Intercalation Mechanism in Lithium/Iron-Phthalocyanine Cells

The cathode reaction mechanism in a lithium//1M LiC10₄/propylene carbonate (PC)//iron-phthalocyanine (FePc) cell was investigated by x-ray diffraction and atomic absorption spectroscopy. For about 12 e^- /FePc depth of discharge, the FePc interlayer distance expanded by about 4.3%. About 9 Li/FePc were found in the cathode pellet by chemical analysis. After recharge, the interlayer distance returned to its initial state and the Li content decreased to 1.4 Li/FePc. These results show that Li⁺ intercalates to at least a 9 Li/FePc ratio and most of the discharge capacity is reversible on the first cycle. The discrepancy between the electrical charge (∼12 e^-/FePc) and the actual Li content in the cathode pellet (∼9 Li/FePc) is caused by side reactions like PC decomposition and FePc anion dissolution during discharge. A different capacity degradation mechanism, C1O₄ ^- intercalation during charging, may also occur.