V. Focused Fundamental Research - EERE - U.S. Department of ...

Newman – LBNL
V.E.5 Analysis and Simulation of Electrochemical Energy Systems (LBNL)
Current, mA/cm 2
0.00
-0.02
-0.04
-0.06
-0.08
-0.10
-0.12
30 min
60 min
6 min
30 sec
-0.14
Reversible
2.0 2.5 3.0
Imaginary impedance, kOhm-cm 2
4
3
2
1
30 min
60 min
30 sec
6 min
0
0 2 4 6 8
Real impedance, kOhm-cm 2
Voltage vs. Li/Li +
Figure V - 202: Nyquist plot of electrode after different lengths of passivation
holds. Longer passivation times cause higher impedance.
Figure V - 201: Steady-state current vs. voltage after different lengths of
passivation holds. Markers are measurements, dashed lines are model fits. 10
Both the exchange current density i0 and the through-film limiting current ilim
decrease. Data is measured at 900 rpm with 1.1 mM ferrocene/ferrocenium
60 min
30 min
hexafluorophosphate.
1
The shape of the curve between 3.15 and 2.5 V is
given by α and i 0 . i lim is determined from the limit as the
curve approaches very low voltages. Both i 0 and i lim
decrease with increased passivation time, and because both
expressions contain the porosity ε, a possible explanation
may be that longer formation times cause thicker but also
less porous films.
2. Impedance characterization. Figure V - 202 shows
impedance measurements of the same films as in Fig. 1 at
open circuit potential and 900 rpm. Each spectrum
exhibits two arcs. The high-frequency arc depends on
passivation time, but the low-frequency arc does not. The
high-frequency arc width increases with more passivation
time. Plotting the imaginary component vs. frequency
(Figure V - 203) shows that, similarly, the low-frequency peak
is independent of passivation time, but the high-frequency
peak decreases with passivation time. The peak frequency
corresponds to the reciprocal of the time constant of the
system, τ = R ct C dl , where R ct is the charge-transfer
resistance and C dl is the double-layer capacitance. As
formation time increases, the time constant increases,
corresponding to a higher charge-transfer resistance or a
slower reaction. These observations agree qualitatively
with the steady-state findings.
Imaginary impedance, kOhm-cm 2
0.1
0.01
0.001
6 min
30 sec
0.0001
10 -1 10 1 10 3 10 5
Frequency, rad/s
Figure V - 203: Bode plot of electrode after different lengths of passivation
holds. The high-frequency peak depends on passivation time, but the lowfrequency
peak does not.
3. Effect of HOPG orientation. Although
comparison of the experimental EIS data with a physicsbased
model shows that EIS does not provide as unique a
fit as the steady-state measurements, the indicators of highfrequency
arc width and time constant agree qualitatively
with steady-state results. Impedance also has significant
experimental advantages over the rotating disk electrode; it
is faster, uses less material, and is less subject to variations
in temperature and bulk concentration. Most importantly, it
permits the use of more materials, including those actually
found in lithium-ion batteries. Previous work has found
that the SEI formation reactions may differ substantially
on the edge and basal planes of graphite; accordingly, the
current task is to use the method developed in this work to
study how passivation differs with graphite orientation. A
preliminary result from this study is shown in Figure V - 204.
Two samples of HOPG, one with an edge fraction of 0.06
(primarily the basal surface exposed) and the other with an
edge fraction of 0.6 (primarily the edge fraction exposed)
FY 2011 Annual Progress Report 645 Energy Storage R&D