Study On The Stability Of Electrolyte Of Non - Water Lithium Air Battery And The Process Of Oxygen Reducing Electrode

With the increasing requirement of higher energy storage system, Li-air batteries, which have the highest energy density(11300 Wh kg-1), have received a great deal of attention since it was proposed. With advantages of safety and simplicity, Li-air batteries based on non-aqueous have been researched as a popular topic. However, before this promising technology becomes a reality, there are many scientific and technical challenges that must be overcome. The challenges come from the instability of nonaqueous electrolyte, poor cycle performance and the obscure relationship between the properties of cathode surface and the discharge performance.The thesis firstly applies cyclic voltammetry, discharge-charge test and ex-situ spectra to study the stability of carbonate(EC\DEC), ether(DME) and amide(NMP) based electrolyte during discharge process in Li-air batteries. Results show that carbonate solvents are easily undergoing decomposition to form Li2CO3 and CH3CH2OCO2 Li. Thus ether solvent could also decompose after a long time exposed to O2 because of autoxidation reaction. Discharge products on cathode in ether based electrolyte are Li2CO3. Continuous consumption of solvents caused both carbonate and ether-based electrolytes incompetence for the non-aqueous Li-air batteries.Amide based electrolyte have the highest stability among these solvents. Products after first discharge in cells using NMP based electrolyte are Li2O2 and high coulombic efficiency(97%) is achieved after first cycle. Due to the superior stability towards O2-, NMP based electrolyte can be used as the fundamental electrolyte in the future O2 reduction process study. Further investigations of the cycle performance of cells utilizing NMP based electrolyte show that in the presence of O2-, NMP could occur decomposition to form Li2CO3 and LiNOx, simultaneously, reduction of NMP on the surface of Li anode also generated by-products. Deterioration of cycle performance are attributed to the decomposition of NMP on both air cathode and Li anode during charge.The thesis study two standard plate electrodes(Au and GC) and take into account heterogeneous electron-transfer of O2, homogeneous disproportionation of O2- and deposition of Li2O2. By using steady-state polarization curve, AC impedance spectrum under cathodic polarization and bulk electrolysis method, the thesis demonstrate obvious differences between characters of O2 reduction process on the two electrodes. Comparing with GC electrode, electron-transfer of O2 on Au is faster while the rate of homogeneous disproportionation is somewhat lower. On the surface of Au electrode, Li2O2 are inclined to grow perpendicular to the surface. Accumulation of O2- on Li2O2 surface results in a significant reduction on Li2O2 resistivity. Because of the higher rate of electron-transfer process and the lower rate of homogeneous disproportionation, Li2O2 on Au surface have a smaller resistivity.In order to understand the relationship between surfaces properties of cathode material and discharge performance, this thesis prepared all carbon nanotubes(CNT) air cathode by impregnation method. Without changes on macro-pore structure, by controlling the heat treatment crafts, CNT surface with different properties are obtained and the effect of surface property on the O2 reduction process have been systematically studied. Results show that introduction of oxygen-containing group could impede electron-transfer process due to the electron-withdrawing ability of group and increase the disproportionation rate. On the surface with higher oxygen content and smaller pores, Li2O2 tends to grow along the CNT surface. On the contrary, growth of Li2O2 on surface with lower oxygen content and larger pores demonstrate stronger vertical growth characterization.According to the resistivity of Li2O2 deposited on CNT surface and morphology of CNT electrode at different discharge depth, this thesis identifies that electron conduction is not the main reasons for the termination of discharge. Discharge performance of CNT electrode is determined by the further reduction ability of O2 on the surface of Li2O2. Higher relative growth index of Li2O2 on CNT surface with lower oxygen content and larger pore width makes Li2O2 have ellipsoid-like structure. The ellipsoidlike structure could favor O2 further reduction on Li2O2 surface and results in a good discharge performance. Above results indicate that the characterization of O2 reduction is determined by the surface properties so that surface property is one of the key factors that affect the discharge performances of CNT electrode.However, current development on non-aqueous electrolyte for non-aqueous Li-air batteries indicates that single electrolyte could not satisfy the stability requirement for air cathode and Li anode in the meantime. Based on the problem, the thesis discusses the application of one novel structure with dual-electrolytes for Li-air batteries. Taking CNT as air cathode, Li metal as anode, using solid-state electrolyte Li1+xAlxGe2-x(PO4)3(LAGP) to limit NMP base electrolyte and carbonate based electrolyte at air cathode and Li anode, respectively, Li-air batteries with dual-electrolytes demonstrate excellent cycle performance. With the limited specific capacity, cells show little deterioration on cycle performance after 40 cycles. The dual-electrolytes structure will be a feasible way for current application of Li-air batteries.