Preparation Of ZIF-based Nanocomposite For The High Performance Lithium-ion Batteries

Lithium-ion batteriy(LIB)is a commonly used energy storage system which is widely used in military and civil space applications.With the increase in the demand for large-capacity battery such as electric vehicles,the existing graphite materials have limited battery applications due to their low theoretical capacity.In this dissertation,several different nanocomposite negative electrode materials were prepared by using zeolite-like imidazole skeleton as the precursor.The main research contents are as follows:(1)ZIF-67-based nano-carbon materials with different structures and morphologies were synthesized by adjusting the amount of protonated TEA added.When the amount of TEA is 4.7 mL,the synthesized dodecahedral structure ZIF-67c is carbonized into a dodecahedral structure surrounded by hollow carbon nanotubes.The particle size is about 300-400 im and the specific surface area is 509 m2 g-1.The pore size is 4.03 nm.The discharge specific capacities of Co@N-CNT-c samples at current densities of 100,200,500,1000,1500,and 2000 mA/g are 959.7?878.5?721.2?649.5?590.4 and 541.2 mAh/g,respectively.After 1000 cycles at 500 mA/g,the discharge specific capacity of Co@N-CNT-c sample is 763.4 mAh/g,and the capacity retention rate is 95%.In-situ TEM analysis further explains why Co@N-CNT-b material has good cycle stability.(2)The calcination temperature and time to oxidize the Co@N-CNT-c material in air were studied.It is found that when calcined at 250? for 20 min,the material remains the original dodecahedral structure surrounded by hollow carbon nanotubes.Elemental Co is oxidized to Co3O4,and the Co3O4 nanocomposites coated with carbon are successfully synthesized,which show the best electrochemical performance.The specific discharge capacity is 975.81 mAh/g at 100 mA/g.After 300 cycles at 500 mA/g,the specific capacity is 447 mAh/g,and the capacity retention rate is 98.5%.(3)ZIF-8 of the dodecahedral structure was successfully synthesized and carbonized to CN-8.The temperature and time were studied to oxidize CN-8 in the air.It is found that when calcined at 250? for 20 min,the material maintains a dodecahedron structure,with slight collapse and small particles on the surface,and elemental Zn is oxidized to ZnO.Carbon-coated ZnO nanocomposites are successfully synthesized and exhibit the best electrochemical performance.The specific discharge capacities at current densities of 100?200?500?1000?1500 and 2000 mA/g,are 747.3?597?537?432?328.1 and 297 mAh/g,respectively.After 300 cycles at 500 mA/g,the discharge specific capacity of 458.6 mAh/g is still maintained,and the capacity retention rate is 97%.