Structural and chemical characterizations of delithiated layered oxide cathodes of lithium-ion cells

Commercial lithium-ion batteries use the layered LiCoO2 cathode, but only 50% of its theoretical capacity could be practically utilized (140 mAh/g). In contrast, 65 and 55% of the theoretical capacities of the analogous LiNi0.85Co0.15O2 (180 mAh/g) and LiNi 0.5Mn0.5O2 (160 mAh/g) could be practically utilized. However, the reason for the differences in capacities among the various layered LiMO2 cathodes has not been fully understood in the literature. With an aim to understand the factors that control the reversible capacity limits of the layered oxide cathodes, this dissertation focuses on the structural and chemical characterizations of the Li1-xMO 2 (M = Co, Co1-yNiy, Co1-yAl y, Co1-yMgy, and Ni1-yMn y, and 0 ≤ (1 - x) ≤ 1) phases obtained by chemically extracting lithium from LiMO2.; Chemical extraction of lithium was accomplished by stirring the layered LiMO2 oxide powders with an acetonitrile solution of the powerful oxidizer NO2BF4 under argon atmosphere, followed by a careful handling and storage of the products to avoid reaction with the ambient. Structural characterization of the various cathodes indicate that while Li1-xCoO2 and the cobalt-rich Li1-x Co1-yMyO2 (M = Ni, Al, and Mg) compositions generally show the formation of P3 and O1 type phases, the nickel-rich Li 1-xNi1-yMyO2 (M = Co and Mn) compositions tend to maintain the initial O3 type structure, but with a smaller c lattice parameter. Factors such as the nature of Mn+, cation disorder between the Mn+ and Li+ planes, and the presence of residual Li+ ions in the Li+ plane influence the structure of the phases formed and the phase relationships. Additionally, chemical lithium extraction technique has been identified as a convenient and faster method to obtain qualitative information on the degree of cation disorder in layered oxides.; Oxygen content analysis of the various delithiated Li1-xMO 2-delta samples indicate that the systems tend to lose oxygen from the lattice at deep lithium extraction due to chemical instability. The chemical instability decreases in the order Li1-xCoO 2-delta > Li1-xNi0.5Mn0.5O 2-delta > Li1-xNi0.75Mn0.25 O2-delta ≈ Li1-xNi0.85Co 0.15O2-delta, which is consistent with the observed charge voltage profiles. The observed loss of oxygen from the Li1-x MO2-delta is due to an overlap of the M3+/4+ :3d band with the top of the O2-:2p band and a consequent oxidation of the oxide ions at deep lithium extraction. The oxygen loss from the lattice is accompanied by a decrease in the c parameter or the formation of new phases. More importantly, the lithium content at which oxygen loss begins to occur correlates well with the reversible limit of lithium extraction and the practical capacities, suggesting that the chemical instabilities may play a crucial role in determining the reversible capacity limits of lithium-ion battery cathodes.