Metal-organic Frameworks Derived Nanocomposites And Their Applications In Anode Materials For Lithium/Potassium-ion Batteries

With the development of electronic products and the popularization of electric vehicles,the demand for lithium-ion batteries(LIBs)has soared in the past few years.However,the traditional graphite anode,due to its low specific capacity,cannot meet the ever-increasing requirements of LIBs with high energy density and high power density.Therefore,it is crucial to develop advanced anodes with high specific capacity and high rate capability.Meanwhile,the utilization of LIBs in large-scale energy storage will be limited by the low nature abundance,uneven distribution and increasingly high cost of lithium and cobalt resources.Potassium-ion battery(PIB)is a promising large-scale energy storage system owing to its merits like bountiful potassium resource,low cost and low redox potential of K+/K.However,due to the large radius of K+,a huge volume expansion will be caused in the process of charging and discharging,resulting in a low specific capacity and poor cycling stability.Therefore,it is urgent to design appropriate electrode materials with good potassium storage performance.Metal-organic frameworks(MOFs)are a series of porous crystalline materials formed by the coordination of metal ions or clusters with organic ligands.MOFs-derived nanomaterials have enormous pore structure,large specific surface area,abundant active sites and good conductivity,so they are extensively explored in energy storage and conversion.This thesis aims to design and prepare nanocomposites with unique structure using metal-organic frameworks as precursors,and use them as advanced anodes for lithium/potassium-ion batteries,as a means to optimize their specific capacity,rate performance and cycling stability.The main contents of the thesis are as follows:1.Transition metal sulfides are widely concerned as promising LIBs anodes owing to their natural abundance,safety and high theoretical capacity.However,poor conductivity and dramatic volume expansion restrict their practical capacity and cycling performance.Herein,we reported a facile MOF-derived self-template approach to prepare hollow CoS polyhedrons decorated with metallic(1T)phase MoS2 nanosheets(denoted as CoS@1T-MoS2)as efficient anodes for LIBs.By virtue of the high intrinsic conductivity of 1T-MoS2,the hierarchical hollow framework,and the synergistic effect of interior CoS and exterior 1T-MoS2,the composite exhibits a large specific capacity of 1269 mA h g-1 over 100 cycles at 0.1 A g-1 and maintains 936 mA h g-1 after 220 cycles under 1 A g-1.The MOF-derived self-templated approach could also be applied to synthesize other nanomaterials with unique hollow structure for electrochemical energy storage and conversion.2.Metal oxides usually exhibit low reversible capacity,inferior rate capability and limited lifespan for PIBs due to the large radius of K+.To solve these problems,a spring-like vanadium monoxide/amorphous carbon(VO/C)composite has been rationally designed.The composite shows good potassium storage performance as PIB anodes,which exhibits 241 mA h g-1 at 1 A g-1 over 1000 cycles and 104 mA h g-1 at 15 A g-1 Meanwhile,a PIB full cell exhibits 168 mA h g-1 at 2 A g-1 and 80%capacity retention over 500 cycles.The performance is superior to most reported transition metal oxides for PIBs.The ex-situ characterizations demonstrate that the spring-like VO/C nanostructure can accommodate the volume variation during cycling.Density functional theory calculations reveal that the hybrid VO/C structure significantly increases the electronic states at Fermi level thus improving the electronic conductivity,and reduces the K+diffusion barrier,which contributes to its large capacity and superior rate performance.This work provides some insights into the rational design of metal oxides-based anodes for high-performance PIBs.3.Carbonaceous materials are promising anodes for PIBs.However,due to the significant volume expansion and structural instability,it is still a challenge to obtain advanced carbon-based PIBs anodes with high capacity,high rate performance and long lifespan.Here,oxygen/fluorine dual-doped porous carbon nanopolyhedra(OFPCN)is reported as a novel anode for PIBs,which provides an impressed capacity of 481 mA h g-1 at 0.05 A g-1 and 218 mA h g-1 after 2000 cycles under 1 A g-1 with 92%capacity retention.Even after 5000 cycles at 10 A g-1,an unprecedented capacity of 111 mA h g-1 is still maintained.To the best of knowledge,such ultrafast potassium storage and unprecedented cycling stability have been seldom reported in PIBs.Kinetics analysis reveals that both diffusion and capacitance process are involved in the potassium storage mechanism.Density functional theory calculations demonstrate that the O/F dual-doped porous carbon has an enhanced K-adsorption ability and can absorb multiple K+with slight structural distortion,which accounts for the high reversible capacity,outstanding rate capablity and excellent stability of the OFPCN anode.These results may shed some light on the design of heteroatom-doped carbon as promising anodes for PIBs.