Preparation Of Zinc Ferrite And Iron Sulfide Based Nanocomposites And Their Lithium Storage Performance

With the prominent increase of energy problems,the development and utilization of new energy materials have attracted more and more attentions.Lithium-ion batteries,as a kind of green energy storage devices,have great potential application in portable electronic products and power vehicles due to their unique advantages.In recent years,the rapid development in the energy storage fields has put forward higher requirements for lithium-ion batteries as power sources.At present,commercial graphite materials can not meet the high demand of modern products because of its low theoretical specific capacity and poor safety.Therefore,it is particularly important to develop a new anode material with high specific capacity and low cost.Among them,iron-based double transition metal oxides and sulfides have many advantages such as high theoretical specific capacities and low costs,which make them one of the most possible anode materials to replace commercial graphite.However,the low conductivity,and the huge volume changes of these anode materials during cycling will lead to capacity decay and poor cycling stability.In addition,the dissolution of sulfur ion into the electrolyte is another problem.These problems can be addressed by preparing nanostructure and coating modification.In this paper,using zinc ferrite?ZnFe₂O₄?and Fe₇S₈ as the main research objects,the hierarchical micro-nano structures were first prepared,then the surface modified ZnFe₂O₄ and Fe₇S₈ nanocomposites were prepared,and their electrochemical performances were discussed and studied systematically.The main contents of this work are as follows:?1?ZnFe₂O₄ nanoclusters with particle size of 40-50 nm were first synthesized by solvothermal method,and then the core-shell structure of TiO₂ coated ZnFe₂O₄?ZnFe₂O₄@TiO₂?was prepared.By adjusting the amount of tetrabutyl titanate,the effects of TiO₂ coatings with different thickness on lithium storage performance of composites were discussed.The results show that ZnFe₂O₄@TiO₂ nanocomposite with TiO₂ coating thickness of 5 nm exbihits the best lithium storage performance.In order to further improve the electrochemical performance of ZnFe₂O₄,N-doped carbon coated composites?ZnFe₂O₄@NC?were prepared using dopamine as coating layer followed by calcination.Its carbon layer thickness is similar to that of TiO₂.The electrochemical performance test results show that ZnFe₂O₄@NC nanocomposites exhibit more excellent long-term cycling stability.The reversible specific capacity can reach 797 mAh g^-1 after 1000 cycles at 2 A g^-1.The results show that the external surface modification layer can alleviate the volume change during cycling,prevent particle pulverization and inhibit capacity decay.In addition,the synergistic effect of ZnFe₂O₄ with hierarchical structure and surface modification layer can also effectively improve the lithium storage performance.The performance of ZnFe₂O₄@NC composites is better,mainly because the coated carbon layer can increase the electronic conductivity of ZnFe₂O₄ based composite,improving the electron transfer efficiency.Besides,the doped nitrogen element can increase the structural defects and contribute to additional lithium storage capacity.?2?The Fe₇S₈ nanoparticles were successfully attached on the carbon nano-networks by freeze-drying process combined with the subsequent calcination in Ar/H₂ atmosphere,forming the Fe₇S₈/C composites,which were used as anode materials for lithium-ion batteries and its lithium storage performance was tested.The results show that the composites exhibit excellent cycling stability and rate capability.At a current density of 2A g^-1,the reversible specific capacity can still maintain at 667 mAh g^-1,showing good long-term cycling stability.In addition,we also studied the sodium storage performance of the composite,which shows excellent cycling performance.After 100 cycles,the reversible specific capacity is still as high as 375 mAh g^-1.Its excellent electrochemical performance is mainly ascribed to the special nanostructure of Fe₇S₈ and the synergistic effect between Fe₇S₈ nanoparticles and carbon nano-networks.?3?The N-doped carbon coated Fe₇S₈ nanocomposites?Fe₇S₈@NC?with a core-shell structure were successfully prepared by a simple three-step synthesis strategy.Fe₇S₈@NC nanocomposites with different carbon contents were obtained by adjusting the dopamine hydrochloride content,and their lithium storage properties were studied.As anode materials for lithium ion batteries,these composites exhibit better cycling stability and rate performance than pure Fe₇S₈.Among them,Fe₇S₈@NC nanocomposite with a carbon weight ratio of 17.8%shows higher capacity and better long-term cycling stability.After1000 cycles at 2 A g^-1,the reversible specific capacity is up to 833 mAh g^-1.Its excellent lithium storage performance is attributed to the core-shell hierarchical nanostructure of Fe₇S₈@NC and the synergistic effect between Fe₇S₈ and conductive N-doped carbon layer.