Structural and electrochemical characterization and surface modification of layered solid solution oxide cathodes of lithium ion batteries
Lithium ion batteries are widely used to power portable electronic devices such as cell phones and laptop computers due to their high energy density. However, the currently used layered LiCoO2 cathode could deliver only 50 % of its theoretical capacity in practical lithium ion cells (140 mAh/g) due to the chemical and structural instabilities at deep charge with (1-x) < 0.5 in Li1-xCoO2. Also, cobalt is relatively expensive and toxic. These difficulties have generated enormous interest in alternative cathode hosts. In this regard, solid solutions between layered Li[Li1/3Mn2/3]O2 (commonly designated as Li2MnO3) and LiMO2 (M = Mn, Ni, Co)) have become appealing as some of them exhibit much higher capacity (~ 250 mAh/g on charging to 4.8 V) with lower cost and better safety compared to LiCoO2. This dissertation investigates the (1-z) Li[Li1/3Mn2/3]O2 - (z) Li[Mn0.5-yNi0.5-yCo2y]O2 (y = 1/12, 1/6 and 1/3 and 0.25 ≤ z ≤ 0.75) layered oxide cathodes, which belong to a solid solution series between layered Li[Li1/3Mn2/3]O2 and Li[Mn0.5-yNi0.5-yCo2y]O2, with an aim to develop a better understanding of the chargedischarge mechanisms and optimize the electrochemical performance of these materials. To accomplish this, the structural and electrochemical characterization of the (1- z) Li[Li1/3Mn2/3]O2 - (z) Li[Mn0.5-yNi0.5-yCo2y]O2 cathodes is carried out. It is found that the amount of oxygen loss is related to the lithium content in the transition metal layer, and the Co and Mn4+ contents play a role in influencing the electrochemical behavior. In addition, the chemically delithiated samples are found to transform to O1 or P3 structure with a vanishing of the superlattice reflections arising from cationic ordering in the transition metal layer due to the incorporation of protons from the chemical delithiation medium, while the electrochemically charged samples retain the initial O3 structure. These layered solid solution oxides exhibit high irreversible capacity (IRC) loss (difference between first charge and discharge capacity) values (up to 100 mAh/g), which have been reduced significantly by modifying the cathode surface with other materials like Al2O3, AlPO4, and F- . For example, compared to an IRC of 75 mAh/g and a first discharge capacity of 253 mAh/g for the pristine Li[Li0.2Mn0.54Ni0.13Co0.13]O2 (y = 1/6 and z = 0.4), the 3 wt. % Al2O3 modified sample exhibits a lower IRC of 41 mAh/g and a higher first discharge capacity of 285 mAh/g, which is two times higher than that achieved with the LiCoO2 cathode. A careful and systematic analysis of the experimentally observed capacity and IRC values suggest that part of the oxide ion vacancies created during first charge is retained in the layered lattice in contrast to the idealized model (elimination of all oxide ion vacancies) proposed in the literature. The surface modification helps to retain even more number of oxide ion vacancies in the lattice, which leads to a lower IRC and higher discharge capacity values. Additionally, bulk cationic and anionic substitutions of Al3+ and F- in Li[Li0.17Mn0.58Ni0.25]O2 (y = 0 and z = 0.5) are found to sensitively decrease the amount of oxygen loss from the lattice.