Metal-Organic Frameworks-Derived Porous Carbon-Based Nanomaterials As Anode Materials For Lithium-Ion Batteries

Metal-organic frameworks (MOFs), an emerging class of materials created from supramolecular assembly of inorganic components (metal ions or metal clusters) with organic components (organic or organometallic complexes), have attracted extensive research interest because of their diverse structural topologies, tunable functionalities, as well as versatile applications in catalysis, biomedical imaging, and gas separation and storage. Additionally, inspired by their high surface area, porosities, and abundant carbon-containing linkers, MOFs have been demonstrated as promising templates or precursors to fabricate porous nanostructured materials via thermolysis. The objective of this dissertation is to synthesize carbon-based nanoparticles and metal oxide nanoparticles with unique structures using various MOFs as precursors or templates.The Li-storage performances of as-prepared nanoparticles were investigated. The details are as follows:1. Theoretical and experimental results reveal that Lithium ion storage capacity for Nitrogen-doped graphene is depended largely on Nitrogen-doping level. However, most of the Nitrogen-doped carbon materials for lithium-ion batteries are reported with a nitrogen content of around10wt%because a higher content Nitrogen atom in a two-dimensional honeycomb crystal lattice can result in its structured unstability. Herewe report Nitrogen-doped graphene analogous particles with a nitrogen content as high as17.72wt%prepared by pyrolysis of a nitrogen containing zeolitic imidazolate framework at800℃under a N2atmosphere. As an anode material for lithium ion batteries, its capacity retained2132mA h g-1after50cycles at a current density of100mA g-1, and785mAh g-1after1000cycles at5A g-1. The remarkable electrochemical performance is attributed to unique Nitrogen-doped graphene analogous particles highly doped with N both within the hexagonal lattice and edges.2. Iron oxides are extensively investigated as anode materials for lithium-ion batteries (LIBs) because of their large specific capacities. However, they undergo huge volume changes during cycling that result in anode pulverization and loss of electrical connectivity. As a result, capacity retention of the iron oxide anodes is poor and should be improved for commercial applications. Herein, we report the preparation of ultrasmall Fe2O3nanoparticles embedded in nitrogen-doped hollow carbon sphere shells (Fe2O3@N-C) by direct pyrolysis of Fe-based zeolitic imidazolate frameworks (Fe-ZIF) at620℃in air. As an anode material for LIBs, the capacity retained1573mAh g-1after50cycles at a current density of0.1C (1C=1000mA g-1). Even undergoing twice high-rate capability test, it can still deliver a remarkably reversible and stable capacities of1142mAh g-1after100cycles at a current density of1C. The excellent electrochemical performance is attributed to the unique structure of ultrasmall Fe2O3nanopartciles uniformly distributed in the shell of nitrogen-doped carbon spheres, which simultaneously solve the major problems of pulverization, facilitate rapid electrochemical kinetics, and effectively avoid the aggregation of Fe2O3nanoparticles during de/lithiation. The novel method developed in this work for the synthesis of functional hybrid materials can be extended to preparation of various MOFs-derived functional nanocomposites owing to the versatility of links and metal centers in MOFs.3. Though MnO has been demonstrated as promising anode materials for lithium-ion batteries (LIBs) in terms of high theoretical capacity (755mAh g-1), low voltage hysteresis comparatively (<0.8V), low cost, and environmental benignity, the application of MnO as practical electrode materials is still hindered by many obstacles, including poor cycling stability and huge volume expansion during the charge/discharge process. Herein, we report a facile and scalable metal-organic framework-derived route for the in-situ fabrication of ultrafine MnO nanocrystals encapsulated in porous carbon matrix, where nanopores increase active sites to store redox ions and enhance ionic diffusivity to encapsulated MnO nanocrystals. As an anode material for lithium-ion batteries (LIBs), these MnO@C composites exhibited a high reversible specific capacity of1221mAh g-1after100cycles at a current density of100mA g-1. The excellent electrochemical performance can be attributed to their unique structure with MnO nanocrystals uniformly dispersed inside a porous carbon matrix, which facilitates to largely enhance the electrical conductivity and effectively avoid the aggregation of MnO nanocrytals, and relieve the strain caused by the volumetric change during the charge/discharge process. This facile and economical strategy will extend the scope of metal-organic framework-derived synthesis for other materials in energy storage applications.4. A facile two-step strategy involving a room-temperature synthesis and subsequent thermal calcination of a binary-metal-based metal organic framework (MOF) has been successfully developed for the preparation of porous MnxFe3-xO4nanocubes. The as-prepared MnxFe3-xO4inherits the morphologies of MOFs and shows a porous structure with a relatively high specific surface area (124m2g-1) due to the release of small gas molecules (CO2, NO2, et al.) in the calcination process in air. The as-prepared porous MnxFe3-xO4nanocubes exhibit greatly enhanced performance for Li storage. The capacity of the porous MnxFe3-xO4retained827mAh g-1after60cycles at a current density of200mA g-1. The enhanced electrochemical performance is attributed to the interconnected porous structures and large amounts of mesopores. A novel and facile route for the large-scale fabrication of2D porous NixCo3-xO4nanosheets is also reported, which involves the thermal decomposition of NixCo1-x hydroxide precursor at450℃in air for2h. The as-prepared2D porous NixCo3-xO4nanosheets exhibit an enhanced lithium storage capacity and excellent cycling stability (1330mA h g-1at a current density of100mA g-1after50cycles). More importantly, it can render reversible capacity of844mA h g-1, even at a high current density of500mA g-1after200cycles, indicating its potential applications for high power LIBs. Compared to pure Co3O4, the reduction of Co in NixCo3-xO4is of more significance due to the high cost and toxicity of Co. The two unique structures mentioned above effectively improve structural stability, reduce the diffusion length for lithium ions and electrons, and buffer volume expansion during the Li+insertion/extraction processes.