Synthesis time using microwave irradiation was varied to elucidate the electrochemical degradation mechanism of LiFePO4 related to the evolution of Fe2P. When the amount of Fe2P was above a critical level, LiFePO4 tended to change into an insulating phase, Li4P2O7. The correlation between structural analysis and electrochemical analysis attributed the initial degradation of LiFePO4 to the low electronic conductivity of Li4P2O7, whereas the deficiency of P and O evolved by Li4P2O7 resulted in the cyclic degradation of LiFePO4. This kind of correlation between structure and electrochemical performance in intercalation materials will significantly contribute to an explanation of their degradation mechanism for their application.