Metal Organic Frameworks As Electrode Materials For Lithium Ion Batteries

One of the most important resources for human society in the twenty-first century is energy.Developing renewable clean energy has become one of our pursuit goals.Lots of battery equipments are needed for the storage and utilization of these energy sources.Lithium ion battery(LIB),as one of the widely used secondary batteries,has the very high energy density and power density.In order to apply LIBs to more energy storage equipments and electric equipments,it is urgent to develop new positive and negative electrode materials with higher energy density.Metal organic frameworks(MOFs),as the star molecules in current research,are promising template materials for synthesis of positive and negative materials because of their monodispersed metal ions and their large pore structure.In this study,a series of electrode materials for lithium ion batteries were synthesized based on the structural properties of metal organic frameworks.Metal organic frameworks exhibit the controllable particle shape and size,and the ligands and metal elements are abundant in their particles.It is for this reason that ZIF-8and ZIF-67 were selected as the templates for synthesizing the metal oxide anode materials in the second chapter.By coating the ZIF-8 with organic biomolecules,the metal and carbon composites were obtained after pyrolysis.The obtained material has stable cycle performance and high efficiency in the first cycle.The obtained ZIF-8@chitosan-800N has a capacity of 750 m Ah g^–1 at a current density of 50 m A g^–1.When the ZIF-67 was doped with Zn or Ni,the core-shell structured oxide materials were obtained after pyrolysis.And the material has higher cycle capacity and rate performance compare with the single metal oxide.Its capacity is up to 1600 m Ah g^–1 at 1000 m A g^–1,and there is little attenuation during 700 cycles.In the third chapter,a layer of ZIF-8 is uniformly formed on the surface of flower shaped ZnO and rodlike MnO₂.Rate performance and the cyclic stability improved after polymerized with ZIF-8.The oxides have a capacity increase of about 30%with respect to the original oxides at the high rate.After pyrolysis,its capacity and cycle stability have been further improved,and its capacity at elevated current densities is enhanced by 40-50%with respect to the original oxide.At the same time,the capacity retention rate is basically100%after 700 cycles(1060 m Ah g^–1).In the fourth chapter,the Si negative electrode material was modified by metal organic frameworks(ZIF-8),and its cycling stability was improved.Firstly,in-situ growth of ZIF-8is on the surface of Si nanoparticles by the method of ball milling.When the material is pyrolyzed and carbonized,the ZIF-8 derived materials can be used to coat the nano Si.After pyrolysis,Si@ZIF-8-700N has a capacity of 1050 m Ah g^–1,and has a low impedance and high rate performance.Secondly,we used MOF as the coating layer to design sandwiched structure(MOF-SC)for protecting Si anode materials.When this MOF-SC method is used to modify the performance of Si,it is coated on the surface of Si just like a cushion that protects Si.The obtained electrodes synthesized by this method have better cycling performance.The initial cycle capacity of the synthesized of C/Si/ZIF-8(micron silicon)is 1700?Ah cm^–2 at 265?A cm^–2.The capacity drops to 850?Ah cm^–2 after 50cycles.C/Si/ZIF-8(nano silicon)electrodes have a capacity of 600?Ah cm^–2 and without any fading after 100 cycles.We also explored ZIF-67,MOF-5,HKUST-1,MIL-53 and NH₂-MIL-53 for modifying Si electrode by MOF-SC method.In the fifth chapter,MOFs were used as templates to synthesize a series of cathode materials:Li CoO₂,Li Ni_xCo_yMn_1-x-yO₂,cathode material of Li-Se battery and sodium ion positive electrode material.Firstly,ZIF-67 is selected as the template to synthesize Li CoO₂,and the metal elements(Al,Mg,Zr)can be doped into the battery materials;secondly,Li Ni_0.8Co_0.1Mn_0.1O₂ and Li Ni_0.6Co_0.2Mn_0.2O₂ are synthesized by using MOF-74 as the templates;thirdly,ZIF-67 is grown on the amino graphite and Se cathode is loaded.The obtained materials are used in the Li-Se battery;fourthly,Na_xCoO₂ is synthesized by using ZIF-67 as the template.Using MOF as the template can make the precursors mixed more uniformly and the obtained cathode materials have higher capacity and better cycle performance.