ARTICLE pubs.acs.org/JPCC
Lithium-Ion Transfer Reaction at the Interface between Partially Fluorinated Insertion Electrodes and Electrolyte Solutions Toyoki Okumura,† Tomokazu Fukutsuka,‡ Keisuke Matsumoto,† Yuki Orikasa,§ Hajime Arai,*,§ Zempachi Ogumi,§ and Yoshiharu Uchimoto† †
Graduate School of Human and Environmental Studies, Kyoto University, Yoshida-nihonmatsu-cho, Sakyo-ku, Kyoto 606-8501, Japan Department of Interdisciplinary Environment, Graduate School of Engineering, Kyoto University, Nishikyo-ku, Kyoto 615-8510, Japan § Office of Society-Academia Collaboration for Innovation, Kyoto University, Center for Advanced Science and Innovation, Uji, 611-0011, Japan ‡
ABSTRACT: Lithium-ion transfer reactions at the interface between insertion electrodes and electrolyte solutions were investigated by using partially fluorinated lithium manganese spinel oxides as model electrodes. LiMn1.8Li0.1Ni0.1O4ηFη (η = 0, 0.018, 0.036, 0.055, 0.073) were synthesized as model compounds. Electrochemical impedance spectroscopy was carried out to evaluate the influence of surface fluorine on the interfacial lithium-ion transfer process. Taking an adion model into account, it is considered that an adion formation process takes place with partial desolvation, and then a process with surface diffusion of the adion occurs followed by the electrode incorporation of the adion with the loss of the remaining solvent. The adion model was also used to compare the interfacial resistances for the lithium-ion transfer process. It was suggested that the fluorine substitution influences the latter process. The importance of structural characterization at the electrode surface on the lithium-ion transfer behavior is clarified.
1. INTRODUCTION Insertion materials have been generally used for positive and negative materials in rechargeable lithium-ion batteries because of their high reversibility. For the application of the rechargeable lithium-ion batteries to electric vehicles and hybrid electric vehicles, high rate (rapid charge/discharge) capability is required to achieve high power density. The electrochemical reaction between the insertion electrode and nonaqueous electrolytes can be divided into some processes: (a) migration and/or diffusion of the ions in the electrolyte, (b) diffusion in the solid phase, that is, the inside of the insertion electrode material, and (c) lithium-ion transfer at the interface between the insertion electrode and the nonaqueous electrolyte. While processes a and b have been studied by many researchers, the study on process c has been dismissed so far, and the elementary steps of the interfacial lithium-ion transfer have not been clarified. P. G. Bruce et al. have reported a two-step model of intercalation (adion mechanism), that is, (1) partial desolvation and adsorption of the ions and (2) surface diffusion, loss of the remaining solvents, and incorporation of the ions in the lattice.1 T. Abe et al. have reported that the activation energy for solvated lithium-ion transfer at the interface is smaller than that for the desolvation process, and the desolvation process is influenced by the interaction between lithium ions and solvent molecules in the electrolyte solution.2,3 Figure 1 shows the “adion model” adopted for the interpretation of the interfacial lithium-ion transfer.4 This model includes (1) the partial desolvation of a lithium ion and r 2011 American Chemical Society
its adsorption process and (2) the surface diffusion and full desolvation of the lithium ion and its lattice incorporation process at the electrode/electrolyte interface.4,5 However, the influence of the electrode surface on the interfacial lithium-ion transfer process has not been clarified yet. In order to investigate the elemental steps of the interfacial lithium-ion transfer, the control of the insertion electrode surface is crucial. In this study, we focus on the oxide compounds with partial substitution of F for O2-. The ionic radius of F is close to that of O2,6 and the degree of electronegativity for fluorine is the largest of all elements. Substitution of F for O2 would change the interaction between the adion (surface lithium ion in this study) and the electrode surface. The effect of the substitution on the electrochemical behavior could be useful for clarifying the important factors for the interfacial lithium-ion transfer process. As model electrodes, a series of partially fluorinated lithium manganese spinel oxide compounds was used.7 Electrochemical impedance spectroscopy (EIS) measurements were carried out to analyze the lithium-ion transfer process at the spinel oxide compound. The EIS observed at various temperatures, different electrode potentials, and different lithium salt concentrations were used to estimate the assignment of the spectra and the process occurring at the interface. Received: March 30, 2011 Revised: May 25, 2011 Published: May 31, 2011 12990
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Figure 1. Schematic illustration of lithium-ion transfer at the interface between insertion electrode and nonaqueous electrolyte solution.
2. EXPERIMENTAL SECTION LiMn1.8Li0.1Ni0.1O4 was synthesized by the solid-state reactions. Li2CO3 (99.99% Kojundo Chemical Lab Industries), NiO (99.9% Furuuchi Chemical Industries), and Mn2O3 obtained by preheating of MnCO3 (99.99% Kojundo Chemical Lab Industries) at 600 °C for 48 h were used as starting materials. The required amounts of the starting materials were mixed and heated at 800 °C for 2 days in air and then cooled with a rate of 0.5 °C/ min. For partial F substitution, the obtained LiMn1.8Li0.1Ni0.1O4 and NH4HF2 were mixed and then fired at 450 °C for 5 h in air,79 leading to the formation of LiMn1.8Li0.1Ni0.1O4ηFη (η = 0.018, 0.036, 0.055, 0.073). The η value was controlled by changing the ratio of NH4HF2 to LiMn1.8Li0.1Ni0.1O4 and estimated with X-ray photoelectron spectroscopy (XPS). There is a possibility that the F spectra observed in the XPS were derived not from fluorine contained in the spinel phase but from LiF, as their spectra are similar to each other. The results of the XRD and Mn K-edge XANES, EXAFS of LiMn1.8Li0.1Ni0.1O4-ηFη, which are to be shown in the other paper,10 have indicated that the valence of Mn ion was reduced by reacting with NH4HF2. We therefore believe that fluorine was substituted for oxygen in LiMn1.8Li0.1Ni0.1O4ηFη. If lithium was removed from Li(1R)Mn1.8Li0.1Ni0.1O4 to form LiF, manganese should be oxidized. Electrochemical measurement was examined using three-electrode cells. A working electrode was a mixture of 75 wt % active material, 20 wt % vapor grown carbon fiber (VGCF), and 5 wt % poly(vinylidine fluoride) (PVdF) coated on an Al current collector. Li foil was used as counter and reference electrodes. Solutions of 0.051 mol dm3 LiClO4/propylene carbonate (PC) and 1 mol dm3 LiClO4/ethylene carbonate (EC) þ diethyl carbonate (DEC) (volume ratio 1:1) were used as electrolytes. The EIS measurements were performed using Versa STAT 3 at various temperatures and potentials in the range of 1030 °C. Before the EIS measurements were performed, the potential of the electrode was changed to a given value by constant current discharge and held at the value to reach the equilibrium state. The frequency ranged from 100 kHz to 10 mHz with an amplitude of 10 mV. The EIS analysis was made with a complex nonlinear least-squares fitting program (Z-view for Windows, Scribner Associates). 3. RESULTS AND DISCUSSION The EIS measurements were carried out for LiMn1.8Li0.1Ni0.1O4ηFη (η = 0, 0.018, 0.036, 0.055, 0.073) electrodes
Figure 2. Nyquist plots of (a) LiMn1.8Li0.1Ni0.1O4 and (b) LiMn1.8Li0.1Ni0.1O3.964F0.036 at 4.0 V (vs Li/Liþ) in PC electrolyte dissolving 1 mol dm3 LiClO4 at various temperatures (from 258 to 283 K).
in order to discuss the lithium-ion insertion mechanism at the interface between the insertion electrode and the electrolyte solution. Figure 2 shows several Nyquist plots for LiMn1.8Li0.1Ni0.1O4 and LiMn1.8Li0.1Ni0.1O3.964F0.036 electrodes at 4.0 V vs Li/Liþ, as examples of the results. In all of the Nyquist plots, two semicircles and an inclined line were observed, and these semicircles became small with the increment of the temperature. The physical and/or chemical phenomena represented by each semicircle were estimated as follows. As there appeared two semicircles in the Nyquist plots, an equivalent circuit shown in Figure 3a was assumed. Each semicircle can be interpreted as a parallel circuit consisting of resistance and capacitance network, and an electrolyte resistance term Rs is connected in series. Hereafter, R1 and CPE1 denote the resistance and constant phase element (CPE) of the semicircle at the high frequency region (process 1), and R2 and CPE2 denote the resistance and CPE of the semicircle at the low frequency region (process 2). A p value of CPE was used for representing the variation of the capacitances of the two semicircles: CPE = 1/(jω)pQ. All the Nyquist plots were fit by using the equivalent circuit shown in Figure 3a. Figure 3b shows typical experimental and calculated Nyquist plots, and the calculated results, whose parameters shown in Table 1, were in good agreement with the actual experimental values. The CPE values of process 1 and 2 were micro-order, and the frequency ranges of these semicircles were similar to the results of the other groups described as the resistance at the interface between the insertion electrode and the electrolyte solution.2,3 The p value indicated the electrode surface roughness since the p values were nearly invariant with the temperature. Here we further assumed that the semicircle represents the lithium transfer behavior at the interface between the insertion electrode and the electrolyte solution, namely, the chargetransfer reaction.11 The charge-transfer resistance Rct at the 12991
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Figure 3. (a) Model of equivalent circuit used to analyze the obtained impedance spectra. Rs represents the electrolyte resistance, and R1 and R2 represent the resistances of semicircles appearing at high and low frequency regions, respectively. (b) Typical Nyquist plot and the corresponding result of fitting.
Table 1. Calculated Fitting Parameters from the Equivalent Circuit of Nyquist Plots of (a) LiMn1.8Li0.1Ni0.1O4 and (b) LiMn1.8Li0.1Ni0.1O3.964F0.036 at 4.0 V (vs Li/Liþ) in PC Electrolyte Dissolving 1 mol dm3 LiClO4 at Various Temperatures (from 258 to 283 K) T/K
Rs/Ω
R1/Ω
(a) Q1 /μF
p1
R2/Ω
Q2/μF
p2
258
256
71
69
0.69
310
337
0.88
263
204
56
73
0.69
211
333
0.89
268 273
166 138
46 40
83 108
0.67 0.65
149 107
339 341
0.89 0.90
278
118
32
61
0.73
83
363
0.88
283
102
26
75
0.71
65
385
0.88
T/K
Rs/Ω
R1/Ω
R2/Ω
Q2/μF
p2
258 263
163 131
41 32
589 389
277 284
0.94 0.94
268
107
26
192
0.64
264
289
0.94
273
90
22
210
0.63
188
309
0.93
278
76
19
235
0.62
136
329
0.92
283
67
15
168
0.67
102
353
0.90
(b) Q1/μF p1 153 163
0.65 0.65
electrode/electrolyte interface could be determined from the modified ButlerVolmer equation.11 According to the Butler Volmer equation, the exchange current density for the charge transfer reaction i0 is expressed as follows ΔG R 1R i0 ¼ AðCo Þ ðCR Þ exp ð1Þ RT where A is a pre-exponential factor and (CO)R and (CR)1R in this study are the activities of lithium in the electrolyte and electrode, respectively. R and ΔG* are a transfer coefficient and
Figure 4. Plots of inverse R1 and R2 at 4.0 V (vs Li/Liþ) against the square root of lithium salt (LiClO4) concentrations for (a) LiMn1.8Li0.1Ni0.1O4 and (b) LiMn1.8Li0.1Ni0.1 O3.964F0.036 in PC electrolyte.
the Gibbs free energy of activation for the charge-transfer process, respectively. R is the gas constant, and T is the absolute temperature. The Rct is inversely related to the exchange current density, and (CR)1R is constant under the potentiostatic condition. Therefore, the eq 1 can be organized as follows: 1 ΔG R µ i0 ¼ AðCo Þ exp ð2Þ Rct RT Equation 2 predicts that the inverse of the charge-transfer resistance Rct1 is proportional to (CO)R, where CO is the lithium concentration in the nonaqueous electrolyte. The relationship between the resistance R1 or R2 and the lithium salt concentration was thus examined. Figure 4 shows the R11 and R21 values obtained by the EIS technique for the LiMn1.8Li0.1Ni0.1O4 and LiMn1.8Li0.1Ni0.1O3.964F0.036 electrodes at 4.0 V vs Li/Liþ at various lithium salt concentrations in the electrolyte. The R11 and R21 values were linearly proportional to the square root of the lithium salt concentration (R = 0.5) for both electrodes. This result suggests that the resistances R1 and R2 observed in the EIS measurements are represented by the ButlerVolmer equation, and thus the two semicircles express the lithium-ion transfer phenomena at the electrode/electrolyte interface. Figure 5 shows the R1 and R2 values for the LiMn1.8Li0.1Ni0.1O4 and LiMn1.8Li0.1Ni0.1O3.964F0.036 electrodes at various electrode potentials. The R1 value was independent of the electrode potential while the R2 value decreased with the increment of electrode potential up to 4.0 V and then increased with increase of electrode potential for both LiMn1.8Li0.1Ni0.1O4 and LiMn1.8Li0.1Ni0.1O3.964F0.036 electrodes since the redox reaction was occurring around 4.0 V. It could be also observed from the discharge profiles, which has been reported before.10 These results indicate that the redox reaction in the electrode material is strongly related to process 2 but not to process 1. It is thus suggested that process 2 occurs at the vicinity of the electrode material when compared to process 1. Figure 6 shows the temperature dependence of R1 and R2 for the LiMn1.8Li0.1Ni0.1O4 and LiMn1.8Li0.1Ni0.1O3.964F0.036 electrodes in contact with the PC or ECþDEC (volume ratio = 1:1) 12992
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Figure 5. Plots of R1 and R2 as a function of the electrode potential for (a) LiMn1.8Li0.1Ni0.1O4 and (b) LiMn1.8Li0.1Ni0.1 O3.964F0.036 in PC electrolyte dissolving 1 mol dm3 LiClO4 at 283 K.
solutions. In this figure, the Arrhenius equation was applied, and the activation energy for each process was obtained from the slope. The activation energy values for R1 for the LiMn1.8Li0.1Ni0.1O4 electrode in PC and ECþDEC were 24 and 17 kJ mol1, respectively. The activation energy values for R2 for the LiMn1.8Li0.1Ni0.1O4 electrode in PC and ECþDEC were 28 and 38 kJ mol1, respectively. The resistance depended on the solvent in both cases. A similar tendency was also observed for the LiMn1.8Li0.1Ni0.1O3.964F0.036 electrode. The activation energy values for R1 in PC and ECþDEC were, respectively, 24 and 15 kJ mol1, and that for R2 in PC and ECþDEC were, respectively, 43 and 32 kJ mol1. In these results, the activation energy values in the PC solution were larger than those in ECþDEC for both charge-transfer processes. These results indicate that the lithium-ion transfer process for R1 is independent of the electrode potential and that for R2 changes with the potential, and both seem to contain the desolvation process reported by Kobayashi et al.5 Accordingly, it is suggested based on the previous literature5 that processes 1 and 2 are an adion formation process with partial desolvation and a process with surface diffusion of the adion followed by electrode incorporation with the loss of the remaining solvent (Figure 1). We then focus on the influence of the fluorine substitution in LiMn1.8Li0.1Ni0.1O4 on the lithium-ion transfer process. Figure 7 shows the activation energy values for R1 and R2 against the F content. The activation energy for R1 was independent of the F content. Since process 1 corresponds to adion formation occurring at a distance from the electrode surface, it is considered that the state at the electrode surface does not influence the activation energy for process 1. On the contrary, the activation energy for R2 depended on the F content, and the activation energy was increased with increase of F content. It is suggested that process 2 proceeds after the surface adsorption of the adion and is influenced by the state at the electrode surface. The dependence of R2 on the F content can be analyzed as follows. First, the affinity between the lithium ion and the electrode surface is discussed. When the lithium ion is in the activated state in the charge-transfer process, surface anion
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Figure 6. Temperature dependence of R1 and R2 for LiMn1.8Li0.1Ni0.1O4 [(a) and (b), respectively] and LiMn1.8Li0.1Ni0.1O3.964F0.036 [(c) and (d), respectively] in 1 mol dm3 LiClO4/PC (black symbol) or 1 mol dm3 LiClO4/ECþDEC (volume ration = 1:1) (red symbol) electrolyte solutions.
Figure 7. Plots of activation energy (E1 and E2) for (a) R1 and (b) R2 of LiMn1.8Li0.1Ni0.1O4ηFη (η = 0, 0.018, 0.036, 0.055, 0.073) at 4 V (vs Li/Liþ).
species could have interaction with the lithium ion. Such interaction for the F ion might be weaker than that for the O2 ion due to the delocalized nature of the F ion, and this could enhance the activation energy for the process. Next, we focus on the bulk structural difference among the electrode materials. The local structures around Mn atoms in LiMn2O4, LiMn1.8Li0.1Ni0.1O4, and LiMn1.8Li0.1Ni0.1O3.964F0.036 have been observed with the X-ray absorption technique for considering the electrochemical properties (capacity and cycle stability), which is to be detailed in the other paper.10 The local distortion in MnO6 octahedrons consisting of the spinel structure has been relaxed by the substitution of lower valence cations as like Ni2þ and Liþ for Mn3þ/4þ due to the decrement of the JahnTeller Mn3þ ion. On the other hand, the substitution of F anion for O2 anion results in the MnO6 octahedral distortion with the increase of the 12993
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JahnTeller Mn3þ ion. These changes of the MnO6 octahedral distortion could also effect on the lithium-ion transfer process at the electrode surface. This consideration well explains the order of the activation energy for R2 for the three electrodes in 1 mol dm3 LiClO4 solution of PC: LiMn2O4 (54 kJ mol1) > LiMn1.8Li0.1Ni0.1O3.964F0.036 (43 kJ mol1) > LiMn1.8Li0.1Ni0.1O4 (38 kJ mol1).12
4. CONCLUSION The adion model can be adapted to the lithium-ion transfer process at the interface between the F-substituted lithium manganese spinel oxide electrode and the organic electrolyte. For analyzing the process occurring at the interface, the electrochemical impedance spectroscopy (EIS) technique is employed at various temperatures, different electrode potentials, and different lithium salt concentrations. It is considered that the adion formation process takes place with partial desolvation, and then the process with the surface diffusion of the adion occurs followed by the electrode incorporation of the adion with the loss of the remaining solvent. The latter process is influenced by the fluorine substitution. The origin of the behavior difference between LiMn1.8Li0.1Ni0.1O4, and LiMn1.8Li0.1Ni0.1O3.964F0.036 can be attributed to their difference both at the surface and in the bulk. It is thus concluded that the lithium-ion transfer behavior at the electrode/electrolyte solution interface is affected not only by the electrolyte solvent species but also by the nature of the electrode. ’ AUTHOR INFORMATION Corresponding Author
*Phone, þ81-774-38-4974; fax, þ81-774-38-4993; e-mail h-arai@ saci.kyoto-u.ac.jp.
’ ACKNOWLEDGMENT This work was partly financially supported by Research and Development Initiative for Scientific Innovation of New Generation Batteries (RISING project) of New Energy and Industrial Technology Development Organization (NEDO). ’ REFERENCES (1) Bruce, P. G.; Saidi, M. Y. J. Electroanal. Chem. 1992, 322, 93. (2) Abe, T.; Fukuda, H.; Iriyama, Y.; Ogumi, Z. J. Electrochem. Soc. 2004, 151, A1120. (3) Abe, T.; Sagane, F.; Ohtsuka, M.; Iriyama, Y.; Ogumi, Z. J. Electrochem. Soc. 2005, 152, A2151. (4) Nakayama, M.; Ikuta, H.; Uchimoto, Y.; Wakihara, M. J. Phys. Chem. B 2003, 107, 10603. (5) Kobayashi, S.; Uchimoto, Y. J. Phys. Chem. B 2005, 109, 13322. (6) Shannon, R. D. Acta Crystallogr. 1976, A32, 751. (7) Matsumoto, K.; Fukutsuka, T.; Okumura, T.; Uchimoto, Y.; Amezawa, K.; Inaba, M.; Tasaka, A. J. Power Sources 2009, 189, 599. (8) Choi, W.; Manthiram, A. Electrochem. Solid-State Lett. 2006, 5, A245. (9) Choi, W.; Manthiram, A. J. Electrochem. Soc. 2007, 154, A614. (10) Okumura, T.; Matsumoto, K.; Fukutsuka, T.; Orikasa, Y.; Arai, H.; Uchimoto, Y.; Ogumi, Z. Dalton Trans. 2011, submitted. (11) Bard, A. J.; Faulkner, L. R. Electrochemcal Methods. Fundamentals and Applications; John Wiley & Sons: New York, 1980. (12) Nakayama, M.; Goto, S.; Uchimoto, Y.; Wakihara, M.; Kitajima, Y.; Miyanaga, T.; Watanabe, I. J. Phys. Chem. B 2005, 109, 11197. 12994
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