A Study On Rechargeable Metallic Magnesium Battery High Performance Cathode Materials

Energy and environment issues are two major challenges for the sustainable development of human society.Lithium ion batteries（LIBs）have been under widespread evaluation as one of the main solutions for transportation power sources due to the due to the excellent electrochemical performance.Nevertheless,low earth’s crust,limited energy density and safety issues make it controversial for the application of lithium ion batteries for next generation of large-scale energy storage.Rechargeable magnesium batteries（RMBs）,due to the dendrite-free electrochemical deposition and earth abundant features,have been considered as the potential candidate for next generation large-scale energy storage system with high safety.However,strong coulombic interaction between magnesium ions and the host material limits the choice of intercalation materials and might lead to sluggishness of electrochemical kinetics.To bring the magnesium metal anode superiority into fully paly and solve the problems of magnesium cathode,this thesis explored two types of cathode materials for Mg-storage,and investigated their Mg-storage mechanism and kinetic performance,and constructed a new type of magnesium sodium hybrid secondary battery.The main research contents are as follows:（1）intercalation-type Cu₂MoS₄ cathode:Cu₂MoS₄ was successfully synthesized by a two-step method,and its morphology formation mechanism was systematically studied.The obtained hollow nanocage Cu₂MoS₄ exhibits excellent electrochemical Mg-storage performance,whose reversible capacity is over 180 m Ah g^?1.And the hollow nanocage Cu₂MoS₄ also shows high rate performance and excellent cycling performance（2500 cycles）.The comparison of different morphology Cu₂MoS₄ reveals the hollow nanostructure could effectively shorten the diffusion path of magnesium ions and improve the electrochemical performance.Further mechanism study indicates Cu₂MoS₄ is an intercalation-type material for Mg²⁺,and the kinetic analysis shows the solid-diffusion process of Cu₂MoS₄ in the magnesium intercalation process could be enhanced via the intrinsic properties of Cu₂MoS₄material and the optimization of the hollow strategy.（2）conversion-type PVPI cathode:polyvinylpyrrolidone iodine（PVPI）was successfully prepared via a reflux method with iodine and PVP.Structural analysis shows that iodine is uniformly distributed in the PVP organic framework with the hydrogen bonding coordination.PVPI exhibits excellent electrochemical Mg-storage performance,which can release a reversible capacity of 200 m Ah g^?1.And it also has superior rate performance and excellent cycle performance（decay rate:0.18%per week）.A mechanism investigation demonstrates the reversible redox reaction between PVP-I₃^–and PVP-I^–during discharge/charge.The attenuation inhibition analysis shows that the iodine and PVP framework are complexed in the form of hydrogen bonds,which can effectively eliminate the shuttle effect and alleviate the self-discharge capacity decay process due to the reaction between the positive electrode and the electrolyte.This type of iodine-based materials with hydrogen bonding can effectively alleviate the shuttle effect,which provides feasible ideas for the design and synthesis of superior-performance iodine-based Mg-storage materials in the future.（3）Mg/Na hybrid battery based on Na₂VTi（PO₄）₃ cathode:The carbon-coated NASCION phase Na₂VTi（PO₄）₃ material was successfully prepared by the sol-gel method.The microstructure of the as-prepared material is nanorods with a diameter of150 nm,and the outer layer is coated with a 4 nm amorphous carbon layer.The results of the constructed Na₂VTi（PO₄）₃ sodium ion full battery reveal that the full cell capacity still decreases even with the presodiation,which could be attributed to the decomposition of the electrolyte.In contrast,the Mg/Na hybrid secondary batteries with Na₂VTi（PO₄）₃ cathode exhibit excellent electrochemical performance,with a specific capacity of 168 m Ah g^-1(50 m A g^-1),and a superior cycling stability over 1000 cycles.Further Na-sodium storage mechanism shows that Na₂VTi（PO₄）₃ could conduct a three electron reversible reaction in the mixed electrolyte,and it also has excellent diffusion kinetics.The present work provides novel ideas and research directions for the subsequent development of alkali metal/magnesium hybrid secondary battery systems.