Effect of Electrode Parameters on LiFePO4 Cathodes

LiFePO 4 electrodes with thicknesses from 15 to 120 μm 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 25 C and still give a capacity of 70 mAh/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 LiFePO 4 electrodes gives a maximum capacity of 1050 mAh at 1-C rate for a 60-μm electrode. This corresponds to a volumetric and gravimetric energy density of 214 Wh/L and 96.5 Wh/kg, respectively. The effective Li diffusivity in the active material is estimated to be of the order of 10 -13 cm 2 /s.