Synthesis Of Bimetallic MOF-derived Carbon Materials And Their Application For Lithium Sulfur Batteries

Due to the high theoretical capacity(～1675 mA h g-1)and high energy density(～2600 W h kg-1),lithium sulfur(Li-S)battery,which is regarded as one of the most promising next generation energy storage devices,has attracted much attention.However,commercial application of current Li-S batteries still suffers from some inherent problems,which include the large volume changes during charge/discharge processes,the dissolution of polysulfides intermediates causing the "shuttle effect" and the insulating nature of sulfur and its discharging products(Li2S/Li2S2).These problems would result in the rapid capacity fading,low Coulombic efficiency and poor cycle stability of Li-S batteries.To address these issues,the aim of this thesis is to optimize the structure of sulfur host materials for improving surface functionality of the cathode to suppress the shuttle effect,and thus enhance the utilization of active materials and cycling stability.(1)We demonstrate a facile strategy to prepare bimetallic Zeolitic Imidazolate Frameworks(ZIFs)-derived "sea urchin"-like Co/N co-doped porous carbon structure(Co-NC)as the sulfur host materials,and the influence of heating time during carbonation process on microstructure,composition and electrochemical performance of Co-NC materials are investigated.The results reveal that the length and diameter of the carbon nanotubes(CNTs)grafted on the surface of Co-NC significantly increase as heating time prolonged.When the heating time is increased to 5 h,the designed CoNC3/S cathode shows the best electrochemical performance.Due to the highest specific surface area and porosity,Co-NC3/S cathode favors the uniform distribution of sulfur.Besides,the integrated CNTs conductive network benefits the diffusion of electrons/ions within the whole electrode and then features low internal resistance of electrode.Thus,when Co-NC3 composite is applied as sulfur host for Li-S batteries,the Co-NC3/S cathode delivers a high specific capacity of 481 mA h g-1 at 3.0 C and maintains 937 mA h g-1 after 65 cycles at 0.2 C.(2)In order to further strengthen the structural advantages of sulfur host materials,the configuration of bimetal MOF(Metal-Organic Framework)precursor is optimized and then CNTs-grafted nitrogen-doped carbon@graphitic carbon nanocages(CNTNC@GC)are prepared via the pyrolysis of core-shell ZIF-8@ZIF-67 by self-assembly method.Firstly,the special hollow structure could accommodate high-content sulfur.Secondly,the adjacent shells of CNT-NC@GC are joined together,the CNTs grafted inside and outside are entangled to form a 3D tough conductive network which could buffer the volume change of the sulfur cathode.Meanwhile,the increase in surface absorption sites from the Co/N co-doping could improve the ability to draw polysulfides into the highly porous carbon matrix and suppress the shuttle effect.This behavior is confirmed by the density functional theory(DFT)calculation.Finally,the embedded Co nanoparticles on the carbon shells/CNT tips,possess the electrocatalytic convention activity to accelerate the redox reaction kinetics of lithium polysulfides.As a result,the designed CNT-NC@GC/S cathode with a high sulfur loading of 79.2 wt%shows good long-term cycling stability with a high capacity retention of 83.6%after 500 cycles at 1.0 C and superior rate capacity with a reversible capacity of 653 mA h g1 at 3.0 C.Most importantly,the cathode can also deliver a stable areal capacity of 4.01 mA h cm-2 when sulfur loading increases to 4.95 mg cm-2,which has great commercial application prospect.