(503g) Electrochemical Characterization of Passivating Films in Lithium-Ion Batteries

Electrochemical
Characterization of Passivating Films in Lithium Ion Batteries

Maureen Tang^a,b

John Newman^a,b

^a Environmental
Energy Technologies Division, Lawrence Berkeley National Laboratory, Berkeley,
CA 94720

^b Department of
Chemical and Biomolecular Engineering, University of California, Berkeley, CA
94720-1462

Lithium-ion batteries are a promising option for
electric vehicles because of their high energy and power density. However, the
high cost and relatively short lifetimes of lithium-ion batteries have so far
prevented widespread automotive application. One reason for the limited
lifetime of a lithium-ion battery is the solid-electrolyte-interphase, or SEI.
The SEI is a passivating film that forms on the graphite anode during the first
few charge cycles. The electrolytes in lithium-ion batteries are
thermodynamically unstable at the potential of graphite. This means that a
battery without an SEI, or with a ?bad? SEI, continuously grows and consumes
electrolyte. The growth of the SEI means that less lithium is available for
energy storage.

Although the SEI has been studied
for many years, scientists still do not understand how it prevents electrolyte
reduction, or what parameters are necessary for the formation of a ?good' SEI.
Because it is sensitive to air, moisture, and impurities, the SEI is very
difficult to characterize. Because formation involves many competing chemical
reactions, the ability of traditional electrochemical techniques to describe
the film is also limited. In this work, we develop a new method to characterize
the SEI using ferrocene, a redox shuttle. By comparing ferrocene kinetics in
the presence and absence of passivating films, the shuttle functions as an
electrochemical probe of the mechanism by which the SEI prevents reaction. The
results of this method contribute to understanding an important failure
mechanism in lithium-ion batteries.

Approach

We
develop two electrochemical methods for characterizing the SEI. Both
experimental methods are coupled with a physics-based model that is used to
interpret experimental results. First, rotating disk electrode experiments are
used to measure the steady-state through-film ferrocenium reduction. The high
stability of both the neutral and cationic forms of ferrocene make it ideal for
surface studies. Second, electrochemical impedance spectroscopy (EIS) is used
to measure the frequency response of the ferrocene reaction in the presence and
absence of the SEI. After developing the method on glassy carbon, a model
surface, the method is extended to highly oriented pyrolytic graphite (HOPG),
which more accurately resembles the carbon found in an actual lithium-ion
battery.

Results
and Discussion

Steady-state
measurements and model fits are shown in Fig. 1. An inert glassy carbon electrode
is held at low voltage, thus reducing the supporting electrolyte and building
an SEI-type film on the electrode.  Varying the time at which the electrode is
held at low voltage controls the film thickness.  After the formation hold, the
electrode is moved to a solution of supporting electrolyte with dilute
ferrocene and ferrocenium hexafluorophosphate.  The markers show the current
measured at 900 rpm after films were built on the electrode for 30 seconds, 6,
30, and 60 minute holds at 0.6 V. Dashed lines are model fits to the passivated
current, and the dotted line is the reversible current, which is seen on the
clean electrode.  Current decreases with passivation time because the electrode
has had longer to grow a ?thicker? film.  The model includes only three adjustable
parameters: an anodic transfer coefficient α_a, an exchange
current density i₀, and a through-film ferrocene limiting current i_lim,
described by the expressions below. 

C^bulk
refers to the concentration in the bulk solution, and the subscripts O and R
represent oxidized ferrocenium and reduced ferrocene, respectively. k is a rate
constant, Do,f is the diffusivity of ferrocenium inside the SEI, F is Faraday's
constant, L is the film thickness, and ε is the film porosity. The shape
of the curve between 3.15 and 2.5 V is given by α and i₀. 
i_lim is determined from the limit as the curve approaches very low
voltages.  Both i₀ 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 films¹. 

Fig.
2 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 (Fig. 3) 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 time
constant of the system, τ = R_ctC_dl, where R_ct
is the charge-transfer resistance and C_dl is the double-layer
capacitance.
corresponding to a higher charge-transfer resistance or a slower reaction. 
These observations agree qualitatively with the steady-state findings. 

Figure
1: Steady-state
current vs. voltage after different lengths of passivation holds.  Markers are
measurements, dashed lines are model fits.  Both i₀ and i_lim
decrease.

Figure
2: Nyquist plot of electrode after different lengths of passivation holds. 
Longer passivation times cause higher impedance.

Fig.
3.  Bode plot of electrode after different lengths of passivation holds.  The
high-frequency peak depends on passivation time, but the low-frequency peak
does not.

Figure
4: Comparison of through-film ferrocene impedance on the edge and basal planes
of graphite.

Although
comparison of the experimental EIS data with a physics-based model shows that
EIS does not provide as unique a fit as the steady-state measurements, the
indicators of high-frequency 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^{2, 3};
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 Fig. 4. 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) were cycled from 3.7
to 0.1 V vs. Li/Li⁺ in a solution of 2 mM ferrocene in 1.0 M LiPF₆
in EC:DEC in order to form an SEI. The impedance spectra at open circuit were
measured both before and after cycling. Fig. 4 shows that, before SEI
formation, impedance spectra on both samples exhibit a straight line of
approximately 45^o slope without any high-frequency semicircles
(dashed lines). The two dashed lines collapse because the kinetics are fast on
both the edge and basal plane. After SEI formation, both samples show an
increased impedance, but the impedance on the edge plane is much higher than
that on the basal plane, despite higher electronic activity on the edge plane. These
preliminary results demonstrate the ability of the developed method to
characterize materials found in actual batteries.

References

1.
M. Tang, J. Newman, J. Electrochem. Soc., 158 (2011)
A530-824-832.

2.
D. Bar-Tow, E. Peled, L. Burstein, J. Electrochem. Soc., 146 (1999)
824-832.

3.
K. Hirasawa, T. Sato, H. Asahina, S. Yamaguchi, S. Mori, J. Electrochem. Soc., 144 (1997)
L81-L84.

Acknowledgements

This
work was supported by the National Science Foundation, the Department of
Energy, and the Japan Society for the Promotion of Science.