Preparation and Characterization of Cathode Materials for Lithium-Oxygen Batteries

A possible future battery type is the Li-air battery which theoretically has the potential of reaching gravimetric energy densities close to those of gasoline. The Li-airbattery is discharged by the reaction of Li-ions and oxygen, drawn from the air, reacting at the battery cathode to form Li₂O₂. The type of cathode material affects the battery discharge capacity and charging potential and with a carbon based cathode many questions are still unanswered. The focus of this Ph.D. project has been the synthesis of reduced graphene oxide as well as the investigation of the effect of reduced graphene oxide as a cathode material, combined with in situ investigations of the formation of decomposition products in and on the cathode. The reduced graphene oxide was synthesized by the modified Hummers method followed by thermal reduction of graphene oxide, while both were investigated by in situ X-ray diffraction. This study revealed an early formation of graphene oxide, new graphene oxide diffraction peaks and an unidentified crystal phase along with a disordered stage of thethermal reduction of graphene oxide. The oxidation time effect on graphene oxide,synthesized by the modified Hummers method, and the following chemically and thermal reduced graphene oxide was investigated. This revealed that trends introduced by changes in oxidation time were observed not only for the graphene oxide but also transcended to both types of reduced graphene oxide. Furthermore the change in oxidation time affected the discharge capacity of the battery as well as the charging potential. In situ X-ray diffraction studies on carbon black cathodes in a capillary battery showed the formation of crystalline Li₂O₂ on the first discharge cycle, the intensity of Li₂O₂ on the second discharge cycle was however diminished. The study furthermore showed how X-rays may affect the Li-O₂ battery, displaying how in situ studies may be invasive. An in situ X-ray diffraction study of a reduced graphene oxide cathode showed formation of both LiOH and Li₂O₂ which also was observed in cells with and without addition of water by XPS. The addition of water to the electrolyte gave indications of additional reactions taking place in the cell. The information provided in this study is useful for a better understanding of reduced graphene oxide both in regards to synthesis and as cathode material in Li-air batteries. The thesis illuminates the importance of considering the synthesis of reduced graphene oxide as this seems to be couple to the abilities as cathode materials in Li-air batteries. It furthermore introduces two types of capillary battery designs optimized for Li-air and in situ X-ray diffraction, but with possibilities within metal-air batteries in general, and it opens up for a discussion of how invasive in situ methods may be.