Investigation of the Lithium Intercalation Behavior of Nanosheets of

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Investigation of the Lithium Intercalation Behavior of Nanosheets of LiV3O8 in an Aqueous Solution H. Heli,*,†,‡ H. Yadegari,§ and A. Jabbari§ †

Laboratory of Analytical and Physical Electrochemistry, Department of Chemistry, Science and Research Branch, Islamic Azad University, Fars, 7348113111 Iran ‡ Young Researchers Club, Science and Research Branch, Islamic Azad University, Fars, 7348113111 Iran § Department of Chemistry, K. N. Toosi University of Technology, Tehran, P.O. Box 16315-1618 Iran ABSTRACT: Nanosheets of lithium vanadium oxide (LiV3O8) are synthesized using a citrate sol
gel combustion route. The physical characterizations are carried out by scanning and transmission electron microscopies (SEM and TEM) and X-ray diffraction (XRD) measurements. Compact nanosheets of the active material are observed by SEM and TEM. XRD data indicate that the prepared nanosheets present pure phase of monoclinic LiV3O8 with p21/m symmetry. The kinetics of electrochemical intercalation of lithium ion into the nanosheets are investigated by cyclic voltammetry (CV), chronoamperometry (CA), and electrochemical impedance spectroscopy (EIS) studies with special emphasis on the application potential as an anodic material for aqueous rechargeable lithium batteries. CV studies of the nanosheets at a slow scan rate of 0.3 mV s
1 between þ250 and
700 mV vs Ag/AgCl demonstrate that nanosheets of LiV3O8 represent well-defined reversible peaks. The nonlinear chemical diffusion of lithium ion into the nanosheets is explored by EIS. The results are discussed on the basis of an equivalent circuit, distinguishing the kinetics of lithium intercalation. The fitting results are in good agreement with the experimental results, and the kinetic parameters of lithium intercalation are obtained with the proposed equivalent circuit. The changes of kinetic parameters of lithium intercalation with potential are also discussed in detail.

1. INTRODUCTION In recent years, the need for portable power sources has accelerated due to the miniaturization of electronic appliances. Accordingly, the lithium-ion battery was invented in the early 1990s, and now it is widely used as power source for portable electronic devices such as laptops and cellular phones due to their high specific energy, high voltage, and low self-discharge rate.1 However, there are many safety problems with these batteries. Usually, lithium-ion batteries contain flammable organic electrolytes, which tend to easily cause intense smoke or even fire in the case of improper use such as overcharging or short-circuiting. Moreover, lithium-ion batteries are relatively expensive due to special cell designing, necessity of a perfectly dry environment during manufacturing steps, and costly organic electrolytes. In spite of the above-mentioned problems, the ecological drawbacks accompanied by nonaqueous battery systems continuously urge the further development of less expensive, safe, and reliable battery systems.2 An alternative rechargeable lithium-ion battery with an aqueous electrolyte was introduced from the middle of 1990s.3
5 Aqueous rechargeable lithium batteries (ARLBs) have lower energy density in comparison with nonaqueous alternatives due to restriction of the cell voltage to the decomposition potential of water. However, ARLB performances can be compared to the performances of the Pb
acid and Ni
Cd batteries. ARLBs do not use poisonous metals (Pb and Cd) and appear to r 2011 American Chemical Society

be environmentally friendly devices. ARLBs have particular advantages including (a) high ion conductivity compared to that the nonaqueous lithium-ion cells, (b) high rate capability, (c) relatively high energy and power density, (d) inherent safety, (e) no environmental pollution, (f) no safety problem even in the case of misuse, and (g) low cost.6 Such batteries are also promising power sources for hybrid electric vehicles.7 Designing the high performance intercalating materials with an appropriate lithium intercalation kinetics is a key issue for the performance of the aqueous battery cell. There are many compounds that can be used as cathode materials for ARLBs, such as LiMn2O4, LiCoO2, LiCo0.19Ni0.81O2, LiNi1/3Mn1/3Co1/3O2, etc.2
5,8,9 However, only a few compounds with proper redox potential are available as anode materials.2,3,5,8
11 Lithium vanadium oxide (LiV3O8) is a promising electrode material because of its high capacity, low cost, long cycle life, and facile preparation as both cathode12,13 and anode2,9,14 materials for lithium-ion batteries. LiV3O8 has a layered structure with two basic structural units of VO6 octahedra and VO5 distorted trigonal bipyramid.15 Lithium ions generally occupy the octahedral sites; however, extra lithium ions that intercalate into the host compound may enter the tetrahedral Received: February 11, 2011 Revised: April 20, 2011 Published: May 12, 2011 10889

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The Journal of Physical Chemistry C sites. Lithium ions in the octahedral sites link to the V3O8 layers via strong ionic bonds, which make the crystal structure of LiV3O8 stable during the intercalation/deintercalation process. It is well understood that the preparation method of LiV3O8 strongly influences its electrochemical properties. In this regard, various synthesis routs, ranging from solid-state reaction,2 sol
gel method,16,17 hydrothermal reaction,18 microwave-assisted solid-state synthesis,19 rheological phase reaction method,20 spraydrying synthesis,21 ultrasonic treatment,22 combustion method,23 and intercalation of inorganic molecules between V3O8 interlayers24 to substitution of lithium ion with other monovalent cations25 have been considered. Recent advanced studies of lithium-intercalated materials take care of nanostructured materials with various morphologies such as nanotubes,26,27 nanowires,28
30 nanorods,31
33 nanosheets,34 and so forth.7 There is no doubt at this time that the nanostructured materials provide short diffusion pathways for lithiumion intercalation/deintercalation from host materials and simultaneously expose high specific surface areas that often provide more active intercalation sites.35,36 However, there are a few studies dealing with application of these nanostructured materials as an anode hosting electrode for aqueous rechargeable lithium batteries.11,37 Following our recent report on the preliminary results of synthesis and potential application of LiV3O8 as an anode material for aqueous lithium-ion batteries as a communication,38 in this work, a detailed study of the intercalation properties of the oxide in an aqueous solution is presented.

2. MATERIALS AND METHODS 2.1. Materials. All chemicals used were of analytical grade from Merck and were used without further purification. All solutions were prepared with doubly distilled water. 2.2. Synthesis of Nanosheets of LiV3O8. Nanosheets of LiV3O8 were synthesized by a citrate assisted sol
gel method as described previously.38 Briefly, stoichiometric amounts of vanadium oxide (V2O5) and lithium oxalate (Li2C2O4) were mixed with distilled water and a solution of citric acid was added to the mixture under constant magnetic stirring. The total amount of metal ions to citric acid ratio was 1:1. Then the solution pH was adjusted to 7.0 by an ammonia solution. At this pH, V2O5 was dissolved completely and the solution's color changed to transparent dirt yellow. The obtained mixture was evaporated at 80
C for a few hours. The color of the solution changed first to light green and then to dark blue. The obtained dark blue gel was further dried overnight in an oven at 120
C to remove the excess water. Finally, the blue solid mass was ground in a mortar and then calcined at 450
C for 20 h to obtain nanosheets of LiV3O8. 2.3. Electrode Preparation. To prepare the working electrode for the electrochemical experiments, the nanosheets of LiV3O8 (80%) was ground with polyvinylidene difluoride (PVDF) (10%) and acetylene black (10%), and dispersed in N-methylpyrrolidone (NMP) by means of an ultrasonic bath to obtain a paste. The resulting composite was supported on a 2.0 mm diameter Pt disk electrode and then heated to 100
C in an oven for several hours. The working electrode contained ≈0.3 mg of the active material. The electrochemical behavior of the electrode was studied in a conventional three-electrode cell containing 1.0 M LiNO3 solution with a Pt disk and an Ag/AgCl/3 M KCl as

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Figure 1. SEM images of LiV3O8 nanosheets with three magnifications.

the counter and reference electrodes, respectively. The supporting electrolyte was 1.0 M LiNO3. 2.4. Apparatus. Electrochemical measurements were carried out using a μ-Autolab potentiostat/galvanostat, type III, FRA2 (The Netherlands). In impedance measurements, the frequency range of 500 kHz to 5 mHz was employed while the ac voltage amplitude was 10 mV and the equilibrium time was 5 s. The system was run by a PC through FRA and GPES 4.9 softwares. Surface morphological studies were carried out using a Model X-30 Philips scanning electron microscope (SEM). The transmission electron microscopy (TEM) was performed using a CEM 902A ZEISS transmission electron microscope, with an accelerating voltage of 80 kV. These techniques provided information about the morphology and size of the particles. Samples were prepared by placing a drop of the particles, dispersed in acetone, on a carbon-covered copper grid (400 mesh) and evaporating the solvent. Powder X-ray diffraction (XRD) patterns were measured by Philips X’Pert (The Netherlands) using Cu KR radiation at 40 kV and 30 mA in the 2θ degree range from 10
to 60
.

3. RESULTS AND DISCUSSION 3.1. Morphological Characterization. Scanning electron microscopy (SEM) micrographs of the as-prepared nanosheets of LiV3O8 with three magnifications are shown in Figure 1. The particle size of LiV3O8 sample (Figure 1A) is in the range 2
5 μm. A closer look on the particles (Figure 1B,C) indicates a layered structure consisting of the packed layers of LiV3O8 sheets a few nanometers thick. Transmission electron microscopy micrographs were measured for more morphological investigation. Micrographs of the as-prepared LiV3O8 sample with two magnifications are shown in Figure 2. The layered structure of LiV3O8 a few nanometers thick is obvious. This layered configuration causes parallel orientation of lithium diffusion pathway that increases the diffusion process efficiency (vide infra). 3.2. Structural Investigation. XRD patterns were used to characterize the phase structure of the nanosheets of LiV3O8. A typical powder XRD pattern of the nanosheets of LiV3O8 is shown in Figure 3. There are a lot of slightly broadened diffraction peaks that can all be indexed to a monoclinic system with p21/m space group.39 The peak at about 2θ = 13.9
is related to the diffraction at the (1 0 0) plane, confirming the layered structure for the 10890

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Figure 4. First three cyclic voltammograms of LiV3O8 nanosheets recorded in 1.0 M LiNO3 solution. The potential sweep rate was 0.3 mV s
1. Inset: ten consecutive cycles (5th
15th cycles) of LiV3O8 nanosheets recorded in 1.0 M LiNO3 solution. The potential sweep rate was 0.3 mV s
1.

Figure 2. TEM images of LiV3O8 nanosheets.

Figure 3. Typical X-ray diffraction pattern of LiV3O8 nanosheets.

LiV3O8 sample. These layers consist of VO5 trigonal bipyramids and VO6 octahedra which are corner sharing with the octahedra. The relative diffraction intensity of (1 0 0) crystal plane is lower than that reported for preferential ordered crystals of LiV3O8.40
42 This indicates that the preferential ordering for the nanosheets is avoided. The preferential ordering can lead to a large diffusion path and hinders the intercalation process. The XRD data suggests that the nanosheets of LiV3O8 have appropriate orientation of lithium diffusion pathways, which is advantageous for the intercalation/deintercalation of lithium ion into/from the nanosheets. This will be approved by electrochemical experiments (vide infra). The lattice parameters of the nanosheets of LiV3O8 were also obtained as a = 6.68 Å, b = 3.60 Å, c = 12.03 Å, V = 275.40 Å3, and β = 107.83
. 3.3. Voltammetric Investigation. The first three cyclic voltammograms of the nanosheets of LiV3O8 recorded in 1.0 M LiNO3 solution in the wide potential range of 250 to
700 mV using a potential sweep rate of 0.3 mV s
1 are shown in Figure 4. In the first cycle, there is one cathodic peak at about
318, a cathodic shoulder at about
200 mV, one anodic peak at

around
235 mV, and a small anodic shoulder at around
190 mV in the voltammogram. During the next cycles, however, only one pair of redox peaks with a midpeak potential of
268 mV is observed in the voltammograms. This well-defined redox peak pair is associated with the intercalation/deintercalation of lithium ion into/from the nanosheets of LiV3O8. The anodic and cathodic shoulders appearing in the first cyclic voltammogram can be attributed to the different occupation sites for the lithium ion. Wang et al.6 investigated the electrochemical intercalation of lithium ion into a LiV3O8 sample as a negative electrode from a saturated lithium nitrate electrolyte and observed three steps (three pair-peaks) during the intercalation process associating to the different phase transformations. Moreover, different phase changes during the intercalation/deintercalation of lithium ion into/from a LiV3O8 sample as a positive electrode material was reported in the nonaqueous electrolytes.43
46 In the case of nanosheets of LiV3O8 prepared here, however, only one pair of redox peaks is observed in the entire potential range. This indicates that no phase transformation occurred during the intercalation/deintercalation process. Similar structural-dependent electrochemical behavior of a LiV3O8 samples was also reported elsewhere.45 On the other hand, the peak potential separation in the voltammogram for the nanosheets of LiV3O8 is ≈100 mV. This value is smaller than that reported for LiV3O8 samples in both aqueous6 and nonaqueous45,46 electrolytes. Decreasing the peak potential separation observed here can be attributed to the nanosized dimension and indicates the improved kinetics of the intercalation/deintercalation process and the high reversibility of the nanosheets of LiV3O8. In addition, consecutive cyclic voltammograms (5
15th cycles) of the nanosheets of LiV3O8 were recorded (Figure 4, inset). It can be seen that the redox currents remain almost unchanged upon cycling, indicating the high cyclability of the nanosheets of LiV3O8 as an anodic material for ARLBs. Cyclic voltammograms of the nanosheets of LiV3O8 in 1.0 M LiNO3 solution recorded at different potential sweep rates of 0.2 to 80 mV s
1 are shown in Figure 5A. Upon increasing the potential sweep rate, the height and area of the redox peaks increase, while the corresponding charges (corresponding to the electrode capacity) remain almost unchanged. This confirms the favorable kinetics of charge transfer even at high potential sweep rates. In addition, both anodic and cathodic peak currents linearly 10891

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Figure 5. (A) Cyclic voltammograms of LiV3O8 nanosheets in 1.0 M LiNO3 solution recorded at different potential sweep rates from the inner to the outer of 0.2, 0.5, 1, 2, 5, 7.5, 10, 20, 40, 60, 80, and 100 mV s
1. (B) Dependency of the anodic peak currents on the square root of the potential sweep rate. (C) Dependency of the cathodic peak currents on the square root of the potential sweep rate.

depend on the square root of the potential sweep rate (Figure 5B,C). This linear dependency indicates that the intercalation/deintercalation process is controlled by lithium-ion diffusion in the host material. Using the slopes of the linear dependencies of peak currents on the square root of the potential sweep rates and on the basis of the Randles
Sevcik equation,47 

Ip ¼ ð2:69  105 Þn3=2 ACLi DLi 1=2 ν1=2

ð1Þ

where Ip, n, A, and ν are the peak current, number of exchanged electrons, surface area of the electrode and potential sweep rate, DLi is the chemical diffusion coefficient of lithium ion, and CLi* is the bulk concentration of lithium ion (0.012 mol cm
3 for LiV3O8 derived from the theoretical density of 3.45 g cm
3), the average value of DLi into the nanosheets of LiV3O8 is obtained as 3.39  10
10 cm2 s
1. 3.4. Chronoamperometric Investigations. Chronoamperometry (CA) is a powerful electrochemical technique for the characterization of ion diffusion in the solid-state host materials.47
49 In a typical CA measurement, the potential of the working electrode is stepped from an initial value, where no electrolysis occurs, to a final value, where a species is electroactive. The resulting current from faradaic processes occurring at the electrode is monitored as a function of time until steady-state conditions are achieved. The expression of the current response to a potential step in a long time domain, assuming onedimensional diffusion transport of a species under finite-space conditions, can be represented as follows:49 IðtÞ ¼ 2ΔQ =τd

¥

∑ exp½
ð2b
1Þ2 π2t=ð4τd Þ b¼0

for t > τd ð2Þ

Figure 6. (A) Typical chronoamperogram of LiV3O8 nanosheets recorded in 1.0 M LiNO3 solution using a step potential of
350 mV. (B) Cottrell plot corresponding to Figure 6A. (C) ln(
I) vs t plot.

where I(t) is the current response to the potential step, ΔQ is the total charge transferred into the electrode during the potential step, τd is the diffusion time constant, t is the elapsed time from the beginning of the potential step, and b is an integer number, with τd ¼ l2 =DLi and

Z

¥

ΔQ ¼

IðtÞ dt ¼
FAlΔC

ð3Þ

ð4Þ

0

where l is the diffusion length, F is the Faraday constant, and ΔC is the variation of the diffusing species concentration. In the present study, a potential of
350 mV (corresponding to the potential of the cathodic peak in the CV plot, Figure 4) was enforced to record the chronoamperogram depicted in Figure 6A. The net current decayed exponentially to a limited value. This indicates that the mass transport of intercalating species occurred only by means of diffusion. A Cottrell plot related to the above-mentioned chronoamperogram is shown in Figure 6B. Three different kinetic regions can be distinguished from the plot. Region K1 (short-term) corresponds to the double layer charging or in other words, relates to the interfacial charging of both the Pt substrate/LiV3O8 and the LiV3O8/electrolyte interfaces. Region K2 (medium term) represents almost constant values for the It1/2, reflecting the fact that the semi-infinite planar diffusion of lithium ions into the nanosheets of LiV3O8 occurs in 10892

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Figure 7. (A) Five consecutive double-step chronopotentiograms (charge/discharge curves) of LiV3O8 nanosheets in 1.0 M LiNO3 solution in the potential range þ200 to
700 mV at a 2C/5 rate or 64 mA g
1. (B) Potential profiles of LiV3O8 nanosheets in 1.0 M LiNO3 solution at different rates (rates for charge and discharge were same).

this region. Region K3 (long-term) represents the finite-space diffusion of lithium ion into the nanosheets of LiV3O8 in the long time domain. Dependency of the response current with time in region K3 can be derived from eq 2 and represented by the following equation:50,51 I ¼ ð2ΔQDLi =l2 Þ expð
π2 DLi t=4l2 Þ

ð5Þ

The plot of ln(I) vs t, which has a linear relation for t > 300 s, is shown in Figure 6C. From the slope of this line and using eq 5, the diffusion coefficient of lithium ion into the nanosheets of LiV3O8 was obtained as 1.89  10
10 cm2 s
1. Interestingly, the diffusion coefficient obtained from the long time domain (region K3) is in closed agreement with the diffusion coefficient obtained from CV technique. 3.5. Charge/Discharge Curves. Five consecutive charge/ discharge curves for the nanosheets of LiV3O8 in 1.0 M LiNO3 solution in the potential range þ200 to
700 mV at a 1.6C rate (or 0.1 A g
1) are depicted in Figure 7A. The charge/discharge curves show a plateau between
250 and
310 mV, corresponding to the redox potential of the cyclic voltammogram (Figure 4). At the same time, there is only one step of intercalation/ deintercalation process for the lithium ion into/from nanosheets of LiV3O8. These chronopotentiograms suggest only one step for the intercalation/deintercalation process without any phase transition step for the synthesized nanosheets of LiV3O8, confirming the results obtained by CV. In addition, a discharge capacity of 63 mAh g
1 is obtained for the nanosheets of LiV3O8 and the electrode material exhibits >95% Columbic efficiency. This discharge capacity value is greater than the value reported by Kohler et al.2 in an aqueous electrolyte.

Figure 7B shows a rate capability test. The nanosheets of LiV3O8 show discharge capacities of 63, 57, and 43 mAh g
1 at charge/discharge current densities of 0.1 A g
1 (1.6C), 0.5 A g
1 (8.8C), and 1 A g
1 (23.2C) between þ200 and
700 mV. 3.6. Electrochemical Impedance Spectroscopy. Electrochemical impedance spectroscopy can access relaxation phenomena over many orders of magnitude and has been employed for the study of the charge transfer kinetics and mass transport phenomena during the intercalation process into the mainly porous electrodes.48,52
54 In the course of lithium intercalation/deintercalation, the processes involved are a combination of (a) electron injection at the host material/current collector interface, (b) diffusion of lithium ions in the solid lattice of the host material, (c) flip-flap of lithium ion across the host material/solution interface, and (d) the lithium-ion transport in the bulk of solution.55 It is assumed that the ion transport in the bulk of solution is fast enough not to control the rate of the intercalation process. Figure 8 represents Nyquist diagrams recorded at various dcoffset potentials in the range of 0 to
450 mV corresponding to the potential of the redox peaks of the cyclic voltammogram (Figure 4) and the plateau region of the charge/discharge curves (Figure 7). The appearance of different signatures in the Nyquist diagrams (denoted as responses at high, medium, and low frequencies) implies different physicochemical processes involved during the intercalation/deintercalation of lithium ions. Nyquist diagrams recorded at dc-potentials of 0 to
120 mV and
360 to
450 mV represent a squashed and depressed semicircle with an approximate unit slope at high frequencies and a diffusional impedance at lower frequencies. Nyquist diagrams recorded at dc-potentials of
150 to
330 mV represent a squashed semicircle at high frequencies, an arc at medium frequencies, and a diffusional impedance at lower frequencies. Some similarity in the patterns of the Nyquist diagrams indicates the same electrochemical mechanism dominated during the process. Moreover, the diffusional impedance corresponds to the mass transport following by accumulation of the intercalative lithium ion in the nanosheets of LiV3O8. The high-frequency response is attributed to the charge (electron) injection across the interface of current collector/ electrode material and related to the V(V)/V(IV) redox transition. This is further confirmed by the fact that the diameter of this semicircle (which is the charge transfer resistance of the electrodic process) depends on the dc-offset potential (see Table 1 in bellow). The large electrode surface area, on the other hand, arises from the porous structure of the electrode56 (vide infra). It should be noted that in some studies on the intercalation of lithium into composite electrodes, a very high-frequency semicircle has been appeared in the Nyquist diagrams.57,58 This semicircle has been attributed to the interface parameters such as electrode porosity, surface film formed on the active material and the current collector, and/or bulk of materials.57,58 This semicircle may be appeared at very high frequencies and was not detectable in the swept frequency range in this work. An important theoretical model for the impedance response of a porous intercalating electrode has been established54 and further theoretically developed for the lithium intercalation in a single particle as well as composite electrodes.59 On the basis of this model, two important parameters that affect the impedance response of the intercalating electrodes are the conductivity of the electrolyte solution and conductivity of the solid host material (denoted respectively as σ and k). The effect of low values of k and σ is known in terms of Ohmic potential drops in 10893

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Figure 8. Nyquist diagrams of LiV3O8 nanosheets in 1.0 M LiNO3 solution recorded at various dc-offset potentials of 0 to
450 mV.

Table 1. Values of the Equivalent Circuit Elements Obtained by Fitting the Experimental Results in the Nyquist Diagrams Represented in Figure 8, the Corresponding Relative Errors, and Values of the Diffusion Coefficient of Lithium Ion at Different Bias CPEdl CPEdl-j

Wo103τD/s

1010DLi/cm2 s
1

3.89 (2.97%) 3.75 (3.31%)

0.73 (3.41%) 0.74 (3.74%)

6.9 (4.07%) 10.08 (4.26%)

36.23 24.80

16.67 (6.23%)

3.21 (4.50%)

0.75 (3.21%)

14 (4.41%)

17.86

16.56 (4.63%)

2.81 (3.35%)

0.74 (3.74%)

19 (4.65%)

13.16

16.03 (4.51%)

2.71 (3.63%)

0.75 (4.13%)

25 (5.48%)

10.00

30.9 (0.54%)

15.76 (3.14%)

2.73 (8.79%)

0.75 (2.58%)

32 (8.83%)

7.82

30.7 (0.56%)

15.31 (7.03%)

2.66 (3.09%)

0.76 (3.58%)

41 (7.12%)

6.09


210

30.4 (0.21%)

15.02 (2.06%)

2.63 (9.86%)

0.76 (1.23%)

49 (3.21%)

5.10


240
270

30.7 (0.40%) 30.6 (078%)

14.75 (4.81%) 14.49 (2.84%)

2.75 (5.15%) 2.54 (5.90%)

0.75 (2.33%) 0.76 (3.36%)

78 (3.18%) 85 (1.55%)

3.20 2.94


300

30.7 (0.79%)

14.26 (2.27%)

2.68 (3.13%)

0.75 (1.75%)

59 (2.07%)

4.24


330

30.2 (0.26%)

14.07 (1.76%)

2.54 (9.89%)

0.72 (1.28%)

43 (7.44%)

5.81


360

30.5 (0.72%)

13.95 (4.63%)

2.77 (3.50%)

0.72 (3.14%)

22 (4.97%)

11.36


390

30.5 (0.69%)

13.85 (4.44%)

3.12 (3.39%)

0.73 (3.60%)

18 (4.54%)

13.89


420

30.1 (0.65%)

14.07 (4.03%)

3.38 (3.08%)

0.74 (3.29%)

13 (4.27%)

19.23


450

30.1 (0.64%)

13.97 (4.09%)

3.62 (3.13%)

0.74 (3.13%)

7.4 (4.31%)

33.78

bias/mV

Rs/Ω

Rct/Ω

0
30

31.2 (0.71%) 31.1 (0.80%)

17.48 (4.03%) 17.08 (4.42%)


60

31.8 (1.0%)


90

31.8 (0.81%)


120

31.6 (0.85%)


150
180

CPEdl-T  10 /Ω 5

the pores filled with the electrolyte and in the solid phase. This causes the high-frequency semicircle (related to the combination of the charge transfer process and the double layer capacitance) to be squashed and a typical medium-frequency impedance response that reflects distributed elements.54 This particular type


1 j

s

of dispersion of the capacitance of porous intercalation electrodes is related to the coupling of interfacial and bulk quantities.60 Along this line, in the Nyquist diagrams represented here, the squashed semicircle at high frequencies is a result of a limitation in the value of k and/or σ. 10894

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The Journal of Physical Chemistry C The charge transfer resistance (Rct) of the electrode reaction is the only circuit element that has a simple physical meaning describing how fast the rate of electron injection during intercalation/deintercalation changes with the electrode potential, while the other parameters are kept constant. The best equivalent circuits that fulfill the impedance characteristics of the intercalation process are shown in Schemes 1 (corresponding to the Nyquist diagrams recorded at 0 to
120 mV and
360 to
450 mV) and 2 (corresponding to the Nyquist diagrams recorded at
150 to
330 mV). These circuits were fitted to the experimental results using a nonlinear least-squares fit. In the circuits, Rs, Rct, and CPEdl characterize the high-frequency response and represent solution resistance, charge transfer resistance and double layer capacitance, respectively. In Scheme 2, parallel combination of CPE1 and R1 characterizes the medium-frequency response. Wo and Cint characterize the low-frequency response indicating lithium-ion diffusion and occupation, respectively. In Figure 8, the symbols represent experimental data and the continuous lines were drawn using the values of the circuit elements obtained by fitting. The values of the equivalent circuit elements obtained by fitting and the corresponding errors are presented in Table 1. Scheme 1

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The good agreement between the fitted and experimental curves (which is also confirmed by the low numerical errors of the fitted values, Table 1) in the high frequency range (Figure 8) led to estimation of the Rct, j, and T values at different dc-offset potentials. These values were plotted as a function of dc-offset potential and shown in Figure 9. CPEdl-j remained almost constant for the different dc-offset potentials. CPEdl-T can be directly related to the double layer capacitance due to the value of j being constant. The double layer capacitance is high at the two entire potentials of intercalation/deintercalation processes. Then, it decreased to low values. This indicates starting the intercalation process. It remains almost constant and then reaches to a maximum at fully charged state. The charge transfer resistance remains almost constant at lowest values at high dcoffset potentials. Then, it increases upon increasing the dc-offset potentials. This behavior has been related to the surface defects present in the electroreactive nanostructured materials.61 During the redox process of the nanosheets, the surface defects present in the nanosheets react with a high rate (the charge transfer resistance is low) and then, the bulk of the particles participate.61 These surface defects produce equal electroreactive sites on the first atomic layer of the nanosheets; therefore, no initial variation in Rct was observed.61 After consumption of these defect sites, the reaction propagates deeper into the bulk of the nanosheets; the reaction is carried out with a lower rate and Rct is increased. From the variation of the values of Rct in the pure charge transfer region, the value of the exchange current density for the redox process of V(V)/V(IV) can be obtained using:47 Rct ¼ RT=nFi0

Scheme 2

ð6Þ

where n is the number of exchanged electrons, i0 is the exchange current density, and the other parameters have their usual meaning. The exchange current density was obtained as 0.047 A cm
2. From the value of the exchange current density and the following equation:47 i0 ¼ nFk0 CLi



ð7Þ

Figure 9. Variations of Rct (A), j (B), and T (C) on dc-offset potential. The data derived from Nyquist diagrams represented in Figure 8. 10895

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where k0 is the standard rate constant; the standard rate constant for the faradaic reaction of V(V)/V(IV) during the lithium intercalation was obtained as 1.29  10
3 cm s
1. At medium frequencies, an arc appeared in the Nyquist diagrams. This arc is characterized by a combination of a constant phase element and a resistance in parallel (CPE1 and R1 in Scheme 1) and is also a consequence of the limitation in the value of k and/or σ (vide supra).54,59 It should be noted that only when the redox reaction starts, σ and k affect the impedance response. Therefore, in the equivalent circuit represented in Scheme 1, Rct and CPE1-R1 are placed in series. The total impedance of the mixed particle porous electrode is represented as54,59,62,63 Ztot ¼ ½r=ðσ þ kÞf1 þ ½2 þ ððσ2 þ k2 Þ=σkÞÞ cosh ζ =ðζ sinh ζÞg

ð8Þ

with ζ ¼ r½ðσ þ kÞχ=σkZ1 0:5

ð9Þ

Z1 ¼ ðRct þ Zdiff Þ=½1 þ jωCdl ðRct þ Zdiff Þ

ð10Þ

Zdiff ¼ ZFLW þ Cint

ð11Þ

and

while

and ZFLW ¼ RFLW fctnhðjωRFLW CFLW Þ0:5 =ðjωRFLW CFLW Þ0:5 g ð12Þ where r is the particles radius, χ is the surface area to volume ratio, ZFLW is a finite length Warburg impedance, RFLW is the resistance of the diffusion of a species through a finite length, CFLW describes the capacitance of the finite space, and Cint characterizes lithium-ion occupation in the lattice sites. In eq 12, ZFLW is the one-dimensional diffusion impedance, which is completely analogous to wave transmission in a finite-length RC transmission line.64 On the basis of this impedance equation, finite values of k and/or σ cause the appearance of both a squashed semicircle with an approximately unit slope at the limit of high frequencies (which is due to the distributed character of the impedance at these finite values rather than to a semi-infinite diffusion54) and an arc in the middle frequencies. The impedance response at low frequencies was composed of two individual time constants for the diffusion process of lithium ion in a finite-space and the capacitive behavior of the characteristic response of a finite blocked diffusion through the electrode (lithium-ion occupation in the intercalating sites) during the intercalation/deintercalation process (eq 12). In eq 12:52,62
64 RFLW CFLW ¼ τd ¼ r 2 =DLi

And any change in the electrode thickness, while r remains constant, causes only Cint to be changed.42 For these reasons, it was assumed that the electrode thickness is the diffusion length in the chronoamperometric studies, while the particle radius is the diffusion length in the impedance measurements. The values of the electrical element components of the low-frequency response (τd) were obtained by a fitting procedure and are reported in Table 1. Using eq 13 and the values of τd, the values of the diffusion coefficient of lithium ion in the nanosheets of LiV3O8 at each individual dc-offset potential were obtained and are reported in Table 1. The diffusion coefficient of lithium ion is potentialdependent and changed smoothly and represent a minimum at
270 mV. Interestingly, this minimum value, which was obtained from Nyquist diagram recorded at dc-offset potential equal to the formal potential (midpeak) of cyclic voltammogram, is almost equal to that obtained from cyclic voltammetry. Similar behavior of the dependency of solid-state diffusion coefficient of lithium ion on the electrode potential has been reported elsewhere.48,65
67 The appearance of a minimum in the dependency of DLi on dcoffset potential was discussed on the basis of different explanations. Barker et al.66 related this minimum to the Li
Li ionic Coulombic repulsions within each structural site in the host lattice. Prosini et al.67 related this minimum to the strong attractive interactions between the lithium ions and the host lattice. Zhang et al.57 related this minimum to the several structural changes and phase transformations of the host lattice and also to the temporary strong Li-host lattice binding.57

4. CONCLUSION Nanosheets of LiV3O8 were synthesized by a new process using citrate and lithium oxalate and vanadium oxide. The electron microscopy investigations indicated that the prepared anodic materials have a layered structure with a mean size of 2
5 μm comprising compact nanosheets. Structural analysis using X-ray powder diffraction showed that the synthesized nanosheets of LiV3O8 were of good crystallinity. The kinetic parameters of the intercalation were studied by means of cyclic voltammetry and potential step chronoamperometry. The lithium-ion diffusion coefficient was determined by analyzing the data of the CV as well as CA measurements. Three kinetic regions were recognized from CA data analysis corresponding to the separate steps of the entire intercalation process. A careful interpretation of the impedance data obtained at different dc-offset potentials allowed the separation of different dominated processes. ’ AUTHOR INFORMATION Corresponding Author

*E-mail addresses: [email protected]. Tel: þ98 728 46 92 114. Fax: þ98 728 46 92 153.

ð13Þ

It should be noted that the active mass of the working electrode comprised particles of average radius r, oriented parallel to the electrode substrate. In these electrodes, lithium-ion diffusion occurs in a direction parallel to the electrode substrate, and the particle radius is the true diffusion length.42,62,63 Here, the impedance of the electrode process in the low-frequency range was modeled by a series combination of Wo and Cint. This combination causes the diffusion time constant to be independent to the active material thickness and it depends on the particle radius. Cint is affected by the active materials thickness.

’ ACKNOWLEDGMENT The financial support of the Iran National Science Foundation (INSF), the Research Councils of Islamic Azad University, Young Researchers Club, and K. N. Toosi University of Technology are gratefully acknowledged. ’ REFERENCES (1) Wu, Y. P.; Dai, X. B.; Ma, J. Q.; Cheng, Y. J. Lithium Ion Batteries: Practice and Applications; Chemical Industry Press: Beijing, 2004. 10896

dx.doi.org/10.1021/jp201382n |J. Phys. Chem. C 2011, 115, 10889–10897

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