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A new equivalent circuit, distinguishing the kinetic properties of Li+ insertion ... with the proposed equivalent circuits as well as the modified Voi...
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J. Phys. Chem. C 2007, 111, 12067-12074

12067

Study on Electrode Kinetics of Li+ Insertion in LixMn2O4 (0 e x e 1) by Electrochemical Impedance Spectroscopy Dongsheng Lu,†,‡ Weishan Li,*,† Xiaoxi Zuo,† Zhongzhi Yuan,† and Qiming Huang† Department of Chemistry, South China Normal UniVersity, Guangzhou 510006, China, and College of Materials Science and Engineering, South China UniVersity of Technology, Guangzhou 510641, China ReceiVed: April 30, 2007; In Final Form: June 14, 2007

Electrochemical impedance spectra (EIS) of Li+ insertion in spinel LixMn2O4 (0 e x e 1) were obtained by using a powder microelectrode. A new equivalent circuit, distinguishing the kinetic properties of Li+ insertion in LixMn2O4 at a lithium-rich state (0.5 e x e 1) from a lithium-depleted state (0 e x < 0.5), is proposed to simulate the experimental EIS. The fitting results are in good agreement with the experimental results, and parameters for the kinetic process of Li+ insertion in LixMn2O4 at different Li+ inserted states can be obtained with the proposed equivalent circuits as well as the modified Voigt-FMG equivalent circuit proposed by Aurbach et al. At the lithium-depleted state, Li+ ions diffuse rapidly and then occupy the available Li+ insertion sites in the LixMn2O4 lattice. Thus, the diffusion process and occupation process occur successively at the lithium-depleted state, and this process can be well-simulated with the modified Voigt-FMG equivalent circuit, in which Warburg impedance and occupation capacitance are in series. At the lithium-rich state, however, the diffusion speed of the Li+ ions decreases due to the repulsive effect from the inserted Li+ ions. The diffusion of Li+ ions in the lattice takes place at the same time of the occupation of Li+ ions because the inserted Li+ ions have to hop and occupy their nearest neighbor vacant sites and vacate their sites for the incoming Li+ ions. Thus, the diffusion process and occupation process occur simultaneously, and Warburg impedance and occupation capacitance should be in parallel. The change of kinetic parameters of Li+ insertion in LixMn2O4 with potential and the influence of immersion time for LixMn2O4 in the electrolyte on the kinetic parameters are discussed in detail.

Introduction Spinel LixMn2O4 (0 e x e 1) as a cathode material for lithium ion batteries has been paid much attention due to its low cost, low toxicity, and relatively high energy density.1-4 Its drawback is serious capacity fading during charge and discharge, especially under high temperatures. To overcome this problem, it is necessary to understand the kinetics of the Li+ insertion/deinsertion process in the spinel. Many techniques have been used, among which electrochemical impedance spectroscopy (EIS) is one of the most informative.5-10 Several models have been proposed to explain the impedance response of the insertion materials of lithium ion batteries, including anode materials such as graphite and cathode materials such as LiMn2O4, LiMnO2, LiNiO2, and LiCoO2.5,11-16 Among these models, the modified Voigt-FMG equivalent circuit, suggested by Aurbach et al.,8,12-13,15 is the best one to account for the Li+ insertion process in the insertion/de-insertion materials. This model reflects the steps involved during Li+ insertion: diffusion of Li+ in solution, Li+ migration through the solid electrolyte interphase (SEI) film, charge-transfer through the electrode/electrolyte interface, and solid state transport of Li+ in the material matrix including solid state diffusion of Li+ in the solid phase and occupation of Li+ in the lattice. However, this model is found to be good for the Lidepleted state but not for the Li-rich state when it is used to simulate the experimental EIS of spinel LixMn2O4. * To whom correspondence should be addressed. Tel. and fax: 86-2039310256; e-mail: [email protected]. † South China Normal University. ‡ South China University of Technology.

The model for Li+ insertion in the Li-rich state of LixMn2O4 should be different from that in the Li-depleted state of LixMn2O4 because there is a strong repulsive interaction between the inserting Li+ ions and the inserted Li+ ions in the lattice of Li-rich LixMn2O4.17 In this work, a new model is proposed to simulate the EIS of Li-rich LixMn2O4 with consideration of the repulsive interaction. Kinetic parameters of Li+ insertion in LixMn2O4 were obtained with the modified Voigt-FMG model as well as the proposed model. In most studies, EIS measurements have been carried out on composite electrodes and thin film electrodes. The present paper presents a series of high-resolution EIS of LixMn2O4 on a powder microelectrode. As compared with a composite electrode or thin film electrode, the main merits of the powder microelectrode are easy preparation, homogeneous polarization, and no organic binder disturbance, etc.18 Experimental Procedures The LiMn2O4 powder microelectrode was used, and its preparation is as follows.18,19 A piece of Pt wire (purity 99.9%, Φ100 µm) was embedded in glass tubing, melted, and polished with SiC paper to obtain a Pt microdisk electrode. The powder microelectrode was obtained by etching the tip of the Pt microdisk electrode in aqua regia so that a microcavity was formed at the tip. Then, the microcavity was packed with a powder mixture of LiMn2O4 (Merck battery grade; particle size: Φ500 nm; specific surface area: 1.22 m2/g measured by liquid N2 absorption/desorption) and graphite (battery grade) (LiMn2O4: graphite 1:1 by weight) by grinding the etched tip

10.1021/jp0732920 CCC: $37.00 © 2007 American Chemical Society Published on Web 07/24/2007

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Figure 1. (a) Cyclic voltammogram of LiMn2O4 powder microelectrode, scan rate 40 µV/s and (b) charge-discharge curve of LiMn2O4 powder microelectrode, constant current 20 nA.

on a glass surface upon which the powder mixture had been sprayed. The active material mass of the electrode was about 5.8 µg. The measurements were performed using a sealed glass cell equipped with two electrodes, a lithium counter electrode (highpurity lithium foil by Foote Mineral Co., Φ1.5 cm), and a LiMn2O4 working powder microelectrode. As the surface area of the counter electrode is much larger than that of the working microelectrode, the counter electrode also can be used as the reference electrode without serious error. The electrolyte (10 mL) was a 1 M solution of LiPF6 in ethylene carbonate-dimethyl carbonate (EC-DMC 1:1 by volume, Merck battery grade). The polymer separator was needless. The cell was assembled in an argon filled dry box with the moisture and oxygen content below 2 ppm. All electrochemical measurements were performed using a PGSTAT-30 electrochemical station (Autolab). The cyclic voltammograms were recorded between 3.5 and 4.5 V at scan rate of 40 µV/s and 2 mV/s, and the charge-discharge curves were measured at a constant current, 20 nA. Prior to EIS measurements, the LiMn2O4 microelectrode was cycled 3 times between 3.5 and 4.5 V at a scan rate of 2 mV/s, and then the potential of the electrode was set to the requested value and held for 40 min (the current declined to less than 5 nA). Then, the Li+ concentration in the LixMn2O4 matrix was considered to have reached the equilibrium state. EIS measurements were carried out at various potentials from 4.30 to 3.65 V, the AC perturbation was (10 mV, and the frequency range was from 105 Hz to 5 mHz. EIS data were analyzed using FRA version 4.8. All experiments were conducted at room temperature. Results and Discussion Figure 1 shows the cyclic voltammogram obtained at a slow scan rate (Figure 1a) and the charge-discharge curve obtained at constant current (Figure 1b). Two pair of redox peaks can be distinguished obviously at 4.01/3.97 and 4.12/4.09 V with a small peak potential difference of 40 and 30 mV, respectively. The charge quantity associated with the two cathodic peaks is almost the same, 2.875 × 10-3 C (4.30 to ∼4.05 V) and 2.900 × 10-3 C (4.05 to ∼3.65 V), respectively. The chargedischarge curve shows two plateaus at about 4.0 and 4.1 V, corresponding to the redox peak potentials of the cyclic voltammogram. These results indicate that Li+ insertion/deinsertion in the LixMn2O4 matrix is a two-step process and that each step delivers half of the total capacity.

Figure 2 shows a family of Nyquist plots obtained from a powder LiMn2O4 microelectrode in the potential range from 4.30 to 3.65 V. Qualitatively, all the spectra can be distinguished in these sections: a potential-independent semicircle in the highfrequency region, a strongly potential-dependent larger semicircle in the medium-frequency region, a Warburg-type element in the low-frequency region, and a steep sloping line at the lower frequencies. These impedance spectra reflect the nature of the overall Li+ insertion process: the high-frequency semicircle is related to Li+ migration through surface film of LixMn2O4, the medium-frequency semicircle is related to charge-transfer through the electrode-electrolyte interface, the Warburg region is assigned to solid state diffusion of Li+ in the LixMn2O4 matrix, while the steep sloping line reflects a capacitive behavior that is related to the occupation of Li+ in inserted sites. The modified Voigt-FMG equivalent circuit (Figure 3) proposed by Aurbach and co-workers12-13 was successfully used to simulate the whole range of insertion potentials of the lithiated graphite electrode as an anode material for lithium ion batteries, and the kinetic parameters were obtained by simulation, including surface film capacitance (Cf) and resistance for Li+ migration through the surface film (Rf), charge-transfer resistance at the bulk-electrolyte interface (Rct), double-layer capacitance (Cdl), diffusion coefficient reflecting the diffusion of Li+ in the solid phase (DLi), and insertion capacitance reflecting the occupation of Li+ into the inserted sites (Cint). This equivalent circuit was believed to be suitable also for describing the process of Li+ insertion into transition metal oxides, such as LiMn2O4, LiMnO2, LiNiO2, and LiCoO2.8,15 When the impedance spectra at the lithium-depleted state of LixMn2O4 (Figure 2a,b) are simulated with Figure 3, good fitting results can be obtained, as shown in Figure 4a. When the impedance spectra at the lithium-depleted state of LixMn2O4 (Figure 2c-f) are also simulated with Figure 3, however, the fitting results are not as good as those at the lithium-depleted state. The disagreement of fitting results from experimental data can be seen in Figure 4b. The deviation of fitting results from experimental data occurs in the lower frequencies. This suggests that the Li+ occupation behavior in the lithium-rich state is different from that in the lithium-depleted state. The two steps of Li+ insertion in LixMn2O4 already are known.20,21 In the first step, Li+ ions occupy every other available tetrahedral site (8a) in the spinel LiMn2O4 structure, until half of the sites are filled. This is in the lithium-depleted state (0 e x < 0.5 in LixMn2O4). In the second step, Li+ ions

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Figure 2. Nyquist plots of LiMn2O4 powder microelectrode at various potentials from 4.30 to 3.65 V.

Figure 3. Equivalent circuit used to describe the Li+ insertion process by Aurbach et al.15

fill the remaining empty 8a sites. This is in the lithium-rich state (0.5 e x e 1 in LixMn2O4), which is different from that in the lithium-depleted state because there exists a repulsive interaction during Li+ insertion in the lithium-rich state. Thus,

the equivalent circuit describing the corresponding process should be different. In spinel LixMn2O4, there should exist channels and vacant sites available for Li+ ions to diffuse and occupy.22-24 It is reasonable to assume that those sites distributed in the LixMn2O4 lattice are equivalent according to the lattice gas model with mean field approximation.25 At the lithium-depleted state, the interactions between Li+ can be neglected because of the greater distance from each other in the LixMn2O4 lattice. Li+ can diffuse freely through the channels and then occupy every other available tetrahedral site, forming a bond to the bridging-type oxygen.6,22 The Li+ ions located at the inserted sites are

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Figure 4. Experimental Nyquist plots and fitting results used by equivalent circuit in Figure 3. (a) Lithium-depleted state (0 e x < 0.5) and (b) lithium-rich state (0.5 e x < 1).

the equivalent circuits of Figures 3 and 6b. The value of Rct decreases with an increase in potential (Figure 9a). A higher potential would facilitate Li+ insertion into the lattice. This is consistent with results reported by Dokko et al.11 Rf and Cf are almost independent of potential (Figure 9b,e, respectively). This can be attributed to the stable surface films formed on LixMn2O4.8,15 Cdl can be discussed by a classical formula14

C)

Figure 5. Schematic illustration of Li+ diffusion and occupation in lattice at lithium-depleted state (0 e x < 0.5). Corresponds to equivalent circuit in Figure 3.

immobile due to the Li-O bonds, while the incoming Li+ ions diffuse through the channels and occupy the remaining vacant sites. In other words, the Li+ diffusion step and Li+ occupation step in the lattice take place successively, as shown in Figure 5. Thus, the element ZW characterizing Li+ diffusion in the lattice and Cint characterizing Li+ occupation in the lattice site should be in series (Figure 3). In the lithium-rich state, every other insertion site has been filled with Li+. The incoming Li+ ions have to overcome repulsive interactions with those Li+ located at the inserted sites. For the Li+ diffusion channel to be available, the Li+ ions located at inserted sites have to hop and occupy their nearest neighbor and empty their sites, accompanied by the breakdown of old Li-O bonds and the formation of new Li-O bonds. The applied potential is responsible for the breakdown of Li-O bonds. At this state, Li+ diffusion and Li+ occupation take place simultaneously, as shown in Figure 6a. Thus, the element ZW characterizing Li+ diffusion in the lattice and Cint characterizing Li+ occupation in the lattice site should be in parallel (Figure 6b). Figure 7 shows a typical experimental Nyquist plot at the lithium-rich state and its corresponding simulation curve with the equivalent circuit of Figure 6b. It can be seen that the fitting results are in very good agreement with experimental data. In fact, two types of inserted sites with different energies usually exist in insertion/de-insertion materials for the diffusion and storage of small ions.21,26 Figure 9 shows the variation of the different parameters with electrode potential, which is obtained from the simulation using

0r A d

(1)

where 0 is the vacuum dielectric constant, r is the dielectric constant of the medium, and A and d are the surface area of active materials in contact with electrolyte and the interplate spacing, respectively. The value of Cdl is proportional to the surface area of the electrode and inversely proportional to the thickness of the double layer. Typical values of Cdl at the liquid-solid interface is on the order of several microfarads per square centimeter. In the present case, Cdl is related to the interface between active material LixMn2O4 and electrolyte. Hence, the Cdl values in such a case can be closer to values of the capacity related to adsorption phenomena, which may be orders of magnitude higher than those of conventional Cdl values. Cdl is higher at lower and higher potentials and minimal at middle potentials (Figure 9d), indicating that the double-layer capacitance Cdl changes during Li+ insertion into LixMn2O4 at higher Li+ contents and lower Li+ contents. This can be attributed to a larger volume change of LixMn2O4 in these cases. DLi can be determined from the impedance data by analyzing the low-frequency Warburg contribution. The relation of DLi with the impedance response can be expressed as11,27

σ)

(dEdx )

Vm

FA(2DLi)1/2

(2)

where Vm is the molar volume of LiMn2O4, which is 140 cm3/ mol.27 F is the Faraday constant, and σ is the Warburg coefficient, which is obtained from simulation of the Warburg impedance spectra at low frequency. The dE/dx value is determined from the slow scan rate cyclic voltammogram curve in Figure 1a,5,28 and A is the electrode surface area, which is determined by multiplying the weight by the specific surface area of LiMn2O4 in the electrode. Figure 9c shows the variation of DLi, calculated by using eq 2, with potential. DLi varies within 10-10 to ∼10-13 cm2/s, which is in the range reported by Aurbach and co-workers.8 The DLi

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Figure 6. (a) Schematic illustration of Li+ diffusion and occupation in lattice at lithium-rich state (0.5 e x e 1) and (b) corresponding equivalent circuit.

Figure 7. Experimental Nyquist plots at lithium-rich state and simulated results used by equivalent circuit in Figure 6b.

value increases with increasing potentials. This suggests that the diffusion process of Li+ is affected by the amount of inserted Li+. The change of Cint with potential is different from Cdl, and its value is much larger that that of Cf and Cdl. There is a maximum value of Cint that appears at the potential in between the two peak potentials appearing in the cyclic voltammogram. This suggests that occupation of Li+ in the lattice is strongly affected by applied potentials at a medium Li+ content in active materials. In an insertion/de-insertion system, considering the interaction of the inserted ions at quasi-equilibrium, the insertion capacitance Cint can be determined by the equation as follows:26

Cint )

[

Le2N 1 g+ kBT x(1 - x)

]

-1

(3)

where L is the electrode thickness, e is the elementary charge, N is a number density proportional to the size of the LixMn2O4 host, kB is Boltzmann’s constant, T is the absolute temperature,

and g (g g -4) is the adimensional insertion parameter that includes the interactions between inserted ions and strain field caused by expansion or contraction of the lattice. The composition of the insertion/de-insertion electrode is described in terms of an occupancy fraction x ) n/N, where n is the number density of inserted atoms. The experimental curve in Figure 9f is consistent with the theoretical curve calculated according to eq 3. When the occupancy fraction of Li+x is 0.5, the maximum value of Cint is obtained corresponding to the peak value of the experimental curve. Figure 8 shows the Nyquist plots measured with the LiMn2O4 powder microelectrode at various potentials after being stored for 24 h. The parameters of the stored electrode, obtained by simulating the experimental data with the equivalent circuits of Figures 3 and 6b, are compared with those of the non-stored electrode and shown in Figure 9. The corresponding parameters after storage and before storage have the same change tendency with potentials. As shown in Figure 9a,b, both Rf and Rct increase. Resistance of Li+ migration through the surface increases, and the charge-transfer process at the LixMn2O4electrolyte interface becomes more difficult. This can be ascribed to the formation of an electrochemical inactive substance and new surface film with a higher resistance on the LixMn2O4 electrode.29,30 The value of Cdl after 24 h of storage tends to decrease, especially at the initial stage of Li+ insertion and at the end of Li+ insertion, as shown in Figure 9d. This is in agreement with the results reported by Dokko et al.11 The decrease in Cdl during storage can be attributed to a thickening of the surface film due to the irreversible electrolyte decomposition occurring on the active materials. Cf increases at all applied potentials after storage, as shown in Figure 9e, and exhibits two maximum peak values corresponding to the CV peaks of Figure 1a. It has been shown, by FTIR, XPS, EDAX, STM, AFM, and XRD, that the pristine Li2CO3 surface film of LixMn2O4 is replaced by a new surface species formed due to interactions between electrolyte and active

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Figure 8. Nyquist plots of LiMn2O4 powder microelectrode at various potentials after 24 h storage from 4.30 to 3.80 V.

compositions during storage,8,15,30-31 and this results in the increase of the surface area of the surface film with porous structure and the increase in Cf after storage. Similar changes of the surface film Cf with applied potentials to Figure 9e was also obtained for the LixCo0.2Ni0.8O2 electrode by Levi and coworkers,14 and this is attributed to a change in the surface film thickness or the bypassing channels of Li+ migration due to the expansion-contraction of the matrix during Li+ insertion. An obvious decrease in the Cint value after storage can be observed in Figure 9f. This can be attributed to a decrease in the sites available for Li+ insertion (a decrease in the N value in eq 3) because of the formation of an inactive substance covered on LixMn2O4 during storage. Figure 10a shows two cyclic voltammograms obtained from a LiMn2O4 powder microelectrode before storage and the same electrode after 24 h of storage. The oxidation peaks shift toward the positive direction, and the reduction peaks shift toward the negative direction after storage. A decrease of the oxidation and reduction peak areas can be observed in the cyclic voltammogram after storage. The separation between the oxidation peak and the reduction peak is characteristic of decreased reversibility, which is embodied by the increase of Rct. To explain the change of kinetic parameters of Li+ insertion in LixMn2O4 with storage time, the variation of the open circuit potential (OCP) of the LiMn2O4 powder microelectrode in the electrolyte solution with storage time was measured. The result is shown in Figure 10b. The OCP of the LiMn2O4 electrode increased at the beginning and reached 4.3 V in 25 h of

immersion, then decreased gradually, and suddenly dropped in 65 h of immersion to about 3.3 V, which is the potential of the Pt substrate versus Li/Li+. The variation of OCP in Figure 10b can be attributed to the dissolution of LiMn2O4 and the formation of defect λ-MnO2. The defect λ-MnO2 forms due to the dissolution of LiMn2O429

2LiMn2O4 + 4H+ f 3λ-MnO2 (defect) + Mn2+ (solution) + 2Li+ (solution) + 2H2O (4) The defect λ-MnO2 is different from λ-MnO2 produced by electrochemical de-insertion of Li+ from LiMn2O4

LiMn2O4 f 2λ-MnO2 + Li+ + e

(5)

Equation 5 is reversible, and the product λ-MnO2 with a cubic spinel structure has a three-dimensional channel structure, which allows easy Li+ diffusion in and out. Equation 4 describing the dissolution of LiMn2O4 in an acidic environment is an irreversible process; the product λ-MnO2 with a defect spinel structure has poor kinetics for Li+ exchange, a high potential, and thermodynamically instability. It is reactive toward the oxidation of solvents as follows:30

λ-MnO2 + xLi+ + xEl f LixMn2O4 + xEl+

(6)

where El denotes the solvent molecule. This would cause the observed decrease in OCP in the electrode during storage. The

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Figure 9. Parameters of insertion of Li+ in LixMn2O4 obtained from simulating the experimental Nyquist plots.

Figure 10. (a) Cyclic voltammogram of LiMn2O4 powder microelectrode before and after storage, scan rate 2 mV/s and (b) variation of OCP of LiMn2O4 powder microelectrode in electrolyte solution with storage time.

water produced by eq 4 causes the decomposition of the electrolyte salt LiPF628

H2O + LiPF6 f POF3 + HF + LiF

(7)

HF produced by eq 7 accelerates eqs 4 and 6, making the LiMn2O4 dissolution process autocatalytic in nature. Finally, the active material shed from the Pt substrate and a sudden potential drop was observed (g-h region in Figure 10b). LiMn2O4 dissolution during storage reduces its electrochemical activity and causes decomposition of the electrolyte solution, which leads to an increase in the electrode reaction resistances (Rct) and a change in the composition of the surface film of LiMn2O4.

Conclusion EIS of a LiMn2O4 powder microelectrode at different potentials is useful to distinguish the steps of the Li+ insertion process, including the diffusion of Li+ in solution, Li+ migration through the surface film of LixMn2O4, charge-transfer through the electrode-electrolyte interface, diffusion of Li+ in the solid phase, and occupation of Li+ in the lattice. The diffusion of Li+ in the solid phase and occupation of Li+ in the lattice occur in series at the lithium-depleted state but in parallel at the lithium-rich state of LixMn2O4. This equivalent circuit model can simulate EIS spectra well at all Li+ insertion states, and all kinetic parameters for the Li+ insertion process in LixMn2O4 can be obtained easily with this model.

12074 J. Phys. Chem. C, Vol. 111, No. 32, 2007 Acknowledgment. This work was financially supported by the National Natural Science Foundation of China (NSFC20373016). Supporting Information Available: Morphology of sample LixMn2O4; XRD results; method of evaluating mass of active material in powder microelectrode; method of evaluating electrode reaction area; first few cyclic voltammograms of sample at a scan rate of 2 mV/s; and EIS experimental and fitting results at different potentials from 4.30 to 3.65 V. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Armstrong, A. R.; Bruce, P. G. Nature 1996, 381, 499. (2) Horne, G. R.; Bergmann, U.; Grush, M. M.; Perera, R. C. C.; Ederer, D. L.; Callcott, T. A.; Cairns, E. J.; Cramer, S. P. J. Phys. Chem. B 2000, 104, 9587. (3) Kaneko, M.; Matsuno, S.; Miki, T.; Nakayama, M.; Ikuta, H.; Uchimoto, Y.; Wakihara, M.; Kawamura, K. J. Phys. Chem. B 2003, 107, 1727. (4) Lee, Y. J.; Park, S.-H.; Eng, C.; Parise, J. B.; Grey, C. P. Chem. Mater. 2002, 14, 194. (5) Nobili, F.; Tossici, T.; Marassi, R.; Croce, F.; Scrosati, B. J. Phys. Chem. B 2002, 106, 3909. (6) Bueno, P. R.; Leite, E. R. J. Phys. Chem. B 2003, 107, 8868. (7) Levi, M. D.; Salitra, G.; Markovsky, B.; Teller, H.; Aurbach, D.; Heider, U.; Heider, L. J. Electrochem. Soc. 1999, 146, 1279. (8) Aurbach, D.; Lei, M. D.; Levi, E.; Teller, H.; Markovsky, B.; Salitra, G. J. Electrochem. Soc. 1998, 145, 3024. (9) Mohamedi, M.; Makino, M.; Dokko, K.; Itoh, T.; Uchida, I. Electrochim. Acta 2002, 48, 79. (10) Striebel, K. A.; Sakai, E.; Cairns, E. J. J. Electrochem. Soc. 2002, 149, 61.

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