Synthesis And Electrochemical Performance Of Sodium Manganese Oxides As Cathodes For Secondary Aqueous Battery

With sodium's high abundance and low cost, and suitable redox potential, rechargeable electrochemical cells based on sodium hold much promise for energy storage applications. Electrode materials with the sodium manganese oxides show excellent reversibility towards alkali metal insertion, but their capacities and cycling stability need to be improved. In this thesis, we were aimed to study about the two structures of the sodium manganese oxides used as cathodes and the impact on the electrochemical performance of changing the components of electrolytes from the perspective of the interaction between the solid electrode and the ions in the electrolyte. In this way, the discharge capacity and stability of the electrode significantly improved. At the same time we improved the rate capability via changing the way to add conductive agents and the species of conductive agents. The main investigations were summarized as follows:1. Sodium manganese oxides with different sodium contents were synthesized by solid state method. Studies show that sodium manganese oxides could store a lot of sodium ions and zinc ions, but the tunnel structure Nao.44Mn02 can store more sodium ions and zinc ions.2. Na0.44MnO2 was used to the subsequent research. Results show that the capacity and cycling stability of Na0.44MnO2 cathodes are enhanced pronouncedly by using the hybrid aqueous electrolyte ?Na2SO4, ZnSO4 and MnSO4?. It rises up to 340mAhg^-1 after 150 cycles at the current density of 100mA g^-1. The energy storage mechanism of as-prepared Nao.44Mn02 in the hybrid aqueous electrolyte is associated with the insertion/extraction of zinc with the help of synergistic effects between zinc and manganese ions and the quasi-reversible depositon-dissolution process of Mn2+ ions. As a result, some deposited MnO2 may prevent the Mn dissolution of Na0.44Mn02 during charge and discharge. At the same time, these deposited MnO2 may provide a certain amount of extra capacity after prolonged cycling. The morphology of Na0.44MnO2 changes from compact rod-like to loose sponge-like porous structure during charge and discharge.3. In order to increase the conductivity of Na0.44MnO2, we compared the types and ways to add conductive agent to improve the discharge capacity and rate capability. Results show when acetylene black is suitable for mechanical mixing, its initial capacity is 62.4 mAh·g^-1 and then rises to 340 mAh·g^-1 after 150 cycles. At the same time carbon nanotube is suitable for the method similar to sol-gel and its initial capacity is 180.9 mAh·g^-1 and then rises to 340 mAh·g^-1 after 90 cycles.