Article pubs.acs.org/JPCC
Surface Modification of Graphitized Carbonaceous Thin-Film Electrodes with Silver for Enhancement of Interfacial Lithium-Ion Transfer Takayuki Doi,† Tomokazu Fukutsuka,*,‡ Kazuhisa Takeda,‡ Takeshi Abe,‡ Kohei Miyazaki,‡ and Zempachi Ogumi† †
Offfice of SocietyAcademia Collaboration for Innovation, Kyoto University, Gokasho, Uji, Kyoto 611-0011, Japan Department of Energy and Hydrocarbon Chemistry, Graduate School of Engineering, Kyoto University, Nishikyo-ku, Kyoto 615-8510, Japan
‡
ABSTRACT: Graphitized carbonaceous thin-film electrodes prepared by plasma-assisted chemical vapor deposition were surface-modified with Ag, and their electrochemical properties were investigated by cyclic voltammetry and ac impedance spectroscopy. Insertion and extraction reactions of lithium ion at the underlying carbonaceous thin-film electrode took place at 0−0.15 V, which was similar to those for an untreated carbonaceous thin-film electrode. In the Nyquist plots, two distinct semicircles were observed at potentials below 1.0 V, which should be assigned to resistances for surface film and interfacial charge-transfer. The resistance for interfacial charge-transfer was much larger than that for an untreated carbonaceous thin-film electrode. The temperature dependence of the charge-transfer resistance showed Arrhenius-type behavior, and the activation energy for interfacial charge-transfer was evaluated. As a result, the Agdeposited carbonaceous thin-film electrode gave an activation barrier lower than highly oriented pyrolytic graphite. These results suggest that the surface modification with Ag should significantly affect charge-transfer kinetics at the carbonaceous thin-film electrode.
■
INTRODUCTION Carbonaceous materials such as graphitic carbon have been generally used as a negative electrode in commercial lithium-ion batteries.1,2 Highly crystallized graphite features an acceptably high capacity and a very flat potential close to Li metal in charge/discharge processes. Graphite has a layered structure with a perfect stacking order of graphene sheets in the c-axis direction and possesses two kinds of characteristic surface, basal and edge planes. This anisotropic structure restricts the paths for intercalation and diffusion of lithium ion to a twodimensional direction; i.e., lithium ion intercalates predominantly at the edge plane and diffuses from the edge plane to the interior parallel to the basal plane (along the a−b plane). Thus, graphite never possesses a structural advantage in lithium-ion transfer kinetics over other insertion electrode materials with three-dimensional network paths of lithium ion. As the lithium-ion transfer kinetics, it should be noted that high activation barriers exist for lithium-ion transfer at the interface between an electrode and electrolyte.3−7 Therefore, interfacial lithium-ion transfer is not a rapid process but should be a ratedetermining step in overall battery reactions. Ogumi et al. reported that the activation energies needed for interfacial lithium-ion transfer are strongly influenced not by the electrode but by the organic solvent in an electrolyte solution.3,5,7 In addition, their results revealed that the activation energies should be closely related to the interaction between lithium ion and solvent in electrolyte, i.e., ion−dipole interaction, and © 2012 American Chemical Society
therefore should be responsible for the desolvation of lithium ion.7,8 On the other hand, they also reported that the activation energies for interfacial charge-transfer at a silicon monoxide (SiO) negative electrode were small (around 30 kJ mol−1) compared to those obtained for other insertion electrodes including graphite (around 50 kJ mol−1 or higher).3,4,6,9 These results suggest that the desolvation of lithium ion should not be responsible for interfacial charge-transfer at an SiO electrode, and therefore SiO should have an advantage in charge-transfer kinetics over other insertion electrode materials. Lithium ion is not inserted into SiO but is incorporated into SiO (Si) to form Li−Si alloys; i.e., alloy formation reactions should be distinguished from simple lithium-ion transfer reactions at insertion electrodes such as graphite. Based on this perspective, in the present work, surface modification of graphite electrode with Ag was carried out. Ag, which shows the highest electric conductivity among metals at room temperature, can alloy with Li to form LiAg, Li9Ag4, etc., at potentials below about 0.2 V vs Li/Li+, and release Li again up to around 0.3 V.10−12 The reversible capacities are known to increase with a decrease in the particle size of Ag.12 Thin films of active materials are very useful to study interfacial reactions because flat thin films, in contrast to Received: March 12, 2012 Revised: May 16, 2012 Published: May 17, 2012 12422
dx.doi.org/10.1021/jp302358v | J. Phys. Chem. C 2012, 116, 12422−12425
The Journal of Physical Chemistry C
Article
granular materials for practical use, can provide a twodimensional surface, and hence well-defined interface can be created between an electrode and electrolyte. We have reported the preparation of graphitized carbonaceous thin-film electrodes by plasma-assisted chemical vapor deposition (PACVD) and their electrochemical properties.13,14 The resultant carbonaceous thin-film electrode is available as a model electrode to study the effect of surface modification. We previously investigated the surface modification by electropolymerization of pyrrole and thiophene on carbonaceous thinfilm electrodes and reported that the reductive decomposition of electrolyte in the first cycle could be suppressed by the surface modification.15,16 In this paper, Ag was layered on graphitized carbonaceous thin-film electrodes by sputter deposition to enhance the charge-transfer kinetics at graphite electrodes.
■
EXPERIMENTAL METHODS Carbonaceous thin-film electrodes prepared by PACVD were used. A detailed procedure of PACVD can be referred to in our previous report.13,17 Starting materials of acetylene and argon gases were used as a carbon source and a plasma assist gas, respectively. Carbonaceous thin films were deposited on substrates of nickel sheets whose temperature was kept at 1023 K. Glow discharge plasma was generated between radio frequency (rf) and ground electrodes using an rf power supply of 13.56 MHz, and the applied rf power was set at 10 W. The flow rates of argon and acetylene were set at 25 and 10 cm3 min−1, respectively. The total pressure of the reaction chamber was kept at 133 Pa. Ag was deposited on the resultant carbonaceous thin films at 1.5 kV and 12 mA for 15 min by a direct-current sputtering method. However, the loading density of Ag was not determined quantitatively due to the small amount of deposits. Electrochemical properties of the Ag-deposited carbonaceous thin-film electrode were studied by cyclic voltammetry and ac impedance spectroscopy using a three-electrode cell. The effective surface area of the Ag-deposited carbonaceous thinfilm electrode was defined by an O-ring. The reference and counter electrodes consisted of Li foil, and hence all potentials in the text reflect V vs Li/Li+. The electrolyte solution was 1 mol dm−3 LiClO4 dissolved in a mixture of ethylene carbonate (EC) and diethyl carbonate (DEC) (1:1 by volume). The cells were assembled in an Ar-filled glovebox. Cyclic voltammetry was carried out between 0 and 3.0 V at a sweep rate of 1 mV s−1. After several potential cycles, the electrode was swept to a given potential (ranging from 1.5 to 0.02 V) and kept at that potential for 0.5 h or more to reach a steady-state. Ac impedance spectra were then obtained by applying a sine wave with an amplitude of 5 mV over a frequency range from 100 kHz to 10 mHz.
Figure 1. Cyclic voltammograms of (a) Ag-deposited and (b) untreated carbonaceous thin-film electrodes, respectively. Electrolyte solution was 1 mol dm−3 LiClO4/EC+DEC. Scan rate was 1 mV s−1. Each inset shows an expanded view of the cyclic voltammogram at potentials between 0 and 0.3 V.
cycle and therefore should be attributed to irreversible reactions such as electrolyte decomposition followed by formation of a surface film. A large cathodic current was observed at around 0 V, and a corresponding anodic current peak appeared at about 0.1 V. These redox currents should be mainly due to the insertion and extraction of lithium ion at the carbonaceous thinfilm electrode. The values of the cathodic and anodic currents were about 57 μA cm−2 at 0 V and 34 μA cm−2 at 0.1 V, respectively. These currents almost remained after the second cycle. A similar redox behavior was observed for the Agdeposited carbonaceous thin-film electrode, as shown in Figure 1a; the values of the redox currents at 0−0.15 V were close to those obtained for the untreated carbonaceous thin-film electrode. Ag incorporates Li to form a Li−Ag alloy at potentials below about 0.2 V and releases Li up to around 0.3 V.12 However, no apparent redox peak originating from Ag was observed in Figure 1a: i.e., no obvious difference was seen in the voltammograms between untreated and Ag-deposited carbonaceous thin-film electrodes, particularly below 0.3 V. Therefore, the redox reactions due to alloying reaction of Ag with Li should be negligible in this work: insertion and extraction reactions of lithium ion at the underlying carbonaceous thin-film electrode can take place free of influence from the deposited Ag. However, the Ag-deposited carbonaceous thin-film electrodes gave irreversible cathodic currents at around 1.6, 0.9, 0.7, and 0.4 V during the initial potential cycle, as shown in Figure 1a, which would be attributed to irreversible reactions such as the reductive decomposition of
■
RESULTS AND DISCUSSION The resultant carbonaceous thin-film electrodes were highly crystallized with lower crystallized surface as reported previously.14 Figure 1a shows cyclic voltammograms of an Ag-deposited carbonaceous thin-film electrode together with those of an as-prepared (untreated) carbonaceous thin-film electrode in Figure 1b for comparison. In the first sweep of the untreated carbonaceous thin-film electrode, irreversible cathodic currents were observed at around 0.9 and 0.6 V, as shown in Figure 1b. These peaks significantly reduced after the second 12423
dx.doi.org/10.1021/jp302358v | J. Phys. Chem. C 2012, 116, 12422−12425
The Journal of Physical Chemistry C
Article
electrolyte. An anodic current peak at about 1.0 V should be attributed to release of Li from an Li−Ag alloy, or possibly stripping of Li which was underpotentially deposited on Ag.11,18,19 A broad peak of a cathodic current appeared at around 0.7 V after the second cycle. This current would be assigned to reverse reactions to those for the anodic current at about 1.0 V, while the detailed reaction mechanism is not yet clear.11,19 Figure 2 shows Nyquist plots of the Ag-deposited carbonaceous thin-film electrode at 0.2 V in EC+DEC-based electrolyte
Figure 2. Nyquist plots of an Ag-deposited carbonaceous thin film at 0.2 V in 0.5 and 1 mol dm−3 LiClO4/EC+DEC. The upper plot is shifted by 250 Ω toward −Z″-axis for clarity.
solution containing 0.5 and 1 mol dm−3 LiClO4. Both the semicircles in the higher and lower frequency regions depended on the concentration of lithium ion in electrolytes: the resistance evaluated from the real part of the semicircles increased with a decrease in the concentration of lithium ion. Hence, these semicircles should be correlated with lithium-ion transfer. Figures 3a and b show Nyquist plots of the Agdeposited carbonaceous thin-film electrode obtained at 1.5−0.8 and 0.6−0.02 V, respectively. Two distinct semicircles were seen at potentials of 1.0 V or lower, while only a semicircle appeared in the higher frequency region at 1.2 V or higher. The semicircle in the higher frequency region was observed at 1.5 V where charge-transfer reaction should not occur. Therefore, this semicircle should be assigned to resistance of a surface film formed on the electrode. The dimension of the semicircle increased with a decrease in the electrode potential and became constant at 0.8 V. This feature is very similar to that seen for an SiO electrode.9 On the other hand, the resistance of the semicircle in the lower frequency region depended on the electrode potentials: the resistance decreased with a decrease in the electrode potentials. Lithium ion was reacted with the electrode at potentials below 1.2 V, as shown in the cyclic voltammogram (Figure 1a). Therefore, it is likely that the resistance can be ascribed to the charge-transfer resistance at the interface between the electrode and electrolyte. It should be noted that two distinct semicircles were observed even at low potentials, which is quite different from those observed for the untreated carbonaceous thin-film electrode;14 the resistance for interfacial lithium-ion transfer rapidly decreased with a decrease in the electrode potential for the untreated carbonaceous thinfilm electrode and apparently only a semicircle was observed at potentials below 0.8 V. Capacitance which is roughly estimated from resistance (Rct) and top frequency ( f top) of semicircles for
Figure 3. Nyquist plots of an Ag-deposited carbonaceous thin-film electrode at (a) 1.6−0.8 V, (b) 0.6−0.02 V in 1 mol dm−3 LiClO4/EC +DEC. Each spectrum is shifted by 250 Ω toward −Z″-axis for clarity.
interfacial charge-transfer using an equation of C = (2πRct f top)−1 differs by 1 order of magnitude: 1 × 10−5 F at 0.9 V for the untreated carbonaceous thin-film electrode, whereas it is 2 × 10−4 F for the Ag-deposited thin-film electrode regardless of the electrode potentials. This result indicates that charge-transfer kinetics should be different between untreated and Ag-deposited carbonaceous thin-film electrodes; interfacial charge-transfer should take place between the carbonaceous thin-film electrode and electrolyte through the Ag layer. This assumption can be verified by discussing charge-transfer kinetics in more depth. The temperature dependence of the interfacial charge-transfer resistance for Ag-deposited carbonaceous thin-film electrode at 0.2 V is shown in Figure 4. Chargetransfer resistance decreased with increasing temperature and showed Arrhenius-type behavior. The activation energy was determined by the least-squares method to be 44.9 kJ mol−1. This value was obviously smaller than highly oriented pyrolytic graphite and various types of carbonaceous thin-film electrodes (50−60 kJ mol−1).3,4 These results revealed that the surface modification with Ag should significantly affect charge-transfer kinetics at the carbonaceous thin film electrode, which supports the above assumption.
■
CONCLUSIONS Surface modifications of a graphitized carbonaceous thin-film electrode with Ag were carried out, and their effects on electrochemical properties as negative electrodes in lithium-ion batteries were investigated by cyclic voltammetry and impedance spectroscopy. Insertion and extraction reactions of 12424
dx.doi.org/10.1021/jp302358v | J. Phys. Chem. C 2012, 116, 12422−12425
The Journal of Physical Chemistry C
Article
(7) Yamada, Y.; Iriyama, Y.; Abe, T.; Ogumi, Z. Langmuir 2009, 25, 12766−12770. (8) Abe, T.; Sagane, F.; Ohtsuka, M.; Iriyama, Y.; Ogumi, Z. J. Electrochem. Soc. 2005, 152, A2151−A2154. (9) Yamada, Y.; Iriyama, Y.; Abe, T.; Ogumi, Z. J. Electrochem. Soc. 2010, 157, A26−A30. (10) Massalski, T. B. Binary Alloy Phase Diagrams, second ed.; ASM International: Materials Park, OH, 1990; pp 51−54. (11) Aurbach, D.; Daroux, M.; Faguy, P.; Yeager, E. B. J. Electroanal. Chem. 1991, 297, 225−244. (12) Park, C. M.; Jung, H.; Sohn, H. J. Electrochem. Solid-State Lett. 2009, 12, A171−175. (13) Abe, T.; Takeda, K.; Fukutsuka, T.; Iriyama, Y.; Inaba, M.; Ogumi, Z. Electrochem. Commun. 2002, 4, 310−313. (14) Abe, T.; Takeda, K.; Fukutsuka, T.; Iriyama, Y.; Ogumi, Z. J. Electrochem. Soc. 2004, 151, C694−C697. (15) Doi, T.; Takeda, K.; Fukutsuka, T.; Iriyama, Y.; Abe, T.; Ogumi, Z. Carbon 2005, 43, 2352−2357. (16) Doi, T.; Takeda, K.; Fukutsuka, T.; Iriyama, Y.; Abe, T.; Ogumi, Z. Tanso 2003, 210, 217−220. (17) Abe, T.; Fukutsuka, T.; Inaba, M.; Ogumi, Z. Carbon 1999, 37, 1165−1168. (18) Kolb, D. M.; Przasnyski, M.; Gerischer, H. J. Electroanal. Chem. 1974, 54, 25−38. (19) Li, L. F.; Totir, D. A.; Gofer, Y.; Wang, K.; Chottiner, G. S.; Scherson, D. A. J. Electrochem. Soc. 1999, 146, 2616−2619.
Figure 4. Temperature dependence of interfacial charge-transfer resistance at Ag-deposited carbonaceous thin-film electrode at 0.2 V in 1 mol dm−3 LiClO4/EC+DEC. Lines were drawn using the leastsquares method.
lithium ion at the underlying carbonaceous thin-film electrode took place at 0−0.15 V, similar to an untreated carbonaceous thin-film electrode, while irreversible capacities in the initial potential cycle increased by the surface modification with Ag. The resistance for interfacial charge-transfer at the Agdeposited carbonaceous thin-film electrode was much larger than that for the untreated thin-film electrode. However, surface modification with Ag resulted in a decrease in the activation barrier for interfacial charge-transfer compared with HOPG. These results indicate that the charge-transfer kinetics at the carbonaceous thin film electrode can be changed by surface modification with Ag and suggest that the charge should transfer between the carbonaceous thin-film electrode and electrolyte through the Ag layer.
■
AUTHOR INFORMATION
Corresponding Author
*Tel. +81-75-3832483; fax +81-75-3832488; e-mail fuku@ elech.kuic.kyoto-u.ac.jp. Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS This work was partially supported by “Research & Development Initiative for Scientific Innovation of New Generation Batteries (RISING) project” from New Energy and Industrial Technology Development Organization (NEDO).
■
REFERENCES
(1) Winter, M.; Besenhard, J. O.; Spahr, M. E.; Novák, P. Adv. Mater. 1998, 10, 725−763. (2) Ogumi, Z.; Inaba, M. Bull. Chem. Soc. Jpn. 1998, 71, 521−534. (3) Abe, T.; Fukuda, H.; Iriyama, Y.; Ogumi, Z. J. Electrochem. Soc. 2004, 151, A1120−1123. (4) Ogumi, Z.; Abe, T.; Fukutsuka, T.; Yamate, S.; Iriyama, Y. J. Power Sources 2004, 127, 72−75. (5) Doi, T.; Iriyama, Y.; Abe, T.; Ogumi, Z. J. Electrochem. Soc. 2005, 152, A1521−1525. (6) Iriyama, Y.; Kurita, H.; Yamada, I.; Abe, T.; Ogumi, Z. J. Power Sources 2004, 137, 111−116. 12425
dx.doi.org/10.1021/jp302358v | J. Phys. Chem. C 2012, 116, 12422−12425
