Studies of manganese based oxides as alternative electrode materials for lithium based energy storage devices

Lithium ion based energy storage devices have captured a significant share of the market for energy storage devices owing to their high energy and power density. However the cost of raw materials and their availability warrants a search for alternative materials with improved energy densities and rate capabilities. Manganese based oxide materials are attractive as alternative choices for electrode materials. However, they encounter a host of issues which result in poor performance in systems based on the Li+ ion shuttle. One of the issues is structural degradation. It is well known that spinel LiMn2O4 undergoes structural degradation when lithium is inserted into the octahedral voids of the structure owing to the change of average valence of manganese from +3.5 to +3. This leads to a Jahn-Teller distortion induced structural relaxation that leads to the formation of a new tetragonal phase which is responsible for poor capacity retention. However if one can manage to keep the average manganese valence to above +3.5 during lithium insertion, it would be possible to prevent JT distortion and associated structural changes thus making it possible to access the voltage offered by the Mn3+/Mn4+ redox couple. This thesis sheds new light on the existing understanding about the effect of substitutions on performance of two well-known spinel compounds LiMn2O4 (substitution with nickel) and Li4Mn5O12 (substitution with titanium). Both substitutions were found to prevent phase transformation during cycling resulting in better capacity retention. Different synthesis methods are employed to prepare samples with different morphologies and the effect of morphology on performance is also investigated. It was found that the performance of the spinel LiNi0.5Mn1.5O4 was affected by the space group of the crystal structure as well as morphology in the voltage range of 2.3 – 3.3V. Nano-structuring was found to play a significant role in improving the performance of spinel LiNi0.5Mn1.5O4. The findings reveal that nickel substitution in LiNi0.5Mn1.5O4 was a more attractive option to pursue. Finally full cell devices were assembled using the best performing material as working electrode to demonstrate the feasibility of manganese based oxide materials as attractive alternative electrode materials for rechargeable energy storage devices based on the lithium ion.DOCTOR OF PHILOSOPHY (MSE