Ensuring the quality of Electrochemical Impedance Spectroscopy measurements on high-power systems

EIS measurements and high-power systems.

The growth of the battery sector is partly due to the increasing need for high autonomy and highly portable everyday electronic devices (smartphones, laptops, etc.). But issues experienced in electric transportation and energy storage sectors are often closely linked to battery development. Batteries made for modern-day applications are often high-power systems. Such systems are considered to be “high-power systems” when they can deliver power higher than 100-500 W. This could be in the form of connected battery cells as a pack, or it might be only one large, single battery cell.

Obtaining good impedance measurements on a high-power battery may prove challenging for certain potentiostat / galvanostats. Indeed, such instruments require capabilities that are sometimes contradictory. On one hand, the technology used to enhance the potentiostat’s power slows its response, on the other hand, high-frequency impedance measurements require a fast galvanostatic system.  This is like requiring a car to be as powerful as a truck and as fast as a Formula One. The ability of the truck to carry heavy goods represents the potentiostat’s ability to apply high current to the cell. The velocity of the Formula One car represents the potentiostat’s ability to apply high-frequency signals.

Figure 1: Analogy of the truck and the Formula One racing car

To solve such a dilemma, as is so often the case, balance is the key. The choice of the electrochemical instrument and its settings must be adapted to the experiments and to the device under test. BioLogic offers a wide range of solutions to address such needs. This article will present the typical impedance graphs obtained for battery stack testing and will then study the quality of the impedance measurements performed on a single high-power battery cell.

The unique EIS QI (Electrochemical Impedance Spectroscopy Quality Indicators) are shown here to demonstrate the quality of EIS measurements performed on a Samsung SDI 94 Ah battery.

A BioLogic high current potentiostat for Electrochemical Impedance Spectroscopy

Galvanostatic Electrochemical Impedance Spectroscopy (GEIS) can be performed on high-power batteries with a BioLogic FlexP external booster driven by an essential range potentiostat. The Nyquist impedance diagram presented in Figure 2 is a typical graph that can be obtained with such a set-up on a stack of batteries. The details of this experiment can be found on Application Note #59.

Figure 2: Nyquist graph of EIS measurements performed with a VMP3 potentiostat linked to a FlexP 0060 external booster.

EIS Quality Indicators: An important Quality Check

The quality of the measurement is quantified by its linearity at high frequencies. Total Harmonic Distortion (THD) is an impedance Quality Indicator (QI), expressed as a percentage, which indicates if the amplitude of the current or potential modulation applied to the system is small enough to consider that the system behaves linearly (see Learning center article How to make reliable EIS measurements with your potentiostat or battery cycler, and Application Notes #64 and #65). If the THD value of the system response is lower than 5 % typically (but not always), it indicates reliable impedance data.

In this article, THD value is not used to characterize the response of the system but the performance of BioLogic potentiostats in EIS measurements, particularly GEIS (Galvano Electrochemical Impedance Spectroscopy) measurements. To this end, the THD I value, i.e., the THD of the stimulus for GEIS measurements, is studied to observe the linearity of the potentiostat and avoid any incorrect interpretation coming from the battery behavior.

Figure 3 shows the quality of EIS measurements performed on a Samsung SDI 94 Ah battery with a BioLogic FlexP 0012 external booster driven by the SP-150 potentiostat.

The potentiostat + booster set-up proved to be within acceptable limits (i.e., the THD of the input signal is lower than a few %) for 10 A amplitude and frequency measurements from 0.1 Hz to 1 kHz. 10 A represents around 10 % of the capacity of the battery (1 C/1 h).

It must be noted that the potentiostat has good EIS performance for frequencies from 2 kHz to 0.1 Hz. For higher frequencies, the input signal causes non-linearities.

AC tests are commonly used to perform tests at a single frequency (typically 1 kHz) to obtain information about battery internal resistance. In our case, for a frequency of 1 kHz, the THD I value is low (2.8 %). This indicates that the SP-150e potentiostat linked to a FlexP 0012 external booster can be used to perform AC tests at 1 kHz.

Figure 3: Quality indicator THD I vs. frequency of EIS measurements performed
with an SP-150e potentiostat linked to a FlexP 0012 external booster.

The same experiments have been carried out with different configurations of BioLogic potentiostats / galvanostats and boosters. The results are summarized in Table I below.
 
Table I: Results of the EIS measurements for a frequency of 1 kHz with AC amplitude of 10 A on the Samsung 94 Ah battery.

It should be noted that the more powerful the instrument, the higher the THD I value. This supports the analogy described at the beginning of this article: it is very difficult to have high power and high speed at the same time for an electrochemical device.

Nevertheless, the THD I value is very low for all these experiments, meaning that high-power BioLogic instruments can perform reliable impedance measurements.