Is small particle size more important than carbon coating? An example study on LiFePO 4 cathodes

Abstract

Based on careful analysis of nine papers by different research groups, we show, for the first time, that in LiFePO 4-based cathode materials the electrode resistance depends solely on the mean particle size. The effect of carbon coating is marginal, it suffices that each LiFePO 4 particle is point-contacted with a reasonable number of carbon black particles usually added in the course of electrode preparation. We present a simple but general theoretical model that consistently explains this unexpected result. The main reason for the relatively small importance of carbon coatings is the fact that the ionic conductivity (ca. 10 −11–10 −10 S cm −1 at RT) is much smaller than the electronic (>10 −9 S cm −1 at RT). The present finding could be of significant importance not only for further optimization of LiFePO 4 cathodes, but also for preparation of other cathode materials in which the ionic conductivity is much lower than the electronic.