Manganese oxide cathodes for rechargeable batteries
Manganese oxides are considered as promising cathodes for rechargeable batteries due to their low cost and low toxicity as well as the abundant natural resources. In this dissertation, manganese oxides have been investigated as cathodes for both rechargeable lithium and alkaline batteries. Nanostructured lithium manganese oxides designed for rechargeable lithium cells have been synthesized by reducing lithium permanganate with methanol or hydrogen in various solvents followed by firing at moderate temperatures. The samples have been characterized by wet-chemical analyses, thermal methods, spectroscopic methods, and electron microscopy. It has been found that chemical residues in the oxides such as carboxylates and hydroxyl groups, which could be controlled by varying the reaction medium, reducing agents, and additives, make a significant influence on the electrochemical properties. The Li/Mn ratio in the material has also been found to be a critical factor in determining the rechargeability of the cathodes. The optimized samples exhibit a high capacity of close to 300 mAh/g with good cyclability and charge efficiency. The high capacity with a lower discharge voltage may make these nanostructured oxides particularly attractive for lithium polymer batteries. The research on the manganese oxide cathodes for alkaline batteries is focused on an analysis of the reaction products generated during the charge/discharge processes or by some designed chemical reactions mimicking the electrochemical processes. The factors influencing the formation of Mn3O4 in the two-electron redox process of d-MnO2 have been studied with linear sweep voltammetry combined with X-ray diffraction. The presence of bismuth, the discharge rate, and the microstructure of the electrodes are found to affect the formation of Mn3O4, which is known to be electrochemically inactive. A faster voltage sweep and a more intimate mixing of the manganese oxide and carbon in the cathode are found to suppress the formation of Mn3O4. Bismuth has also been found to be beneficial in the one-electron process of gMnO2 when incorporated into the cathode. The results of a series of chemical reactions reveal that bismuth is blocking some reaction paths leading to the unwanted birnessite or Mn3O4. Barium is also found to play a similar role, but it is less effective than bismuth for the same amount of additive. Optimization of the additives has the potential to make the rechargeable alkaline cells based on manganese oxides to successfully compete with other rechargeable systems due to their low cost, environmental friendliness, and excellent safety features.