Preparation Of Transition Metal Composites And Their Electrochemical Energy Storage Properties

With the rapid growth of the world economy and the continuous expansion of the economic scale,the human demand for energy has risen sharply.Lithium-ion batteries(LIB)are favored as an important technical means in the field of energy storage due to their high reversible capacity and high energy density.However,the reserves of lithium ore resources are scarcity and expensive,which leads to the prominent cost problem of lithium-ion batteries.In contrast,sodium resources are abundant and inexpensive.Lithium and sodium are both alkali metal elements.They have similar physical and chemical properties,and also have similar electrochemical behaviors.The standard electrode potential of sodium is higher than lithium,and it has better safety performance as an energy storage material.Therefore,sodium-ion batteries(SIB)have broad application prospects and are promising to replace LIB in the field of energy storage.As one of the important components of sodium-ion battery,the negative electrode material plays a crucial role in the performance of sodium-ion battery.Carbon-based anode materials were shown to have no large sodium intercalation capacity.Transition metal oxides,sulfides and other anode materials have high sodium storage capacity.But they have large volume expansion and low electrical conductivity problems,which lead to poor cycle stability.Combining carbon-based materials with transition metal oxides and sulfides can effectively compensate for the shortcomings of poor capacity when carbon materials are used alone,as well as the disadvantages of poor electrical conductivity and poor cycle performance when transition metal compounds are used alone,thereby improving the electrochemical performance of sodium-ion batteries.The specific research contents of this paper are as follows:(1)Synthesis and exploration of carbon cloth supported transition metal NiMn-MOF materials and their application in sodium-ion batteries.By a solvothermal method,nanosheet-like NiMn LDH was used as a precursor,which was converted into NiMn-MOF nanosheets through a ligand exchange reaction and loaded on the surface of carbon fiber cloth.The carbon fiber cloth substrate has good electrical conductivity.NiMn-MOF nanoflowers are synthesized from NiMn LDH nanosheets and organic ligands,with high specific surface area,increased contact with electrolyte,uniform distribution of active substances,accelerated sodium ion transport,and can buffer the volume during cycling swell.At the same time,this self-supporting structure avoids complex grinding and tedious coating processes,does not need to add conductive agents and binders,and can be used directly as electrodes.The material still maintained a capacity of 510 m Ah g^-1 after 100cycles at a current density of 100 m A g^-1.(2)Synthesis and exploration of carbon-coated cobalt-doped vanadium oxide nanomaterials and their application in sodium-ion batteries.The surface of cobalt-doped V₂O₃ nanoparticles is coated with carbon by solvothermal method,and then calcined to obtain the composite material.The cycling stability and electrical conductivity of the material are improved due to the coating effect of carbon and the doping effect of cobalt.After 200 cycles at a current density of 0.1 A g^-1,the specific capacity of the composite remained at 350 m Ah g^-1.And it can still exert a high discharge specific capacity of 250m Ah g^-1 at a large current density of 1 A g^-1.(3)MoS₂ uniformly intercalated in nitrogen-doped three-dimensional porous carbon network(3D MoS₂@N-CN)composite was prepared by freeze-drying and hydrothermal methods using NaCl crystal as template.The 3D N-CN-stabilized porous structure provides a suitable pathway for ion/electron diffusion,suppresses the agglomeration of MoS₂nanoparticles,improves electrical conductivity,and promotes rapid electron/ion diffusion.Due to these structural advantages,the MoS₂@N-CN electrode exhibits high discharge specific capacity and stable cycling performance.After 100 cycles at a large current density of 1 A g^-1,the capacity remains at 214 m Ah g^-1.