOFs（MIL-53）were selected as the precursor,and ferric oxide（Fe₂O₃@NC）with a nitrogen-doped carbon-coated structure was prepared by calcination as the anode material.In addition,the effects of different calcination temperatures on the morphology,composition and battery performance of Fe₂O₃@NC were explored.Finally,the Fe₂O₃@NC-500 sample obtained by calcination at 500°C achieved reversible specific capacities of 875.4 and 342 m Ah g^-1 at current densities of 100 and 500 m A g^-1,respectively,showing good cycle stability and rate performance.（2）Iron molybdate（FeMoO₄）has high stability,safety,and a theoretical specific capacity as high as 993 m Ah g^-1,so it is expected to become a high-performance anode material.However,the complex preparation process makes related research still in its infancy.Here,using phosphomolybdic acid（PMA）as the phosphorus source,Fe-based MIL-53 modified by solvothermal method（NH₂-MIL-53）was calcined to obtain phosphorus-dopedβ-iron molybdate/γ-tiron oxide@nitrogen-doped carbon（Fe Mo O₄/Fe₂O₃@N-C）composite anode material.The effects of different amounts of PMA on the composition,morphology and electrochemical performance of the final materials were explored.The best sample achieved reversible specific capacities of 1046 and 862 m Ah g^-1 at current densities of 100 and 500mA g^-1,respectively.