Factors Affecting the Rate Performance of LiFePO4
To assess the origin of capacity loss observed during the first cycles, a study was conducted to evaluate the effect of carbon content in the composite cathode on the electrochemical performance of LiFePO4. Carbon is usually added into the composite cathode to increase the electronic contact between the active material and the electronic conductor. The conductivity of the composite cathode is expected to increase by incrementing the amount of carbon. On the other side Chen and Dahn (J. Electrochem. Soc. 149:A1184–A1189, 2002) showed that the presence of carbon, even below 1.0 wt%, causes a significant tap density decrease. The amount of carbon that is needed to optimize the electrode performance depends on the particle size of the active material. Depending upon the performance requirements, the particle size need not necessarily to be nanometric. Dahn concluded that it was important for the LiFePO4 producer to carefully note the effect of carbon coating on capacity, rate capability, and tap density. In order to investigate the factors affecting rate performance of LiFePO4 with a well defined grain size and to maximize the electrode performance, a series of electrodes were prepared with variable active material/carbon ratios. Undoped, nano-crystalline LiFePO4 was used to prepare composite electrodes containing 10, 15, and 20 wt% carbon. The electrodes were tested as cathodes in non-aqueous lithium cells (Zane et al., Electrochim. Acta 49:4259–4271, 2004). By increasing the carbon content, an increase in the overall electrochemical performance was observed. Impedance spectroscopy was used to investigate the ohmic and kinetic contributions to the cell overvoltage. It was found that increasing the carbon content leads to a reduction of the cell impedance as a consequence of the charge transfer resistance reduction. The poor performance exhibited at very high discharge rates was related to the high value of the charge transfer resistance. For currents larger than 3C rate, a severe capacity fade affected the electrodes. It was concluded that the electronic contact at the LiFePO4/carbon interface plays a decisive role in material utilization at different discharge rates and affects the capacity fade upon cycling. A further decrease of the charge transfer resistance in high carbon content cathodes (20 wt% carbon) was obtained by improving the powder mixing procedure. The cell performance of well mixed, high carbon content electrodes was better than our previously obtained results in terms of higher capacity retention both for different discharge rates and repeated cycling.
KeywordsComposite Film Electrochemical Performance High Carbon Content Composite Cathode High Discharge Rate