Preparation And Fundamental Research Of Co-based Metal-organic Frameworks Derivatives For Aluminum-Ion Batteries

With the adjustment of energy structure,the need for high-performance energy storage devices is becoming urgent.Lithium-ion batteries(LIBs)have been dominated commercial markets due to their high specific capacity.However,with the mass production and wide applications of LIBs,the demerits are becoming obvious in the meantime,such as expensive raw materials and safety issues.Researchers are moving their attention to next-generation rechargeable metal-ion batteries,such as Na-ion,Mg-ion,and Al-ion batteries.Among all these metal-ion battery systems,Al-ion batteries(AIBs)have received attentions from industrial and academic community,due to its high safety,inexpensive raw material resource,and high capacity of Al negative electrode.According to these reported Al-ion battery systems,it has been proved that the charge/discharge specific capacity and working voltage highly depend on the choice of positive electrode materials.Thus,developing high-performance and stable positive electrode materials has always been the hot research topic.Scientists have proven that metal-organic frame works-derived(MOFs-derived)materials have promising applications in energy storage and conversion area,where MOFs-derived materials can serve as active materials,host materials,or catalysts.The research work in this dissertation will provide investigation and exploration of applications of cobalt-based MOFs-derived(ZIF-67-derived)materials in AIBs,and the obtained main results and progress are listed as follows:(1)ZIF-derived Co₃O₄ materials were prepared via a high temperature calcination of ZIF-67.The as-prepared Co₃O₄ materials inherited the polyhedron shape from ZIF-67 precursor,and the size of polyhedron is uniform.Electrical conductivity is improved with the addition of multiwall carbon nanotubes(MWCNTs).The charge\discharge specific capacity of the 1st cycle can reach to 249.4,and 266.4 mAh g^-1.The specific capacity can maintain at 125 mAh g^-1 after 150 cycles,the coulombic efficiency(CE)maintains at 99%.X-ray photoelectron spectroscopy(XPS)survey towards to the charge/discharge product confirm that the oxygen element in Co₃O₄ doesn't participate in electrochemical reaction,cobalt element is transfered between Co³⁺and Co⁰ during cycling,which provides capacity during charge/discharge process.(2)Co₃S₄ materials are prepared via a solvothermal method of ZIF-67 precursor,X-Ray Diffraction(XRD)patterns and XPS survey confirm that no impurity is introduced.Co elements are showing Co2+and Co³⁺states,and S elements are showing S2-and S-.The specific surface area of Co₃S₄ is much lower than that of ZIF-67,and the pore size is similar.From electrochemical tests results,Co₃S₄ materials have a relatively high specific capacity and flat voltage plateau.A pair of redox peaks(1.07 V,and 0.78 V)is discovered in cyclic voltammetry(CV)curve.The charge\discharge specific capacity of the 1st cycle at a current density of 100 mA g^-1 can reach to 876,and 875 mAh g^-1.The charge/discharge voltage plateaus are 0.9,and 0.65 V,the voltage hysteresis is reduced to 0.25 V.Rate performance test at higher current density exhibits that the specific capacities are reduced with increased current density,but voltage plateaus are still clear.After severe fading of specific capacity in first 15 cycles,the charge/discharge specific capacity can maintain at 215,190 mAh g^-1 after 130 cycles at a current density of 200 mA g^-1,CE maintains at 89%.(3)S@ZIF-67 composite positive electrode materials are prepared via a direct sulfur loading method during the nucleation process of ZIF-67.The S@ZIF-67 have a specific capacity of 180 mAh g^-1 at a current density of 100 mA g^-1.S@ZIF-67-700 composite material is prepared via high-temperature S-loading treatment(155?),which combine sulfur with carbonized ZIF-67-700.S@ZIF-67-700 is showing a flat charge/discharge voltage pleteaus at 1.5,and 0.7 V,the voltage hysteresis is reduced to 0.8 V.Due to the high sulfur loading(70 wt%),charge/discharge specific capacity of the 1st cycle can reach to 623,and 415 mAh g^-1.The charge/discharge specific capacity can maintain at 170 mAh g^-1 after 160 cycles at a current density of 300 mA g^-1.First-principles calculation results prove that the spontaneous adsorption behaviors exist between ZIF-67/ZIF-67-700 and sulfur/polysulfides.The ZIF-67 and ZIF-67-700 can combine with sulfur and polysulfides,and inhibite active material dissolution and shuttling effect.The average binding energies of ZIF-67 and ZIF-67-700 are-0.78,and-2.46 eV,which confirm stronger adsorption behavior of ZIF-67-700 to sulfur or polysulfides.(4)Based on the ZIF-derived Co₃S₄ materials,combining with reduced graphene oxide(rGO),Co₃S₄/rGO is prepared as host materials for sulfur positive electrode.High-temperature S-loading treatment is carried out to synthesized S@Co₃S₄/rGO composite.Electrochemical tests reveal improved performance of S@Co₃S₄/rGO positive electrode with flat working voltage plateau and high specific capacity.The charge\discharge specific capacity of the 1st cycle at a current density of 300 mA g^-1 can reach 1267,and 1414 mAh g^-1.With the aid of Co₃S₄/rGO host materials,the voltage hysteresis phenomenon of sulfur positive electrode in AIBs is obviously alleviated.In the 1st cycle,the charge/discharge voltage plateaus are 1.48,and 0.85 V,voltage hysteresis is reduced to 0.6 V.The charge/discharge specific capacity can maintain at 440,and 380 mAh g^-1 after 250 cycles at a current density of 300 mA g^-1,CE maintains at?86%.After subtracting the specific capacity contribution from Co₃S₄,the specific capacity for sulfur is calculated to 532,and 487 mAh gs-1(based on the weight of sulfur element).The average binding energies of graphene and Co₃S₄ are-3.624,and-2.386 eV,which confirm strong combination to sulfur and polysulfides,and inhibiting solution of polysulfides and harmful shuttle effect.The ab initio molecular dynamic(AIMD)calculations confirm stable behavior of the proposed adsorption models.