Li-air batteries (LABs) have been attracting much attention during the past several years. This system has one of the highest energy densities among the energy storage systems. To realize the successful application of Li-air batteries in the electrical vehicles, it is essential to produce suitable cathodes and highly efficient catalysts for this system. Understanding the underlying reaction mechanisms and reasons for the capacity degradation is also helpful in producing much better performing Li-air batteries. This thesis focuses on developing MnO₂ based cathode/catalyst for Li-air batteries. Reduced graphene oxide was firstly applied to solve the problem of poor conductivity of the catalyst and further the nanosized MnO₂ catalyst was directly fabricated onto the current collec...[ Read more ]

Li-air batteries (LABs) have been attracting much attention during the past several years. This system has one of the highest energy densities among the energy storage systems. To realize the successful application of Li-air batteries in the electrical vehicles, it is essential to produce suitable cathodes and highly efficient catalysts for this system. Understanding the underlying reaction mechanisms and reasons for the capacity degradation is also helpful in producing much better performing Li-air batteries. This thesis focuses on developing MnO₂ based cathode/catalyst for Li-air batteries. Reduced graphene oxide was firstly applied to solve the problem of poor conductivity of the catalyst and further the nanosized MnO₂ catalyst was directly fabricated onto the current collector. The objectives also include providing the explanations for the capacity degradation, deciding the suitable electrolytes and understanding the reversible performance of the MnO₂ catalyst.

The electrochemical performance of rechargeable Li-air batteries containing reduced graphene oxide (rGO)/α-MnO₂ composite and neat α-MnO₂ electrode is studied. The rGO/α-MnO₂ composite exhibits a specific capacity as high as 558.4 mA h g^-1 at a current density of 100 mA g^-1, indicating its potential to make a good cathode material. The composite electrode also presents a relatively moderate degradation of capacities with increasing cycles, compared to the neat α-MnO₂ electrode. rGO functions as the conducting medium to connect the α-MnO₂ nanorods, thus improving the Li ion transfer. The mechanisms responsible for the capacity degradation in the composite electrodes are studied after a series of interrupted charge/discharge cycles, detecting Li₂O₂ and LiF as the main reaction products formed on the electrode surface. In particular, the LiF layer is identified to be an important component of reaction products, which serves as barrier to reactions between the Li ions and electrons with the electrode, giving rise to detrimental effects on the cyclic and capacity performance of Li-air batteries.

In-situ grown MnO₂ nanoparticles were fabricated on the Ni mesh surface and this material was tested as the cathode in the Li-air batteries. With this method, nanosized catalysts were directly connected to the current collector without the addition of carbon materials and binders. The Ni surface was fully coated with a layer of catalyst and functioned as reaction sites during the charge/discharge process. The merits of the good contact were successfully demonstrated with the highly reversible cyclic performance exhibited by the MnO₂/Ni cathode. The CV measurements were employed in order to determine its catalyzing effect and the charging efficiency throughout the tests. Ex-situ EIS, XPS and SEM analysis were also conducted corresponding to different charge/discharge stages. Li₂O₂ was determined as the main reaction product after a discharge process and its decomposition after a further charge process indicated the effectiveness of the MnO₂ catalysts. Furthermore, the mechanism of this reversible performance derived from the synthesized cathode was proposed and explained with the assistance of the above characterization tools.