Electrochemical impedance spectroscopy (EIS) is strongly requested for analysis of the battery health.  The capacity and the internal resistance of large-scale lithium ion batteries (LIBs) become higher and lower, respectively, for the application to electric vehicles and large-scale power storage systems. EIS using conventional FRA – potentiostat systems is not easy to measure the impedance of the LIB because of its low internal resistance. In our previous study, application of square wave potential/current for input signals of EIS was investigated in simple electrochemical reaction to verify a new technique called “Square-potential/current electrochemical impedance spectroscopy (SP-EIS, SC-EIS)” which is a method for EIS without using the FRA systems [1]. And then, we applied SC-EIS to evaluate a state of commercial LIB [1]. In this study, introduction of SC-EIS to battery assessment system of kW-class LIB systems was developed, and impedance of LIB modules and cubicles was measured.

Battery assessment system was redesigned from commercial energy storage system with LIB.  A commercially available LIB with a nominal capacity of 20 Ah (SCiB TM, developed by Toshiba Co.) was used. One module was assembled using 24 cells (2 parallel cells, 12 series cells). One cubicle was assembled using 5 modules. This battery assessment system with 50 kW has two cubicles, which were 11kWh with 240 cells. Power controller in the battery assessment systems was used for generating input signal source. On the basis of the technique of Fourier transform in ref. [1][2], EIS was carried out using square current input at SOC = 50 %. Frequency of square current was 50m 0.5, 5.0, and 50 Hz.  Amplitude of peak to peak and sampling frequency were 10 A and 100 kHz, respectively.

The impedance measurement of LIBs was carried out using battery assessment system with SC-EIS technique. In this system, impedance results of 10 modules and a pair of cubicle could be obtained by one measurement. Figure 1(a) shows Nyquist plots of one module with 24cells. Figure 1(b) shows Nyquist plots of two cubicles with 10 modules (240 cells).  Using 0.5, 5.0 and 50 Hz of square current as input signal, the each harmonics of module and a pair of cubicle could be measured with wide range of over 1 decade. Using 50 mHz, only four harmonics could be measured. Using these four frequencies, very wide range of 50m Hz – 1.3 kHz could be measured by using this method. Removing inductance value of wiring from the measured module impedance, the value was similar to totaling impedance of LIB in module. This result indicates possibility of impedance analysis of average one cell from the results of module impedance. Moreover, removing inductance value of wiring from the measured a pair of cubicle impedance, the value was similar to totaling impedance of 10 modules.  This result indicates possibility of impedance analysis of average cell/module from the results of cubicle impedance. Thus, this battery assessment system using SC-EIS has a possibility for analysis of battery health of LIB in kW-class energy storage systems.

References

1) T. Yokosihma et. al, Electrochem. Acta, 180, 922 (2015).

2) T. Osaka, et. al, Bull. Chem, Soc. Jpn., 55, 36 (1982).

Acknowledgements

This work was partly supported by "Development of Safety and Cost Competitive Energy Storage System for Renewable Energy" from NEDO, Japan.