Synthesis Of Carbon Coated Fe₃O₄ And Iron Vanadates Nanocomposites And Their Electrochemical Performances For Lithium Ion Batteries

Recently,with the continuous development of science and technology as well as social progress,the energy problem has gradually gained great attention by scientists.As a new type of portable energy storage device,lithium-ion batteries（LIBs）have been applied in many fields such as electric vehicles and portable electronic products because of its outstanding electrochemical performance.The commercial graphite anodes have lower specific capacity(372 mAh g^-1)and poor safety problems,limiting their applications.Transition metal oxides as anode materials have attracted extensive attention due to the high specific capacity,abundance in earth.However,the huge volume changes during cycling will lead to capacity decay and poor cycling performance.Besides,its low conductivity will affect the electron transfer efficiency,which limits their large-scale applications.In this paper,iron oxide and iron vanadate were prepared via a facile solvothermal method.By adjusting the experimental parameters,two kinds of carbon coated composites with different hierarchical structures could be obtained via the polymerization reaction and subsequent calcination.We prepared the composites via a solvothermal method,adjusted the microstructure of the nano-materials,and modified them with dopamine by situ polymerization.After the subsequent calcination process,N-doped carbon coated composites were obtained.These composites were acted as anode materials,and their lithium storage performance were studied systematically.The main contents of this work are as follows:（1）The composites of N-doped carbon coated Fe₃O₄ nanoclusters（Fe₃O₄@NC）were prepared by a facile preparation process.The microwave-assisted solvothermal method was first employed to obtain Fe₃O₄ nanoclusters,and then using dopamine as carbon sources,the N-doped carbon coated Fe₃O₄ nanoclusters were obtained by calcining in vacuum.By changing the amount of dopamine and calcination temperature,a series of products with different carbon coating thickness and crystallinity were obtained,and their phases and microstructure were tested and analyzed.The composites were acted as anode materials for LIBs,and tested their electrochemical performance.The composite synthesized by adding 0.08 g dopamine hydrochloride and calcining at 500 ^oC exhibits ultra-long cycle life and good rate capability.Its reversible capacity can retain at 445 mAh g^-11 after 2000 cycles even at a current density of 2 A g^-1.The results indicate that the hierarchical Fe₃O₄@NC composites are promising high performance anode materials for LIBs.（2）A facile synthesis strategy was used to fabricate the carbon coated new spinel Fe_1.5V_1.5O₄ nanorods.The initial obtained precursor was prepared by a facile solvothermal process.By adjusting the reaction,reaction time and the atomic ratio of Fe to V,the influence of different synthesis conditions on the phase and morphology of the precursor was discussed.After it was treated by different methods,hierarchical Fe_1.5V_1.5O₄@C nanorods consisted of Fe_1.5V_1.5O₄ primary nanoparticles with different particle sizes could be obtained.The prepared materials were acted as anode materials for LIBs,and their electrochemical performance were tested.As anode materials for LIBs,the hierarchical Fe_1.5V_1.5O₄@C nanorods composed of smaller primary particles show higher reversible capacity,better cycling performance and rate capability.Its reversible capacity can retain at 753 mAh g^-11 and 709 mAh g^-11 even cycled at 0.5 A g^-11 and 2 A g^-11 after 100 cycles and1000 cycles,respectively.The excellent lithium storage performance of carbon coated Fe_1.5V_1.5O₄ nanorods is dependent on the spinel crystal structure,unique hierarchical structure,and synergistic effect of active materials and carbon layer in the composites.The results of CV curves imply that the overall capacity upon cycling is determined by both of the surface-controlled capacitive contribution and diffusion-controlled Li⁺intercalation process.