iMSine - “Intelligent” multi-sine excitation in electrochemical impedance spectroscopy
Speeding up your battery testing
W.O.Strunz and S.Feihl
Batteries and related energy storage systems have the potential to play a significant role in the global process of energy productions towards renewable energy sources. Within this application note we want to address the researcher related to this topic to take advantage of the presented method of intelligent multiple frequency excitation (iMSine).
Electrochemical impedance spectroscopy (EIS) is one of the most prominent techniques for the evaluation of electrochemical systems like batteries, fuels cells and solar cells. EIS allows the tracking of charge carrier processes within the different components of an electrochemical system, such as electrodes and electrolytes.
The systems under investigation are sequentially excited by applying small sinusoidal waves of a quantity, such as current. This is done within a given frequency range (i.e. 100 mHz to 100 KHz). The response of the system is a frequency dependent, alternating signal, i.e. alternating voltage signal. The frequency dependence of the responding current can be attributed to specific processes occurring either at the interfaces between, or inside of the different phases which are present in the investigated system.
Despites their scientific potential EIS measurements suffer from increasing time consumption when data recording occurs in a frequency range below 1 Hz. Here, at lower frequencies a complete EIS measurement may take at least several minutes. But sometimes it is wanted, and even necessary, for the user to record a high number of measurements in a short time. For example, when batteries are in the focus of research. Charging and discharging of batteries cause continuous changes of both the battery voltage and current. Since batteries have the great potential to play a significant role within the renewable energy revolution this topic is very meaningful to the research community.1,2 At the end reducing the time consumption of the EIS measurement is the goal for accelerating the data acquisition. Exciting the system in a multi sine mode is one way to do so. This means the multiple excitation of several frequencies in a simultaneous way, especially in the low frequency range of the system. The time consumption of the EIS measurement at a low frequency range is then significantly reduced compared to single sine excitation mode. But nevertheless, this method bears drawbacks related to its accuracy. In conclusion, the analysis of the corresponding data may become less reliable then, leading to wrong interpretations.
In order to overcome such problems the ZAHNER THALES software provides a method which includes, next to the ability of reducing recording time by simultaneous frequency excitation, also a correction mode. This helps to correct drift phenomena already during the measurement progress. This method has been called by the ZAHNER technicians “intelligent Multi Sine” (iMSine) excitation. Next to the excitation of multiple frequencies in the range of lower frequencies, a permanent, automatic signal correction occurs online during EIS measurement. This happens by means of correlation analysis of the Fourier-transform of not excited parts of the harmonic content in the frequency range, which is applied during the EIS measurement. Practically, this includes that the lowest frequency (known as the ground wave) does not belong to the set of excitation frequencies.
Thus, the following Fourier-transform of the corrected data set gives a more accurate image of the measured EIS back to the user. This a very unique feature, provided up to now solely by the ZAHNER THALES software.
Electrochemical impedance spectroscopy
For the purpose to simulate drift phenomena in battery stacks a commercially available, regular Li-ions based battery with an energy volume of 800 mAh and a voltage of 3.6 V was used. The Li-ions battery was carefully contacted via wires and connected to a ZAHNER-elektrik electrochemical workstation in order to perform accurate EIS measurements. All EIS measurements were performed in a frequency range of 100 mHz to 100 kHz whilst the battery was discharged with 50 mA DC. The AC excitation amplitude was set to 20 mA. In order to demonstrate the time saving effect a variety ofdifferent sine modes and numbers of sinusoidal excitation waves are compared in Table 1.
Figure 1: Connections scheme for providing EIS measurement on a commercially available Li-ions battery.
Scheme 1: Scheme of activation of drift compensation procedure under setup. From upper left to lower right: Enter setup menu in THALES EIS application program – Edit actual settings – AC-mode settings: more – LF drift compensation: 1 = activated, 0 = deactivate.
|Sine mode||Time consumption / min : sec|
Table 1: Measurement settings of drift phenomena and estimated time consumption.