J. Phys. Chem. C 2009, 113, 20135–20138
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Li+-Ion Transfer through the Interface between Li+-Ion Conductive Ceramic Electrolyte and Li+-Ion-Concentrated Propylene Carbonate Solution Fumihiro Sagane, Takeshi Abe,* and Zempachi Ogumi Department of Energy and Hydrocarbon Chemistry, Graduate School of Engineering, Kyoto UniVersity, Nishikyo-ku, Kyoto 615-8510, Japan ReceiVed: September 07, 2009; ReVised Manuscript ReceiVed: September 29, 2009
Lithium-ion transfer through the lanthanum lithium titanate/LiClO4-concentrated propylene carbonate (PC) solution interface was investigated by the ac impedance method. Charge-transfer resistance (Rct) across the solid/liquid interface was observed and the interfacial activation energy was calculated from temperature dependency of Rct. The activation energies for interfacial Li+-ion transfer were invariable for 1 mol dm-3 and Li+/PC ) 1/3 (by molar ratio) solution, but about 10 kJ mol-1 higher for Li+/PC ) 1/2 solution. Raman spectra showed that the contact ion pair was formed in the Li+/PC ) 1/2 solution. These results suggest that the activation energy for the cleavage of the ion-ion interaction process was higher than that for the desolvation process; that is, the cleavage of the ion-ion interaction was the rate-determining step for the solid/concentrated solution. 1. Introduction Lithium-ion secondary batteries are widely used in portable electronic devices, such as cell phones and laptop computers, because of their extensively high energy densities. Recently, lithium-ion secondary batteries have been proposed as the power sources for hybrid electric vehicles or electric vehicles. For use in these large electric devices, lithium-ion secondary batteries must possess sufficient high-power densities; that is, fast charge and discharge reactions must be required.1 The charge and discharge reactions in the lithium-ion secondary batteries are based on the Li+-ion transfer between positive and negative electrodes via lithium-ion conductive electrolyte. Therefore, rapid Li+-ion transfer processes consisting of Li+-ion transfer in the electrodes and electrolytes, and Li+ion transfer through the electrode/electrolyte interfaces, must take place. So far, we focused on the Li+-ion transfer at the electrode/ electrolyte interface since Li+-ion diffusion in electrodes and electrolyte can be fasten by using the fine particles of batteryactive materials and by decreasing the electrodes’ thickness and separator. In particular, we employed two Li+-ion conductive electrolytes to fabricate a model interface of ceramic electrolyte/ liquid (polymer) electrolyte.2,3 In these systems, no redox reactions occur at the interface, and therefore, it is very easy for us to study what will influence the interfacial Li+-ion transfer. We fabricated a four-probe cell and studied the Li+ion transfer at the ceramic electrolyte/liquid electrolyte interface by ac impedance spectroscopy.3 Consequently, we clarified that the Li+-ion transfer at the interface possessed large activation energies and that the desolvation process from liquid (polymer) electrolyte to solid electrolyte is responsible for the large activation energies. The desolvation process is directly correlated with ion-solvent interaction. Therefore, the coordination condition and solvent species will play an important role in the Li+ion transfer at the interface. * Corresponding author. Tel.: +81-75-3832487. Fax: +81-75-3832488. E-mail:
[email protected].
The above results motivated us to study the interfacial Li+ion transfer under Li-salt concentrated solution since the coordination condition of Li+ ion must be varied. Indeed, unique electrochemical properties of concentrated solution were reported by Jeong et al.4 Generally, Li+-ion intercalation into a graphite electrode is known to be difficult in a propylene carbonate (PC)-based solution since the solvent decomposition and intensive exfoliation of graphene layers continue to occur without the formation of a stable surface film (SEI).5,6 However, Jeong et al. clearly showed the electrochemical Li+-ion intercalation/deintercalation at the graphite electrode by use of LiN(C2F5SO2)2-PC concentrated solution (Li+/PC ) 1/2 by molar ratio). In this work, Li+-ion transfer through the solid electrolyte/ electrolyte solution with various concentrations was studied by ac impedance spectroscopy. From the concentration dependency of the activation energies, the rate-determining steps on the interfacial ion transfer were discussed. 2. Experimental Section Lanthanum lithium titanate (LLT) was used as the Li+-ion conductive ceramic electrolyte. The ceramic pellet (10 mm diameter, 1 mm thickness) was prepared by a traditional solidstate reaction.7 Propylene carbonate (PC) solutions containing LiClO4 or lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) were used as liquid electrolytes and their concentrations were 1 mol dm-3 and Li+/PC ) 1/3 and Li+/PC ) 1/2 by molar ratio (3.95 and 5.93 mol dm-3, respectively). All Li salts and PC solvent were purchased from Kishida Chemical and used without further purification. The solid/liquid interface was fabricated by using the fourelectrode electrochemical cell (see our previous report2) and ac impedance measurements were conducted. Both counter and reference electrodes were Li metal. The area of the solid/liquid interface was kept at 0.20 cm2 by using an O-ring. All impedance measurements were conducted with Solartron 1260 and 1287 over a frequency region of 100 kHz to 100 mHz, with applied voltage amplitude of 20 mV. All experiments were conducted under an Ar atmosphere.
10.1021/jp908623c CCC: $40.75 2009 American Chemical Society Published on Web 10/23/2009
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Sagane et al.
Figure 2. Temperature dependency of the interfacial resistances for LiClO4-PC solutions (square: 1 mol dm-3; circle: Li+/PC ) 1/3; triangle: Li+/PC ) 1/2).
TABLE 1: Summary of the Interfacial Activation Energies for LiClO4-PC/LLT and LiTFSI-PC/LLT molarity (Li+/PC) -3
Figure 1. (a) Complex impedance plot for a cell of Li/1 mol dm-3 LiClO4-PC/LLT/1 mol dm-3 LiClO4-PC/Li (four-electrode measurement, T ) 298 K). (b) Concentration dependency for impedance (square: 1 mol dm-3; circle: Li+/PC ) 1/3; triangle: Li+/PC ) 1/2).
Raman spectra of the electrolyte solutions were recorded to investigate the coordination conditions in the solutions, using a triple monochromator (Jobin-Yvon, T-64000). Excitation was carried out with a 514.5 nm line (50 mW) from an Ar ion laser (NEC, GLG2265). A quartz cell was used and sample solutions were sealed in an Ar atmosphere. 3. Results and Discussion A complex impedance plot for a cell of Li/1 mol dm-3 LiClO4-PC/LLT/1 mol dm-3 LiClO4-PC/Li is shown in Figure 1a. Two semicircles appear with the characteristic frequencies of ca. 1 kHz and ca. 10 Hz. Three impedance components can be considered in this system: (1) Li+-ion transfer through the solid/liquid interface, (2) Li+-ion conduction in LLT, and (3) ion conduction in the PC solutions. The interfacial impedance at the Li metal/PC solution is not observed because of the fourelectrode measurements. Among these components, the ion conductions in the PC solution and bulk LLT are so fast that the semicircles ascribed to these impedances do not appear in the present frequency regions. The semicircle in the higher frequency region was ascribed to the grain boundary resistance of LLT, by comparison with the result of impedance measurement of the Au/LLT/Au system. On the basis of the results, the semicircle with the characteristic frequency of ca. 10 Hz was ascribed to the interfacial resistance of Li+-ion transfer at the interface of LLT/PC solution. The LiClO4-salt concentration dependency of the complex impedance plots are shown in Figure 1b. The charge-transfer resistance through the interface increased with the increase of the salt concentration. The result is somehow strange since the charge-transfer resistances should decrease with the increase of the charge densities. In fact, we previously reported that the
1 mol dm (1/11.6) 3.95 mol dm-3 (1/3) 5.93 mol dm-3 (1/2)
LiClO4-PC/LLT
LiTFSI-PC/LLT
-1
56 kJ mol-1
57 kJ mol-1
57 kJ mol-1
68 kJ mol-1
64 kJ mol-1
53 kJ mol
charge-transfer resistances decreased with the increase of the Li-salt concentration in PC over the concentration range of 0.1-1 mol dm-3.3 However, in a higher Li-salt concentration region, the dissociation degrees become smaller, resulting in a smaller number of mobile Li+ ion. In addition, a higher Li-salt concentration leads to higher viscosity of the solution, and therefore, the wettability between solid electrolyte and electrolyte solution will become poor. Thus, the interfacial resistance will increase in the high Li-salt concentrated solution. Figure 2 shows the temperature dependency of the interfacial resistances for LLT/LiClO4-PC solutions. By the least-squares method, the activation energies of the Li+-ion transfer through the LLT/LiClO4-PC solution interface were evaluated. The results are shown in Table 1. The activation energies for 1 mol dm-3 and Li+/PC ) 1/3 solutions were almost identical. These two values show good agreement with our previous literature (56.2 kJ mol-1 for LLT/1 mol dm-3 LiCF3SO3-PC).3 Therefore, it can be concluded that the activation energy is responsible for the desolvation process of Li+ ion from PC solvent. However, the activation energy for Li+/PC ) 1/2 is higher than those for 1 mol dm-3 and Li+/PC ) 1/3 by ca. 10 kJ mol-1. Similar results are obtained for the LiTFSI system as listed in Table 1. The higher activation energies cannot be explained simply by the desolvation process since all solutions used in this study are composed of the same solvent (PC), and therefore, solvation abilities in these three solutions should be the same. Hence, the higher activation energy will not be responsible for the desolvation process and another mechanism should be considered. To investigate the coordination conditions in the concentrated solutions, Raman spectroscopy was employed. Figure 3 shows the Raman spectra of pure PC solvent and LiClO4-PC solutions. The peaks in Figure 3a correspond to the ring deformation band of PC. The band is useful for obtaining information on the
Li+-Ion Transfer through LLT/LiClO4-PC Interface
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Figure 4. Schematic diagrams of the coordination condition for LiClO4-PC: (a) solvated Li+ ion, (b) solvent-shared ion pair, and (c) contact ion pair.
Figure 5. Raman spectra of LiTFSI-PC solutions at different salt concentrations: pure PC and concentrations of 1, 3.95, and 5.93 mol dm-3, from bottom to top. Figure 3. Raman spectra of LiClO4-PC solutions at different salt concentrations: pure PC and concentrations of 1, 3.95, and 5.93 mol dm-3, from bottom to top.
interaction between PC solvent and Li+ ion.8,9 In pure PC, the strong band peak at 712 cm-1 corresponding to the ring deformation of free PC appeared. As the concentration of Li salt increases, the band shifts to higher wavenumber. In Li+/ PC ) 1/2 solution, the peak appeared at 723 cm-1, corresponding to the ring deformation band of solvated PC. It indicates that most of the PC molecules solvate Li+ ions in the solution. Figure 3b shows the Raman spectra of the LiClO4-PC solutions in the range 900-1000 cm-1. The peaks around 940 and 960 cm-1 correspond to the symmetric stretching vibration of ClO4- ion and the -O-C(dO)-O- symmetric stretching of PC, respectively. The former peak is known to be sensitive to ion associations and James and Mayes10 give the following assignment to the peaks: (i) free solvated anions (933.8 cm-1), (ii) solvent-shared ion pairs (939.3 cm-1), and (iii) contact ion pairs (947.7 cm-1). In 1 mol dm-3 solution, a sharp peak at 933 cm-1 was observed. This means that Li+ ion is solvated only by PC. With the increase of LiClO4 salt concentrations, the relative peak intensity at 933 cm-1 began to decrease and a new peak at 939 cm-1 appeared. In Li+/PC ) 1/3 solution, the peak at 939 cm-1 was mainly observed and the peak at 933 cm-1 existed as a shoulder, which implies that the solvent-shared ion pair was formed mainly in the Li+/PC ) 1/3 solution. As for Li+/PC ) 1/2 solution, a free anion peak at 933 cm-1 was no longer observed and a new peak around 946 cm-1 appeared. By consideration of the assignment, free solvent anions, solventshared ion pairs, and contact ion pairs mainly exist in 1 mol dm-3, Li+/PC ) 1/3 solution, and Li+/PC )1/2 solution, respectively. The schematic diagrams of the coordination condition in each solution are illustrated in Figure 4. Figure 5 shows the Raman spectra of the LiTFSI-PC solution in the range 650-800 cm-1. A Raman shift around 720 cm-1 arises from PC, as described above. The band around 750 cm-1
is attributed to the symmetric deformation band of CF3 in TFSI anion.9 Although the latter band is affected by the coordination condition of TFSI anion, it is somewhat difficult to identify the solvent-shared ion pairs and contact ion pairs clearly. Indeed, the only relative intensity changed with Li-salt concentration and no band splitting was observed in Figure 5. Therefore, the following discussions were focused on the LiClO4-PC system. An interesting correlation between the interfacial activation energies and the coordination of Li+ ion was found. When the concentration of Li+ ion ranges from 1 mol dm-3 to Li+/PC ) 1/3, Li+ ion is surrounded only by PC as depicted by Figure 4a,b and their activation energies were almost the same value, around 55 kJ mol-1. In contrast, the contact ion pairs are formed in Li+/PC )1/2 solution, indicating that ClO4- ion also interacts with Li+ ion directly as can be seen in Figure 4c. Therefore, in Li+/PC ) 1/2 solution, both the cleavage of the ion pair and the desolvation from PC are required for the interfacial Li+-ion transfer. It is well-known that the rate-determining step reflects the activation energy. When it is more difficult for the desolvation process to occur than cleavage of the ion pair in Figure 4c, the activation energy should be around 55 kJ mol-1, similar to the coordination conditions of Figure 4a,b. On the basis of these considerations and results, it is concluded that the higher activation energy should be ascribed to the cleavage of the ion-ion interaction between Li+ ion and ClO4- ion. So far, we have proposed that the desolvation process is the ratedetermining step for the Li+-ion transfer through the solid/liquid interface.3 And presently, we show another possibility of the rate-determining step for the interfacial Li+-ion transfer, that is, the cleavage of ion-ion interaction in the concentrated electrolyte solution. 4. Conclusion Lithium-ion transfer through the interface between lithiumion conductive ceramic electrolyte and concentrated electrolyte solution was studied by ac impedance spectroscopy. In the LiClO4-PC case, Li+/PC ) 1/2 solution showed higher inter-
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facial activation energy than the other low-concentration solutions. Raman spectra indicated that Li+ ion formed the contact ion pair in Li+/PC ) 1/2 solution, whereas Li+ ion was surrounded only by PC solvents in the other solutions. From the results, we concluded that the cleavage of the ion-ion interaction was the more difficult process compared to the desolvation process for the solid/concentrated solution. It means that the desolvation process we proposed previously was not always the rate-determining step of the charge-transfer reactions. For the concentrated solution, the cleavage of the ion-ion interaction was also a candidate for the rate-determining step. As discussed in the present paper, the ion-ion interaction between Li+ ion and anion seems to give very interesting characteristics for the charge-transfer process at the interface between solid and liquid electrolytes. To understand the interaction in detail, we have studied the Li+-ion transfer through the ceramic electrolyte/ionic liquid interface, and results will be reported elsewhere.
Sagane et al. References and Notes (1) Iwahori, T.; Ozaki, Y.; Funahashi, A.; Momose, H.; Mitsuishi, I.; Shiraga, S.; Yoshitake, S.; Awata, H. J. Power Sources 1999, 81-82, 872. (2) Abe, T.; Ohtsuka, M.; Sagane, F.; Iriyama, Y.; Ogumi, Z. J. Electrochem. Soc. 2004, 151, A1950. (3) Abe, T.; Sagane, F.; Ohtsuka, M.; Iriyama, Y.; Ogumi, Z. J. Electrochem. Soc. 2005, 152, A2151. (4) Jeong, S. K.; Inaba, M.; Iriyama, Y.; Abe, T.; Ogumi, Z. Electrochem. Solid-State Lett. 2003, 6, A13. (5) Besenhard, J.; Fritz, H. J. Electroanal. Chem. Interfacial Electrochem. 1974, 53, 329. (6) Dey, A.; Sullivan, B. J. Electrochem. Soc. 1970, 117, 222. (7) Inaguma, Y.; Chen, L.; Itoh, M.; Nakamura, T. Solid State Ionics 1994, 70/71, 196. (8) Battisti, D.; Nazri, G.; Klassen, B.; Aroca, R. J. Phys. Chem. 1993, 97, 5826. (9) Wang, Z.; Gao, W.; Huang, X.; Mo, Y.; Chen, L. J. Raman Spectrosc. 2001, 32, 900. (10) James, D.; Mayes, R. Aust. J. Chem. 1982, 35, 1775.
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