Sodium Manganate Na-ion Electrode Materials And Their Hybrid Supercapacitor Properties

Sodium manganate as electrode material for lithium ion battery and sodium ion battery has been studied extensively owing to the aboundance of sodium resourse and low price. Sodium (2.83%) is more abundant than lithium (0.0065%) in earth’s crust. If an energy storage device based on sodium compounds as electrode material can be developed, it will be more competitive than lithium ion battery. The research work of this dissertation is carried out following international frontier. Tunnel structured Na4Mn9O18was studied by solid state and sol-gel preparation methods, and layer structured Na0.7MnO2.05was prepared by sol-gel method. The synthesis conditions and electrochemical performance of the sodium manganate materials were investigated in this work. The structure and morphology of the samples were investigated by TGA, XRD and SEM techniques. The electrochemical behaviors of hybrid capacitor composed of sodium manganate as positive and active carbon (AC) as negative electrodes were studied. The conclusions can be drawn as follows:1. The Na4Mn9O18material can be prepared by solid state method or sol-gel method. The effect of temperature on structure, morphology and electrochemical behavior of the samples was studied. The diameter of the sample synthesized from solid-state method was larger than that from the sol-gel method. The sample prepared by solid state method can be agglomerated while the sample prepared by sol-gel method at800℃has a particle diameter of0.2～1μm. The initial specific capacitance was nearly200F g-1in1mol L-1Na2SO4electrolyte. After500cycles, the samples synthesized from sol-gel method show better electrochemical performance with the capacitance retention of88%while the sample synthesized from sol-gel method has the capacitance retention of71.5%.2. We used Na4Mn9O18as positive electrode, AC as negative electrode, Ni grid as current collector, celgrad3501as separator and1mol L-1Na2SO4as electrolyte to assemble hybrid supercapacitor. The results indicated that when the mass ratio of Na4Mn9O18to AC was1:2, the hybrid capacitor exhibits highest capacitance. The hybrid capacitor has shown a combination of double-layer capacitance and faradaic reaction characteristics. After4000cycles, the capacitance retention was81.5% indicating excellent cyclic behavior.3. Finely tuned small size (～1μm) layered sodium manganese oxides Na0.7MnO2.05were synthesized at700℃by a sol-gel method. The structure and morphology of the sample were gradually transformed to Na0.55Mn2O4·1.5H2O during the cycle process. Cracks and ripples can be observed on the surface of crystallite particles after100cycles. Electrochemical tests show the newly formed material exhibits higher capacitance.4. Hybrid supercapacitor was assembled with Na0.7MnO2.05as cathode and AC as anode. When the mass ratio of Na0.7MnO2.05to AC was1:1.5, the hybrid capacitor displayed highest capacitance. The energy density of the device was15.2Wh kg-1at30mA g-1current. The capacitance retention was80%after5000cycles with a coulombic effiency of100%. The newly formed Nao.55Mn2O4·1.5H2O shows good cyclic performance. The hybrid supercapacitor has small resistance with an electrolyte resistance of0.8Ω and a charge transfer resistance of1.5Ω.