Study On Controllable Synthesis And Electrochemical Properties Of Micro/Nano Cobaltites

As a new type of energy storage device, electrochemical capacitors have beendrawn a widespread concern. It is an energy storage component between theelectrostatic capacitors and batteries, which has the advantages of the high energydensity of chemical batteries and the high power density of traditional capacitors.Electrode materials play a crucial role in electrochemical capacitors and application.Cobaltite materials have good electrical conductivity, but the use as an electrodematerial for electrochemical capacitors has rarely been studied.The aim of this thesis is to prepare a series of cobaltite materials using thesol-gel method, precipitation method and high temperature solid-phase syntheticroute by changing the factors such as initial concentration, reaction temperature,calcination temperature, the ratio of reactants, the kind of solvents and so on. X-raydiffraction （XRD）, field emission scanning electron microscopy （FESEM）,transmission electron microscopy （TEM）, energy dispersive X-ray spectroscopy（EDS）, cyclic voltammetric measurements （CV）, galvanostatic charge-dischargemeasurements and other testing methods were used to characterize the samples andinvestigated its electrochemical properties. In addition, it also explored therelationship between the phase, morphology, size of some samples and itselectrochemical performance. The main content of the thesis is as follows:1. Sol-gel approach for controllable synthesis and electrochemical propertiesof NiCo₂O₄crystals. We have prepared three NiCo₂O₄crystals with differentmorphology, using the sol-gel method, citric acid as a chelating agent by adjustingthe factors such as the precursor evaporation time of precursor, initial concentration,the kind of solvent and so on. The specific capacitance values of NiCo₂O₄crystalswith different morphology can be calculated by the galvanostatic charge-dischargetechnique. The result shows that at a high mass loading （5.6mg/cm2）, cyclestability of submicron-sized NiCo₂O₄/Ni electrode is excellent and its specificcapacitance value is highest, reaches as much as217F/g. In addition, byinvestigating the parameters （calcination temperature）, the best calcinationtemperature of preparing the NiCo₂O₄crystal has been obtained.2. Precipitation method for synthesis and electrochemical properties of MCo₂O₄（M=Mn, Ni, Zn） crystals. NiCo₂O₄materials with special morphologieshave been prepared by using the precipitation method, dimethylglyoxime, nickelnitrate and cobalt nitrate as raw material with a molar ratio of6:1:2. The specificcapacitance values of NiCo₂O₄crystals with this morphology can be calculated bythe galvanostatic charge-discharge technique and reaches185.2F/g, which shows agood specific capacitance. In addition, by co-precipitation-Roasting, sodiumoxalate as a precipitant, we obtained MCo₂O₄（M=Mn, Ni, Zn） powder materialunder different calcination temperature, which are spinel cubic phase structure. Themorphology of each sample varies by field emission scanning electron microscope.3. Solid-phase method for synthesis and electrochemical properties of a seriesof Mn-Co-O materials NiCo₂O₄crystals. A series of Mn-Co-O materials have beenprepared by adjusting the initial mole ratios and using nickel acetate and cobaltacetate as raw materials via solid-phase method, which have been calcined at800℃in air. The galvanostatic charge-discharge results reveal that MnCo₂O₄crystal hasthe maximum specific capacitance value up to44.9F/g and good cyclingperformance.