High Energy Density Cathode Active Materials for Lithium-ion Batteries

Renewable energy sources such as solar energy, wind and hydroelectric power are increasingly being developed as essential energy alternatives to alleviate the deleterious effects of greenhouse gases in the globe. Large scale energy storage is an indispensable component of renewable energy sources and in this context, Li-ion batteries (LIBs), due to their high energy and power densities and long cycle life, have spurred great interest. This dissertation presents an account of investigations leading to advanced materials which overcome the deficiencies of high energy density Li-ion battery cathodes.;Chapter 1 discusses the fundamental aspects of generic battery systems and elaborates on the current state of the art of rechargeable Li batteries.;Chapter 2 deals with the discovery of the material 0.3Li 2Mn3.0.7LiNi0.5Co0.5O2 (LLNC) that allowed us to conclude which segment of the lithium rich layered composite metal oxide is responsible for structural transformation from the layered to spinel phase during charge/discharge cycling. Using X-ray absorption (XAS) spectroscopy we elucidated the oxidation states of the K edges of Ni and Mn in the two materials with respect to different charge and discharge states. The XAS data along with electrochemical results revealed that Mn atoms are not present in the LiMO2 structural segment in LLNC. Electrochemical cycling data from Li cells further revealed that the absence of Mn in the LiMO2 segment significantly improves the rate capabilities of LLNC with good capacity maintenance during long term cycling. Removing the Mn from the LiMO2 segment of lithium rich layered metal oxides appears to be a holistic strategy for improving the structural robustness and rate capabilities of these high capacity cathode materials for Li-ion batteries.;Chapter 3 examines the effect of alkali ion doping (Na +) into the cathode material of the composition 0.3Li2Mn 3.0.7LiMn0.33Ni0.33Co0.33O2 (LLNMC). The 5 wt. % Na doped material, formulated as 0.3Li2Mn 3.0.7Li0.97Na0.03Mn0.33Ni0.33 Co0.33O2, was compared to its counterpart without Na doping. The discharge rate capability of the LLNMC was greatly improved at both room temperature and 50 °C with the Na doping. The Na doped material exhibited significantly higher electronic conductivity than its un-doped analog as evidenced by dc electronic conductivity data and AC impedance of Li cells. Charge/discharge cycling results of Li cells containing these cathode materials at 50 °C indicated that the voltage decay of LLNMC, accompanied by a layer to spinel structural conversion, was mitigated with Na doping.;The results of a comprehensive electrochemical and structural investigations of a wide range of lithium rich layered metal oxide cathode active materials, xLi2Mn3.(1-x)LiMnaNibCocO2 (where x, a, b and c vary) are reported in Chapter 4. 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.;In Chapter 5, the results of a new synthesis technique, called self-ignition combustion (SIC), that dramatically enhanced the rate capabilities of a lithium rich layered metal oxide compound we prepared are discussed. In this chapter, we report a high rate Li-rich layered manganese nickel cobalt (MNC) cathode material of the composition 0.5Li2Mn 3.0.5LiMn0.5Ni0.35Co0.15O2, termed SIC-MNC cathode material for Li-ion batteries with discharge capacities of 200, 250, and 290 mAh/g at C, C/4 and C/20 rates, respectively. This high rate discharge performance combined with little capacity fade during long term cycling is unprecedented for this class of Li-ion cathode materials. (Abstract shortened by UMI.).