A Search for the Optimum Lithium Rich Layered Metal Oxide Cathode Material for Li-Ion Batteries

We report the results of a comprehensive study of the relationship between electrochemical performance in Li cells and chemical composition of a series of Li rich layered metal oxides of the general formula xLi₂MnO₃ · (1-x)LiMn0.33Ni0.33Co0.33O₂ in which x = 0,1, 0.2, 0,3, 0.5 or 0.7, synthesized using the same method. In order to identify the cathode material having the optimum Li cell performance we first varied the ratio between Li₂MnO₃ and LiMO₂ segments of the composite oxides while maintaining the same metal ratio residing within their LiMO₂ portions. The materials with the overall composition 0.5Li₂MnO₃ · 0.5LiMO₂ containing 0.5 mole of Li₂MnO₃ per mole of the composite metal oxide were found to be the optimum in terms of electrochemical performance. The electrochemical properties of these materials were further tuned by changing the relative amounts of Mn, Ni and Co in the LiMO₂ segment to produce xLi₂MnO₃ · (1-x)LiMn0.50Ni0.35Co0.15O₂ with enhanced capacities and rate capabilities. The rate capability of the lithium rich compound in which x = 0.3 was further increased by preparing electrodes with about 2 weight-percent multiwall carbon nanotube in the electrode. Lithium cells prepared with such electrodes were cycled at the 4C rate with little fade in capacity for over one hundred cycles.