The present study examines, for the first time, the evolution of the electrochemical impedance spectroscopy (EIS) of a lithium iron phosphate (LiFePO4) battery in response to degradation under various operational conditions. Specifically, the study focuses on the effects of operational temperature and compressive force upon degradation. In addition, the evolution of the corresponding parameters with the proposed equivalent circuit model (ECM) is estimated in order to elucidate the influence of the specific electrochemical mechanisms upon the degradation of the LiFePO4 battery. This systematic analysis reveals that the degradation of electrochemistry significantly depends upon the operational temperature and the compressive force. Specifically, the operational temperature is found to affect the degradation of all components in the battery, whereas the compression only affects the ohmic resistance and interfacial impedance. Moreover, these two conditions can each accelerate and decelerate the degradation rate to generate distinct degradation pathways depending upon the cycle. The origins of these distinct degradation pathways are further elucidated via differential voltage (DV) analysis. This comparative study shows for the first time that EIS is more effective than DV for elucidating the effects of compressive force upon the degradation. This is because the high-frequency response of EIS is more sensitive to the mechanical constraints that affect the ohmic resistance and interfacial layer impedance. Finally, the absolute impedance magnitude, consisting of the double-layer capacitance, the charge transfer resistance, and the Warburg impedance, is suggested as a novel and distinct health indicator based on Grey relational analysis because this indicator is more highly correlated to the state of health compared to the other independent parameters in the proposed ECM.