Electrochemical Properties Of Ionic Liquids And Optimization Of Electrocatalytic Activity For Nanorod Manganese Dioxide

With the threat of energy crises and the requirement for environmental safty, electrochemical power without environmental pollution trigged the worldwide interest, lithium air cell, the ultimate environmentally friendly electrochemical power source, becomes a hopeful and excellent candidate duo to its theoretical energy density of5200Wh/Kg first being reported by Abraham in1996.Lithium air cell comprises four parts:the metallic lithium anode, the electrolyte, membrane and the air cathode. Because of the cathodic active material O2without stockpile, the anodic metallic lithium which is the lightest and electropositive, simple structure and lightweight properties, lithium air cell is suitable for the alternative energy sources for electrical vehicles in the fiiture. At present, most of lithium air cells use the commercial organic electrolyte which is adopted in lithium ion cells. The papers reported the low discharge capacity and irreversible cell reaction duo to the volatility of organic eleltrolyte and not oxidized lithium carbonate(Li2CO3) and lithium alkylcarbonate (R-O-(C=O)-OLi, R=alkyl group) generated by the decomposition of carbonate species. While the lithium air cells adopted by the aqueous electrolyte system and solid electrolyte system are not beneficial to the improvement of cell’s power at high current density, because the resistance of solid membrane is so high that it is difficult for the transfer of lithium ion. Despite of the several merits, lithium air cell exists the problems of lower power density, energy density.In order to improve the discharge properties of lithium air cell, in this work, we researched the activity of oxygen reduction for manganese dioxide catalyst and chose three different kinds of pyrrolidinium-based room temperature ionic liquids with higher oxygen-saturated solubility, electrochemical stability and lithium ion conductivity. The improvement of discharged properties for lithium air cell is achieved by increasing the dynamics performance of oxygen reduction to electrode. Firstly, we optimized and prepared the manganese dioxide catalyst with higher activity of oxygen reduction and measured the solubility of oxygen, lithium ion conductivity and oxygen-electrochemical properties in N-butyl-N-methyl pyrrolidinium bis (trifluoromethanesulfonyl) imide(PYR14TFSI), N-methoxyethyl-N-methyl pyrrolidinium bis (trifluoromethanesulfonyl) imide (PYR1(2oi)TFSI) and N-propyl-N-methyl pyrrolidinium bis (trifluo-romethanesulfonyl) imide(PYR13TFSI) with the change of lithium bis (trifluoromethanesulfonyl) imide (LiTFSI) concentration. This work was also expected to find out the relationship between properties of RTILs electrolyte and discharge curve. The results are described below.(1) The preparation of nanorod manganese dioxide and optimization of oxygen reduction activity. The nanorod manganese dioxide was prepared via chemical precipitation method at normal pressures. By analysis of X ray diffraction (XRD) and scanning electron microscope (SEM), we researched the crystal structure and morphology of nanometer manganese dioxide by controlling the reactive temperature and time. Using the cyclic voltammetry and chronoamperometry, we stydied the activity of oxygen reduction for different crystal structure and morphology of nanometer manganese dioxide in1MLiPF6+EC:DMC:EMC (1:1:1by vol) organic electrolyte. The XRD and SEM show that nanorod manganese dioxide(α-) could be prepared under the condition of temperature higher than90℃and reaction time more than6h. The crystal structure and proportion of nanometer manganese dioxide grow and the morphology becomes better and better with the increase of reaction temperature and time. The characterization of cyclic voltammetry (CV) shows that the oxygen reduction activity for nanometer manganese dioxide increases with the increase of reaction temperature and time and the cathodic reaction is irreversible. The result of oxygen reduction activity tested by chronoamperometry is consistent with that measured by cyclic voltammetry for nanometer manganese dioxide less than a period of time (120s). More than the time(120s), the reduction current decayed faster than other samples for the sample A prepared at the condition of100℃18h. From the total damped time, the damped time of sample A is just lesser than that of sample D(90℃18h), B(100℃12h) and E(90℃12h). Sample A is considerd as the most perfect catalyst of oxygen reduction activity.(2) The study of the properties of pyrrolidinium-based ionic liquids electrolyte and discharged character for lithium air cell. In order to improve the discharged power density and energy density for lithium air cell, Three different kinds of pyrrolidinium-based ionic liquids were characterized by cyclic voltammetry and cell discharge/charge test and other measurements in order to study the oxygen-saturated solubility, electrolytes conductivity and oxygen-electron chemical properties with varying Li-salt concentration. The results show that the electrolytes conductivity and oxygen-saturated diffusion coefficient in RTILs electrolytes are inversely proportional when LiTFSI is added. The oxygen-saturated solubility shows down first and then up during the increase of Li-salt concentration in RTILs. The activity of oxygen reduction is best in PYRi(2oi)TFSI and worst in PYR14TFSI in the presence of0.6mol/kg LiTFSI. The discharge capacity for lithium air cell with PYR]4TFSI, PYR1(201)TFSI and PYR13TFSI in the presence of0.6mol/kg LiTFSI is1068,1084and1249mAhg-1carbon, respectively. Lithium air cell with0.6mol/kg LiTFSI-PYR13TFSI has the highest the discharge plateau and obviously higher than the discharge capacity and discharge plateau of the lithium air cell with organic electrolyte (EC:DMC:EMC=1:1:1by vol).