Internal Resistance Analysis of Na/NiCl2 Cells using Electrochemical Impedance Spectroscopy

A pulsed discharge regime (GITT) was used to investigate the ohmic internal resistance of Na/NiCl2 cells, also known as ZEBRA Cells. Two ways were chosen to determine the internal resistance, on the one side the voltage response of the corresponding current enables a calculation. On the other side the potentiostatic electrochemical impedance spectroscopy (EIS) was used to measure cell spectra including the ohmic internal resistance. By comparing both methods the feasibility of ohmic internal resistance (IR) calculation can be evaluated. The cells were investigated within five different operating temperatures (300 °C–180 °C with 30 K steps). Results show that the ohmic internal resistance of a Na/NiCl2 is increasing when the operating temperature is decreasing, mainly due to the decrease of ionic conductivity of separator and the secondary electrolyte NaAlCl4. As a direct consequence, there is a significant capacity loss. Therefore, the operating temperature can be identified as capacity limiting factor.