| By the wide application of lithium ion batteries(LIBs)in portable devices,more int Acompanied erest emerges onto batteries implemented in electric vehicles with higher energy and power densities and higher safety standard.Meantime,Due to the abundance of sodium resource,developing sodium ion batteries(SIBs)with potential value in great energy storage system becomes more important.However,Graphite materials,which have been widely used as the anodes in commercial LIBs,only deliver a low theoretical specific capacity of 372 mAh g-1 and exhibit considerably poor rate performance.For the next generation LIBs and SIBs,one of the major challenges is hence to search for alternative anodes with high energy density and high rate capability.Transition metal oxides(TMOs)stand out due to their higher theoretical capacity,lower cost and controllable systhesis.Unfortunately,the practical application of TMOs is still hindered by the problems of sluggish kinetics for slow Li-ion/electron transfer,large voltage hysteresis and poor capacity retention during cycling caused by large volume change,especially at high current densities.An attempt to overcome these significant drawbacks is the exploitation of nanostructured materials,and another popular method is to improve the performance involves coating or encapsulation of the active materials with carbon.However,both strategies seem still far away from practical industrial application due to their complex synthetic processes.Besides,trasition metal chalcogenides(TMCs)beome popular in terms of better electrical conductivity than metal oxides.Nevertheless,existing reported TMCs exhibit lack of stable cycle performance and competable rate capability.Thus,this thesis focus on how to improve the electrochemical performance of TMOs electrodes and explore new TMCs electrodes as well.Chapter 1 introduces the research background and work priciples of lithium ion batteries and sodium ion batteries.After a generalized summarization for the anode materials including insertion,convertion and alloying types,a summury on TMOs and TMCs as electrode materials is provided.Chapter 2 is a introduction of experiment reagents,fabrication equipment and test methods,followed by a detailed description of material composition,phase and morphology characterization methods and electrochemical technology.In chapter 3,we proposed that amorphous materials with the nature of isotropy and out of boundry could benefit for relieving the stress generated during volume change and fast ions diffusion,which could improve electrochemical performance of TMOs with a conversion type reaction.The First-principles calculations show that amorphous Fe2O3 exhibits a lower reaction’s change in Gibbs free energy(AG)with respect to its crystalline counterpart.Besides,amorphous Fe2O3 exhibits more reversible reactions,better cycle stability,higher rate capability and lower polarizations.Furthermore,the cycle performance of amorphous Fe2O3 shows three stages from initial decline,stabilizing to final rise.Deep analysis explains that the origin of capacity rise is caused by strengthened capacitive-like behavior.Based on this work,the incorporation of Ag nanoparticles into Fe2O3 anode is found to give a considerable enhancement in rate capability and cycling stability.In Chapter 4,a generized and scalable way to systhesis 2D TMO nanomaterials is introduced to synthesize high-quality 2D Co3O4 nanosheets under mild hydrothermal reactions followed by a quick heat treatment process.Notably,this approach could be extended to the synthesis of other analogue nanosheets,including binary and ternary TMOs(NiO and NiCo2O4).As one of the most efficient nanostructures for enhanced lithiation/delithiation or sodiation/desodiation reactions,2D nanosheet structures can provide significantly shortened ion and electron diffusion pathways,sufficient electrode/electrolyte contact,abundant electrochemically active sites and high mechanical flexibility to endow long-term cycling stability.All these three materials delivered high capacity and durable lithium/sodium storage performances.We also studied the effect of solvent species on product morphology and pointed out the necessary existence of ethylene glycol.In Chapter 5 we explored the electrochemical performance and reaction mechanisam of a-NiS as a new electrode material for SIBs.Firstly hierarchical hollow NiS spheres with porous shells composed of nanoparticles are designed and synthesized by tuning the reaction parameters.The formation mechanism of this unique structure is systematically investigated,which is clearly revealed to be Ostwald ripening mechanism on the basis of the time-dependent morphology evolution.The hierarchical hollow structure provides sufficient electrode/electrolyte contact,shortened Na+diffusion pathways,and high strain-tolerance capability.The hollow NiS spheres deliver high reversible capacity,excellent rate capability and good cycling stability.The exsitu-TEM anlyasis evaluates a two-step conversion reaction mechanisam during discharge process of NiS electrode.In Chapter 6,we fabricated TiSe2 bulk materials by solid state reaction.And this TiSe2 precursor was found to be extremely easy to be exfoliated to nanosheets under sonication or grinding.It was the first time to be found that,TiSe2 nanosheets deliver excellent cycle stability and rate capability with a high coulumbic efficiency.The Insitu-XRD analysis accompanied upon cycling shows that TiSe2 electrodes go through a multi-step insertion/disertion process,and the final discharge product is NaTiSe2.What’s more,the capacity rise during cycling test may be caused by pseudocapacitance contribution and further exfoliation of nanosheets.Finally,in Chapter 7,the study progress and achievements of this thesis are summarized.Some suggestions and prospects on future research are presented. |
