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

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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.
2796 Journal of The Electrochemical Society, 146 (8) 2794-2798 (1999) S0013-4651(98)02-016-5 CCC: $7.00 © The Electrochemical Society, Inc. Figure 3. (a) Impedance responses recorded at the graphite electrode in 1.0 M LiClO 4 in PC/EC at an x value of 0.33 in Li x C 6 at an open-circuit potential of 0.20 V; (b) an equivalent circuit describing the impedance responses shown in (a). The charge-transfer resistance, R CT , is related to the exchange current (i 0 ) by the equation 23a R CT � RT/(nFi 0 ) [3] The exchange current densities were calculated using Eq. 3 for various x values in Li x C 6 as listed in Table I. The result is shown in Fig. 4. The exchange current densities range between 1.4 and 2.4 mA/cm 2 and decrease monotonously with an increase in the x value of Li x C 6 with some scattered points. The dependence of the exchange current on the amount of Li � is readily expected because of the different equilibrium potentials at the interface. While we found no reported exchange current density data for lithium intercalation into the graphite electrode in the literature, there are reports 6,24 about the charge-transfer resistance at various carbon electrodes determined by ac impedance methods. These values were reported to range between 5 and 20 � depending on the carbon types, which are in good agreement with ours listed in Table I. A similar but more drastic change in the exchange current has been reported by Colson et al. 25 for sodium intercalation in sodium molybdates. This result suggests that the interfacial charge-transfer process is associated with the electron transfer rather than the Li � transfer. Shown in Fig. 5 are the impedance spectra recorded at fresh electrodes (without preintercalation) in 0.1, 0.2, 0.5, 0.8, and 1.0 M LiClO 4 in the PC/EC mixed solvent at an applied potential of 0.20 V with no crown ethers added. The equivalent circuit presented in Fig. 3b also applies to the data shown in Fig. 5. Values obtained for the various circuit elements at different LiClO 4 concentrations are listed in Table II. As expected, the solution resistance (R s ) estimated from the high-frequency intercept and the charge-transfer resistance (R 2 ) obtained from the larger semicircle for the Li intercalation decrease as the Li � concentration increases in solution. The exchange current also increases with an increase in the Li � concentration as shown in Fig. 6. From the dependence of i 0 on the Li � concentration, one can calculate the transfer coefficient (�) for the Li intercalation process represented by Eq. 1. The relationship between the exchange current, i 0 , and the concentrations is 23 i0 � nFk0C Li� (l��) � CLi While this equation is for solution species, it should be applicable to the interfacial electron transfer as the equilibrium potential at the elec- Table I. Values obtained for simulation of the elements in equivalent circuit shown in Fig. 6 at various x in Li x C 6 . a,b Open-circuit Q1 Q2 Q3 x in LixC6 potential, V R1 , � R2 , � Y, S n Y S n Y, S n x2 0.000 — 2.01 10.77 1.35 � 10 �4 0.858 6.82 � 10 �3 0.614 0.125 0.543 1.3 � 10 �3 0.166 0.20 2.91 11.20 6.70 � 10 �4 0.676 6.53 � 10 �3 0.613 0.141 0.555 1.7 � 10 �4 0.330 0.20 3.02 14.65 6.27 � 10 �4 0.702 6.51 � 10 �3 0.619 0.167 0.588 2.5 � 10 �4 0.429 0.077 3.44 14.35 2.40 � 10 �4 0.808 5.91 � 10 �3 0.652 0.266 0.514 1.9 � 10 �4 0.444 0.080 4.10 11.70 8.36 � 10 �4 0.649 5.32 � 10 �3 0.675 0.216 0.401 4.1 � 10 �4 0.576 0.070 4.42 12.99 7.89 � 10 �4 0.661 4.97 � 10 �3 0.679 0.253 0.490 4.3 � 10 �4 0.680 0.060 2.96 9.46 6.13 � 10 �4 0.704 5.63 � 10 �3 0.624 0.203 0.453 2.0 � 10 �4 0.740 0.072 4.46 15.83 3.75 � 10 �3 0.547 5.10 � 10 �3 0.762 0.219 0.394 2.7 � 10 �4 1.000 0.061 4.37 18.68 4.37 � 10 �4 0.928 6.14 � 10 �3 0.717 0.188 0.481 2.6 � 10 �4 a Rs values are constant to 13.50 ∀ 1.0 �. b Impedance measurement under dc potential stepped to �0.2 V. Figure 4. The exchange current plotted vs. x in Li x C 6 . [4]
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