Electrochemical Impedance Spectroscopy Studies:
Electrochemical impedance spectroscopy (EIS) was measured after every 20/25 charge-discharge cycles at room temperature and in fully charged state. EIS is essential for evaluating the power loss due to cycling and analyzing the failure mechanism. The Nyquist plots of the cells discharged at different temperatures are illustrated in Figure 6 a-d. All the cells initial showed similar impedance values (0.05 – 0.06 Ω) prior to cycling. However, after 130 cycles the impedance values are significantly higher: 0.41, 0.20, 0.13, and 0.38 Ω at -10, 0, 25, and 40 °C, respectively. The increase in impedance is consistent with capacity degradation after 130 cycles as shown in Figure 5a.