The Application Of MOFs-derived Carbon-based Composites In Lithium Batteries

Recently,the rapid development of electric automobile industry proposes higher requirement on the energy density of lithium batteries.Due to its limited energy density,the conventional lithium-ion batteries with graphite as anodes and LiFePO4 or Li(Ni,Co,Mn)O2 as cathodes can not meet the requirement.Therefore,it is necessary to exploit new high-energy-density anode materials or even new battery systems,such as lithium sulfur batteries.As the key parts of lithium batteries,both electrodes and separators have great influence on the whole properties of lithium batteries,thus,to develop high-energy-density lithium batteries should mainly focus on the design and fabrication of electrode and separator materials.MOF-derived carbon-based composites possess the special advantages,such as light weight,excellent electron/ion conductivity,aboundant pores and even-dispersed active sites.In addition,due to their controllable morphologies/components,abundant sources and adjustable pore sizes,these carbon-based composites are also suitable for mechanism researches,thus causing extensive attentions recent years.Aiming at the key problems of lithium-ion and lithium sulfur batteries,this dissertation design and fabricate a series of new MOFs-derived carbon-based composites by using MOFs as precursors and apply them as electrodes or separator coatings in lithium batteries to investigate their effects on the electrochemical properties.Furthermore,the relationship between structure/component and properties is also uncovered by using advanced methods,such as theoretical calculation and in situ TEM.The detail works are as follows:1?Aiming at the large-volume-change problem of high-energy-density anode materials,inspired by the geological plate-movement in nature,a "slippage" strategy was proposed to achieve excellent electrochemical properties of anode materials with low porosities and high tap densities:ZnS-QDs@mNC with a high tap denstiy of 0.86 g cm-3 and low total pore volume of 0.092 cm-3 g-1 was fabricated by sulfurization of ZIF-7 as precursors under high temperature.It is demonstrated that the carbon nanosheets of ZnS-QDs@mNC can slide against each other during lithiation process via in situ TEM.The slippage behaviour can make full use of the limited gaps between the carbon nanosheets to reduce the volume expansion of ZnS-QDs@mNC significantly(the volume expansion is only 6.5%after full lithiation).Therefore,as anodes of lithium-ion batteries,ZnS-QDs@mNC possesses excellent cycling stability and rate performance.Even at a high loading of 2.76 mg cm-2,cycling stability can still be achieved.This work provide a novel approach to achieve long-term cycling stability of high-energy-density anode materials with high tap densities and low porosities.2?Aiming at the large-volume change and poor electrical conductivity of high-energy-density anode materials,the design of hollow nano-structure with double carbon coating was provided:H-Co3O4/NC@C composed of hollow nano-Co3O4 coated by double carbon layers was developed by using ZIF-67 as precusors.The hollow nano-Co3O4 particles are beneficial for reducing lithium-ion pathway distances and relasing volume-change stress.The double carbon coating can not only improve electrical conductivity,but also strengthen the structure stablity:the interior porous carbon layer effectively accommodates the volume change,and the outside compact carbon layer benefits the formation of stable SEI.Therefore,as anodes of lithium ion batteries,H-Co3O4/NC@C has excellent rate performance and cycling stability.3?Aiming at the shuttle effect of polysulfides in lithium sulfur batteries,inspired by the mechnism behind that natural spider webs can effectively capture insects,we proposed the combination of physical confinemnt and chemical interaction:a spider-web-like composite Co/mSiO2-NCNTs,composed of the hollow mSiO2 nanospheres/Co nanoparticles threaded by interwoven NCNTs,was fabricated by calcination of ZIF-67 as precusors.When coated on commercial separators and then used in Li-S batteries,Co/mSiO2-NCNTs can effectively anchor and reutilize polysulfides through both physical and chemical interaction as natural spider webs capture insects.In additon,the combination of experiments and molecular dynamics simulation demonstrates the reversible adsorption/desorption of polysulfides on the mSiO2.Such behavior can not only inhibit the shuttle effect,but also contribute to the full utilization of active materials.Furthermore,Co and N doping of Co/mSiO2-NCNTs perform a synergistic effect on the conversion of polysulfides,which contritbutes to accelerate the reaction rate of polysulfides.Therefore,compared with the commercial separator,Co/mSiO2-NCNTs coated separator can significantly improve long-term cycling stability and rate performance of lithium sulfur batteries.This work comfirms that the combination of rational structure design and component arrangement is an effective approach to solve the shuttle effect problem.4?Aiming at the low sulfur utilization at high sulfur loading with lean eletrolyte,the synergistic regulation of polysulfides conversion and depositon was proposed:by using Co-MOFs nanosheets(CNSs)arrays grown on the carbon fiber cloth(CFC)as precursors,a Co,N doped carbon-based composite(Co,N-CNTs-CNS/CFC)with a hierarchical structure was fabricated via one-step calcination.Possessing abundant active sites and fast electron/ion pathways makes Co,N-CNTs-CNS/CFC can effectively anchor and convert polysulfides.The combination of experiment and DFT calculation confirms that both Co and N doping can enhance the interaction between Co,N-CNTs-CNS/CFC and polysulfides,thus accelerating the reaction kinetics of polysulfides.Besides,the special hierarchical structure can induce 3D Li2S depostion,obviously improving the sulfur utilization.Thus,after loaded with certain polysulfides,Co,N-CNTs-CNS/CFC as sulfur cathodes,exhibits outstanding cycling stability and rate performance.Even when the sulfur loading is increased to 10.20 mg cm'2 with the low electrolty/sulfur(E/S)of 6.94 ?L mg-1,cycling stability can still be maintained with a high areal capacity of 7.42 mA h cm-2.This work provides a new approach to achive high sulfur utilization under high sulfur loading with lean electrolyte.