The Preparation Of Li₄Ti₅O₁₂ And MnO₂Nanostructure-Arrays And Their Energy Storage Performance

With the rapid consumption of non-renewable energy, clean energy leads to more and more attention. The energy storage device with large capacity, long cycle life, high operating voltage and no pollution has been showing very important position. With the rapid development of electronic products, miniaturization and portability become the trend, thus causing the development micro-battery, as well as the concern of per unit area energy storage capacity. In recent years, variety of nanostructure arrays directly grown on a conductive substrate/film has caused nanomaterials researchers’great concern. The large-area ordered arrays of nanostructures using physical and chemical methods directly on metal substrate, use the characteristics of the nano-ordered arrays of large surface area combine its geometric advantage (such as one-dimensional single crystal nanowires directional transmission of electrons, the gap between the ordered nanowire is good penetration and diffusion of ions) to improve storage performance. In this thesis, Using chemical methods grow the orderly structure of the nano-lithium titanate (Li4Ti5O12) nanowires array and birnessite manganese dioxide (MnO2) nanosheets array on the flexible conductive substrate (Ti). In energy storage applications, we efficient use the structural characteristics of the zero-strain lithium titanate (Li4Ti5O12) and the two-dimensional layered structure features birnessite MnO2, focused on lithium-ion batteries and supercapacitors. We study the lithium-ion battery charge and discharge performance and cycle performance with synthesized Li4Ti5O12nanowire arrays as the negative electrode and supercapacitors rapid charge and discharge performance and cycle performance with birnessite MnO2nanosheet arrays as electrode material. The prepared nano-ordered-array is expected as micro-battery/capacitor electrode materials for micro-electronic devices. The main experimental results are as follows:(1)Li4Ti5O12as a "zero strain" of lithium intercalation materials with the advantages of structural stability, excellent cycling performance, highly reversible redox reaction, is a new anode material to substitute traditional carbon. Using a series of simple hydrothermal reaction, proton exchange, hydrothermal ion exchange, annealing at high temperature we prepared single-crystal nanowire materials Li4Ti5O12on the conductive substrate (Ti foil). We can confirm the material composition of synthesized materials and crystal structure from SEM, XRD, TEM. We explore Li4Ti5O12nanowire growth factors from comparative experiments to get the best synthesis conditions. The Li4Ti5O12nanowire posses the initial discharge capacity162.9μAh/cm2, the first charge capacity about150μAh/cm2and the coulombic efficiency93%at the current density of13μA/cm2; Its initial discharge capacity is145.1μAh/cm2and charge capacity reach133μAh/cm2coulombic efficiency91.7%at current density of32.5μA/cm2; when the current density increases to65μA/cm2its first discharge capacitor132.9μAh/cm2and the charge capacity is129μAh/cm2, coulombic efficiency reached97%, showing good reversibility. After200cycles, the capacity still has150μAh/cm2at13μA/cm2, almost no attenuation; After200cycles the discharge capacitance remains70%of the initial discharge capacitance at32.5μA/cm2and65μA/cm2.(2)With simple hydrothermal method combined with gas protection annealing we synthesized birnessite type (MnO2) nanosheets arraw using SEM, XRD to characterize its morphology, material composition and crystal structure. The electrolyte is1M Na2SO4. Electrochemical tests were performed on a CHI660C electrochemical workstation at room temperature with a three-electrode system. A platinum electrode and a calomel electrode (SCE) served as the counter electrode and reference electrode, respectively. MnO2nanosheets grown on Ti foil were directly used as working electrode and subjected to electrochemical measurements involving cyclic voltammetry (CV) and galvano-static charge-discharge tests. The scan rate is10mV/s,20mV/s,40mV/s,60mV/s,80mV/s,100mV/s respectively. The CV generally showed a symmetrical rectangle, does not appear very sharp redox peaks showing capacitor characteristics; In the electrochemical tests, MnO2nanosheets supercapacitor delivered a specific capacitance of72mF/cm2at a current density of1000μA/cm2. Meanwhile, this electrode exhibits excellent cycling lifespan with only approximately10%loss of specific capacitance after4000cycling tests.In order to increase the material’s conductivity and per unit area, we use carbon modify the materials. In the electrochemical tests, the carbon-modified MnO2nanosheets supercapacitor delivered a specific capacitance of135mF/cm2at a current density of1000μA/cm2. After4000cycling tests, MnO2nanosheets supercapacitor still delivered a specific capacitance of127mF/cm2. The capacity per unit area of carbon modified two-dimensional nanosheet arrays of MnO2has been greatly enhanced, with good cycling performance.