Preparation Of V₂CT_x/MnO₂ Composites And Performance Of Rechargeable Magnesium Batteries

As the current development of lithium-ion batteries has encountered bottlenecks such as high cost,lack of resources,and low safety,in order to meet the sustainable development of energy,the research and development of new secondary energy storage batteries has received more and more attention from researchers.Among them,magnesium ion batteries are the most competitive.Magnesium has large reserves and low cost.The divalent nature of magnesium can provide greater volume specific capacity.At the same time,the reduction potential of magnesium is low,which corresponds to the high output voltage and high energy density of magnesium ion battery.Most importantly,the dissolution and deposition of magnesium as a negative electrode during charge and discharge is reversible and does not produce dangerous dendrites like lithium metal batteries.The development of magnesium ion batteries is largely affected by the cathode materials.The key to the performance of magnesium ion batteries lies in the magnesium storage capacity of the cathode materials.However,due to the divalent nature of magnesium and the large charge density,the strong interaction of the cathode material anions during the insertion and extraction of magnesium ions leads to slow electrode reaction kinetics.Therefore,it is of great significance to design and develop high-performance cathode materials.In this thesis,the two-dimensional transition metal carbide V₂CT_x with high conductivity and large specific surface area is used as the substrate,which is compounded with the transition metal oxide MnO₂ with high voltage and large capacity.The synergistic effect of the two materials is used to solve the problems of self-stacking,small interlayer spacing and less magnesium ion embedding of V₂CT_xmaterials.At the same time,the disadvantages of low conductivity and poor circulation of MnO₂ are improved.The specific research contents are as follows:（1）Different crystal forms of MnO₂ were grown in situ on the hydrofluoric acid etched multilayer V₂CT_x sheets by hydrothermal method,and the physical and electrochemical properties of different crystal forms of composites were tested.The results show that the in-situ growth of MnO₂ with different crystal forms in multi-layer V₂CT_x layers can effectively inhibit the self-stacking of the layers,expand the interlayer spacing,provide additional ion channels,and improve the wettability of the electrode material and the electrolyte,which can accommodate more magnesium ions.When different crystalline MnO₂ composite multilayer V₂CT_x materials are used as cathode materials for magnesium ion batteries,the first cycle discharge specific capacity ofδ-MV composites is higher than that of other crystalline forms at a current density of 50 m A g^-1,and the capacity retention rate reaches 70.3%after 100 cycles.After cycling at high current density,the rate performance ofδ-MV composites is still better than that of other crystal forms,indicating that among the four different crystal forms of composite multilayer V₂CT_x materials,the structural stability ofδ-MV composites is the best,showing good reversibility.（2）The composite materials of V₂CT_x and MnO₂ with different electrical properties were prepared by electrostatic self-assembly technology.The composite materials were tested by XRD,SEM and specific surface area,and the electrochemical properties of the composite materials were analyzed.The results show that when the mass ratio of dl-MnO₂ and dl-V₂CT_x is 30%,the dl-MV composite has the largest interlayer spacing and specific surface area of V₂CT_x,the most abundant ion transport channels and the most redox active sites.The composite materials with different proportions were used as the positive electrode of magnesium ion battery.The dl-MV-30%composite had the best cycle performance at a current density of 50 m A g^-1.The first cycle discharge specific capacity was 150.5 m Ah g^-1,and the capacity retention rate was as high as 87%after 100 cycles of charge and discharge.Cyclic voltammetry and rate performance tests showed that the dl-MV-30%composite had better magnesium storage performance.