Intercalation of Lithium Ions into Graphite Electrodes Studied by AC ...

of conducting analysis of heavily convoluted frequency dispersion
data by deconvoluting the complex responses into those of simple
subcomponents. This approach combined with the general nonlinear
least-squares fitting procedure allowed us to construct equivalent circuits
whose simulated responses describe actually measured data
well. From this simulation, values of various circuit components
were obtained.
In the present work, a constant phase element (CPE or Q) is used
for equivalent circuits except for resistors, R. The general expression
for the admittance response of the CPE is 19
YCPE � Yc�n cos(n�/2) � jYC�n sin(n�/2) [2]
where � is the angular frequency, which is 2�f with f being frequency
and j � (�1) 1/2 . Depending on the n value, the CPE can have
a variety of responses. If n � 0, it represents a resistance with R �
Y �1
c ; if n � 1, a capacitance with C � YC, and if n � 0.5, a Warburg
response.
Results and Discussion
Chronopotentiometric responses.—A major problem encountered
during the electrochemical lithium intercalation reaction in the PCbased
electrolytes is the excessive electrolyte decomposition reaction
during the first lithiation process. Much effort has been expended to
overcome these problems. 7-10,20 Fong et al. 10 reported that introducing
a cosolvent, EC, into the PC-based electrolyte improves the reversibility
of Li/graphite cells. Other workers8,9,20 suggested that
crown ethers reduce the degree of PC decomposition reactions when
used as an additive. We examined effects of the electrolyte composition
on the PC decomposition by recording chronopotentiograms in
four different electrolyte solutions (see Fig. 1) under otherwise-identical
experimental conditions. In Fig. 1, the period during which the
potential plateau is maintained at around �0.8 V corresponding to the
PC decomposition reaction21 changes with the electrolyte composition.
In 0.1 M LiClO4-PC/EC with 0.1 M 12-crown-4 added, the period
for the plateau is the shortest. A serious capacity loss is observed
in the 0.1 M LiClO4-PC solution, as can be seen from the long potential
plateau corresponding to the PC decomposition reaction. Similar
results were reported by Shu and co-workers. 9 Fong et al. 10 concluded
that the PC decomposition reaction at the graphite electrode is
associated with Li� solvated with PC molecules which become cointercalated
into the graphite layers. The addition of EC or crown ethers
to the electrolyte may change the solvation structure and appears to
suppress the cointercalation process, resulting in the reduction of the
PC decomposition reaction.
According to Fong et al., 10 the PC decomposition reaction results
in the formation of passive films on the graphite electrode surface. Reversible
Li intercalation still takes place on the film-covered graphite
Figure 1. Chronopotentiometric results obtained at graphite electrodes with an
applied current of 0.2 mA in (a) 0.1 M LiClO 4 in PC, (b) 0.1 M LiClO 4 in
PC/EC (50:50), (c) 0.1 M LiClO 4 in PC/EC (50:50) with 0.1 M 18-crown-6
added, and (d) 0.1 M LiClO 4 in PC/EC with 0.1 M 12-crown-4 added.
Journal of The Electrochemical Society, 146 (8) 2794-2798 (1999) 2795
S0013-4651(98)02-016-5 CCC: $7.00 © The Electrochemical Society, Inc.
Figure 2. Chronopotentiometric results obtained at the graphite electrode at an
applied current of 0.2 mA in 0.1 M LiClO 4 in PC/EC during (a) first and (b)
second cycles.
surface even after the surface is passivated. Figure 2 shows the
chronopotentiograms recorded at the graphite electrode for the first
two consecutive runs. It is clearly seen in this figure that the length of
the plateau at about �0.8 V is significantly shorter during the second
than the first run. This means that the PC decomposition reaction
mainly takes place during the first intercalation cycle. After the first
cycle, the dominant process is the lithium intercalation reaction. For
this reason, we used a 1.0 M LiClO4 solution in PC/EC (50:50) as an
electrolyte in order to minimize the effect of solvent decomposition
reactions on the electrochemical measurements and ran each electrochemical
experiment after the first cycle, during which the PC decomposition
is a predominant reaction. For kinetic measurements, crown
ethers were not added to the electrolyte solutions.
AC impedance studies.—Shown in Fig. 3 are (a) a typical electrochemical
impedance spectrum at a preintercalated graphite electrode
with x � 0.330 at an open-circuit potential and (b) an equivalent
circuit obtained by fitting the impedance responses. The GIC,
LixC6 , with various x values was prepared by passing a given amount
of cathodic charge. The x value is then calculated from the increase
in mass of the electrode from the Faraday law using the amount of
current applied and the duration of the current flow. 18 The irreversible
capacity loss was not taken into account in the calculation
of x values, assuming that the loss is not significant compared to the
total amount of Li� intercalated.
The impedance responses shown in Fig. 3a consist of a depressed
semicircle in the high-frequency range (50 kHz-0.35 Hz) and a linear
portion with a slope close to unity in the low-frequency range
(0.41-0.005 Hz). The features shown here are in good agreement
with those reported in the literature6,22 under similar experimental
conditions. The depressed semicircle is shown to consist of two arcs
from the curve-fitting procedure. The small arc in the high-frequency
range (50 kHz-150 Hz) is attributed to the formation of a passive
film on the graphite surface. 22 The large semicircle in the mediumfrequency
range (0.5-145 Hz) is ascribed to the charge-transfer reaction
of Li intercalation into graphite. 6,22 The linear portion observed
in the low-frequency region (0.41-0.005 Hz) is characteristic of a
diffusion-limited process, which is discussed in more detail later.
The equivalent circuit presented in Fig. 3b describes the impedance
spectra shown in Fig. 3a; solid lines are calculated responses using the
circuit shown in Fig. 3b. Values obtained from the simulation for various
circuit elements shown in Fig. 3b at various x values in LixC6 are
listed in Table I. The equivalent circuit consists of two parallel RC circuits
in series, one for the passive film formation and the other for
lithium intercalation, respectively, as pointed out previously. Three
CPEs, Q1 � Q3 , are included in the equivalent circuit. From Table I
we see that Q3 is basically the Warburg impedance with n � 0.5. The
charge-transfer resistance (R2) associated with Li intercalation varies
depending on the composition of the graphite electrode.