Design, Improvement and Fundamental Studies on Aprotic Lithium-Oxygen Batteries
thesisposted on 21.10.2015 by Jing Gao
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In the family of rechargeable batteries, the Li-O2 battery has the largest theoretical energy density and thus is considered to be a promising candidate for the next generation of the rechargeable battery. However, there are challenges in the development of Li-O2 batteries, such as the high overpotential and the short cycle-life. Several aprotic Li-O2 battery designs have been reported with a low overpotential; however, their high cost limits the application of those battery designs. Moreover, there is not a full understanding of the discharge products so far. Therefore, our goal is to develop an aprotic Li-O2 battery combining a good performance with a low cost and investigate the mechanism of the cell reactions as well. The objectives of this dissertation are to design a new cathode for the aprotic Li-O2 battery to reduce the overpotential, and to investigate the mechanisms of the discharge reactions in an aprotic Li-O2 battery. In the presented work, the SPPd6 cathode was designed based on a nanoscale Pd catalyst via thermal evaporation and was evaluated with Swagelok testing cells, which reduced the charge overpotential from more than 1V to about 0.2V and increased the cycle-efficiency from 62~70% to 80~84%. Other advantages were also found on this new-kind cathode, such as less degradation and prolonged working life. Behaviors of a SPPd6-based cell were investigated to get an understanding of the issues in an aprotic Li-O2 battery, including the drop of discharge voltage and the capacity fading. Discharge products were probed on the discharged active carbon cathode to study the mechanism of the oxygen reduction reaction (ORR). The results agree with the disproportionation reaction for the formation of Li2O2. An electrochemical-peeling strategy was developed to investigate the composition and structure of the toroid, by which the structure of a toroid with a LiO2-like compound on the surface and Li2O2 in the core was revealed.