Study On Sodium Storage Mechanism And Performance Of Metal Compound Anode Materials

With the consumption of non-renewable energy?oil,natural gas and so on?and the aggravation of environmental pollution,utilization of renewable energy becomes imminent,especially solar energy and wind energy.The main problem of renewable energy utilization is the storage and transmission of electric energy,while low cost and long life rechargeable battery system is the key to achieve energy storage.However,current rechargeable battery can not meet the demand of large-scale energy storage.Therefore,it is urgent to develop low cost and long life sodium-ion battery.In addition,the exploration of sodium ion battery is mainly focused on the performance of electrode material.As high capacity and low cost anode materials,metal compound are potential candidates for sodium-ion batteries.However,the cycle stability and rate ability of many metal compound are really terrible,and their sodium storage mechanism and the reason for capacity fading is not clear,which is bad for performance improvement.The main research object in this paper are metal compound.After exploring the sodium storage mechanism and the reason of capacity fading,several strategies are proposed to optimize the electrochemical properties of these materials.The main contents of this paper are as follows:?1?FeS₂ is a high capacity and low cost anode materials for sodium-ion batteries.However,the cycle stability of FeS₂ is poor.In order to improve the cycle stability of FeS₂,FeS₂/porous carbon nanofibers were prepared by electrospinning and high temperature sulfuration.Compared with pure FeS₂ material,the electronic conductivity,structural stability and chemical stability of FeS₂/porous carbon nanofibers had been improved significantly,so FeS₂/porous carbon nanofibers exhibited excellent cycle stability and rate performance.In addition,the structure evolution,morphology evolution and composition evolution of were studied by ex-situ XRD,TEM and in-situ UV visible absorption spectrum.The sodium storage mechanism of FeS₂ was composed by insertion and conversion reaction,and the protection effect of porous carbon was also proved.?2?micron-sized FeS₂ material has many advantages,such as high tap density and good processing property.However,the cycle stability of micron-FeS₂ is poor.In order to improve the performance of FeS₂,the sodium storage mechanism of micron-sized FeS₂ was studied.Then a synthetic solution was proposed.The results showed that suitable voltage range will suppress the pulverization of FeS₂ particles,and a high specific capacity could also achieved at the same time.PAA-Na binder could restrain the structure collapse and the pulverization of the particles;graphene coating could not only increase the electronic conductivity of the material,but also prevent the loss of active materials.The performance of FeS₂ electrode has been greatly improved by the synthetic solution.The initial capacity of modified FeS₂ electrode was about 524 mA h g-1,and the capacity retention was about 87.8%after 800 cycles.Even in a high current density of 5000 mA g-1,the capacity of modified FeS₂ electrode was up to 323 mA h g-1.In addition,the chemical valence evolution of FeS₂ was studied by in-situ X-ray fine structure absorption spectrum during the electrochemical process,and the sodium storage mechanism of FeS₂ was also been verified.?3?MnS is a high capacity anode material for sodium-ion battery.However,few researches was performed,and the sodium storage process is not clear.In order to improve the cycle and rate performance of MnS,MnS/porous carbon nanofibers were designed.In the fiber structure,MnS particles were dispersed in the porous carbon matrix,and the surface of the particles is covered by a compact carbon layer.Porous carbon could restrain the volume change during the electrochemical process,and cross-linked carbon fiber conductive network could improve the electronic conductivity of MnS.What's more,abundant pore structure could promote the transmission of sodium ion and the permeability of the electrolyte.Besides,compact carbon layer can inhibit the loss of active material.The initial capacity of MnS/porous carbon nanofibers were 523.6 mA h g-1,and the capacity retention was about 90.6%after 1000 cycles.At a high current density of 2000 mA h g-1,the capacity of MnS/porous carbon nanofibers is 301.4 mA h g-1.In addition,in-situ UV visible absorption spectrum was performed to characterize the loss of sulfides during the electrochemical process,which was a novel techniques for sodium-ion batteries.?4?A new K0.8Ti1.73Li0.27O4 anode material for sodium-ion batteries was reported.The slice of K0.8Ti1.73Li0.27O4 is easy to peel off during sodium insertion process.In this study,carbon coated K0.8Ti1.73Li0.27O4 is carried out by vapor deposition technique.The results showed that the capacity of carbon coated K0.8Ti1.73Li0.27O4 is about 119.6 mA h g-1,and the capacity is up to 137.7 mA h g-1 after 250 cycles,which showed excellent cycle stability.In addition,the sodium storage mechanism of K0.8Ti1.73Li0.27O4 was studied by ex-situ XRD,and the shrink of the interlayer spacing during the electrochemical process was also found.