Construction And Electrochemical Magnesium Storage Properties Of Nickel-based Selenides

Lithium ion batteries（LIBs）are widely used all over the world,but considering the current problems of commercial LIBs in terms of economy and safety,it is of great importance to develop new battery systems.Magnesium ion batteries（MIBs）as a new type of multivalent ion batteries due to its high theoretical specific capacity,low reduction potential,abundant magnesium reserves and other advantages have received widespread attention in the second life battery energy storage system.However,its diffusion kinetics is very slow due to the strong interaction forces between Mg²⁺and the anion in the cathodes.Therefore,it is difficult to find cathodes with high energy density and long cycle life.In this thesis,nickel-based selenides were constructed as the cathode of MIBs,then their electrochemical magnesium storage properties and reaction mechanism were further explored.The main research contents are as follows:The effect of d-p orbital hybridization on Mg adsorption behavior was calculated by using density functional theory（DFT）,which could show that Mg atoms inserted in NiSe can not only intensify the electronic interactions within the structure but also enhance the charge transfer kinetics.The d-p orbital hybridization can promote local charge rearrangement and electron polarization,which is beneficial to the adsorption and storage of Mg atoms.NiSe submicron particles were prepared by the solution method and used as the cathode for electrochemical magnesium storage tests to investigate the electrochemical properties.At 500 mA g^-1,the specific discharge capacity of NiSe after stabilization was about 54.9 mAh g^-1 and it still exhibited a reversible capacity of 63.5 mAh g^-1 over 680 cycles,showing good electrochemical performance and cycling stability.Controlling the feed ratio to regulate the reaction rate of nickel and selenium,nickel-based selenides with different stoichiometric ratios were prepared by the solution method.The effects of varying parameters such as solvent volume,heating time and nickel source on the synthesized materials were investigated.The synthetic samples of NiSe₂,Ni₃Se₄ and Ni_0.85Se were used as the cathode for MIBs to investigate the differences in their electrochemical magnesium storage properties.The results show that the NiSe₂ and Ni₃Se₄ have good electrochemical properties.And Ni₃Se₄ exhibits better rate performances and cycling stability.At a current density of300 mA g^-1,the specific charge/discharge capacities of NiSe₂ and Ni₃Se₄ were 104.7and 110.1 mAh g^-1 after 190 cycles,which have good electrochemical performance.NiSe₂ micro-octahedra with nitrogen atom doping was prepared by hydrothermal method followed by pyrolysis.Compared with the NiSe₂ without nitrogen atom doping,the N-NiSe₂ micro-octahedra electrode used as the cathode material for MIBs shows a good rate performance and cycle stability due to the doping of nitrogen atoms.At 300 mA g^-1,the reversible specific charge/discharge capacity of N-NiSe₂micro-octahedra is about 98.4 mAh g^-1 after 400 cycles,while the specific capacity of NiSe₂ is only 46.4 mAh g^-1 after 400 cycles.The DFT calculation results showed that the doping of nitrogen atoms not only improved the diffusion kinetics of Mg²⁺,but also increased more active sites and improved the electronic properties of NiSe₂.The thesis has 35 pictures,2 tables,and 256 references.