Study On The Structure Control Of Molybdenum Disulfide And Its Cathode Performance Of Magnesium-lithium Hybrid Ion Batteries

Due to combining the advantages of lithium batteries（LIBs）and magnesium batteries （MIBs）with low cost,high safety,and high energy density,Magnesium-lithium hybrid-ion batteries（LMIBs）are expected to become the next generation of high-efficiency energy storage batteries.Currently,LMIBs cathode materials can only achieve the intercalation of Li⁺,so that their electrochemical performance is excessively dependent on the content of the added Li salt in the electrolyte,resulting in problems such as low reversible charge-discharge capacity and low energy density.Cathode materials that can realize Li⁺/Mg²⁺co-intercalation with a higher specific capacity and energy density,and have received extensive attention from scientific researchers.As a typical two-dimensional material,MoS₂ has unique interlayer structure and open ion transmission channel,which can make it have great application prospects in the field of energy storage.However,due to the small radius and high charge density of Mg²⁺,it is subject to a large Coulomb force in the host material and leads to a sluggish diffusion rate.Therefore,this thesis improves the diffusion kinetics of MoS₂ electrode materials through the optimization of the material structure and composition,including expanding the interlayer spacing,constructing heterostructures,and designing anion vacancies or defects to obtain the high-performance of LMIBs cathode materials that can realize the Li⁺/Mg²⁺co-intercalation.First,using Mo-MOF as a precursor,a hollow nitrogen-doped carbon nanofiber supported expanded MoS₂ was synthesized by coating polypyrrole,low-temperature heat treatment,and solvothermal vulcanization.Among them,the interlayer spacing of MoS₂ was expanded from 0.62 nm to 0.94 nm.Expanding the interlayer spacing is beneficial to increase ion transmission rate and increase ion insertion active sites;in addition,the coating of polypyrrole not only enables the electrode material to maintain the hollow tubular morphology,but also increases the content of nitrogen-doped carbon in the electrode material,which is beneficial for the electrode material to exhibit a larger specific surface area and superior electronic conductivity.Thanks to the above advantages,the electrode can still achieve Li⁺/Mg²⁺co-intercalation even at a current density of 1000 m A g^-1,showing excellent electrochemical performance.In addition,after cycled for 2000 cycles at 1000 m A g^-1,the reversible capacity of this material still remains 134.4 m Ah g^-1.The extended layer MoS₂ can achieve Li⁺/Mg²⁺co-intercalation and is conducive to the improvement of electrochemical performance.However,the interaction between the intercalated Li⁺and Mg²⁺in the layers and the related mechanisms are not clear.Therefore,in this thesis,a simple hydrothermal method was used to synthesize a graphene-supported expanded layer of MoS₂ nanoflower material,in which the interlayer spacing of O-MoS₂ was expanded to 0.885 nm.The combination of expanded interlayer spacing and graphene are beneficial to increase the electronic conductivity,improve the diffusion rate of ions between layers,increase the number of active sites,and improve electrode cycling stability.In addition,it is proved by means of electrochemical performance test and density functional theory calculation that Li⁺intercalation not only increases the number of subsequent Mg²⁺intercalation,but also significantly reduces the diffusion energy barrier of Mg²⁺between layers and improves the ion diffusion coefficient.The reversible capacity of this material still remains 123.3m Ah g^-1 after cycling 2000 cycles at 1000 m A g^-1.The introduction of guest substances between MoS₂ layers can support the expansion of the interlayer spacing,so that the distance between the host material and the intercalated ions is increased,and the Coulomb force is reduced.However,these guest substances will also partially occupy the ion transport channel and affect the diffusion rate of ions.The alternately stacked MoS₂ and graphene van der Waals heterostructure is constructed by electrostatic adsorption self-assembly and heat treatment.The modification of graphene oxide by polyethyleneimine helps MoS₂ grow along the plane direction on the graphene surface,forming a two-dimensional heterostructure.Electrochemical tests and density functional theory calculations prove that the diffusion energy barrier of Li⁺and Mg²⁺in this new type of ion transport channel（between MoS₂and graphene layers）is significantly smaller than that between the two layers of MoS₂.This structure can not only enhance electronic conductivity and improve structural stability,but also more effectively reduce the transmission resistance of ions between layers and improve the dynamic characteristics of ion diffusion.The reversible capacity of this material still remains 145.8 m Ah g^-1 after cycling 2200 cycles at 1000 m A g^-1.For the MoS₂ monolayer in the MoS₂/G,further improving its inherent conductivity and weakening the interaction force with the intercalated ions are the keys to effectively increasing battery capacity,improving cycle stability,and rate performance.A simple hydrothermal method and heat treatment strategy were used to synthesize a van der Waals heterostructure with anion-rich molybdenum selenium sulfide nanosheets and graphene alternately stacked.When S:Se is 1:1,the number of anion vacancies in the material is the largest.Density functional theory calculations prove that the introduction of selenium and the increase in the number of anion vacancies are beneficial to reduce Li⁺and Mg²⁺diffusion barriers,improve electrode conductivity,and increase ion diffusion rate.The reversible capacity of this material still remains 164.6 m Ah g^-1 after cycling 3000 cycles at 1000 m A g^-1.