Study On Preparation And Electrochemical Performance Of Selenide As Cathode Material For Magnesium-based Battery With High Performance

The rechargeable battery with metallic magnesium as an anode is a new type of green energy storage system.Although the energy density of lithium-ion batteries can reach as high as 300 Wh kg^-1,the low safety,high cost,and environmental pollution of lithium batteries hinder up their development.Compared with Li metal,Mg metal anode has unique characteristics of lower cost,more abundant earth resource,higher volume specific capacity(3833 m Ah cm^-3)and higher security.However,due to the high charge density of magnesium ions and serious solvation effect,Mg²⁺is difficult to（de）intercalate into most of cathode materials and its diffusion rate is sluggish.Therefore,finding a suitable cathode material with reversible（de）intercalation of Mg²⁺is one of important development directions for Mg-based batteries.In this thesis,selenides are used as cathode materials for Mg-based batteries on the basis of Prussian blue analogue.The structure,properties,electrochemical performance and energy storage mechanism of selenides are systemtically investigated.The main contents are as follows:（1）Preparation and properties of N-doped carbon coated Ni_0.6Co_0.4Se₂@r GO composite.The low-temperature pyrolysis method is adopted to realize the carbonization-selenization Prussian blue analogue（PBA）with one-step and successfully obtain sub-micron cubic N-doped carbon-coated bimetallic Ni_0.6Co_0.4Se₂@r GO composite material.At a low current density of 25 m A g^-1,its first discharge specific capacity is increased to 693 m Ah g^-1 and its energy density reaches up to 489 Wh kg^-1,which is higher than those of the current cathode materials for magnesium-lithium hybrid ion batteries.Even at 500 m A g^-1,Ni_0.6Co_0.4Se₂@r GO yields discharge specific capacity of 230 m Ah g^-1.In the APC-Li Cl/THF electrolyte,the charge and discharge process of Ni_0.6Co_0.4Se₂@r GO material involves in intercalation and conversion reaction mechanism,which is mainly controlled by pseudocapacitance.Ni_0.6Co_0.4Se₂@r GO composite shows excellent cycle stability and rate performance.（2）Preparation and properties of cubic morphology Cu_2-xSe@（Cu,Co）Se₂composite.Cu_2-xSe@（Cu,Co）Se₂ with cubic morphology was successfully synthesized by precipitation method and carbonization-selenization method.In the APC-Li Cl electrolyte,the initial discharge capacities of Cu_2-xSe@（Cu,Co）Se₂ electrode are 370 m Ah g^-1 and 345 m Ah g^-1 at the current density of 100 and 500 m A g^-1,respectively.After 100cycles,the capacity retention ratio of Cu_2-xSe@（Cu,Co）Se₂ are 88%and 67%,respectively comparing to the third cycle.Their coulombic efficiencies are close to 100%.In the APC electrolyte,Cu_2-xSe@（Cu,Co）Se₂ can achieve the discharge specific capacity of～100 m Ah g^-1 at 25 m A g^-1 after 100 cycles and has the coulombic efficiency of 100%.The electrochemical reaction of Cu_2-xSe@（Cu,Co）Se₂ is kinetically controlled by diffusion in the APC-Li Cl electrolyte,while the electrochemical reaction of Cu_2-xSe@（Cu,Co）Se₂ is mainly affected by pseudocapacitance control in the APC electrolyte.Cu_2-xSe@（Cu,Co）Se₂ composite displays high reversibility and excellent rate performance in the APC-Li Cl electrolyte.In summary,Ni_0.6Co_0.4Se₂@NC@r GO and Cu_2-xSe@（Cu,Co）Se₂composites are promising cathode materials.