Study On The Lithium/Sodium Storage Performance Of Metal Organic Frameworkderived MgFe₂O₄ Anode Material

As anode materials for lithium/sodium ion batteries,MFe₂O₄（M=Mg,Co,Ni,Zn,Mn）have a lot of advantages such as high specific capacity,low cost and nontoxic.These dual metal oxides can combine the advantages of both metals,such as enlarged capacity and modified stability during the process of cycling.The objective of this dissertation is to modify the MgFe₂O₄ by using MOF Prussian blue（PB）as self-sacrificial templates and Fe source.The MOFs-derived MgFe₂O₄ particles often keep a uniform size,and possess interior cavities and porous shells.The relationships between the morphology,microstructure and lithium/sodium storage performance of as-prepared submicron crystals were studied in detail and the energy storage mechanism of MgFe₂O₄ particles in lithium ion battery was explored.The details are as follows:1.We synthesized regular hollow MgFe₂O₄ submicron cubes by using MOF PB as templates.The regular MgFe₂O₄ particles are characterized by⁵70 nm in size and shells of¹00 nm in thickness after calcination,which consists of crystallites with an average size of²4 nm.To confirm the advantages on the anode performance of hollow MgFe₂O₄ submicron cubes,we made a comparison between such hollow structure and the solid MgFe₂O₄ composite synthesized via sol-gel method.It indicates that the MOF-derived MgFe₂O₄ submicron cubes have higher electrochemical performance and electric conductivity,which maintained a discharge capacity of 646 mAh g^-1 in the100th cycle at 50 mA g^-1.At a current density of 1.0 A g^-1,the capacity of MgFe₂O₄submicron cubes faded in the first several tens cycles.Then,it starts to increase and reaches a high discharge capacity of 781 mAh g^-1 after 550 cycles.On this condition,some MgFe₂O₄ particle still kept its cubic structure.Therefore,MgFe₂O₄ submicron cubes may resist the deformation during cell fabrication and cycling.Additionally,MgO nanocrystallites existed in both discharged and charged samples.As one of the intermediates,they can buffer the pulverization the other intermediate Fe or Fe₂O₃during cycling and hence promotes the rate performance and cycle capability of hollow MgFe₂O₄ anode material.2.MOF-derived hollow Mg Fe₂O₄ submicron cubes were assembled into sodium battery at different ratios（sample A（7:2:1）,sample B（8:1:1））.The rate performance and conductivity of sample A are better than sample B.It indicaes that with the increase of the ratio of Super-P carbon black,the diffusion rate of sodium and rate capacity of the MgFe₂O₄ submicron cubes electrode can be improved.Sample A exhibited a good capacity of 406 mA h g^-1 at a current density of 50 mA g^-11 and maintained a reversible capacity of 131 mA h g^-11 after 500 cycles.After cell disassembly and 100th charge,some MgFe₂O₄ particle still kept its cubic structure,which illustrated its good structural stability.