Layered lithium nickel manganese cobalt dioxide as a cathode material for lithium-ion batteries

With the development of new applications such as hybrid, plug-in and fully electric vehicles, the market for lithium ion batteries is growing rapidly. However, the majority of the commercial cathodes in lithium ion batteries still use lithium cobalt dioxide, LiCoO2, in which the element cobalt is not only expensive but toxic. In addition, the low energy density limits this material for large scale applications.;The motivation of the work reported in this dissertation is to investigate the mixed layered oxides LiNiyMnyCo1-2yO 2 as the cathode material for Li-ion batteries. The combination of these three transition metals provides higher capacity, lower prices and improved safety features compared with LiCoO2 and thus are being studied and characterized using different techniques in order to completely understand the processing-structure-property relation in this four-cation oxide cathode material.;First we utilized the co-precipitation method to synthesize the low-cobalt material: LiNi0.45Mn0.45Co0.1O2 with an initial discharge capacity at about 180 mAh/g. XRD data shows it has the layered structure of alpha-NaFeO2 and a cation disorder of around 6% between Li+ and Ni2+ ions. We then investigated the influence of the lithium content in this mixed oxide. As the lithium content increases, the Li+-Ni2+ cation disorder extent decreases which was confirmed by XRD analysis and magnetization studies. An appropriate amount of inter-slab Ni2+ ions help pinning the transition metal layers and thus improve the structure stability on lithium cycling.;The maximum manganese content in the stoichiometric LiMO2 (M=Ni, Mn, Co) is 50% without the formation of a second phase. Part of the Mn4+ ions are reduced to 3+ in order to balance the charge without inducing a Jahn-Teller distortion. Increasing the lithium and manganese contents simultaneously leads to the formation of a layered structure with Li : Mn ordering in the transition metal layers. The first discharge capacity is improved substantially to 200 mAh/g. In-situ XRD was applied to observe the structural changes of this Li and Mn rich compound during cycling. Solid-state NMR was combined with Rietveld refinement to reveal the transition of ordering pattern in the transition metal layers; the corresponding electrochemical performance can be explained successfully.