Kazunori Donoue

Study of LiFePO4 by Cyclic Voltammetry

A systematic study of LiFePO 4 with cyclic voltammetry (CV) 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 ×10 -14 and 1.4 X 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 carbonate=3:7 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.

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.

Impurities in LiFePO4 and Their Influence on Material Characteristics

Li 3 PO 4 impurity was found in hydrothermally grown LiFePO 4 samples made with excess Li. The impurity dissociates in water, leading to an apparent alkaline nature of the LiFePO 4 sample. The impurity can be removed by washing the sample in a neutral buffer solution, after which an increase in specific capacity of the LiFePO 4 sample was observed. Li 3 PO 4 , as an inactive component within the active material, reduces the energy density of LiFePO 4 . Additional experiments were performed to study the reactivity of LiFePO 4 in different environments. It is found that LiFePO 4 decomposes in a strong alkaline solution and the decomposition can be slowed down by carbon coating the material. After prolonged storage in water, the specific capacity of carbon-coated LiFePO 4 is reduced and Fe dissolution is observed. Material degradation is thought to be due to interactions among LiFePO 4 , impurities, and water.