EIS measurements: Potentio (PEIS) or Galvano (GEIS) mode, that is

EIS measurements: Potentio (PEIS) or Galvano (GEIS) mode, that is
Application Note ð49
10302013
EIS measurements: Potentio (PEIS) or
Galvano (GEIS) mode, that is the question
I
Introduction
Electrochemical Impedance Spectroscopy
(EIS) measurements are more often performed
under potentiostatic control than under galvanostatic control. In most cases, potentio and
galvano modes are equivalent (PEIS or GEIS
techniques, in EC-Lab® software) and result in
the same impedance diagrams provided that
a sine current amplitude is “equivalent” to the
voltage sine amplitude. However, in certain
conditions, typically when the system evolves
during the measurement, results from the two
techniques may be different.
For example, in corrosion applications, the polarization resistance is often determined under
potential control around the open circuit voltage (OCV). This is an appropriate approach if
the corrosion potential does not change during
the measurement. However, if the corrosion
potential drifts, a potentio controlled experiment defined to run at OCV could result in a
measurement being performed at an anodic or
cathodic potential with respect to the true OCV.
Under galvano control, drift is not a problem
as the desired zero-current condition is maintained throughout the recording, ensuring the
measurement is performed at the true corrosion potential [1]. In battery applications, it is
often of interest to determine the variation of
internal resistance during discharge/charge. In
this case it may also be appropriate to use the
galvano control in EIS measurements [2].
Both, potentio and galvano controls (PEIS and
GEIS techniques, respectively) are available in
our EC-Lab® software. In this application note,
a comparison between potentio and galvano
control in EIS measurements is presented on
a commercial Li-ion button battery. Two cases
are considered: first, PEIS and GEIS measurements are made around the OCV, and second
an example is presented where the use of the
galvano control is required.
II
EIS measurements
the OCV
around
EIS measurements were performed in potentio
and galvano mode on a commercial Li-ion button cell (nominal capacity 120 mA h). After full
charge, the battery was discharged under C/10
regime, during 10 min and, after a 40 min rest,
the EIS measurements were performed (under
potentio or galvano control) around the OCV.
This cycle is repeted 11 time for each mode.
The frequency range for both measurements
was between 100 kHz and 100 mHz, with the
sine amplitudes of 10 mV and 5 mA for PEIS
and GEIS, respectively. Figs. 1 and 2 show the
parameter settings for these measurements.
Fig. 1: Parameter setting windows for the PEIS
technique.
Bio-Logic Science Instruments, 1 Rue de l’Europe, 38640 Claix, FRANCE
Tel: +33 476 98 68 31 - Fax: +33 476 98 69 09 www.bio-logic.info
1
Application Note ð49
10302013
of the PEIS Nyquist plot during the discharge.
As shown in Fig. 4 PEIS and GEIS show the
same result in the whole range of frequencies,
for both experimental conditions. The fact that
these results are identical under potentio and
galvano control confirms empirically that the
GEIS amplitude used was appropriate. Had
the results been different, a new GEIS amplitude could be tested and checked until a better value was found. As a rough approximation, the GEIS amplitude can be chosen from
the current modulus obtained in potentio mode,
|I| 1 . In most cases, this value is bigger than
the minimum amplitude necessary to obtain the
right measurements.
Fig. 2: Parameter setting windows for the GEIS
technique.
The battery voltage changes with time during discharge, rest, PEIS and GEIS measurements, as shown in Fig. 3. During rest, the battery voltage is allowed to stabilize before the
EIS measurements.
Fig. 3: Voltage vs. time during C/10 discharge
and rest (blue circles), PEIS (green circles) and
GEIS (red circles) techniques.
Fig 4 (Top) shows the first cycle of both PEIS
and GEIS and Fig. 4 (Bottom) the evolution
Fig. 4: Top: EIS diagrams under potentio control (PEIS technique, red points) and under
galvano control (GEIS technique, blue circles).
Bottom: EIS diagram evolution (under potentio
control) during the discharge.
1
This variable is available unchecking the option Hide Additional Variables in the Data file and plot selection windows [3].
Bio-Logic Science Instruments, 1 Rue de l’Europe, 38640 Claix, FRANCE
Tel: +33 476 98 68 31 - Fax: +33 476 98 69 09 www.bio-logic.info
2
Application Note ð49
10302013
The change of internal resistance with the potential or State of Charge (SoC) is studied
by EIS measurements during discharge (or
charge). This resistance is determined by fitting the EIS graphs with an Equivalent Electric
Circuit. EC-Lab® software provides a powerful user-friendly tool to analyze the successive
impedance measurements: Z Fit [4–6]. Z Fit
also (automatically) determines and plots the
values of the electric circuit components for a
series of impedance diagrams. Fig. 5 shows
the result of the fitting process for the first potentio controlled cycle with the equivalent circuit : L1 + R1 + Q2 /R2 + Q3 /(R3 + Q4 ), and the
progression of the internal resistance R1 with
the battery potential. We can observe that R1
value increases during the discharge.
grams during continuous charge or discharge.
To illustrate this case, the EIS measurements
under discharge in galvano control mode were
performed after full charge. The imposed current was −12 mA (C/10 regime) and the corresponding sine amplitude was 5 mA.
Fig. 6 shows the variation of potential as a
function of time, during the EIS measurements
and Fig. 7 shows the EIS graph during continuous discharge. As one can observe, the
low frequency behavior is not the classical restricted diffusion behavior expected on a Li-ion
battery.
Fig. 6: Battery potential during the discharge.
Fig. 5: Top: Result of the fitting process with
Z Fit. Bottom: Echange of R1 with the battery
potential.
III
EIS during continuous discharge (or charge)
In the previous section, we showed an example
where the EIS measurements under potentio
or galvano control are equivalent. However,
galvano control is needed if the user is interested in studying the variation of the EIS dia-
Fig. 7: EIS measurements under galvano control with Ia = − 12 mA.
Bio-Logic Science Instruments, 1 Rue de l’Europe, 38640 Claix, FRANCE
Tel: +33 476 98 68 31 - Fax: +33 476 98 69 09 www.bio-logic.info
3
Application Note ð49
10302013
Under continuous discharge the system
changes during the measurement, so only the
high and middle frequencies (between 100 kHz
and 5 Hz) were considered in the fitting procedure. Therefore the EIS measurements were
fitted with Z Fit, using the equivalent circuit
L1 + R1 + Q2 /R2 + Q3 /R3 , with results shown
in Fig. 8.
Usually the difficulty is to find the sine current
amplitude equivalent to the sine voltage amplitude. As a rule of thumb, for batteries we
recommend a current amplitude of about 10%
of the discharge/charge current. As mentioned,
we can also use (as rough approximation) the
value of the current modulus obtained from the
PEIS measurement.
As shown in this note, galvano control is the
most appropriate to follow the change of the
internal resistance of an operating cell. Potentio mode only allows to study the cell under discontinuous discharge. The galvanostatic control is also recommended to EIS measurements on low internal resistance batteries.
Fig. 8: Result of the fitting process with Z Fit.
References
As we can observe in Fig. 9 the internal resistance behaviors are not the same around the
OCV or in operating condition.
[1] A. Guyader, F. Huet, and R. P. Nogueira. Corrosion, 65(2):136, 2009.
[2] J.-P. Diard, B. Le Gorrec, and C. Montella.
J. Power Sources, 70:78, 1998.
[3] Ec-lab software user’s manual.
[4] Application note # 18. Staircase Potentio Electrochemical Impedance Spectroscopy and automatic succesive Z Fit analysis. (www.biologic.info/potentiostat/notesan.html).
Fig. 9: Variation of the internal resistance R1 as
a function of the battery potential: around the
OCV (red triangles from PEIS experiment) and
under continuous discharge (blue circles from
GEIS experiment).
IV Conclusion
In most cases, performing EIS measurements
in potentio or galvano control are equivalent.
[5] Application note # 18. Using ZFit for
multiple cycles analysis.
(www.biologic.info/potentiostat/notesan.html).
[6] A. Pellissier, N. Portail, N. Murer, B. MolinaConcha, S. Benoit, and J.-P. Diard. Z Fit a
powerful tool for multiple impedance diagram
fitting. EIS2013-9th International Symposium
on Electrochemical Impedance Spectroscopy,
poster(Okinawa, Japan), 2013.
Belen Molina Concha, Ph. D.
Jean-Paul Diard, Pr. Hon.
Bio-Logic Science Instruments, 1 Rue de l’Europe, 38640 Claix, FRANCE
Tel: +33 476 98 68 31 - Fax: +33 476 98 69 09 www.bio-logic.info
4
Was this manual useful for you? yes no
Thank you for your participation!

* Your assessment is very important for improving the work of artificial intelligence, which forms the content of this project

Download PDF

advertisement