Anal. Chem. 2005, 77, 1696-1700
Pulse Voltammetric and ac Impedance Spectroscopic Studies on Lithium Ion Transfer at an Electrolyte/Li4/3Ti5/3O4 Electrode Interface Takayuki Doi,* Yasutoshi Iriyama, Takeshi Abe, and Zempachi Ogumi
Department of Energy & Hydrocarbon Chemistry, Graduate School of Engineering, Kyoto University, Nishikyo-ku, Kyoto, 615-8510, Japan
Pulse voltammetry and ac impedance spectroscopy were used to study the lithium ion kinetics at a lithium ion insertion electrode consisting of Li4/3Ti5/3O4 thin films in an organic electrolyte. In the cyclic voltammogram, two redox peaks appeared at around 1.56 V vs Li/Li+ due to the insertion and extraction of lithium ion at the electrode. Differential pulse voltammetry gave a large reduction current at ∼1.56 V during a cathodic scan due to lithium ion insertion into the electrode. From the peak current and potential, the charge-transfer resistance was evaluated by quantitative analysis using approximate equations for irreversible reactions. In the Nyquist plot, one semicircle was observed at 1.56 V, which was assigned to the chargetransfer resistance due to lithium ion transfer at the electrode/electrolyte interface. The value of the chargetransfer resistance at 1.56 V was almost identical to that evaluated by differential pulse voltammetry with an identical characteristic relaxation time. This result shows that both dc differential pulse voltammetry and ac impedance spectroscopy are useful for elucidating the phase transfer kinetics of lithium ion at insertion electrodes. Charge transfer at a solid/liquid interface has long been studied in the form of electrochemical reactions at the interface between metal electrodes and liquid electrolytes. Electron transfer at the interface has been extensively studied by various electrochemical methods. In the case of insertion electrodes, ion transfer at the electrode/electrolyte interface may take place. A typical example is charge/discharge reactions in lithium ion batteries. Lithium ion batteries use electrochemical insertion reactions of lithium ion from an electrolyte into an insertion electrode, which are intrinsically simple and reversible:1 lithium ion in the electrolyte is inserted into the electrode and extracted from the electrode to the electrolyte. This electrochemical insertion is accommodated by a host/guest solid-state redox reaction involving electron transfer from a current collector to a host electrode together with the insertion of mobile guest ions from an electrolyte into the host electrode. The host structure can be maintained after the insertion of guest ions. In contrast to the vast studies on electron * Corresponding author. Tel.: +81-75-383-2483. Fax: +81-75-383-2488. Email:
[email protected]. (1) Winter, M.; Besenhard, J. O.; Spahr, M. E.; Novak, P. Adv. Mater. 1998, 10, 725-763.
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transfer at the metal electrode/electrolyte interface, there have been few studies on ion transfer, even though ion transfer plays an important role in lithium ion batteries, nickel-hydrogen batteries, etc. There are a variety of lithium ion insertion materials, such as transition metal oxide and chalcogenide.2 Among them, spinel Li4/3Ti5/3O4 shows minimal variation of the cubic unit cell during the electrochemical insertion/extraction of lithium ion,3 which takes place at an extremely constant potential of ∼1.56 V vs Li/ Li+.4 Thus, lithium ion, which is heavily solvated by solvent molecules in a liquid electrolyte, is able to transfer from the electrolyte into the electrode topochemically and vice versa. As mentioned above, spinel Li4/3Ti5/3O4 gives almost zero strain during charge and discharge reactions; therefore, this material is ideal for studies on ion transfer kinetics at an electrode/ electrolyte interface. Ion transfer at the insertion electrodes was first studied by Ho et al.5 They studied lithium ion transfer at a WO3 electrode/ electrolyte interface by ac impedance spectroscopy, and Rct was evaluated by using the Randles equivalent circuit.5 We previously studied lithium ion transfer at an insertion electrode/electrolyte interface by ac impedance spectroscopy using various kinds of insertion materials, such as highly oriented pyrolytic graphite,6 nongraphitizable carbon,7 carbonaceous thin films,8 LiMn2O4,9 and LiCoO2.10 These results showed that the resistance of lithium ion transfer was very large and that there were high activation barriers at the electrode/electrolyte interface for lithium ion transfer. Typical Nyquist plots included a semicircle that was assigned to charge-transfer resistance due to lithium ion transfer at the (2) Bruce, P. G. Solid State Electrochemistry, 1st ed.; Cambridge University Press: Cambridge, 1995; Chapter 7. (3) Ohzuku, T.; Ueda, A.; Yamamoto, N. J. Electrochem. Soc. 1995, 142, 14311435. (4) Colbow, K. M.; Dahn, J. R.; Haering, R. R. J. Power Sources 1989, 26, 397402. (5) Ho, C.; Raistrick, I. D.; Huggins, R. A. J. Electrochem. Soc. 1980, 127, 343350. (6) Abe, T.; Fukuda, H.; Iriyama, Y.; Ogumi, Z. J. Electrochem. Soc. 2004, 151, A1120-1123. (7) Doi, T.; Miyatake, K.; Iriyama, Y.; Abe, T.; Ogumi, Z.; Nishizawa, T. Carbon 2004, 42, 3183-3187. (8) Ogumi, Z.; Abe, T.; Fukutsuka, T.; Yamate, S.; Iriyama, Y. J. Power Sources 2004, 127, 72-75. (9) Yamada, I.; Abe, T.; Iriyama, Y.; Ogumi, Z. Electrochem. Commun. 2003, 5, 502-505. (10) Iriyama, Y.; Inaba, M.; Abe, T.; Ogumi, Z. J. Power Sources 2001, 94, 175182. 10.1021/ac048389m CCC: $30.25
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electrode/electrolyte interface.11 The semicircle was fitted using a parallel equivalent circuit of the charge-transfer resistance (Rct) and double layer capacitance (Cdl), which correspond to faradaic and nonfaradaic impedance, respectively. Constant phase elements (Zcpe) could also be used in place of Cdl to express distorted semicircles.12 The value of Rct was determined from the fitted curve;13 however, the double layer for the insertion electrode, which is characterized by Cdl or Zcpe, has not yet been clarified. Differential pulse voltammetry (DPV) is a highly attractive and especially sensitive method for the study of diverse electrochemical processes.14 Since the differential current measurement can subtract out the background current, the faradaic current is welldiscriminated from the charging current, which is consumed to charge the double layer at the electrode/electrolyte interface. Therefore, DPV makes it possible to analyze faradaic processes without the contribution of an unexplained factor of Cdl. In the present study, we examined lithium ion transfer kinetics at a Li4/3Ti5/3O4 electrode by differential pulse voltammetry for the first time. To the best of our knowledge, there have been no previous studies on lithium ion kinetics at insertion electrodes using pulse voltammetry. In addition, ac impedance spectroscopy was used for comparison. We also discuss the correlation between these two methods. EXPERIMENTAL SECTION Preparation and Characterization of Li4/3Ti5/3O4. Li4/3Ti5/3O4 thin film was prepared on a Pt substrate by a sol-gel method as described in the literature.15 Since we used thin films, no binders were needed. The film thickness was 100 Hz) and a straight line with a slope of 45° to the real axis in the lower frequency range (
