Cathode Materials Derived From MOFs For Aqueous Zn-ion Batteries And Its Electrochemical Mechanism

Because of the advantages of safe working environment,convenient storage and conversion,environment benignity and abundant raw material resources,aqueous zinc ion secondary batteries have attracted attentions as a promising green energy storage system.The cathode material is an important component of the aqueous zinc ion battery,but there are some issues,such as low discharge capacity,poor structural stability and unclear electrochemical mechanism,which severely impede its practical application.Development of cathodes with good kinetic properties and stable structure is an effective way to improve the electrochemical performance of aqueous zinc ion secondary batteries.Metal-organic frameworks(MOFs)have high specific surface area and their porosity can induce abundant active sites and high specific surface area in MOFs-derived cathodes,thus improving the ion diffusion rate and electron conductivity.Therefore,they are widely investigated in energy materials and devices.To address the problems of cathodes for aqueous zinc ion secondary batteries,this thesis makes full use of the structural characteristics of MOFs-based materials to construct a series of manganese-based,nickel-manganese bimetallic and vanadium-based cathode materials to improve their electrochemical properties and further investigate the corresponding mechanisms,shedding lights on the application of these materials in aqueous zinc ion batteries.1.Research on MOFs-derived manganese-based cathode materials and zinc storage performanceMn-based oxide cathode materials are prepared by a one-step hydrothermal method using Mn-BTC as the precursor.The effects of material morphology and structure on their electrochemical properties are systematically investigated by varying the ratios of raw materials.The results show that the hierarchical structure composed of nano-stems and nano-sheets derived from Mn-MOF is beneficial to the structural stability of the electrode.Specifically,the high specific surface area and the rich pore structure of the MF-5 electrode are effective in improving the ion transport properties of the material,resulting in good rate performance and high reversible discharge capacity with a specific discharge capacity of 322.5 m Ah·g-1 at a current density of 0.1 A·g-1 and a capacity of 274 m Ah·g-1 at 1.0 A·g-1.Nano-MnO2 anchored on Co/MOF-derived carbon is further designed as cathodes for zinc ion batteries.It is shown that the porous carbon layer improves the structural stability and electronic conductivity of the cathode material,leading to the coupling of pseudocapacitance and double electric layer in the electrode reaction.The ion migration mechanism of H+/Zn2+ co-insertion is revealed during the electrochemical reaction,and the H+/Zn2+ ion cross-insertion is accompanied by the adsorption of anions/cations,which is important in improving the electrochemical performance.As a result,,the C-Mn O2 material achieves a capacity retention of 91% at 1 A·g-1 after 1000 cycles and exhibite a discharge capacity of 120 m Ah·g-1 at 5 A·g-1.2.Research on nickel-manganese bimetallic oxides derived from MOFs and the corresponding electrochemical performanceTo address the low capacity of manganese-based monometallic oxides,bimetallic organic framework material Ni-PTA-Mn was constructed by hydrothermal method in the thesis.Benefiting from the abundant active sites and hydrogen bonding network,the electronic conductivity was promoted.Ni-PTA-Mn exhibited an initial discharge specific capacity of 139 m Ah·g-1 when used directly as the cathode of aqueous Zn-ion batteries.Nickel-manganese bimetallic oxide cathode materials containing Ni O/Mn2O3 were constructed based on the conductive Ni-PTA-Mn material.It is shown that the high specific surface area and abundant phase boundaries result in a pseudo-capacitive behavior,which can highly improve the high discharge capacity and good cycling stability.Specifically,the NM-650 cathode prepared by calcination at 650 ℃ has a capacity of 247 m Ah·g-1 after 100 cycles at a current density of 0.1 A·g-1,and maintains a discharge capacity over 90 m Ah·g-1 after 1000 cycles at 1.0 A·g-1.3.Research on vanadium-based oxides derived from MOFs and zinc storage performanceCompared with the three-dimensional framework of Mn-based oxide materials,layered vanadium-based oxides have more valence variations and zinc storage sites.Based on the previous work,a hierarchical carbon-coated V2O3-x-CC cathode with oxygen vacancy defects is designed,and its electrochemical performance and the corresponding mechanism are investigated by theoretical calculations and experimental characterization.Electrochemical tests show that the V2O3-x-CC cathode has excellent cycling stability,exhibiting a reversible specific capacity of 587 m Ah·g-1 at 0.1 A·g-1 and a capacity retention rate of 71% after 5000 cycles.Meantime,the cathode exhibits excellent rate performance with a discharge specific capacity of 367 m Ah·g-1 at 20 A·g-1.Experimental and DFT calculations show that oxygen vacancy defects provide extra H+/Zn2+ co-insertion sites in the vanadium-based oxide/carbon composite.Meanwhile,the hierarchical carbon structure can provide additional sites and channels for ion,especially H+,adsorption.Dynamic monitoring by in-situ p H meter indicates that there are hysteresis effect of H+ desorption,double layer capacitance of anions/cations and the synergistic effect of pseudo-capacitance of redox reactions during charging and discharging processes.These findings further clarify the mechanism of ion de-insertion in vanadium-based materials and provide a theoretical basis for the application of this system in aqueous zinc ion batteries.