textManganese 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.Materials Science and Engineerin