Structure Design And Energy Storage Mechanisms Of Magnesium Battery Cathode Materials

Since 1991,lithium-ion batteries have developed rapidly due to their high energy density and long cycle life,then dominated the market for portable electronic devices.However,with the wide application of energy storage equipment,problems such as shortage of lithium resources have become increasingly searious.In addition,the lithium dendrite problems also bring some safety risks.In recent years,magnesium ion batteries have been gaining popularity due to their advantages such as high volume specific energy,abundant resources and high safety.However,magnesium ion batteries have developed relatively slow and are still in the initial stage of research.This is mainly due to the strong interaction between divalent Mg²⁺ and positive host materials,which makes most materials exhibit slow diffusion kinetics and thus have poor electrochemical performance.In addition,the mature electrolyte in the lithium-ion battery system will produce a passivation film on the surface of the magnesium negative electrode,which hinders the reversible deposition of Mg²⁺,and ultimately leads to poor coulombic efficiency.Finding suitable cathode materials and electrolyte is still the key to the development of magnesium ion batteries.Transition metal chalcogenide compounds are attractive cathode materials for magnesium ion battery systems due to their relatively low ionization and high theoretical capacity.However,in the process of electrochemical cycling,they have some problems such as large volume expansion and poor cycling stability,which limits their development as cathode materials for magnesium ion batteries to a certain extent.To solve the above problems,three different transition metal chalcogenides(Ni₃Se₄,VS₄ and VS₂)were selected as the research objects respectively,and three kinds of micro-nano structure materials were constructed as the cathode of magnesium ion battery through different strategies,then their electrochemical performance and reaction mechanism were further explored.Finally,we have successfully applied some experience in the research of sulfur compound cathode materials to the design and synthesis of oxide cathode materials(NH₄)₂V₆O₁₆·1.5H₂O.By introducing crystal water between layers not only weakens the electrostatic interaction between magnesium ions and materials,but also reduces the migration energy barrier,so the electrochemical performance is significantly improved.Through the research of this paper,the obtained main research results are as follows:(1)Ni₃Se₄ particles with a size of about 10-20 nm were successfully synthesized by one-step hydrothermal method and used as cathode material for magnesium ion batteries.The electrochemical properties and reaction mechanism of Ni₃Se₄ particles in APC electrolyte were investigated.First-principles calculations show that Ni₃Se₄ has a high Mg²⁺ diffusion barrier.Based on this,the diffusion path of Mg²⁺ can be effectively shortened by nano-design of Ni₃Se₄.Finally,good electrochemical performance is obtained.The material has a capacity retention rate of75 % after 100 cycles at a current density of 50 m A^-1,and exhibits good rate properties.At the same time,the material with large particle size was synthesized by solvothermal method,and its electrochemical cycling performance was investigated,which further confirmed the above conclusion.Subsequently,the mechanism of Magnesium storage of Ni₃Se₄ was investigated by off-position X-ray diffraction(XRD),X-ray photoelectron spectroscopy(XPS)and first-principles calculation.The results showed that the material underwent a solid solution structure change during the Mg²⁺ insertion process,and all the charge transfer occurred on Ni cation,while the Se anion did not change during the reaction process.(2)Although it is possible to obtain good electrochemical performance by reducing the particle size of the material,the first coulomb efficiency of the material is greatly reduced due to the side reaction and agglomeration between the nanomaterials and the electrolyte in the cycle process.VS₄@r GO was synthesized by CTAB cationic-assisted hydrothermal method.The introduction of graphene can promote the uniform dispersion of nanoparticles on the surface of graphene and effectively inhibit its agglomeration.Then,the electrochemical properties and reaction mechanism of VS₄@r GO in MACC electrolyte were studied.As a comparison,VS₄ was synthesized by hydrothermal method,and its electrochemical performance was tested under the same conditions.The results show that VS₄@r GO composites exhibit better cycle stability and rate performance when used as cathode material for magnesium ion batteries.This is mainly because the synergistic effect of VS₄ nanoparticles and conductive graphene sheets is conducive to the transport of Mg²⁺ in the material,thus improving the kinetic properties.(3)The transition metal sulfide layers of the layered structure are connected with each other by relatively weak van der Waals forces,which is conducive to the diffusion of magnesium ions between layers,while the higher conductivity of VS₂ is conducive to electron transport.The particle size and electrical conductivity of the material are critical to improve the dynamic properties of the cathode materials for magnesium ion batteries.Nano-sheet VS₂ was synthesized by hydrothermal method as cathode material for magnesium ion batteries.At the same time,considering the volatility and flammability of APC electrolyte,the electrolyte was optimized.By adding Py₁₄NTF₂ ionic liquid into APC electrolyte,the introduction of ionic liquid improves the electrical conductivity and safety,which makes VS₂ show excellent electrochemical performance.At the current density of 100 m Ag^-1,the first reversible capacity is 438 mAh ^-1 and has good cycle stability.(4)Although transition metal chalcogenides have obtained better electrochemical performance as cathode materials for magnesium ion batteries,there is still a low voltage,and the stability voltage of chloride electrolyte in the condition of the switch cell is only 2.2 V,which limits the development of high voltage materials.In order to further improve the reaction voltage of cathode materials,high voltage magnesium ion battery is realized.(NH₄)₂V₆O₁₆·1.5H₂O nanoribbon were successfully prepared by one-step hydrothermal method as cathode material of magnesium ion battery.Due to the support effect of interlaminar ammonium ions and the classical shielding effect of interlaminar crystal water,excellent electrochemical performance was obtained,with a capacity of 100 m A·hg^-1 at a current density of 100 m Ag^-1.In addition,the average discharge voltage is 2.65V,indicating its great potential as a high voltage cathode material.Meanwhile,(NH₄)₂V₆O₁₆ nanosheet cathode material was synthesized and compared with the material containing water of crystal in detail.By combining GITT,in-situ XRD characterization and first-principles calculation,the positive effect of crystallization water and the mechanism of magnesium storage in the material were investigated.In summary,through the optimized design of transition metal thioides composite laminated oxide materials,four new synthesis methods of nanomaterials have been successfully regulated and developed,and applied to the cathode materials of magnesium ion batteries.The effects of particle size,composite mode,electrode electrolyte interface regulation and structure construction on the physical and chemical properties of the materials were summarized.The reasons for the improvement of material properties and the reaction mechanism were explored in detail by combining experiments with first-principles calculations.It provides some guidance for the development of cathode materials for magnesium ion batteries in the future.