Effect of Particle Size on Iron-Phthalocyanine Cathodes in Secondary Lithium Cells

To overcome small capacities at high discharge rates in Li/iron-phthalocyanine (FePc) secondary cells, the effect of particle size on FePc is investigated. Large particle FePc is obtained by sublimation and gas evaporation is used to prepare FePc ultrafine particles. The discharge capacity at 0.1 mA/cm² increases with increasing particle size (ultrafine particles < starting powders < single crystals). The relatively low discharge capacity of ultrafine particles at low discharge rate may be due to the low crystallinity. The order of discharge capacity, however, is reversed at high discharge rates over 0.3 mA/cm². This tendency is consistent with the result of Li diffusivity measurements. The large discharge capacity of ultrafine particles at high discharge rates must be a product of the large surface area. From these results, ultrafine FePc particles are found to be a good organic cathode material at high discharge rates.