Introduction of Square-Current Electrochemical Impedance Spectroscopy (SC-EIS) to Diagnosis Technology of Laminated Lithium-Ion Battery

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 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 diagnosis technology of laminated LiB was investigated. A commercially available laminated LIB with a nominal capacity of 5 Ah was used. For LIB module investigation, one module was assembled using four cells (two series cells, two parallel cells). High precision bipolar power supply and conventional large-scale charge-discharge test system were mainly used for input signal source. On the basis of the technique of fourier transform in ref. [2][3], EIS was carried out using square current input at SOC = 50 %. Frequency of square current was 0.5, 5.0, 50 Hz. Amplitude of peak to peak and sampling frequency were 800 mA and 100 kHz, respectively. Figure 1 shows input current waveform at 50 Hz, output voltage waveform, and Nyquist plots of 5Ah LIB using bipolar power supply and large scale charge – discharge test system. Dots show the results of SC-EIS. Open symbols show the reference results of conventional impedance spectroscopy. The waveform generated by charge-discharge system is not ideal square shape compared with the waveform generated by bipolar power supply. Thus, effect of waveform on the EIS was investigated. In the case of ideal square waveform generated by bipolar power supply, fine Nyquist plots could be obtained by means of SC-EIS as well as that by means of the FRA system even in the case of LIBs. 200th harmonic could be measured at frequency of (0.5 Hz, 5 Hz, 50 Hz). Only using these three frequencies, very wide range of 0.5 Hz – 10 kHz could be measured by using this method. In the case of not ideal square waveform generated by large-scale charge-discharge test system, fine Nyquist plots up to 3 kHz could be obtained by means of SC-EIS as well as that by means of the FRA system even in the case of LIBs. In usual, frequency response up to 2 kHz is needed for diagnosis technology of LIBs. Thus, this technique is could be applied to cell checking system only using not expensive instruments. We also applied this technique to cell module. As a result, we successfully obtained fine Nyquist plots of both module and each single cell by once measuring. Moreover, some error in the modules could be picked out by using this measurement technique successfully. From these results, new techniques called “Square- Current Electrochemical Impedance Spectroscopy (SC-EIS)” demonstrated to be suitable method for EIS without using FRA systems. The SC-EIS must be a method of great candidate for diagnosis technology of battery management systems. References [1] T. Yokoshima et al., Program of the 9th Int'l Symp. on Electrochemical Impedance Spectroscopy, S01-07, S01-08 (2013). [2] T. Osaka, et al., Bull. Chem, Soc. Jpn. , 55 (1982) 36. [3] T. Osaka, et al., DENKI KAGAKU , 50 , (1982) 295. Acknowledgements This work was partly supported by "Development of Safety and Cost Competitive Energy Storage System for Renewable Energy" from NEDO, Japan.