| As a novel type of electrochemical energy storage material,the study on nitrogen-containing composites based on zeolitic imidazole frameworks(ZIFs)derivatives was carried out in this thesis.Followings are the main acquired results:1.The as-prepared ZIF-67 materials were used as precursors to attain porous metal-nitrogen-carbon(Co-N-GC)composites.When applied as sulfur hosts,the as-prepared porous Co-N-GC exhibited excellent rate and cycling performances in the Li-S batteries.The S@Co-N-GC cathode with a high sulfur content of 70 wt%could deliver an extremely high initial discharge capacity of 1670 mAh g-1 at the current rate of 0.05 C,which is almost the theoretical capacity of S,and a reversible charge capacity of 1651 mAh g-1,with a high coulombic efficiency of 98.9%.A specific capacity of 565 mAh g-1 could be retained even at a high current rate of 5 C.Besides,the S@Co-N-GC cathode could maintain a specific capacity of 850 mAh g-1 after 200 cycles at the current rate of 0.2 C,with a slow capacity decay rate of 0.023%per cycle.When the current rate was increased to 1 C,it could also maintain at 625 mAh g-1 after prolonged 500 cycles.2.The reaction mechanism of lithium-sulfur battery was deeply studied and the concept of "multi-functional,dual-catalysis" was proposed for the first time.The experiment results confirmed that the catalyzing for S redox,entrapping of polysulfides and an ideal electronic matrix were successfully achieved synchronously in the Co-N-GC composite.On one hand,the large specific surface area and abundant pores are beneficial to the physical adsorption of polysulfides.On the other hand,Co-N played a dual-catalyzing role in the S reduction and oxidation processes.Thus Co,doped-N and N-GC matrix acted according to their own role in the synergistic system,which effectively suppressed the shuttle effect,overcame the problems such as active material loss,capacity fading and improved electrochemical reaction dynamics,significantly improve the electrochemical performance of Li-S batteries.3.Aquasi-2D Co@N-C composite with honeycomb architecture was designed and prepared based on the study of the Co-N-GC compocite.When applied as a high loading sulfur host in Li-S batteries,the cellular Co@N-C cathode exhibited excellent electrochemical performance.4.The cellular flake with large surface area and honeycomb architecture can encapsulate much more sulfur,leading to high sulfur content(HSC)of 93.6 wt%,and by stacking these HSC flakes,a high sulfur loading(HSL)electrode of 3.6-7.5 mg cm-2 can be realized owing to their high layer bulk density.When the areal sulfur mass loading in the cathode was controlled to be 3.6 mg cm-2,the 90S/Cellular Co@N-C electrode exhibited great rate and cycling performance.When the current rate was as high as 10 C,a reversible specific capacity of 290 mAh g-1 could also be acquired.In addition,the 90S/Cellular Co@N-C cathode could still possess 75.0%of the initial capacity even over 850 cycles at the current rate of 2 C.Even when the areal mass loading of sulfur was controlled to be as high as 7.5 mg cm-2,the cathode could also exhibit great rate capability up to 2 C and good cycle stability at 1 C.5.A one-dimensional carbon nitride nanotube(t-ACN)was synthesized via an efficient template method and tested as anode material for Li-ion batteries.The electrode could deliver an exceptional high initial specific discharge capacity of 2229 mAh g-1 and a reversible charge capacity of 1224 mAh g-1 at the current density of 0.05 Ag-1.When the current densities were gradually increased to 0.1,1,5 and 20 A g-1,the reversible capacities were around 1139,1028,875 and 631 mAh g-1.Even at a relatively high current density of 100 A g-1 although it took only 10 seconds to discharge and charge,the t-ACN electrode could still deliver a favorable capacity of 357 mAh g-1,suggesting its ultra-high power characteristic.Besides,the t-ACN electrode exhibited excellent cycling stability.It delivered a favorable capacity of 374 mAh g-1 after prolonged 24000 cycles even at an ultrahigh current density of 50 A g-1,where the discharge or charge process took only 28 s,with a slow capacity decay rate of only 0.0005%per cycle.6.The lithium storage mechanism of t-ACN was preliminarily discussed.By distinguishing the capacities of two different lithium storage mechanisms,we studied the processes of Faradaic and surface lithium storage and proposed the idea and feasibility of designing electrode material with high capacity and high power.7.In addition to be used as a high capacity anode for LIBs,the t-ACN can also be used in a symmetric battery and a total battery with high power and energy densities.The symmetric device exhibited an energy density of 54 Wh kg-1 at the current density of 100 A g-1,where only a 2-3 seconds discharging-charging time was needed.Moreover,the device showed a much lower self-discharging rate than commercial supercapacitors in the fully lithiated state.Last but not least,when the t-ACN material is coupled with LiFePO4@C to give a fully-rechargeable Li-ion battery,a high power density of 16 kW kg-1 could be delivered with a high energy density of 160 Wh kg-1 at the current rate of 100 C,where the discharge or charge process took only 10 seconds.The ARC test confirmed that this carbon nitride nanotube also exhibited a much better thermal stability thus safer than commercial graphite. |
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