Fabrication And Electrochemical Performance Study Of Transition-Metal Based Anode Materials For Sodium-Ion Batteries

Nowadays,as the world economy keeps advancing,people's needs of energy resources are also increasing.As a result,traditional fossil energy resources are faced with a serious problem of running out.Meanwhile,their products of combustion will cause air pollution and further contribute to greenhouse effect.Recently,electrochemical energy storage devices,such as lithium-ion batteries?LIBs?,have been widely researched and developed,which have great potential as one of the candidates of next-generation green energy resources.However,the lithium reserve on the earth is not high.The lithium resource,such as lithium carbonate,is expensive,which will lead to high cost of the LIBs.Compared to lithium resource,sodium resource is more abundant and costs much less.Moreover,both sodium and lithium belong to alkaline metals,which share similar physical and chemical properties.Sodium-ion batteries?SIBs?work under the same principal with LIBs.Hence,SIBs have good potentials in application in large-scale energy storage devices.Anode materials are one of the key conponents in SIBs,which play important roles in the electrochemical performance of SIBs.Nevertheless,the traditional graphite anode materials have been proved to have very low sodium storage capacity,which is urging us to develop anode materials with high capacity and long-term cyclability.Transition metal-based anode materials,such as transition metal oxides/sulfides,have large sodium storage capacities,which are future candidates of SIB anode materials.Due to the conversion reactions during cycling,transition metal oxides/sulfides will suffer form volume expansion after cycling,which will bring about electrode pulverization,causing cell degradation.Meanwhile,the electronic conductivity of this kind of materials are also not very satisfying.Based on the above concern,this dissertation primarily attends to the designing of nanostructured transition metal oxides/sulfides combined with highly conductive materials as the composite anode materials of SIBs.This dissertation is mainly composed of the following four parts.?1?The fabrication and electrochemical performance of self-supported FeCo₂O₄nanoflowers.The FeCo₂O₄ nanoflowers were coated on the suface of nickel foam through hydrothermal method.These nanoflowers were self-assembled from nanosheets.Their high specific areas can buffer the volume expansion during cycling.Meanwhile,the contact between the electrode and the electrolyte was favored,which is beneficial to the transportation of sodium ions.The nickle foam substrate has good electronic conductivity.This self-supported structure doesen't need ball milling or slurry casting processes,which can also avoid the use of conductive agents and binders,leading to more uniform distribution of the active matters.It can be disrectly used as the electrode.After 100 cycles at 50 mA g^-1,it could still maintain a capacity of 422mA h g^-1.Its rate performance showed that a capacity of 333 mA h g^-1could be delivered at 1 A g^-1.?2?The fabrication and electrochemical performance of the CoFe₂O₄nanoparticles-polypyrrole nanotubes composites?CFO-PPy-NTs?.The polypyrrole nanotubes was prepared by soft-template method.Then the CoFe₂O₄ nanoparticles were uniformly coated on the surfaces of polypyrrole nanotubes through solvothermal method.This uniform distribution can avoid further agglomeration of the nanoparticles.The sizes of the nanoparticles are around 5 nm,which can effectively buffer the volume expansion.At the meantime,the composite nanotubes can form highly conductive network to favor the electronic transpotation,resulting in good long-term cyclabiliy at high current density and good rate performance.After 200 cycles at 100 mA g^-1,it could still maintain a capacity of 400 mA h g^-1.Even at higer current density of 1 A g^-1,it could still deliver a capacity of 220 mA h g^-1 after 2000 cycles.The rate performance exhibited that a capacity of 189 mA h g^-1could be delivered at a high current density of10 A g^-1.?3?The fabrication and electrochemical performance of Fe₇S₈ nanoparticles anchored on N-doped graphene nanosheets.The Fe₇S₈ nanoparticles was uniformly anchored on the sufaces of N-doped graphene nanosheets through a solvothermal method.Due to the conversion reaction during cycling,the Fe₇S₈ nanoparticles went through a particle fining process,leading to capacity increase during initial cycles.After140 cycles at 400 mA g^-1,the capacity increased to 827.7 mA h g^-1.After 500 cycles,the capacity was stabilized at 393.1 mA h g^-1.During the later period of cycle life,owing to the formation of large amout of solid-electrolyte interphase?SEI?,the capactiy decreased and was gradually steadied.The existence of N-doped graphene can enhance the electronic conductivity,leading to a better rate performance of the material,which could still still deliver a capacity of 400.2 mA h g^-11 at 10 A g^-1.?4?The fabrication and electrochemical performance of few-layered disordered MoS₂/N-doped carbon composite?FLD-MoS₂/NC?.Firstly,the MoS₂/polypyrrole precursor was prepared through solvothermal method.Then,after post heating process,the FLD-MoS₂/NC was obtained.Large amounts of few-layered MoS₂ were uniformly and randomly distributed in the N-doped carbon matrix.The interlayer distance was greatly enlarged.The composite has exhibited a quasi-amorphous characteristic.This unique structure can increase the active sites for sodium ion storage,which will increase the capacity.Meanwhile,the enlarged interlayer distance and the quasi-amorphous characteristic can greatly buffer the strain resulting from repeating sodiation/desodiation process.The existence of the carbon matrix can enhance the conductivity of the materials,and also buffer the volume expansion.This material displayed good electrochemical performance.After 1000 cycles at 1 A g^-1,2 A g^-1,4 A g^-1,5 A g^-11 and 6 A g^-1,it could deliver capacities of 419.3,380.5,324.8,286.1 and226.7 mA h g^-1,respectively.The capacity retentions were 74.3%,77%,75.4%,72.3%and 83.5%,respectively.Its excellent rate performance showed that even at high current density of 10 A g^-1,it could still deliver a capacity of 257.7 mA h g^-1.