Correlation of Arrhenius behaviors in power and capacity fades with cell impedance and heat generation in cylindrical lithium-ion cells

A series of cylindrical 18650 lithium-ion cells with an MAG-10|1.2M LiPF₆ ethylene carbonate (EC):ethyl methyl carbonate (EMC) (w:w = 3:7)|Li_xNi_0.8Co_0.15Al_0.05O₂ configuration were made and tested for power-assist hybrid electric vehicle (HEV) applications under various aging conditions of temperature and state-of-charge (SOC). The cells were intermittently characterized for changes in power capability, rate capacity, and impedance as aging progressed. The changes of these properties with temperature, as depicted by Arrhenius equations, were analyzed. We found that the degradation in power and capacity fade seems to relate to the impedance increase in the cell. The degradation follows a multi-stage process. The initial stage of degradation has an activation energy of the order of 50-55kJ/mol, as derived from power fade and C₁ capacity fade measured at C/1 rate. In addition, microcalorimetry was performed on two separate unaged cells at 80% SOC at various temperatures to measure static heat generation in the cells. We found that the static heat generation has an activation energy of the order of 48-55 kJ/mol, similar to those derived from power and C₁ capacity fade. The correspondence in the magnitude of the activation energy suggests that the power and C₁ capacity fades were related to the changes of the impedance in the cells, most likely via the same fading mechanism. The fading mechanism seemed to be related to the static heat generation of the cell.