Study On The Construction And Electrochemical Performance Of Metal Oxide(Sulfue) Nanostructures

Lithium-ion batteries?LIBs?have been considered as one of the most promising energy storage system because of their high energy density,long lifetime and environmental benignity.However,due to its shortcomings,such as low theoretical?372 mAh g-1?,low energy density,potential safety hazard,graphite is unable to meet the needs of developing high performance,high safety of lithium ion battery.Therefore,the research and development of novel and high-performance anode materials have become a very important work.Among numerous alternative materials,transition metal oxides?sulfides?have attracted considerable attention owning to their high theoretical capacity,rich resources,environment-friendly property,high security,etc.However,they are suffered from the problems of poor electrical conductivity,low initial coulombic efficiency and large volume change during charge and discharge processes,which result in poor cycling performance.In addition,transition metal sulfides,represented by molybdenum disulfide,showexcellent electrical performance of catalytic activity for hydrogen evolution.But molybdenum disulfide possesses less active sites and poor conductivity.So molybdenum disulfide still presents inferior performance compared with Pt-based catalysts.Graphene has been extensively applied on nanoelectron device,electrochemical storage and energy conversion,due to its high charger mobility,large surface area,good flexibility and the others.In this thesis,according to the characteristic and the existing defects of transition metal oxygen?sulfur?compounds and combining the advantages of graphene materials,we have studied the properties of electrochemical lithium storage and electrocatalytic hydrogen evolution from the aspects of material nanocrystallization,electrode structure andmorphology control,respectively.The main contents are summaried as following:1.Sandwiched graphene paper@Fe₃O₄ nanorod array@graphene?GPFG?composites are firstly designed and fabricated as integrated electrodes for highperformance lithium-ion batteries.The Fe₃O₄ nanorod array,directly grown on the conductive graphene paper,assures fast reaction kineticsand high reversibility.The packing graphene not only accelerates the electron transfer but also buffers the volume strain.Therefore,the electrode delivers reasonably large and stable lithium storage,good rate capability,and an amazingly long cycling life.Unfortunately,the packing graphene layer was only physically deposited on the surface of nanorod array and showed the fragile mechanical stability,which was easily crushed during the electrode management process.Then,carbon film is employed as the packing layer to fabricate novel pie-like free-standing electrode of graphemepaper@Fe₃O₄ nanorod array@carbon?GFC?.The carbon film with good mechanical stability is in situ transformed from the depositedpolyaniline?PANI?film,which well encapsulates the nanorod array and prevents their dissolution.Based on the combinedstructural advantages,the present pie-like electrode with interior array architecture therefore exhibits excellent electrochemical lithium storage properties,especially large,stable,and durable cyclicperformance at high current density?852 mA g-1 after 1000 cycles at2 A g-1?,making it a promising candidateas high-performance integrated electrode.2.Using polyhedral CoSn?OH?₆ as a precursor,we construct a hybrid paper of graphene entrapped SnO₂–Co₃O₄ nanocubes?G/SnO₂–Co₃O₄?as a flexible integrated anode for LIBs.The rationally selected composition,polyhedral nanostructures,and proper construction of the specific architecture successfully trigger a synergistic lithium storage effect,assuring the partial reversible conversion of SnO₂ and interfacial lithium storage of graphene,which are responsible for the remarkably high reversible capacity of 1665 mAh g-1 after 100 cycles at 100 mA g-1.Large and stable cyclicperformance as well as long cyclic life is also obtained at a high current density of 1 A g-1?1208 mAh g-1 after 1000 cycles?.More importantly,the free-standing electrode can well retain its flexiblefeature even after 300 repeated electrochemical cycles,making it a promising candidate for a high-performance and stable electrode for flexible LIBs.3.Mesoporous FeCo₂O₄ octahedra are synthesized for the first time with evaporation-inducedself-assembly method as advanced anode materials for lithium-ionbatteries.The partial metal substitution in the spinel MTMOs can improvethe lithium storage properties.The tailored anisotropy octahedral structure will supply structural stability and improvethe cyclic performance.The mesoporous feature can shorten the pathlength of charge transfer,promote the electrolyte flux,supply more surface area to absorb lithiumion,as well as supply void surface to buffer the volume strain.Endowed with the combined benefits of multicomponent effect,octahedral structure stability,and mesoporous features,the sample delivers excellent electrochemical performance,including large,fast,and stable cyclic performance?1101mAh g-1 at 1 A g-1 after 200 cycles?,as well as the best high-rate performance ever reported for FeCo₂O₄?518 m Ah g-1at 10 A g-1?.4.We have rationally designed and successfully fabricated the graphene quantum dots doped MoS₂ nanosheets via a hydrothermal process.XRD,SEM,TEM,and Raman techniques are employed to reveal the formation and the structural features.The doping with GQDs simultaneously endows MoS₂ with structural and compositional advantages for high performance lithium storage,including expanded interlayer distances between layers,fast electrochemical kinetics,and additional stability to buffer the volume variation.When evaluated as an anode material for lithium-ion batteries,GQDs/MoS₂ delivers high capacity,improved cyclic performance and excellent rate capability.The doping of GQDs into MoS₂ nanosheets plays the key role in enhancing the catalytic activity by creating abundant defect sites both in the edge plane and the basal plane,as well as enhancing the electrical conductivity.Endowed with this,the sample delivers remarkably improved catalytic properties forelectrochemical hydrogen evolution reaction,including the large cathodic current?10 m A cm-2at a small overpotential of 200 mV and 74 mA cm-2at a small overpotential of 300 m V?and low onset potential?140 mV?.