A systematic study of LiFePO 4 with cyclic voltammetry ͑CV͒ was conducted using thin electrodes with a loading of 4 mg/cm 2 . Peak current of the CV profile was proportional to the square root of scan rate under 0.2 mV/s. Results were analyzed using a reversible reaction model with a resistive behavior. This resistance was consistent with other resistances obtained from electrochemical impedance spectroscopy and charge-discharge curves. Apparent Li diffusion constants of 2.2 ϫ 10 −14 and 1.4 ϫ 10 −14 cm 2 /s were obtained at 25°C for charging and discharging LiFePO 4 electrodes in 1 M LiPF 6 ethylene carbonate/diethyl carbonateϭ3:7 by volume, respectively. Activation energies of the apparent diffusion constants and electrode resistance are about 0.4 eV. These parameters are good indicators for assessing the effectiveness of material modifications such as surface coating and doping.

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LiFePnormalO4 electrodes with thicknesses from 15to120μm were coated on Al current collectors. The electrochemical characteristics of these electrodes depend strongly on film thickness, with the largest rate capability for the thinnest film—a 15-μm electrode can be discharged at a current rate of 25C and still give a capacity of 70mAh∕g . This shows great promise for high-power applications such as hybrid electrical vehicles. Increasing the amount of carbon in the electrode, decreasing the packing density, or using an electrolyte with lower viscosity and higher ionic conductivity improved the rate performance. This suggests that the thickness effect is caused by a larger electrode resistance and a slower Li-ion conduction through the electrolyte for thicker films. Electrode thickness in turn affects the energy density of a battery, because the percentage of inactive materials increases with decreasing film thickness. An energy density prospect for a 18650-type battery with these LiFePnormalO4 electrodes gives a maximum capacity of 1050mAh at 1-C rate for a 60-μm electrode. This corresponds to a volumetric and gravimetric energy density of 214Wh∕L and 96.5Wh∕kg , respectively. The effective Li diffusivity in the active material is estimated to be of the order of 10−13cm2∕s .

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Tin anodes deposited electrochemically on a copper foil current collector are studied to develop a next-generation lithium-ion battery with higher energy density. Better cycle performance through ten initial cycles under full charge and discharge conditions was attained by annealing tin electrodeposited on a rough surface copper foil. The annealing process was found to change the main active material from Sn to Cu6Sn5 with some minor compounds. Furthermore, a microcolumnar structure of the active material portion was found to be self-organized in accordance with the surface profile of the foil during the first charge-discharge cycle. Advantages of these structural features are discussed in terms of the initial charge and discharge performance, including specific capacity and coulombic efficiency measured by using a three-electrode cell. © 2003 The Electrochemical Society. All rights reserved.