Electrochemical Impedance Analysis of a Hierarchical CuO Electrode

On the basis of the analysis of the variation of kinetic parameters, several ... TMOs have much higher theoretic capacities and better rate properties...
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Electrochemical Impedance Analysis of a Hierarchical CuO Electrode Composed of Self-Assembled Nanoplates J. Y. Xiang, J. P. Tu,* Y. Q. Qiao, X. L. Wang, J. Zhong, D. Zhang, and C.D. Gu State Key Laboratory of Silicon Materials and Department of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, China ABSTRACT: Although 3d transition metal oxides (TMOs) are well-known as promising anodes for Li ion batteries, little is known about the mechanism of electrode process kinetics. In this work, impedance behavior of the flower-like hierarchical CuO electrode is first investigated to understand the kinetics that influences the performances of TMOs toward lithium. The electrochemical impedance spectra are measured at different discharge and charge states during cycling. A modified twoparallel diffusion path model is set up to account for the Nyquist plots. The kinetic parameters in the model that represent the migration of lithium ions through surface-passivating film, charge transfer on active material/electrolyte interfaces, and diffusion of lithium ions in solid material are discussed in detail. On the basis of the analysis of the variation of kinetic parameters, several promising approaches are proposed to improve the electrochemical performances of copper oxides, which can also be applicable to all the 3d transition metal oxides.

’ INTRODUCTION Since Tarascon et al.1 first reported the excellent electrochemical performance of CoO toward lithium, 3d transition metal oxides (TMOs, where M is Fe, Co, Ni, and Cu) have been widely investigated as promising anodes for lithium ion batteries.2-5 The mechanism of Li reactivity in TMOs differs from the classical Li intercalation/deintercalation or Li-alloying process but involves the formation and decomposition of Li2O. The electrode reaction of TMOs is shown as follows Mx Oy þ 2yLiþ þ 2ye - S yLi2 O þ xM

ð1Þ

Compared to the current commercial carbonaceous anodes, TMOs have much higher theoretic capacities and better rate properties, which will hopefully realize the wide application of Li ion batteries in various areas as next-generation electronic devices, electric vehicles, solar energy storage, etc. For instance, the CuO film prepared by spray pyrolysis exhibited high reversible capacity (625 Ah kg-1) even over 100 cycles.6 However, the Coulombic efficiencies and cycling performances of pure TMOs are disappointing. It is commonly attributed to the poor conductivity of active material and incomplete decomposition of Li2O during cycling. Thus, surface modification and nanostructure fabrication are developed to improve the electrical conductivity and enhance the electrochemical activity of TMOs.7-9 Although various novel structures and original experiments have been reported, it is still short of theory supports. To effectively overcome the shortcomings of TMOs, it is necessary to understand the kinetics of lithium ion migration through surface film, charge transfer on solid/electrolyte interfaces, and further diffusion in electrode material since these processes indeed govern the polarization and reaction rate of TMOs. It would be desirable to find out what kinetics and relevant r 2011 American Chemical Society

properties such as surface resistance, charge transfer resistance, and Liþ diffusion coefficient can improve the electrochemical performances of TMOs. Electrochemical impedance spectroscopy (EIS) is a commonly used technology to analyze the electrode process kinetics. In recent years, EIS has been rigorously applied in explaining the topotactic insertion reaction10-15 and also widely used to account for the insertion reaction containing first-order phase transition.16-20 However, for the conversion-reaction type material like TMOs, the application of the present EIS models is much more difficult and limited. Moreover, to our knowledge, there are few works concerning the electrochemical impedance features of TMOs, especially the nanostructured oxides. Therefore, in this work, we take hierarchical CuO possessing of 2D nanoplates as an example and first describe its impedance responses at different discharge and charge states. A modified two-parallel diffusion path model is introduced to investigate and explain the corresponding electrode reaction kinetics. Furthermore, on the basis of EIS analysis, we show how the information can be used to optimize the electrochemical performances of CuO electrode and even other TMOs.

’ EXPERIMENTAL SECTION The preparation of hierarchical CuO is straightforward. An amount of 0.2 g of Cu(Ac)2 3 H2O was dissolved in 80 mL of deionized water, and 1.5 mL of 28 wt % commercial ammonia was added under constant stirring to adjust the pH value to 10.5. The mixture was then transferred into a stainless steel autoclave Received: August 31, 2010 Revised: November 6, 2010 Published: January 19, 2011 2505

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Figure 1. (a) XRD pattern and (b) SEM image of the hierarchical flower-like CuO.

with a Teflon linear of 100 mL capacity and heated at 150 °C for 8 h. After the autoclave was air-cooled to room temperature, black precipitation and colorless solution were obtained. The resulting precipitation was separated by centrifugation, washed with deionized water and ethanol several times, and dried at 60 °C for 12 h. The structure and morphology of the as-prepared CuO were analyzed by X-ray diffraction (XRD, Philips PC-APD with Cu KR radiation) and field emission scanning electron microscopy (FESEM, Hitachi S-4800), respectively. The XRD pattern (Figure 1a) confirms the as-prepared sample is monoclinic CuO with a space group of C2/c (JCPDS 48-1548), and the SEM image (Figure 1b) shows the flower-like CuO particles and exhibits a hierarchical system, in which the skeleton is made up of a large number of two-dimensional (2D) petals. Each petal is about 1 μm in length, 200 nm in width, and 20 nm in thickness. The working electrodes were prepared by a slurry coating procedure. The slurry consisted of 85 wt % CuO particles, 10 wt % acetylene black, and 5 wt % polyvinylidene fluoride (PVDF) dissolved in N-methyl pyrrolidinone (NMP) and was incorporated on nickel foam with 12 mm diameter. After drying at 90 °C for 24 h in vacuum, the sample was pressed under a pressure of 20 MPa. The galvanostatic discharge-charge tests were performed with coin-type cells (CR2025) on a LAND battery program-control test system at a rate of 0.1 C (1 C = 670 mAh g-1) in the voltage range of 0.02-3.0 V (versus Li/Liþ). Cyclic

Figure 2. (a) CV curves of the flower-like CuO electrode. (b), (c) Galvanostatic discharge-charge curves of the flower-like CuO electrode.

voltammogram (CV) and electrochemical impedance were investigated in a three-electrode cell on a CHI660C electrochemical workstation. Test cells were assembled in an argon-filled glovebox with metallic lithium foils as both reference and counter electrodes, 1 M LiPF6 in ethylene carbonate (EC)-dimethyl carbonate (DMC) (1:1 in volume) as electrolyte, and a polypropylene (PP) microporous film (Cellgard 2300) as a separator. CV measurements were recorded between 0 and 3.0 V at a scan rate of 0.1 mV s-1, and EIS measurements were carried out in the frequency range from 100 kHz to 0.01 Hz under AC stimulus with 5 mV of amplitude and no applied voltage bias. The impedance data were fitted using the ZsimpWin computer program. All the tests were conducted at room temperature (25 ( 1 °C). 2506

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’ RESULTS AND DISCUSSION Cyclic Voltammogram and Galvanostatic DischargeCharge Curves. It is commonly accepted that the CV curves

and discharge-charge curves can provide relevant information about the contributing steps of the entire conversion process,21 thus we present the CV and discharge-charge curves of the asprepared CuO before discussing its electrochemical impedance behaviors. Figure 2a shows the first three CV curves of the flowerlike CuO electrode. The three peaks during the first cathodic scanning correspond to the reductive reaction from CuO to an intermediate copper oxide phase [Cu1-xIICuxI]O1-x/2 (0 e x e0.4), to Cu2O, and further decomposition into Cu and Li2O, respectively.22 The corresponding reactions are expressed as follows x CuO þ xLiþ þ xe - f Cu1 - x II Cux I O1 - x=2 þ Li2 O ð0exe0:4Þ ð2Þ 2

1 Cu1 - x II Cux I O1 - x=2 þ ð1 - xÞLiþ þ ð1 - xÞe - f Cu2 O 2 1-x Li2 O ð0exe0:4Þ þ ð3Þ 2 1 1 Cu2 O þ Liþ þ e - f Cu þ Li2 O 2 2

ð4Þ

The two wide peaks during the anodic scanning are indexed to the formation of Cu2O and the partial oxidation of Cu2O to CuO, respectively. In the subsequent cycles, the cathodic peaks shift to higher potentials, while the anodic peaks stay almost at the same locations. The galvanostatic discharge-charge curves of the hierarchical flower-like CuO electrode are shown in Figure 2b. The discharge and charge plateaus correspond well with the cathodic and anodic peaks in the CV curves. The initial discharge and charge capacities are 1009 and 662 mAh g-1, respectively. After five cycles, the reversible capacity is 621 mAh g-1, sustaining 92.7% of the theoretic capacity (670 mAh g-1). Thus, it is approximately considered that the electrode reaction during the fifth cycle is complete. Some significant voltages during the dischargecharge process are labeled in Figure 2c. The impedance response of the electrode at these states will be carefully investigated in this work. Nyquist Plots and Equivalent Circuit Model. Figures 3a-c show the Nyquist plots of the electrode at different discharge states in the fifth cycle. The shape of Nyquist plots is quite different along with the increasing discharge depth. At the discharge/charge states ranging from 3.0 to 1.34 V, the Nyquist plots consist of a depressed semicircle where a high-frequency semicircle (HFS) and a medium-frequency semicircle (MHS) overlap each other and a long low-frequency line (LFL). As the electrode is discharged to 1.22 V, the depressed semicircle becomes a little smaller, and the separation of HFS and MFS becomes more distinct. Besides, the inclined line moves toward the imaginary axis. When the electrode potential is below 0.47 V, MFS enlarges significantly, and LFL shows a decreasing tendency and moves toward the real axis. The EIS of the electrode at different charge states during the fifth cycle is also presented in Figures 3d-f. The change of Nyquist plots with increasing potential exhibits a reversible behavior of that with the decreasing potential during the discharge process.

Generally, the depressed semicircle in high frequency is related to the resistance of the surface-passivating layer. The small semicircle in medium frequency is indexed to the resistance of charge transfer on solid/electrolyte interfaces, and the following inclined line corresponds to the Warburg impedance of Liþ diffusion in solid material. The characteristics of Nyquist plots at different states can reflect the changes of kinetics during the discharge-charge process. For instance, the diffusion of the lithium ion must be different in [Cu1-xIICuxI]O1-x/2 (0 e x e 0.4), Cu2O, and Cu for the different LFL behaviors at discharge states of 2.02, 1.01,and 0.47 V, respectively. However, the Nyquist plots shown in Figure 3 are somewhat complex for analyzing, especially in the high-medium frequency region. It is difficult to accurately describe the kinetic steps such as lithium ion migration through the surface-passivating layer and charge transfer on the solid/electrolyte interface only by comparing the shape of Nyquist plots. Therefore, an appropriate equivalent circuit model is necessary to set up and account for the electrochemical impedance of the hierarchical CuO electrode. Due to the plate-shaped morphology of the as-prepared CuO electrode, the diffusion route of Liþ must be different from those of homogeneous spherical particles. As shown in Figure 3g, the diffusion length of route “c” is obviously shorter than that of routes “a” and “b”. Aurbach et al.23 have used the two-parallel diffusion path model to perfectly describe the kinetics of a slabshaped particle and nonhomogenous composite electrode. Therefore, in this work, a modified two-parallel diffusion path model is set up, and the corresponding equivalent circuit is presented in Figure 3h, where Rel indicates the solution resistance; Rsl(i) and Csl(i) stand for the resistance of migration and capacity of the surface-passivating layer, respectively; Rct(i) and Cdl(i) designate the charge transfer resistance and double-layer capacitance, respectively; and ZW(i) represents the diffusioncontrolled Warburg impedance (i = 1, 2). The calculated data from the model are displayed in Figures 3a-f. The excellent fitness between the simulated curves and the experimental Nyquist plots indicates the accuracy of the two-parallel diffusion path model. The electrical parameters in the equivalent circuit can be calculated using the ZsimpWin computer program. Therefore, the investigation of electrochemical process kinetics would be more exact and straightforward. Surface Resistance. It is widely accepted that the surface resistance of anode material results from the formation of solid electrolyte interfaces (SEIs).24,25 SEI is known as a passivating layer, which contains ethylene oxide based oligomers, LiF, Li2CO3, and lithium alkyl carbonate (ROCO2Li).26 Figures 4a and b display the variation of surface resistance Rsl with different discharge and charge states in the fifth cycle. It is seen that the value of Rsl keeps almost a constant above 0.47 V, indicating the SEI film formed in the former cycle remains persistent and stable. As the potential decreases to 0.15 V, Rsl increases remarkably, suggesting the growth of a thicker SEI film or the formation of new SEI on some fresh electrode surface. During the charge process, Rsl decreases reversibly in the potential region from 0.02 to 0.91 V, indicating that the as-produced SEI film during the discharge process is partially dissolved or decomposed. The surface resistance of the electrode during cycling at the same discharge depth (0.02 V) is presented in Figure 4c. During the first few cycles, the value of Rsl is vibrated and shows a total trend of increasing. Theoretically speaking, the surface resistance at the fully discharged state should be changeless if the SEI film formed in the initial cycle is integrated and persistent. The 2507

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Figure 3. Continued 2508

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Figure 3. Nyquist plots of the CuO electrode (a-c) at different discharge states and (d-f) at different charge states during the fifth cycles. [Arrows in (b) and (e) indicate the data obtained at 100 kHz, 100 Hz, and 1 Hz, respectively. Symbols and lines in (c) and (f) indicate experimental and calculated data, respectively.] (g) Schematic illustration for the kinetics routes of Liþ in a CuO nanoplate. (h) Equivalent circuit model for the hierarchical CuO electrode.

notable variation in surface resistance indicates the repetitive decomposition and formation of SEI film, which can be ascribed to two aspects. On one side, the CuO nanostructures deliver relative high reactivity that can promote the partial decomposition

of SEI during the charge process. On the other side, the electrode volume change that occurred during the electrode reactions (eqs 2-4) may destroy some as-formed SEI films and produce a fresh electrode surface, where new SEI film will be rebuilt in the 2509

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Figure 4. (a), (b) Variation of surface resistance with different discharge and charge states during the fifth cycle calculated from fitting the Nyquist plots, respectively. (c) Variation of surface resistance during cycling at the same discharge depth (0.02 V) calculated from fitting the Nyquist plots.

next discharge process. After eight cycles, the value of Rsl remains almost changeless, indicating an integrated and stable SEI film formed on the surface of CuO. The persistent SEI film can avoid the destruction from organic molecules in electrolyte and improve the cycling stability of the electrode. It is important to point out that the cycling performance and Coulombic efficiency of the electrode during the first 15 cycles (Figure 4d) are in good agreement with the above discussion. The Coulombic efficiency increases from 65.6% to 95.6% during the first eight cycles and then stays at 99.5%-99.7% in the subsequent cycles. As is known to all, the formation of SEI consumes lithium ions and reduces the Coulombic efficiency. It is thus confirmed that the relatively low efficiency in the first few cycles results from the sustainable decomposition and formation of SEI. Charge Transfer Resistance. The variation of charge transfer resistance Rct with different discharge states during the fifth cycle displays an interesting undulated shape as shown in Figure 5a. The value of Rct is relatively low at the discharge states of 2.24, 1.34, and 0.93 V, which are the exact potentials in the middle of the three discharge plateaus (Figure 2c). The value of Rct is relatively high at the discharge states of 1.54, 1.01, and 0.47 V, which correspond to the states at the end of the three discharge plateaus. The phenomenon can be correctly explained by Levi’s classical equation of Rct27 Rct ¼

1 0:5 fFk0 Ac0:5 O cR

ð5Þ

where f designates the usual electrochemical constant (equal to F/RT with F and R being the Faraday and gas constants,

respectively, and T the absolute temperature); k0 denotes the heterogeneous rate constant; and cR and cO are the concentration of reductive and oxidative components in the electrode, respectively. In this work, cR and cO represent the concentration of reacted Li in Li2O and unreacted Liþ. It should be noted that in each reaction step as listed in eqs 2-4, respectively, the total concentration of cR and cO should be a constant (cO þ cR = cT). Mathematically analyzing, Rct has a maximum value when cOfcT or cRfcT and exhibits a minimum value when cO = cR. Therefore, Rct has a relatively low value during the reaction process and delivers a relatively high value before and after the electrode reaction. It is confirmed that eq 5 can be used to correctly interpret the experimental data, and the charge transfer resistance calculated according to the two-parallel diffusion path model is more accurate than simply comparing the shape of Nyquist plots. Figure 5b shows the charge transfer resistance of the CuO electrode during cycling at the same discharge depth (1.34 V). As seen in this figure, Rct decreases slightly in the first five cycles and then increases dramatically in the subsequent cycles. After about 25 cycles, the value of Rct becomes changeless. Since the process of charge transfer occurs on active material/electrolyte interfaces, it is generally accepted that the surface morphology of the electrode is one of the key points to affect the impedance of charge transfer.28,29 Therefore, the surface morphology of the electrode after cycling is investigated (inset of Figure 5b). It is observed that the original nanoplates have partially changed into nanowires after five cycles. In our previous study, it was reported that plate-shaped CuO is self-assembled by parallel CuO 2510

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Figure 6. Schematic representation of the conversion-reaction model for the plate-shaped CuO electrode.

is expressed as follows ZW ¼ RD þ RL þ σω - 1=2

Figure 5. (a) Variation of charge transfer resistance with different discharge depth during the fifth cycle calculated from fitting the Nyquist plots. (b) Variation of charge-transfer resistance during cycling at the same discharge depth (1.34 V) calculated from fitting the Nyquist plots.

nanowires during preparation. So accordingly, the appearance of CuO nanowires should result from the disassembly of CuO nanoplates, which increases the surface area of the electrode. The sufficient contact between the active material and electrolyte leads to the enhanced electrode activity and reduces the impedance of charge transfer.30 It is thus evident that the decrease of Rct in the first few cycles is attributed to the increase in surface area from the transformation of electrode morphology. After 25 cycles, the SEM image shows that the surface of CuO nanoplates and nanowires is seriously blocked by the accumulating decomposition products of the electrolyte, which results in the decrease of the contact area of CuO/electrolyte. The growth of Rct in the subsequent cycles should be ascribed to the decreasing active surface area of the electrode. Lithium Ion Diffusion Process. EIS is also an important method to evaluate the diffusion coefficient of a lithium ion in electrode material. The diffusion coefficient of lithium ion DLiþ can be calculated according to the following equation R2 T 2 D ¼ ð6Þ 2 2A n4 F 4 C2 σ2 where n is the number of electrons per molecule during reaction; A is the active surface area of the electrode; R is the gas constant; T is the absolute temperature; F is the Faraday constant; C is the concentration of Liþ; and σ is the Warburg factor. σ has a relationship with Warburg impedance Zw, which

ð7Þ

The above equations are commonly used to calculate the Liþ diffusion coefficient of “solid-solution” electrodes like carbon, LiFePO4, Li2MnO4, etc., where lithium ions intercalate into and deintercalate out of the crystal lattices.12,15,31 However, it should be mentioned that copper oxide is known as a typical “conversion-reaction” electrode material, where lithium ions do not intercalate/deintercalate but take part in the electrode reaction and produce new phases during cycling. The schematic illustration of the conversion-reaction mechanism for a signal CuO plate is presented in Figure 6. Since Li2O is amorphous and inactive toward Li,32 it is considered that the diffusion of Liþ does not occur in Li2O. During the conversion-reaction processes (steps B, D, and F), two active phases coexist in the electrode. At these states, Liþ diffusion can occur through the phase boundary and also within each phase. Hence, the measured DLiþ would be the resultant average value, which may be treated as an apparent rather than true value. Only after the transformation (steps C, E, and G) does the electrode possess a single active phase, at which time eqs 6 and 7 can be used to accurately calculate the lithium ion diffusion coefficient. Therefore, we only investigate the Liþ diffusion behavior at the end of the three discharge plateaus (potential: 1.45, 1.01, and 0.15 V, respectively). Here the electrode reactions are considered to be sufficient and complete as mentioned above. Figure 7a shows the relationship between Zre and ω-1/2 in the low-frequency region (0.1-0.01 Hz) as the electrode is discharged to 1.45, 1.01, and 0.15 V, respectively. Linear behaviors with different slopes are observed in the three curves. Then, the Warburg factor σ and lithium ion diffusion coefficient DLiþ can be calculated according to eqs 6 and 7, respectively. During the calculation, it is supposed that 0.4 Liþ per molecule takes part in the first step of electrode reactions (x = 0.4 in eq 2). The results presented in Table 1 show that DLiþ decreases markedly with the increase of discharge depth. At the end of the third step (eq 5) which corresponds to the formation of Cu and Li2O, DLiþ is only 6.86  10-12 cm2 s-1. Bearing in mind that CuO is a conversionreaction electrode material, the electrode reaction mechanism is not the intercalation/deintercalation of Liþ but the direct reaction between Liþ and active material. Therefore, although 2511

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Figure 8. Cycling performances of the CuO electrode discharged with different cutoff potentials.

Theoretic Proposals for Electrochemical Performance Optimization. The cutoff potential of discharge should be

Figure 7. (a) Relationship between Zre and ω-1/2 in the frequency region of 0.1-0.01 Hz. (b) Warburg impedance at different discharge depth during the fifth cycle calculated from fitting the Nyquist plots.

Table 1. Warburg Factor and Diffusion Coefficient of Lithium Ions in Different Materials materials

[Cu0.6IICu0.4I]O0.75 þ Li2O Cu2O þ Li2O Cu þ Li2O

σ (Ω s-1/2)

124.8

24.58

DLiþ (cm2 s-1)

5.05  10-9

5.32  10-10 6.86  10-12

19.42

[Cu1-xIICuxI]O1-x/2þ (0 e x e 0.4), Cu2O, and Cu are of different crystal lattices, it does not affect the diffusion behavior of Liþ. However, as the electrode reactions are carried on, more and more Li2O is produced in the electrode. To some degree, the accumulation of passivating Li2O phases hampers the diffusion of lithium ions. It is thus suggested that the content of Li2O in the matrix might influence the Liþ diffusion coefficient. More Li2O phases in the electrode lead to the lower diffusion coefficient of Liþ. The above analysis is directly based on Nyquist plots in the low-frequency region, so the calculated results reflect the total Liþ diffusion coefficient of the CuO electrode. However, as mentioned in Figure 3g, the diffusion length of route “c” in the 2D plate-shaped CuO is much shorter than those of route “a” and “b”, which might result in different Liþ diffusion impedance. To investigate Warburg impedance of the different diffusion route separately, Zw(1) and Zw(2) are calculated according to the equivalent circuit model. As shown in Figure 7b, the difference between Zw(1) and Zw(2) is a bit large (about 1-2 orders of magnitude). The notable difference confirms that the Liþ diffusion with short diffusion length is much easier than that with long diffusion length.

below 0.1 V. As shown in Figure 4a, SEI is formed near 0.15 V during the discharge process. Formation of stable and persistent SEI on the surface of the electrode can prevent the destruction of active material from the electrolyte and improve the cyclability of the electrode. Figure 8 gives the comparison on cycling performances of the electrode discharged with different cutoff potentials. The electrode that discharged in the range of 0.02-3.0 V exhibits much better capacity retention than that discharged in the range of 0.2-3.0 V. The fast capacity fading of the latter one is ascribed to the incomplete formation of SEI film on the electrode surface, which causes the active material to degrade seriously after cycling. Fabrication of porous, hollow, and hierarchical structures is favorable for enhancing the electrode kinetics since large electrode surface area can facilitate the charge transfer on solid/ electrolyte interfaces and reduce the corresponding impedance. The large surface area of the electrode must be active and not blocked by the accumulating decomposition products of electrolyte during cycling. The lithium ion diffusion coefficient DLiþ of the CuO electrode is relatively low compared to carbonaceous materials and silicon anodes.18,33,34 To eliminate this shortcoming, 3D nanostructured CuO should be paid more attention than the 2D and 1D ones since the diffusion length in 3D nanostructures is short and not restricted in one or two specified directions. When the diffusion length reduced to some extent, the lithium ion diffusion coefficient would not be the most important factor to influence the electrode process kinetics. In other words, the 3D nanostructured electrode can abate the negative effect of the low diffusion coefficient of Liþ and enhance the electrode process kinetics.

’ CONCLUSIONS In this work, we first analyzed the electrochemical impedance response of the conversion reactions in hierarchical CuO electrode composed of 2D nanoplates by a modified two-parallel diffusion path model. It is confirmed that the SEI film is formed at the potential range of 0.47-0.15 V during the discharge process, and it takes about eight cycles to form integrated and persistent SEI film on the surface of the CuO electrode. The charge-transfer resistance is greatly influenced by the electrode surface morphology during cycling. For the nanoplate-shaped CuO, Rct decreases 2512

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’ AUTHOR INFORMATION Corresponding Author

*Tel.: þ86-571-8795-2856. Fax: þ86-571-8795-2573. E-mail: [email protected].

’ REFERENCES

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