Doped LiFePO4 Cathodes for High Power Density Lithium Ion Batteries

Olivine LiFePO4 has received much attention recently as a promising storage compound for cathodes in lithium ion batteries. It has an energy density similar to that of LiCoO 2, the current industry standard for cathode materials in lithium ion batteries, but with a lower raw materials cost and an increased level of safety. An inherent limitation of LiFePO4 acknowledged by researchers studying this material is that its low intrinsic electronic conductivity limits its applicability in commercial systems. Through a doping process, however, its electrochemical performance at high current rates can be improved to levels above that of commercially available lithium batteries. The increase in performance is brought about by a concurrent increase in the electronic conductivity and a reduction of the final particle size. The experimental data suggest that cells formulated with this doped cathode material may produce power densities high enough for consideration as a future battery system for hybrid electric vehicles and other high rate applications. 1. Background Information 1.1 Basic Principles of Battery Systems In the most basic sense, a battery is a device that converts stored chemical energy into electrical energy through a spontaneous chemical reaction. The reaction occurring in a cell is an oxidation-reduction reaction. One species in the cell oxidizes, thereby giving up the electron used to reduce the other species in the cell. In the charged state, the redox reaction is prevented from occurring by physically separating the oxidizable species from the reducible species with an electronically insulating material. When the two species are electrically connected through an external circuit, the reaction proceeds spontaneously and the electrons flowing through the external circuit provide a current that can be used to power a resistive load. A battery is composed of a cathode, an anode, and an electronically insulating but ionically conductive electrolyte. The cathode is the species that undergoes reduction during the discharge of the battery, and the anode is the species that undergoes oxidation. Thus, during discharge, electrons flow externally from anode to cathode. Since electrons are flowing away from the anode, it is therefore at a negative electrical potential when compared to the cathode, and is therefore described as the negative electrode. The cathode is thereby referred to as the positive electrode. There are two types of battery systems: primary and secondary. Primary batteries are those where the chemical reaction is irreversible, and therefore cannot be recharged by forcing the chem'cal reaction to proceed in the opposite direction. On the other hand, secondary battery systems are rechargeable. By supplying an electrical current to move electrons from cathode to anode, the chemical reaction is reversed and the battery is "recharged." The lith.um batteries used in portable electronics applications such as mobile phones and laptop computers are secondary battery systems. In a lithium battery, the anode and cathode are separated by a lithium ion conductive electrolyte, usually either an organic solvent or cosolvent mixed with a soluble lithium salt, or a solid polymer material that provides adequate ionic conductivity. The most common commercial anode material is carbon, which can alloy with lithium to form the compound LiC6. The most commonly used cathode material in presently available commercial batteries is LiCoO 2. A common organic electrolyte system is a mixture of ethylene carbonate with either diethyl carbonate or dimethyl carbonate with I M of a dissolved lithium salt, often LiPF6. A diagram of a discharging lithium ion cell is displayed in Figure 1.