| In recent years, the nonaqueous lithium air batteries have received much attention since they can theoretically store 5-10 times more energy than current Li-ion batteries which could realize the high theoretical energy density that is even compatible with that of gasoline. In addition, a typical rechargeable nonaqueous lithium air battery is comprised of a Li-metal anode, Li conducting organic electrolyte and a porous catalytic cathode. Futhermore, the mechanism of lithium air batteries are based on the reaction of oxygen with lithium in non-aqueous electrolyte. However, the reactions in the lithium air system happen at the liquid-solid-gas three-phase interface. Therefore, there are many factors affecting the performance of the lithium air batteries. In our work, we aim to develop high energy density lithium air battery by investigating the integral part of the cells. Moreover, Ex-situ X-ray diffraction (XRD), Fourier transform-infrared spectrophotometer (FT-IR) and scanning electron microscope (SEM) technologies are employed to investigate the operating mechanism of lithium air battery during the discharge/charge cycles, which are very important for developing Li-air battery. In this paper, our research focuses on carbon materials for the air catalytic electrode, a type of ionic-liquid based electrolyte and the effect of ambient humidity. There are three sections in this thesis as follow:1. The research of carbon materials for air electrodeHere, a novel leaf-like GO with a carbon nanotube (CNT) midrib is developed by using vapor growth carbon fiber (VGCF) through the conventional Hummers method. Furthermore, the leaf-like GO with a carbon nanotube (CNT) midrib was used as the active material for O2 catalytic electrode in the investigation of Li-air battery. In order to further demonstrate the advantages of leaf-like GO for Li-air battery, conventional GO prepared by Hummers method, commercial CNT and their direct mixture (GO/CNT weight ration:1:1) were employed as the catalysts for Li-air batteries. Furthermore, the leaf-like GO effectively unites the advantages from CNT and GO since the CNT provides a natural electron diffusion midrib for the leaf-like GO, and the large surface area of GO makes the leaf-like GO have more active sites for O2 reduction reaction. Therefore, the leaf-like GO-based Li-air battery displays excellent cyclic ability and capacity performance. According to the above comparision, we can conclude that the materials with the high electronic conductivity and many active sites may facilitate the Li2O2/O2 conversion, and thus improve the performance of the Li-air batteries.In addition, the porous structure of the air electrode materials also affects the performance of the Li-air batteries. According to recent reports, the ordered mesoporous channels in these materials can effectively improve the electrolyte immersion and facilitate Li+diffusion and electron transfer. More importantly, the macropores channels can provide a space for O2 diffusion and O2/Li2O2 conversion. Therefore, an ordered hierarchical mesoporous/macroporous carbon is synthesized through a templating approach. Since the reactions in the lithium air system happen at the liquid-solid-gas three-phase interface and the hierarchical porous structure of the MMCSAs can provide an ordered three-phase reactive interface, thereby enhancing the performance of the Li-air batteries. In addition, the MMCSAs could maintain its ordered hierarchical mesoporous/macroporous structure over the discharge/charge cycles, which may improve the cycling life of the Li-air batteries. Furthermore, all the Li-air batteries using the different wt.% MMCSAs (x=5,10,30, 50 and 80) at a current density of 50 mA g-1 exhibit an improved discharge capacity and a higher operating voltage than that for pure super P-based Li-air batteries. Considering both the capacity and the cycling capability, we found that the optimized wt.% of the MMCSAs in the O2 catalytic electrode for Li-air batteries is 30%. In addition, the polarization between the discharge and charge for the 30 wt. %MMCSA-based Li- air batteries is much lower than that for pure super P-based Li-air batteries at different current densities. To further determine why the MMCSAs can greatly improve the cycle and rate capacities, we also employ ex-situ XRD, FT-IR and SEM technologies to investigate the operating mechanism of lithium air battery during the discharge/charge cycles.2. The investigation for a type of ionic-liquid based electrolyteThe electrolyte of lithium air batteries generally consists of a solvent and a salt, and instability of the solvent forms the bottleneck for the development of lithium air batteries. Furthermore, organic electrolytes are currently the most studied system of lithium-air batteries. However, recent investigations have demonstrated that the highly reactive oxygen radical anion O2·- can decompose most organic electrolytes, including the organic carbonate solutions commonly used in lithium-ion batteries. Ionic-liquid based electrolyte has been considered as one of the promising electrolyte candidates for rechargeable Li-air batteries, owing to its inherent characteristics of non-volatility, non-flammability, and O2-stability.The electrochemical performance of Li-air batteries using y-MnOOH nanorods as a catalyst is investigated in a new type of ionic-liquid based electrolyte consisting of EMIMBF and LiNTf2. The achieved results indicate that the Li-air batteries display perfect cycle performance and high capacity. For comparision, a-MnO2 nanorods/carbon composite or pure Super P is employed as air electrode materials for Li-air batteries in the ionic-liquid based electrolyte consisting of EMIMBF and LiNTf2. We can find that both the operating voltage and capacity of Li-air batteries using y-MnOOH nanorods/carbon composite catalyst are superior to that of the Li-air battery using α-MnO2 nanorods/carbon composite catalyst or pure Super P catalyst. Therefore, we can conclude that the ionic-liquid electrolyte of EMIMBF4/LiNTf2 coupled with the catalyst of y-MnOOH nanorods can effectively improve the electrochemical performance of Li-O2 batteries. The long cycle life should be ascribed to 02-stability and wide electrochemical stable potential window of ionic-liquid based electrolyte. Furthermore, y-MnOOH catalyst facilitates O2 reduction in ionic-liquid based electrolyte, and thus enhances the operating voltage and capacity of Li-air battery. This indicates that proper ionic-liquid electrolyte combined with high performance catalyst is quite important for building advanced rechargeable Li-air batteries.3. The effect of humidity on electrochemical performance of lithium oxygen batteriesFor developing practical Li-air batteries which can breathe O2 from environment, it is very important to investigate the ambient humidity effect on the performance of Li-air batteries. In this work, we compare the performance of Li-air batteries in pure/dry O2, pure O2 with a relative humidity (RH) of 15% and ambient air with a RH of 50%, and analyze the ambient humidity effect on the reactions in the carbon-based catalytic electrode. Electrochemical investigation indicates that discharge capacities of Li-air batteries increased with growth of RH value, but cyclic ability and rate performance are influenced in an opposite way. Ex-situ X-ray diffraction (XRD), Fourier transform-infrared spectrophotometer (FT-IR) and scanning electron microscope (SEM) investigations suggest that ambient humidity affects not only the Li2O2/O2 conversion, LiCO3/CO2 conversion and LiOH formation but also the morphology of discharge products in porous catalytic electrode over charge/discharge cycle. In addition, the effect of the ambient humidity on Li-anode also greatly aggravates the electrochemical performance difference of these batteries, which should also be investigated in near future. These results may be important for developing Li-air battery. |
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