Synthesis Of Iron-based Metal Organic Frameworks And Their Application In Lithium-ion Batteries

Lithium-ion batteries have been widely used in various applications(such as mobile communication devices,digital devices,new-energy vehicles),on the account of large energy density,high output voltage and long service life.However,commercialized lithium-ion batteries are unable to fully satisfy the increasing demands of next-generation electrical equipments.The key point for improving the electrochemical performance of lithium-ion batteries lies in exploring new electrode materials.Metal-organic frameworks(MOFs)is a kind of organic-inorganic hybrid porous materials built by self-assembly of metal-connecting nodes and organic-bridging ligands.Most of inorganic units(metal ions and metal oxides)and organic ligands in the MOFs are redox active sites during the electrochemical process.The highly regular channel structures of MOFs facilitate diffusion of lithium ions.Moreover,MOFs with good stability can maintain their structural integrity during discharge-charge process.Therefore,MOFs could be used as a new promising class of electrode materials for lithium-ion batteries.Based on hydrothermal/solvothermal method,two different kinds of iron-based metal organic frameworks(MIL-88A and MIL-88B)with a submicron size were successfully prepared in this study.Further pyrolysis of MIL-88 A under air or nitrogen atmosphere result in the formation of iron oxide or iron oxide/carbon hybrids.Polypyrrole-coated MOFs were synthesized via in situ chemical oxidative polymerization of pyrrole in the presence of MIL-88A or MIL-88B.The samples were systematically characterized using X-ray powder diffraction,infrared spectroscopy and scanning electron microscope observations.Then,the potential applications of iron-based MOFs in lithium-ion batteries were investigated.1.Rod-shaped MIL-88 A particles with an average diameter of 500 nm and length of 2?4?m were hydrothermally synthesized from a mixture of FelCl3·6H2O and fumaric acid.When used as anode material in Li-ion battery,the pure MIL-88A exhibited some degree of lithium-storage capacity(an initial discharge capacity of 140.5 mA·h·g-1).But it showed large initial capacity loss and poor cycling performance.Besides,adsorption characteristics of MIL-88A for gases generated within lithium-ion batteries were also studied using the grand canonical Monte Carlo(GCMC)method.It was found that MIL-88A exhibited a high adsorption capacity for gas generated from Li-ion battery at ambient temperature.There is a negative correlation between equilibrium adsorption capacity and the kinetic diameter of H2,CO2 and PF5.MIL-88A thus would be a promising material for adsorption removal of gas generated from Li-ion battery.2.Iron oxide(Fe2O3)or iron oxide/carbon(FemOn/C)hybrids could be easily obtained via direct thermolysis of MIL-88A under air or nitrogen atmosphere due to its low thermal stability.Results showed that,iron oxide exhibited better overall electrochemical performance than that of iron oxide/carbon hybrids when used as anode material in Li-ion battery.Fe2O3 derived from MIL-88A displayed a high capacity of 817 mA·h·g-1 after 60th cycle at a current density of 100 mA·g-1.Compared with pure MIL-88A,the polypyrrole-coated MIL-88A(MIL-88A@PPy)showed lower interfacial charge-transfer resistance,resulted in better cycle performance and good rate capability.3.In order to explore the effectiveness of polypyrrole coating in improving electrochemical performance of other iron-based MOFs,MIL-88B was synthesized by solvothermal reaction of terephthalic acid and FeCl3.And then the MIL-88B particles were coated with polypyrrole via in situ surface polymerization of pyrrole monomer.Result showed that poly pyrrole coating could effectively improve the initial charge-discharge coulombic efficiency of MIL-88B(The initial charge-discharge coulombic efficiency increased from 61.8%to 84.9%at a current density of 100 mA·g-1).In addition,the interfacial charge-transfer resistance was reduced after coating with polypyrrole,resulting in a drastic improvement of cycle performance.After 75 charge-discharge cycles,MIL-88B@PPy could maintain 81%of its initial capacity while only 13%of the initial capacity was maintained for MIL-88B.