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Current oscillations in anodic electrodissolution of copper in lithium-ion battery electrolyte
Electrochimica Acta 50 (2005) 2423–2429
Current oscillations in anodic electrodissolution of copper in lithium-ion battery electrolyte Qingzhou Cui, Howard D. Dewald∗ Department of Chemistry and Biochemistry, Clippinger Laboratories, Ohio University, Athens, OH 45701, USA Received 9 July 2004; received in revised form 28 September 2004; accepted 14 October 2004 Available online 23 November 2004
Abstract Current oscillations were observed during electrodissolution of copper in nonaqueous lithium-ion battery electrolyte under potentiostatic conditions using copper foil electrodes. Mixed-mode oscillations were observed over certain ranges of stir rate and applied potential. A nonlinear dynamics technique (return map) was applied to characterize the oscillations. The dynamic stir conditions of the electrolyte influenced the frequency and pattern of the oscillations. The amplitude of the oscillations increased with increasing potential. Also, cyclic voltammetry (CV) showed that the oscillatory current was correlated to the oxidation of the copper electrode. © 2004 Elsevier Ltd. All rights reserved. Keywords: Oscillations; Lithium-ion battery; Current collector; Copper; Nonaqueous electrolyte solution
1. Introduction Nonlinear behavior, such as periodic or chaotic oscillations of current or potential, has been reported in many electrochemical systems. Especially, current oscillations have been recognized for a long time in electrodissolution of metals in various electrolyte solutions, with Fe, Cu, Ni, Co having been extensively reported [1]. Oscillations during copper oxidation have been reported in various electrolytes. Hudson and Bassett studied current oscillations during copper electrodissolution in acidic chloride solution [1]. Tsitsopoulos et al. reported oscillations during copper electropolishing in phosphoric acid [2]. Dewald et al. studied current oscillations on a rotating copper disk electrode in acetic acid/acetate buffer [3]. Oscillations in the anodic dissolution of a copper disk were also reported in an aqueous NaCl electrolyte containing SCN− [4]. Copper electrodissolution in trichloroacetic acid solution was recently reported [5]. Most of these reported oscillations were observed to take place in acidic aqueous solutions, and thus related to the hydrogen ion in the pro∗
Corresponding author. Tel.: +1 740 593 2979; fax: +1 740 593 0148. E-mail address: [email protected] (H.D. Dewald).
0013-4686/$ – see front matter © 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2004.10.064
posed mechanisms [1–4]. Current oscillations in nonaqueous solution have not been reported. Copper foil is used as the anodic current collector in most lithium-ion batteries. Copper stability is an important issue for the lithium-ion battery industry, and has been the subject of recent investigations [6,7]. In this study, current oscillations were observed and characterized in a common lithium-ion battery electrolyte composed of 1 M LiPF6 in a ternary mixture of propylene carbonate (PC)–ethylene carbonate (EC)–dimethyl carbonate (DMC) [1:1:3 (v/v/v)]. A nonlinear dynamics analysis method (return map) along with cyclic voltammetry was used in characterizing the oscillations.
2. Experimental 2.1. Materials The copper electrodissolution was carried out in a homemade three-electrode cell [6]. The cell was manufactured from an end sealed 25 mm Ace-thredTM (Ace Glass Inc.) glass connector and an O-ring sealed threaded TeflonTM plug
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that had been machined with electrode ports. The working electrodes (WE) were battery-grade copper foil that were prepared as ‘flags’ with a working area of 1 cm × 1 cm and connected to a 22-gauge nickel wire. The 12 m thick copper foil (grade LP1/LP3) was used as received from Saft, which had obtained it from Fukuda Metal Foil and Powder Co. The electrodeposited foil (one matte side and one shiny side) had a purity of 99.9% with the major trace element being Cr at ≤130 ppm. The flag of a WE was rinsed with acetone and air-dried before use. The reference electrodes (RE) were prepared by rolling and pressing an approximately 1 cm × 1 cm lithium foil (Cyprus Foote Mineral Company) onto the tip of a nickel wire and assembled in a dry box in electrolyte solution in a glass tube containing a 6 mm diameter porous VycorTM tip (Bioanalytical Systems, BAS, MF-2042). The auxiliary electrode (AE) was a 0.5 cm diameter, 23 cm long platinum wire coil (BAS, MW-1033). The electrolyte was 1 M LiPF6 in a ternary mixture of propylene carbonate (PC)–ethylene carbonate (EC)– dimethyl carbonate (DMC) [1:1:3 (v/v/v)]. The electrolyte was obtained from EM Industries/Merck K.G.a.A and was prepared from 99.98% purity solvents (<20 ppm H2 O, as determined by a Karl Fischer titration) and Stella LiPF6 . The electrolyte was guaranteed at <80 ppm HF and was analyzed at Saft as <50 ppm using an acid–base titration. Electrolyte was frozen before degassing for 30 min and then thawed. The procedure was repeated three times. 2.2. Cell preparation and apparatus Cell assembly was performed in a dry box. The moisture and oxygen contents of the dry box were both less than 1 ppm. A volume of 10.0 ml electrolyte was used in the electrochemical cell. The O-ring sealed threaded TeflonTM plug was machined to fit the electrodes, and the cell was sealed further with Parafilm. The working electrode and the auxiliary electrode were placed vertically parallel to each other, and the distance between them was around 0.5 cm. The cell was mounted on a C3 cell stand (BAS). Stir rates between 0 and 800 rpm could be obtained with a magnetic stir bar. The
diameter of the cylindrical stir bar was 0.2 cm, and its length was 0.8 cm. Experiments were performed with the CHI-604A electrochemical system (CH Instrumental Inc.), which was interfaced to a computer. A potentiostatic signal was applied under static and dynamic electrolyte solution conditions. Current–time (I–t) measurements were acquired and stored via the computer for analysis. The sampling interval for measurement was 0.01 s. Cyclic voltammetric (CV) measurements were also performed with the CHI-604A system. During CV the potential was scanned positively from 2.95 to 3.80 V, then scanned negatively back to 2.95 V. A second cycle was run. The scan rate was 20 mV s−1 . The open circuit voltage (OCV) of the cell was 3.20 ± 0.10 V.
3. Results and discussion The copper foils (12 m thick and 1 cm × 1 cm) used in each experiment had a mass of only 15.8 ± 0.2 mg. Since the potential applied in the system was higher than the OCV of the copper electrode, the electrode was gradually consumed by oxidation. At high applied potentials the copper electrode was quickly oxidized (dissolved) and limited the duration of an experiment. For example, at a potential of 3.90 V an electrode typically lasted only 400 s. The longest experiment, which lasted until an electrode was totally oxidized, showed that current oscillations were present whenever potential was applied, and the same pattern was sustained throughout the measurement. Thus short duration experiments lasting 15 s were conducted to study the oscillatory behavior and minimizing electrodissolution of the electrode. Fig. 1 shows the current–time response for potentiostatic excitation signals under static and dynamic conditions at a potential of 3.80 V. Under static conditions, no current oscillations were observed (Fig. 1a). When a stir rate of 250 rpm was applied, a sustained, periodic current oscillatory behavior was observed (Fig. 1b). The current decreased with time as a result of the changing surface morphology of the copper electrode.
Fig. 1. Current–time responses for anodic dissolution of copper foil electrodes in 1 M LiPF6 /PC:EC:DMC [1:1:3 (v/v/v)] electrolyte under (a) static and (b) dynamic [250 rpm] conditions. Applied potential = 3.80 V vs. Li/Li+ .
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Fig. 2. Current oscillations at different stir rate of (a) 0 rpm, (b) 50 rpm, (c) 150 rpm, (d) 250 rpm, (e) 350 rpm, (f) 450 rpm, (g) 550 rpm, and (h) 650 rpm. Applied potential = 3.80 V vs. Li/Li+ .
3.1. Stir rate effect The solution stir rate determined the pattern of oscillations, and also determined their frequency and amplitude. Fig. 2 shows a time series of current response over a range of stir rates. The highest stir rate that can be obtained with the instrument is 800 rpm. At stir rates higher than 650 rpm, the copper foil kept fluttering, which induced an irregular current response that was no longer stable. Only oscillations at stir rates lower than 650 rpm were studied. Oscillatory patterns can be clearly observed for several stir rates as shown in Fig. 2, in which all the oscillations were obtained at a potential of 3.80 V on the same electrode. The potential of 3.80 V was selected for study because it was not too high that the copper would be quickly dissolved. From Fig. 2a, no current oscillations were observed under static conditions, as previously shown in Fig. 1a. Periodic oscillations were
present when a stirring was applied to the solution. At a stir rate of 50 rpm, a periodic, single peak oscillation was present (Fig. 2b). At 150 rpm, a minor peak was developed to the left of the major peak (Fig. 2c), and increased at a stir rate of 250 rpm (Fig. 2d). A single major peak was observed again at 350 rpm (Fig. 2e). As the stir rate was further increased to 450 rpm, a minor peak was observed to the right of the major peak (Fig. 2f), and it kept increasing with the stir rate increasing to 550 rpm (Fig. 2g) and 650 rpm (Fig. 2h). To compare the oscillatory patterns of oscillations at different stir rates, time series data (I–t) were used to generate return maps. Regular return maps are frequently constructed by plotting successive (N + 1)th current minima against the Nth current minima. Return maps are mostly used to indicate chaos from oscillations [1,8]. Return maps not only indicate chaotic behavior, but also are useful in characterizing the pattern of oscillations. However, the decreasing current
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Fig. 3. Return maps for characterizing the pattern of current oscillations under stir rates of (a) 50 rpm, (b) 150 rpm, (c) 250 rpm, (d) 350 rpm, (e) 450 rpm, and (f) 550 rpm. NB: Different x- and y-axes scale in (d).
over time in this study made it hard to use return maps to identify oscillatory patterns. In this study, instead of measuring current minima, oscillatory peak amplitude (I) was measured. The amplitude was defined as the difference between the previous peak maximum and the current minimum, and those peak amplitudes were used to generate the return maps. The return maps at stir rate of 50, 150, 250, 350, 450, and 550 rpm are shown in Fig. 3. The return map for a stir rate of 650 rpm was similar to 550 rpm. From these return maps, the changing pattern under different stir rates became apparent. The distance from the center of the cluster to the origin (0, 0) provides the oscillatory amplitude, and the distribution of points in the cluster could indicate nonlinear behavior. Table 1 summarizes the oscillatory frequencies and amplitudes as a function of solution stir rate. The major peaks
in each time series were used to take an average for obtaining oscillatory amplitude. Arbitrarily, ten peaks in the middle of the 15 s experiment were selected for calculating the average amplitude. All the experiments were repeated with new electrodes and fresh electrolyte, and the trend of pattern changes had high reproducibility except that the ratio of minor peak versus major changed a small amount. The data in Table 1 show that the oscillatory amplitude increased from 44 to 227 A in the range of 50–350 rpm, and then decreased from 227 to 48 A when the stir rate was increased from 350 to 650 rpm. The trend was visibly observed in the return maps shown in Fig. 3. Oscillatory frequencies were obtained by arbitrarily measuring the number of peaks per second in a 5 s window in the middle of an experiment. From the results, the correlation of the frequency to the stir rate was clearly observed. At a stir rate of 50 rpm, the ratio (stir rate/peak
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Table 1 Current oscillation of copper foil electrode at different stir rates in 1 M LiPF6 /PC:EC:DMC [1:1:3 (v/v/v)] electrolyte Stir rate (rpm)
Peak amplitude (A)
Stir rate (Hz)
Peak frequency (Hz)
Ratio (stir rate/peak frequency)
0 50 150 250 350 450 550 650
No oscillation observed 44 138 159 227 86 78 48
0.8 2.5 4.2 5.8 7.5 9.2 10.8
1.6 2.5 4.2 5.8 7.5 9.2 10.8
1:2 1:1 1:1 1:1 1:1 1:1 1:1
Applied potential = 3.80 V vs. Li/Li+ .
frequency) is 1:2, which ratio is present only when no minor peaks were present. At the other stir rates when minor peaks were present, only the major peaks were counted for obtaining peak frequencies. The ratio of stir rate to peak frequency changed to 1:1. If the minor peaks were also counted in the peak frequency calculation, the ratio was still 1:2.
3.2. Potential effect The dependence of the oscillatory amplitude on potential was examined over the range of 3.50 to 3.90 V at stir rates of 250 and 350 rpm. Fig. 4 shows the oscillations observed at a stir rate of 250 rpm. A minor peak and a major peak were
Fig. 4. Current oscillations at different applied potential of (a) 3.55 V, (b) 3.60 V, (c) 3.65 V, (d) 3.70 V, (e) 3.75 V, (f) 3.80 V, (g) 3.85 V, and (h) 3.90 V vs. Li/Li+ . Stir rate = 250 rpm.
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Table 2 The characterization of oscillations at different potentials in 1 M LiPF6 /PC:EC:DMC [1:1:3 (v/v/v)] electrolyte Potential (V)
3.50 3.55 3.60 3.65 3.70 3.75 3.80 3.85 3.90
Stir rate = 250 rpm
Stir rate = 350 rpm
Amplitude (A)
Frequency (Hz)
Amplitude (A)
19 59 87 107 147 163 208 229
No oscillation observed 4.2 38 4.2 96 4.2 129 4.2 153 4.2 198 4.2 220 4.2 245 4.2 307
Frequency (Hz)
current was observed above a potential of 3.50 V corresponding to oxidation of Cu0 to Cu2+ . When the potential scan was reversed, two reduction peaks were obtained. The two peaks were assigned to the reduction of Cu2+ to Cu+ and Cu+ to Cu0 , respectively [9]. 3.3. Discussion
5.8 5.8 5.8 5.8 5.8 5.8 5.8 5.8
present in each experiment, and the ratios between them had greater variation with increasing potential than with varying stir rates at constant potential. The oscillatory amplitudes and frequencies are summarized in Table 2. As before, 10 peaks in the middle of the experiment were used for calculating the average amplitude; only the major peak was considered for calculating amplitude when a minor peak was also present. At potentials higher than 3.90 V, the copper foil was quickly consumed. At potentials lower than 3.55 V, no apparent oscillations were observed. Based on these experiments, the following correlations can be made: (1) the amplitude of the oscillations increased with an increase in potential; (2) the potential change did not affect the oscillatory frequencies. Next, cyclic voltammetry (CV) was performed. Fig. 5 shows the CV of a Cu foil electrode in 1 M LiPF6 in a ternary mixture of propylene carbonate (PC)–ethylene carbonate (EC)–dimethyl carbonate (DMC) [1:1:3 (v/v/v)]. The experiment was highly reproducible in that the two consecutive runs were nearly indistinguishable. From Fig. 5, it can be seen that in the initial positive scan, a sharp rise in oxidation
Fig. 5. Cyclic voltammograms (two consecutive scans) of a copper foil electrode in 1 M LiPF6 /PC:EC:DMC [1:1:3 (v/v/v).]. E− = 2.95 V; E+ = 3.80 V. OCV = 3.30 V. Scan rate = 20 mV s−1 .
The origin of oscillatory behavior in copper, as well as most electrochemical oscillations is not well understood. In aqueous solution, most studies have suggested that oscillatory behavior arises from an intricate process involving surface reactions, film formation/deformation, and mass transfer. Especially, the film formation/deformation process has been suggested by most researchers as causing current oscillations. Dewald et al. studied the current oscillations on a rotating copper disk electrode in acetate buffer [3], and described that oscillations were associated with the formation of a porous salt film. The authors suggested that the complex reactions on the electrode surface were similar to the Oregonator model, which is an autocatalytic mechanism of oscillating reactions. In their model, the high current state represents low surface oxide coverage, while the low current state represents high surface oxide coverage. The transition of the electrode surface coverage from high to low is commensurate with current oscillations. Oscillations in the anodic dissolution of a copper disk were also reported in an aqueous NaCl electrolyte containing SCN− [10]. The authors discussed how a black oxide film attached to the electrode surface plays an important role in the current oscillations. The autocatalytic Brusselator model was suggested for the surface film formation in the study. In aqueous electrolytes, most proposed models about the origin of oscillations show reaction mechanisms in which H+ and Cu+ have some role in film formation/deformation [1–4]. Most researchers reported that anions, such as OH− , Cl− , PO4 3− , SCN− , reacted with Cu+ or Cu2+ , with precipitate formation on the electrode surface. Precipitate dissolution was affected by the concentration of H+ . This precipitation/dissolution process induced current oscillations. In nonaqueous organic electrolyte, if H+ was present it could possibly influence precipitation with electrolyte anions on the electrode surface. The autocatalytic models, such as Oregonator and Brusselator, may be still valid for explaining the origin of oscillations. In aged electrolyte, studies have shown that the decomposition of LiPF6 in organic carbonates can produce PF5 , which in turn can form POF3 and HF. This process could ultimately lead to the formation of phosphoric acid [11,12]. The acid produced by such a process could contribute to current oscillations. However, with the fresh electrolyte used in a lithium-ion battery, the concentration of H+ is presumed zero. Thus the mechanism concerning film formation/deformation may not be applicable for explaining the origin of oscillations. This was confirmed by the apparent absence of surface film formation or deformation on the copper electrode.
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In this study, dynamic situations were obtained by rotating a magnetic stirrer. One question is whether the magnetic field affects oscillatory behavior. To answer the question, the characteristics of the oscillations are considered. A characteristic of the oscillations is that they did not require any onset time. They started as soon as an appropriate potential was applied, and were sustained throughout the experiment. This is different from film mechanisms, in which oscillations begin after film formation or dissolution. Another characteristic is that the oscillation did not show up in static solutions, in which no magnetic effect was applied. These observations imply that the origin of the oscillations might be related to the magnetic field. It has been reported that oscillatory behavior can be strongly influenced by imposed magnetic fields through interactions with the electrolyte flow structure [4,13,14]. The existence of the magnetic field enhances the mass transport rate by magnetohydrodynamic (MHD) flow, which changes the current oscillatory behavior. Fahidy and co-workers have studied the MHD effect on electrolysis, and sufficiently elucidated the relationship between the current amplitude and magnetic field [15–17]. However, it should be noted that all these studies were conducted in a magnetic field with fixed directions. A rotating magnetic field, which changed its direction all the time, was not applied to study current oscillations. In this regard, an interpretation of oscillations (and a possible mechanism) awaits more diverse and elaborate experiments. Further studies in the authors’ laboratory will continue to address the effects of a rotating magnetic field on mass transport, as well as, use of a rotating electrode. Also, a nonmagnetic stirrer, which will delete the effect of the magnetic field, will be applied to the oscillations. Moreover, the oscillations will also be characterized under different electrolyte compositions and in presence of several impurities such as H2 O and HF. Temperature effects will also be studied. It is hoped that these studies of current oscillations
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in nonaqueous solution can provide some clues about the origin of the oscillations. Acknowledgments This work was supported in part by the Saft Research and Development Center, Cockeysville, MD. Frederick R. Lemke is thanked for the use of the dry box. Mingming Xu is thanked for his assistance and helpful discussions. References [1] J.L. Hudson, M.R. Bassett, Rev. Chem. Eng. 7 (1991) 109. [2] L.T. Tsitsopoulos, I.A. Webster, T.T. Tsotsis, Surf. Sci. 220 (1989) 391. [3] H.D. Dewald, P. Parmananda, R.W. Rollins, J. Electrochem. Soc. 140 (1993) 1969. [4] Z.H. Gu, A. Olivier, T.Z. Fahidy, Electrochim. Acta 35 (1990) 933. [5] L.A. Li, S.H. Chen, H.T. Wu, H.T. Cui, J. Serb. Chem. Soc. 69 (2004) 33. [6] M. Zhao, S. Kariuki, H.D. Dewald, F.R. Lemke, R.J. Staniewicz, E.J. Plichta, R.A. Marsh, J. Electrochem. Soc. 147 (2000) 2874. [7] M. Zhao, Ph.D. Dissertation, Ohio University, Athens OH, 2001. [8] S. Kariuki, H.D. Dewald, J. Thomas, R.W. Rollins, J. Electroanal. Chem. 486 (2000) 175. [9] U. Bertocci, D.R. Turner, in: A.J. Bard (Ed.), Encyclopedia of Electrochemistry of the Elements, Marcel Dekker, New York, 1974, p. 383 (Chapter II-6). [10] Z.H. Gu, S.J. Xia, T.Z. Fahidy, Electrochim. Acta 41 (1996) 2837. [11] S.E. Sloop, J.K. Pugh, S. Wang, J.B. Kerr, K. Kinoshita, Electrochem. Solid State Lett. 4 (2001) A42. [12] M.J. O’Neil (Ed.), The Merck Index: An Encyclopedia of Chemicals, Drugs, and Biologicals, 13th ed., Merck, Whitehouse Station, NJ, 2001, p. 1318. [13] Z.H. Gu, T.Z. Fahidy, J.P. Chopart, Electrochim. Acta 37 (1992) 97. [14] C. Wang, S. Chen, Electrochim. Acta 43 (1998) 2225. [15] K. Kim, T.Z. Fahidy, J. Electrochem. Soc. 142 (1995) 4196. [16] T.Z. Fahidy, Chem. Eng. J. 72 (1999) 79. [17] T.Z. Fahidy, J. Appl. Electrochem. 32 (2002) 551.
Current oscillations in anodic electrodissolution of copper in lithium-ion battery electrolyte Qingzhou Cui, Howard D. Dewald∗ Department of Chemistry and Biochemistry, Clippinger Laboratories, Ohio University, Athens, OH 45701, USA Received 9 July 2004; received in revised form 28 September 2004; accepted 14 October 2004 Available online 23 November 2004
Abstract Current oscillations were observed during electrodissolution of copper in nonaqueous lithium-ion battery electrolyte under potentiostatic conditions using copper foil electrodes. Mixed-mode oscillations were observed over certain ranges of stir rate and applied potential. A nonlinear dynamics technique (return map) was applied to characterize the oscillations. The dynamic stir conditions of the electrolyte influenced the frequency and pattern of the oscillations. The amplitude of the oscillations increased with increasing potential. Also, cyclic voltammetry (CV) showed that the oscillatory current was correlated to the oxidation of the copper electrode. © 2004 Elsevier Ltd. All rights reserved. Keywords: Oscillations; Lithium-ion battery; Current collector; Copper; Nonaqueous electrolyte solution
1. Introduction Nonlinear behavior, such as periodic or chaotic oscillations of current or potential, has been reported in many electrochemical systems. Especially, current oscillations have been recognized for a long time in electrodissolution of metals in various electrolyte solutions, with Fe, Cu, Ni, Co having been extensively reported [1]. Oscillations during copper oxidation have been reported in various electrolytes. Hudson and Bassett studied current oscillations during copper electrodissolution in acidic chloride solution [1]. Tsitsopoulos et al. reported oscillations during copper electropolishing in phosphoric acid [2]. Dewald et al. studied current oscillations on a rotating copper disk electrode in acetic acid/acetate buffer [3]. Oscillations in the anodic dissolution of a copper disk were also reported in an aqueous NaCl electrolyte containing SCN− [4]. Copper electrodissolution in trichloroacetic acid solution was recently reported [5]. Most of these reported oscillations were observed to take place in acidic aqueous solutions, and thus related to the hydrogen ion in the pro∗
Corresponding author. Tel.: +1 740 593 2979; fax: +1 740 593 0148. E-mail address: [email protected] (H.D. Dewald).
0013-4686/$ – see front matter © 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2004.10.064
posed mechanisms [1–4]. Current oscillations in nonaqueous solution have not been reported. Copper foil is used as the anodic current collector in most lithium-ion batteries. Copper stability is an important issue for the lithium-ion battery industry, and has been the subject of recent investigations [6,7]. In this study, current oscillations were observed and characterized in a common lithium-ion battery electrolyte composed of 1 M LiPF6 in a ternary mixture of propylene carbonate (PC)–ethylene carbonate (EC)–dimethyl carbonate (DMC) [1:1:3 (v/v/v)]. A nonlinear dynamics analysis method (return map) along with cyclic voltammetry was used in characterizing the oscillations.
2. Experimental 2.1. Materials The copper electrodissolution was carried out in a homemade three-electrode cell [6]. The cell was manufactured from an end sealed 25 mm Ace-thredTM (Ace Glass Inc.) glass connector and an O-ring sealed threaded TeflonTM plug
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that had been machined with electrode ports. The working electrodes (WE) were battery-grade copper foil that were prepared as ‘flags’ with a working area of 1 cm × 1 cm and connected to a 22-gauge nickel wire. The 12 m thick copper foil (grade LP1/LP3) was used as received from Saft, which had obtained it from Fukuda Metal Foil and Powder Co. The electrodeposited foil (one matte side and one shiny side) had a purity of 99.9% with the major trace element being Cr at ≤130 ppm. The flag of a WE was rinsed with acetone and air-dried before use. The reference electrodes (RE) were prepared by rolling and pressing an approximately 1 cm × 1 cm lithium foil (Cyprus Foote Mineral Company) onto the tip of a nickel wire and assembled in a dry box in electrolyte solution in a glass tube containing a 6 mm diameter porous VycorTM tip (Bioanalytical Systems, BAS, MF-2042). The auxiliary electrode (AE) was a 0.5 cm diameter, 23 cm long platinum wire coil (BAS, MW-1033). The electrolyte was 1 M LiPF6 in a ternary mixture of propylene carbonate (PC)–ethylene carbonate (EC)– dimethyl carbonate (DMC) [1:1:3 (v/v/v)]. The electrolyte was obtained from EM Industries/Merck K.G.a.A and was prepared from 99.98% purity solvents (<20 ppm H2 O, as determined by a Karl Fischer titration) and Stella LiPF6 . The electrolyte was guaranteed at <80 ppm HF and was analyzed at Saft as <50 ppm using an acid–base titration. Electrolyte was frozen before degassing for 30 min and then thawed. The procedure was repeated three times. 2.2. Cell preparation and apparatus Cell assembly was performed in a dry box. The moisture and oxygen contents of the dry box were both less than 1 ppm. A volume of 10.0 ml electrolyte was used in the electrochemical cell. The O-ring sealed threaded TeflonTM plug was machined to fit the electrodes, and the cell was sealed further with Parafilm. The working electrode and the auxiliary electrode were placed vertically parallel to each other, and the distance between them was around 0.5 cm. The cell was mounted on a C3 cell stand (BAS). Stir rates between 0 and 800 rpm could be obtained with a magnetic stir bar. The
diameter of the cylindrical stir bar was 0.2 cm, and its length was 0.8 cm. Experiments were performed with the CHI-604A electrochemical system (CH Instrumental Inc.), which was interfaced to a computer. A potentiostatic signal was applied under static and dynamic electrolyte solution conditions. Current–time (I–t) measurements were acquired and stored via the computer for analysis. The sampling interval for measurement was 0.01 s. Cyclic voltammetric (CV) measurements were also performed with the CHI-604A system. During CV the potential was scanned positively from 2.95 to 3.80 V, then scanned negatively back to 2.95 V. A second cycle was run. The scan rate was 20 mV s−1 . The open circuit voltage (OCV) of the cell was 3.20 ± 0.10 V.
3. Results and discussion The copper foils (12 m thick and 1 cm × 1 cm) used in each experiment had a mass of only 15.8 ± 0.2 mg. Since the potential applied in the system was higher than the OCV of the copper electrode, the electrode was gradually consumed by oxidation. At high applied potentials the copper electrode was quickly oxidized (dissolved) and limited the duration of an experiment. For example, at a potential of 3.90 V an electrode typically lasted only 400 s. The longest experiment, which lasted until an electrode was totally oxidized, showed that current oscillations were present whenever potential was applied, and the same pattern was sustained throughout the measurement. Thus short duration experiments lasting 15 s were conducted to study the oscillatory behavior and minimizing electrodissolution of the electrode. Fig. 1 shows the current–time response for potentiostatic excitation signals under static and dynamic conditions at a potential of 3.80 V. Under static conditions, no current oscillations were observed (Fig. 1a). When a stir rate of 250 rpm was applied, a sustained, periodic current oscillatory behavior was observed (Fig. 1b). The current decreased with time as a result of the changing surface morphology of the copper electrode.
Fig. 1. Current–time responses for anodic dissolution of copper foil electrodes in 1 M LiPF6 /PC:EC:DMC [1:1:3 (v/v/v)] electrolyte under (a) static and (b) dynamic [250 rpm] conditions. Applied potential = 3.80 V vs. Li/Li+ .
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Fig. 2. Current oscillations at different stir rate of (a) 0 rpm, (b) 50 rpm, (c) 150 rpm, (d) 250 rpm, (e) 350 rpm, (f) 450 rpm, (g) 550 rpm, and (h) 650 rpm. Applied potential = 3.80 V vs. Li/Li+ .
3.1. Stir rate effect The solution stir rate determined the pattern of oscillations, and also determined their frequency and amplitude. Fig. 2 shows a time series of current response over a range of stir rates. The highest stir rate that can be obtained with the instrument is 800 rpm. At stir rates higher than 650 rpm, the copper foil kept fluttering, which induced an irregular current response that was no longer stable. Only oscillations at stir rates lower than 650 rpm were studied. Oscillatory patterns can be clearly observed for several stir rates as shown in Fig. 2, in which all the oscillations were obtained at a potential of 3.80 V on the same electrode. The potential of 3.80 V was selected for study because it was not too high that the copper would be quickly dissolved. From Fig. 2a, no current oscillations were observed under static conditions, as previously shown in Fig. 1a. Periodic oscillations were
present when a stirring was applied to the solution. At a stir rate of 50 rpm, a periodic, single peak oscillation was present (Fig. 2b). At 150 rpm, a minor peak was developed to the left of the major peak (Fig. 2c), and increased at a stir rate of 250 rpm (Fig. 2d). A single major peak was observed again at 350 rpm (Fig. 2e). As the stir rate was further increased to 450 rpm, a minor peak was observed to the right of the major peak (Fig. 2f), and it kept increasing with the stir rate increasing to 550 rpm (Fig. 2g) and 650 rpm (Fig. 2h). To compare the oscillatory patterns of oscillations at different stir rates, time series data (I–t) were used to generate return maps. Regular return maps are frequently constructed by plotting successive (N + 1)th current minima against the Nth current minima. Return maps are mostly used to indicate chaos from oscillations [1,8]. Return maps not only indicate chaotic behavior, but also are useful in characterizing the pattern of oscillations. However, the decreasing current
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Fig. 3. Return maps for characterizing the pattern of current oscillations under stir rates of (a) 50 rpm, (b) 150 rpm, (c) 250 rpm, (d) 350 rpm, (e) 450 rpm, and (f) 550 rpm. NB: Different x- and y-axes scale in (d).
over time in this study made it hard to use return maps to identify oscillatory patterns. In this study, instead of measuring current minima, oscillatory peak amplitude (I) was measured. The amplitude was defined as the difference between the previous peak maximum and the current minimum, and those peak amplitudes were used to generate the return maps. The return maps at stir rate of 50, 150, 250, 350, 450, and 550 rpm are shown in Fig. 3. The return map for a stir rate of 650 rpm was similar to 550 rpm. From these return maps, the changing pattern under different stir rates became apparent. The distance from the center of the cluster to the origin (0, 0) provides the oscillatory amplitude, and the distribution of points in the cluster could indicate nonlinear behavior. Table 1 summarizes the oscillatory frequencies and amplitudes as a function of solution stir rate. The major peaks
in each time series were used to take an average for obtaining oscillatory amplitude. Arbitrarily, ten peaks in the middle of the 15 s experiment were selected for calculating the average amplitude. All the experiments were repeated with new electrodes and fresh electrolyte, and the trend of pattern changes had high reproducibility except that the ratio of minor peak versus major changed a small amount. The data in Table 1 show that the oscillatory amplitude increased from 44 to 227 A in the range of 50–350 rpm, and then decreased from 227 to 48 A when the stir rate was increased from 350 to 650 rpm. The trend was visibly observed in the return maps shown in Fig. 3. Oscillatory frequencies were obtained by arbitrarily measuring the number of peaks per second in a 5 s window in the middle of an experiment. From the results, the correlation of the frequency to the stir rate was clearly observed. At a stir rate of 50 rpm, the ratio (stir rate/peak
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Table 1 Current oscillation of copper foil electrode at different stir rates in 1 M LiPF6 /PC:EC:DMC [1:1:3 (v/v/v)] electrolyte Stir rate (rpm)
Peak amplitude (A)
Stir rate (Hz)
Peak frequency (Hz)
Ratio (stir rate/peak frequency)
0 50 150 250 350 450 550 650
No oscillation observed 44 138 159 227 86 78 48
0.8 2.5 4.2 5.8 7.5 9.2 10.8
1.6 2.5 4.2 5.8 7.5 9.2 10.8
1:2 1:1 1:1 1:1 1:1 1:1 1:1
Applied potential = 3.80 V vs. Li/Li+ .
frequency) is 1:2, which ratio is present only when no minor peaks were present. At the other stir rates when minor peaks were present, only the major peaks were counted for obtaining peak frequencies. The ratio of stir rate to peak frequency changed to 1:1. If the minor peaks were also counted in the peak frequency calculation, the ratio was still 1:2.
3.2. Potential effect The dependence of the oscillatory amplitude on potential was examined over the range of 3.50 to 3.90 V at stir rates of 250 and 350 rpm. Fig. 4 shows the oscillations observed at a stir rate of 250 rpm. A minor peak and a major peak were
Fig. 4. Current oscillations at different applied potential of (a) 3.55 V, (b) 3.60 V, (c) 3.65 V, (d) 3.70 V, (e) 3.75 V, (f) 3.80 V, (g) 3.85 V, and (h) 3.90 V vs. Li/Li+ . Stir rate = 250 rpm.
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Table 2 The characterization of oscillations at different potentials in 1 M LiPF6 /PC:EC:DMC [1:1:3 (v/v/v)] electrolyte Potential (V)
3.50 3.55 3.60 3.65 3.70 3.75 3.80 3.85 3.90
Stir rate = 250 rpm
Stir rate = 350 rpm
Amplitude (A)
Frequency (Hz)
Amplitude (A)
19 59 87 107 147 163 208 229
No oscillation observed 4.2 38 4.2 96 4.2 129 4.2 153 4.2 198 4.2 220 4.2 245 4.2 307
Frequency (Hz)
current was observed above a potential of 3.50 V corresponding to oxidation of Cu0 to Cu2+ . When the potential scan was reversed, two reduction peaks were obtained. The two peaks were assigned to the reduction of Cu2+ to Cu+ and Cu+ to Cu0 , respectively [9]. 3.3. Discussion
5.8 5.8 5.8 5.8 5.8 5.8 5.8 5.8
present in each experiment, and the ratios between them had greater variation with increasing potential than with varying stir rates at constant potential. The oscillatory amplitudes and frequencies are summarized in Table 2. As before, 10 peaks in the middle of the experiment were used for calculating the average amplitude; only the major peak was considered for calculating amplitude when a minor peak was also present. At potentials higher than 3.90 V, the copper foil was quickly consumed. At potentials lower than 3.55 V, no apparent oscillations were observed. Based on these experiments, the following correlations can be made: (1) the amplitude of the oscillations increased with an increase in potential; (2) the potential change did not affect the oscillatory frequencies. Next, cyclic voltammetry (CV) was performed. Fig. 5 shows the CV of a Cu foil electrode in 1 M LiPF6 in a ternary mixture of propylene carbonate (PC)–ethylene carbonate (EC)–dimethyl carbonate (DMC) [1:1:3 (v/v/v)]. The experiment was highly reproducible in that the two consecutive runs were nearly indistinguishable. From Fig. 5, it can be seen that in the initial positive scan, a sharp rise in oxidation
Fig. 5. Cyclic voltammograms (two consecutive scans) of a copper foil electrode in 1 M LiPF6 /PC:EC:DMC [1:1:3 (v/v/v).]. E− = 2.95 V; E+ = 3.80 V. OCV = 3.30 V. Scan rate = 20 mV s−1 .
The origin of oscillatory behavior in copper, as well as most electrochemical oscillations is not well understood. In aqueous solution, most studies have suggested that oscillatory behavior arises from an intricate process involving surface reactions, film formation/deformation, and mass transfer. Especially, the film formation/deformation process has been suggested by most researchers as causing current oscillations. Dewald et al. studied the current oscillations on a rotating copper disk electrode in acetate buffer [3], and described that oscillations were associated with the formation of a porous salt film. The authors suggested that the complex reactions on the electrode surface were similar to the Oregonator model, which is an autocatalytic mechanism of oscillating reactions. In their model, the high current state represents low surface oxide coverage, while the low current state represents high surface oxide coverage. The transition of the electrode surface coverage from high to low is commensurate with current oscillations. Oscillations in the anodic dissolution of a copper disk were also reported in an aqueous NaCl electrolyte containing SCN− [10]. The authors discussed how a black oxide film attached to the electrode surface plays an important role in the current oscillations. The autocatalytic Brusselator model was suggested for the surface film formation in the study. In aqueous electrolytes, most proposed models about the origin of oscillations show reaction mechanisms in which H+ and Cu+ have some role in film formation/deformation [1–4]. Most researchers reported that anions, such as OH− , Cl− , PO4 3− , SCN− , reacted with Cu+ or Cu2+ , with precipitate formation on the electrode surface. Precipitate dissolution was affected by the concentration of H+ . This precipitation/dissolution process induced current oscillations. In nonaqueous organic electrolyte, if H+ was present it could possibly influence precipitation with electrolyte anions on the electrode surface. The autocatalytic models, such as Oregonator and Brusselator, may be still valid for explaining the origin of oscillations. In aged electrolyte, studies have shown that the decomposition of LiPF6 in organic carbonates can produce PF5 , which in turn can form POF3 and HF. This process could ultimately lead to the formation of phosphoric acid [11,12]. The acid produced by such a process could contribute to current oscillations. However, with the fresh electrolyte used in a lithium-ion battery, the concentration of H+ is presumed zero. Thus the mechanism concerning film formation/deformation may not be applicable for explaining the origin of oscillations. This was confirmed by the apparent absence of surface film formation or deformation on the copper electrode.
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In this study, dynamic situations were obtained by rotating a magnetic stirrer. One question is whether the magnetic field affects oscillatory behavior. To answer the question, the characteristics of the oscillations are considered. A characteristic of the oscillations is that they did not require any onset time. They started as soon as an appropriate potential was applied, and were sustained throughout the experiment. This is different from film mechanisms, in which oscillations begin after film formation or dissolution. Another characteristic is that the oscillation did not show up in static solutions, in which no magnetic effect was applied. These observations imply that the origin of the oscillations might be related to the magnetic field. It has been reported that oscillatory behavior can be strongly influenced by imposed magnetic fields through interactions with the electrolyte flow structure [4,13,14]. The existence of the magnetic field enhances the mass transport rate by magnetohydrodynamic (MHD) flow, which changes the current oscillatory behavior. Fahidy and co-workers have studied the MHD effect on electrolysis, and sufficiently elucidated the relationship between the current amplitude and magnetic field [15–17]. However, it should be noted that all these studies were conducted in a magnetic field with fixed directions. A rotating magnetic field, which changed its direction all the time, was not applied to study current oscillations. In this regard, an interpretation of oscillations (and a possible mechanism) awaits more diverse and elaborate experiments. Further studies in the authors’ laboratory will continue to address the effects of a rotating magnetic field on mass transport, as well as, use of a rotating electrode. Also, a nonmagnetic stirrer, which will delete the effect of the magnetic field, will be applied to the oscillations. Moreover, the oscillations will also be characterized under different electrolyte compositions and in presence of several impurities such as H2 O and HF. Temperature effects will also be studied. It is hoped that these studies of current oscillations
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in nonaqueous solution can provide some clues about the origin of the oscillations. Acknowledgments This work was supported in part by the Saft Research and Development Center, Cockeysville, MD. Frederick R. Lemke is thanked for the use of the dry box. Mingming Xu is thanked for his assistance and helpful discussions. References [1] J.L. Hudson, M.R. Bassett, Rev. Chem. Eng. 7 (1991) 109. [2] L.T. Tsitsopoulos, I.A. Webster, T.T. Tsotsis, Surf. Sci. 220 (1989) 391. [3] H.D. Dewald, P. Parmananda, R.W. Rollins, J. Electrochem. Soc. 140 (1993) 1969. [4] Z.H. Gu, A. Olivier, T.Z. Fahidy, Electrochim. Acta 35 (1990) 933. [5] L.A. Li, S.H. Chen, H.T. Wu, H.T. Cui, J. Serb. Chem. Soc. 69 (2004) 33. [6] M. Zhao, S. Kariuki, H.D. Dewald, F.R. Lemke, R.J. Staniewicz, E.J. Plichta, R.A. Marsh, J. Electrochem. Soc. 147 (2000) 2874. [7] M. Zhao, Ph.D. Dissertation, Ohio University, Athens OH, 2001. [8] S. Kariuki, H.D. Dewald, J. Thomas, R.W. Rollins, J. Electroanal. Chem. 486 (2000) 175. [9] U. Bertocci, D.R. Turner, in: A.J. Bard (Ed.), Encyclopedia of Electrochemistry of the Elements, Marcel Dekker, New York, 1974, p. 383 (Chapter II-6). [10] Z.H. Gu, S.J. Xia, T.Z. Fahidy, Electrochim. Acta 41 (1996) 2837. [11] S.E. Sloop, J.K. Pugh, S. Wang, J.B. Kerr, K. Kinoshita, Electrochem. Solid State Lett. 4 (2001) A42. [12] M.J. O’Neil (Ed.), The Merck Index: An Encyclopedia of Chemicals, Drugs, and Biologicals, 13th ed., Merck, Whitehouse Station, NJ, 2001, p. 1318. [13] Z.H. Gu, T.Z. Fahidy, J.P. Chopart, Electrochim. Acta 37 (1992) 97. [14] C. Wang, S. Chen, Electrochim. Acta 43 (1998) 2225. [15] K. Kim, T.Z. Fahidy, J. Electrochem. Soc. 142 (1995) 4196. [16] T.Z. Fahidy, Chem. Eng. J. 72 (1999) 79. [17] T.Z. Fahidy, J. Appl. Electrochem. 32 (2002) 551.