Self-Assembled Synthesis of Hierarchical Waferlike Porous Li–V–O

ARTICLE pubs.acs.org/JPCC

Self-Assembled Synthesis of Hierarchical Waferlike Porous Li
V
O Composites as Cathode Materials for Lithium Ion Batteries Y.Q. Qiao, J.P. Tu,* X.L. Wang, J. Zhang, Y.X. Yu, and C.D. Gu State Key Laboratory of Silicon Materials and Department of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, China

bS Supporting Information ABSTRACT: Wafer-like porous xLiV3O8-yLi0.3V2O5 (Li
V
O) composites are synthesized by a facile self-assembled synthesis using a glycine-assisted solution route followed by a low-temperature reaction. The crystalline compounds have a uniform shape and distribution of sizes, and the primary particles are platelike in shape with 150
500 nm in length and 80
200 nm in width. Such porous materials have many advantages such as good electrical contact among the particles and easy for electrolyte to penetrate the active materials, thus facilitating improvement in the electrochemical performance of Li-intercalation cathode. Among these Li
V
O composites, the one synthesized at 400 °C, which has 27.06 wt % Li0.3V2O5, exhibits the highest initial discharge capacities of 300.5, 265.7, and 237.0 mAh g
1 at current densities of 20 (C/15), 50 (C/6), and 120 mA g
1 (0.4 C) between 2.0 and 4.0 V, respectively. The good electrochemical performance of the as-synthesized composite can be attributed to the porous structure, thus highly favors the solid-state diffusion kinetics and enhances the capacity of the Li
V
O electrode. The ease of synthetic preparation of this novel porous Li
V
O composite together with its good electrochemical performance shows promise application in lithium ion batteries.

1. INTRODUCTION With the rapid consumption of fossil fuels and aggravating global warming due to the burning of fossil fuels for energy, seeking clean and renewable energy sources to support the sustainable development of the global economy and society is perhaps the greatest challenge in today’s energy-based society. The lithium-ion battery is the most promising energy storage and conversion system for a wide range of applications, such as portable electronic devices, electric vehicles (EVs), hybrid electric vehicles (HEVs), and stationary energy storage from renewable energy resources.1
4 LiCoO2 demonstrably represents the most successful category of commercial cathode material; however, its high cost, relative toxicity of cobalt and safety concerns of this material may limit the commercial usefulness in future.5 In recent years, olivine-type LiFePO4 has been extensively studied because it is a prosperous alternative to replace the commercial cathode materials for lithium-ion batteries.1,5,6 Lithium vanadium oxide, LiV3O8, is also regarded as a promising cathode material in rechargeable lithium batteries, which has been investigated extensively during the last decades because of its potentially high specific capacity, low cost, good safety, and high electronic conductivity.7
12 LiV3O8 has a layered monoclinic structure (P21/m) which consists of two structural units of VO6 octahedra and VO5 trigonal bipyramids. These two units are interconnected to each other by corner-sharing oxygen atoms to form V
O layers.10
12 r 2011 American Chemical Society

Lithium ions generally reside in the octahedral interstices between the layers, while the extra lithium ions involved in the charge/discharge processes are accommodated at the tetrahedral sites.10 More than 3 lithium ions per formula can be inserted into this compound.8 Lithium ions housing in octahedral sites before Li insertion are no hindrance to incoming lithium ions from occupying empty tetrahedral sites, thus showing a good structural reversibility during the intercalation/deintercalation processes.8,10,13 For LiV3O8, an extensive amount of study has found that the electrochemical properties of this compound are strongly connected with the preparation method or synthetic condition. Up to now, various methods have been employed to prepare LiV3O8 such as solid-state reactions,8,9,13 low-temperature reaction routes,14,15 hydrothermal reactions,16,17 sol
gel processes,11,18,19 rheological phase reaction methods,20 spray-drying syntheses,21 spray pyrolysis,22 combustion methods,23 freeze-drying,24
26 ultrasonic methods,27 RF magnetron sputtering,28,29 and so on. The final products are different in many ways such as microstructure, particle size distribution, crystallinity, and phase purity, which have a significant influence on the electrochemical performance. For instance, LiV3O8 nanorods could be obtained by low-temperature thermal codecomposition,10 surfactant-assisted Received: August 19, 2011 Revised: October 1, 2011 Published: November 14, 2011 25508

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Figure 1. TGA/DSC curves of the precursor for Li
V
O composites recorded from room temperature to 700 °C at a heating rate of 10 °C min
1 in air.

polymer precursor,12 hydrothermal reaction,16 hydrothermal-solsolid-state,30 and electrospinning combined with sol
gel31 routes; however, the discharge capacities and cycling performances of those nanorods were still in relatively remarkable differences. Actually, the LiV3O8 nanorods showed excellent electrochemical performances which could be attributed to their one-dimensional structure, since the one-dimensional nanorods had a large surface-to-volume ratio, short Li diffusion distances and facile strain relaxation during the intercalation/deintercalation processes.32,33 Recently, Shi et al.28 prepared LiV3O8 thin film with a mixed amorphous-nanocrystalline microstructure and found that this film showed a high initial discharge capacity of 382 mAh g
1 at a current density of 10 μA cm
2. As is well-known, porous aggregates of electrode materials show great advantages such as good contact with electrolyte, high specific surface area, improving Li+ permeation and easier to bind than isolated nanosized particles, which can reduce the polarization and decrease the structure stress during the charge/discharge processes.34
40 Thus, the cathode materials with porous structure have been considered to be an efficient way to improve their electrochemical performance. Liu et al.25 reported that porous LiV3O8 synthesized by a combination of a freeze-drying method and post-treatment under Ar delivered a very high insertion capacity of 347 mAh g
1 at a current density of 50 mA g
1 in the voltage range of 1.5
4.5 V. Recently, another kind of porous LiV3O8 was synthesized by using a tartaric acidassisted sol
gel process which displayed a maximum discharge capacity of 320 mAh g
1 at a current density of 40 mA g
1 between 1.5 and 3.5 V.41 In this work, we describe the self-assembled syntheses of waferlike porous xLiV3O8-yLi0.3V2O5 composites by using a glycine-assisted solution route followed by a low temperature reaction. It is necessary to demonstrate here that these porous composites are composed of LiV3O8 and Li0.3V2O5 phases. The presence of Li0.3V2O5 in LiV3O8 has also been reported by other groups.10
12,19,21,42 A few reports have shown the presence of both Li0.3V2O5 and LiV2O5 impurities to degrade electrochemical performance.43,44 However, many other works present a minor influence on the electrochemical performance10
12,19,42 or even an improved performance of LiV3O8 in the presence of other active phases.21,45,46 Tran et al.21 synthesized Li1.1V3O8 by a spray-drying method and observed that the Li1.1V3O8

Figure 2. Rietveld-refined XRD patterns of Li
V
O composites synthesized at different temperatures: (a) 350 °C (32.09 wt % Li0.3V2O5) and 400 °C (27.06 wt % Li0.3V2O5) and (b) 450 °C (26.33 wt % Li0.3V2O5) and 500 °C (14.48 wt.% Li0.3V2O5). (c) XRD pattern of LiV3O8 compound synthesized at 550 °C.

contained Li0.3V2O5 phase had better electrochemical performance compared to the phase-pure compound. Dubarry et al.45,46 prepared Li1+nV3O8/β-Li1/3V2O5/C nanocomposites by carboreduction and found that the electrochemical performances of these nanocomposites were significantly better than that of pure LiV3O8. Here, we defined the xLiV3O8-yLi0.3V2O5 as L
V
O composites. The morphology, crystal structure, electrochemical performance, and kinetics properties of our novel synthesized 25509

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Figure 3. SEM images of waferlike porous Li
V
O composites synthesized at different temperatures: (a) 350 °C, (b) 400 °C, (c) 450 °C, and (d) 500 °C.

Figure 4. N2 adsorption
desorption isotherm of the Li
V
O composite synthesized at 400 °C. The inset shows pore size distribution, calculated from desorption branch of isotherm, using Barrett
Joyner
Halenda method.

hierarchical waferlike porous Li
V
O composites were systematically investigated.

2. EXPERIMENTAL SECTION 2.1. Synthesis. All chemicals were used directly without further purification. The waferlike porous Li
V
O compounds were synthesized using a glycine-assisted solution route followed by a low-temperature reaction. In a typical synthesis, 2.06 g of LiOH 3 H2O, 15.89 g of NH4VO3, and 5.00 g of glycine were added into 50 mL of deionized water under magnetic stirring for 1 h at room temperature. Then, the resulting white suspension was heated in an oven at 90 °C until the solution was dried. The as-obtained precursor was calcined at 350
550 °C in air for 6 h to yield the final products. 2.2. Material Characterization. Differential scanning calorimety and thermogravimetric analysis (DSC-TGA) of the precursor was measured on a SDT Q600 apparatus in the temperatures ranging from 25 to 600 °C at a heating rate of 10 °C min
1 in

air. The morphologies and structures of the as-synthesized powders were characterized using field emission scanning electron microscopy (FESEM, FEI SIRION), X-ray diffraction (XRD, Philips PC-APD with Cu Kα radiation), and high-resolution transmission electron microscopy (TEM, Tecnai G2 F30 S-Twin). The unit cell lattice parameters were obtained by Rietveld refinement of the powder XRD data using the software Maud.47,48 2.3. Electrochemical Measurements. Electrochemical performances of Li
V
O composites were investigated using CR2025 coin-type cell. A metallic lithium foil served as the anode. The cathode consisted of 75 wt % active material, 15 wt % acetylene black, and 10 wt % polyvinylidene fluoride (PVDF) on aluminum foil. LiPF6 (1 M) in ethylene carbonate (EC):dimethyl carbonate (DMC) (1:1 in volume) was the electrolyte, and a polypropylene microporous film (Cellgard 2300) was the separator. The cells were assembled in an argon-filled glovebox with H2O and O2 concentrations below 1 ppm. The charge
discharge tests were conducted on LAND battery programcontrol test systems (Wuhan, China) between 2.0 and 4.0 V by applying from 20 to 480 mA g
1 (C/6 to 1.6 C) current densities at room temperature. Cyclic voltammetry (CV) tests were performed on CHI660C electrochemical workstation in the potential range of 2.0
4.0 V (vs Li/Li+) at a scan rate of 0.1 mV s
1. For electrochemical impedance spectroscopy (EIS) measurements, the test cells were with the metallic lithium foil as both the reference and counter electrodes. EIS measurements were performed on CHI660C electrochemical workstation over a frequency range of 100 kHz to 10 mHz at both stages of charge and discharge (2.0
4.0 V) by applying an AC signal of 5 mV. The galvanostatic intermittent titration technique (GITT) was employed at a pulse of 10 mA g
1 for 10 min and with 40 min interruption between each pulse.

3. RESULTS AND DISCUSSION 3.1. Structure and Morphology Analysis. To determine the calcining temperature, DSC-TGA analysis is introduced to study the precursor and the results are illustrated in Figure 1. Three main weight-loss regions are observed in the TGA curve. 25510

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Figure 5. (a) TEM and (b) HRTEM images of waferlike porous Li
V
O composite synthesized at 400 °C.

The first one between 25 and 271 °C is attributed to the release of adsorbed water and NH3. The second steep weight loss, which occurs between 271 and 314 °C, is related to the decomposition of glycine, NH4VO3, and LiOH. It has been previously reported that the decomposition of glycine mainly occurs in the temperature range of 230
300 °C.49,50 Therefore, a big absorbing heat peak at 297 °C on the DSC curve can be mainly ascribed to the decomposition of glycine. The weight loss that occurs between 314 and 386 °C in the TGA curve can then be ascribed to the formation of the Li
V
O compounds. Above 386 °C, there is no obvious weight loss in the TGA curve. Therefore, we choose the calcining temperatures as 350, 400, 450, 500, and 550 °C. The XRD patterns and the Rietveld refinement of the Li
V
O powders are shown in Figure 2. Only the main (hkl) lines are indexed for clarity. The refinement parameters for Li
V
O compounds are listed in Table S1 of Supporting Information. It can be seen that the composites synthesized in the temperature range of 350
500 °C are all composed of LiV3O8 and Li0.3V2O5 phases. The LiV3O8 compound has a

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layered monoclinic structure and belongs to the P21/m space group (JCPDS 72
1193). The XRD pattern of Li0.3V2O5 is refined on the basis of reported monoclinic structure (JCPDS 73
1670). The weight percentage of the two phases within the composites was estimated using a Rietveld refinement of the XRD patterns. It is found that the calcining temperature has a great influence on the content of Li0.3V2O5 in the composites. The composite synthesized at 350 °C has a high percentage of Li0.3V2O5 phase (32.09 wt.%). For the ones synthesized at 400 and 450 °C, 27.06 wt % and 26.33 wt % Li0.3V2O5 phase are present in the two composites, respectively. At a higher temperature of 500 °C, the composite contains only 14.48 wt % Li0.3V2O5. However, with further increasing the calcining temperature to 550 °C, all of the reflections are attributed to the monoclinic phase LiV3O8 that lacks any impurity phase, as shown in Figure 2c. The compound was found to have a very high intensity (100) plane and slightly lower a value (Table S1 of Supporting Information), indicating the preferred orientation along the (100) plane and a slightly smaller interlayer spacing in the structure which would lead to a longer diffusion path for the lithium ions and thus depress the electrochemical performance.10
12,30 The SEM images of precursor powder formed using a glycineassisted solution route are shown in Figure S1 of Supporting Information. Interestingly, the precursor powder exhibit micrometer-sized aggregates of cubic particles (Figure S1a). The SEM image of a single precursor clearly demonstrates that the cubic particle is solid (Figure S1b). After the calcination at different temperatures, the particles still remain cubic morphology (Figure 3). For the composite synthesized at 350 °C, its surface is relatively smooth and dense, and only some micropores exist which can be attributed to the volatilization of gases during the calcination, as shown in Figure 3a. With increasing the temperatures to 400 and 450 °C, it can be seen from parts b and c of Figure 3 that the Li
V
O cubic particles are waferlike in shape with pore distribution over the whole area of the particles forming a waferlike porous structure. The Brunauer
Emmett
Teller specific surface area for the powder synthesized at 400 °C was measured to be 41.5 m2 g
1, and the pore size ranged from 20 to 50 nm (Figure 4). It is comparable to the results of mesoporous spherical LiFePO4/C materials.6 With further increasing the calcining temperature to 500 °C, the particles are still maintained in waferlike morphology but become more unconsolidated, as shown in Figure 3d. These porous structures can promote good electrical contact among the particles and help the electrolyte to penetrate the materials, thus highly favoring the solid-state diffusion kinetics and enhancing the capacity of the Li
V
O electrode materials. However, with an increase in the calcining temperature to 550 °C, the porous Li
V
O particles are found to melt into compact LiV3O8 agglomerations, as shown in Figure S2 of Supporting Information. The porous Li
V
O composite synthesized at 400 °C is further analyzed using TEM after grinding and ultrasonication, as shown in Figure 5. The primary particles are platelike in shape with sizes of 150
500 nm in length and 80
200 nm in width (Figure 5a). From the HRTEM image (Figure 5b), the lattice fringes are clearly visible in the body part of the particle, indicating a well crystallinity of the Li
V
O composite. The 11.24 Å spacing and 3.34 Å spacing correspond to the (001) and (
201) planes of monoclinic LiV3O8, respectively. On the basis of the above results, it is necessary to understand the formation mechanism of the hierarchical waferlike porous Li
V
O composites, and Scheme 1 illustrates a schematic 25511

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Scheme 1. Schematic Illustration of Synthetic Process of Hierarchical Waferlike Porous Li
V
O Composites

drawing of the main process of preparing the composites. We proposed that the formation of precursor of the hierarchical waferlike porous Li
V
O composites was a simplified selfassembled synthetic process by using glycine as a chelating agent and also a crystal control additive. First, multiple reactants were mixed in deionized water under magnetic stirring for 1 h at room temperature. Then, the resulting white suspension was transferred into an oven at 90 °C. After heating for 2 h, the white suspension was changed into a clear solution. With prolonging the heating time, the color of the clear solution became yellowish and then changed from yellowish to dark yellow. When continued to keep heating with the evaporation of H2O from the liquid, black cubic precursor was formed by self-assembly through the combinative action of chelation and recrystallization of glycine. Herein, glycine has an important function in controlling the morphology of the precursor. The hydroxyl group and amino group in glycine can chelate with metal ions during the heating process. In the process of recrystallization, the crystallization progress in the metal chelated glycine caused a precursor to grow in a required direction, thus forming a regularly shaped precursor. Subsequently, in the calcining step, the cubic precursor could decompose and release H2O, CO2, NH3, and NOx gases, resulting in the formation of pores in the particles. Consequently, the hierarchical waferlike porous Li
V
O composites were obtained. 3.2. Electrochemical Performance. For potential Li-ion battery application, electrochemical performances of the hierarchical waferlike porous Li
V
O composites as cathode materials were examined. Figure 6a shows the initial charge
discharge curves of porous Li
V
O composites synthesized at different temperatures at a current density of 50 mA g
1 between 2.0 and 4.0 V. As shown in this figure, one main charge plateau around 2.8 V and six discharge plateaus around 3.6, 3.4, 2.8, 2.7, 2.6, and 2.5 V are observed in the charge
discharge curves. The plateaus around 3.4 and 2.6 V belong to the active phase

Li0.3V2O5 as observed in the XRD patterns (Figure 2). Compared to the others, the composite synthesized at 400 °C has long charge
discharge plateaus and low plateau voltage separations, indicating that it has low electrochemical polarization and excellent reversibility in the charge
discharge processes.10,37,51 The composite synthesized at 400 °C exhibits an initial discharge capacity of 265.7 mAh g
1, which is higher than those synthesized at 350 °C (236.9 mAh g
1), 450 °C (228.7 mAh g
1), and 500 °C (208.3 mAh g
1) and much higher than pure LiV3O8 synthesized at 550 °C (144.0 mAh g
1). After 50 cycles, the discharge capacities of the Li
V
O composites synthesized at 350, 400, 450, 500, and 550 °C are 82.0, 219.1, 166.2, 164.6, and 142.1 mAh g
1, respectively, as shown in Figure 6b. It should be pointed out that both LiV3O8 and Li0.3V2O5 are considered as active phases; thus the capacity of the two compounds should be treated as a whole. In fact, it is difficult to calculate the accurate capacity of Li0.3V2O5 that contributes to the total capacity of the composites. Here, we just can give an estimate of the capacity of Li0.3V2O5 in the Li
V
O composites. Assuming the accommodation of up to three equivalents of Li+ per formula unit (Li1+3V3O8),8 about 1.76 Li+ ions can stored in Li0.3V2O5 during the intercalation/ deintercalation processes. Thus, Li0.3V2O5 can offer a discharge capacity about 61.8 mAh g
1 to the total discharge capacity for the initial discharge capacity of the composite synthesized at 400 °C. At a higher current density of 120 mA g
1, this composite can still deliver the high initial discharge capacity of 237.0 mAh g
1, as shown in Figure 6c. For the compound synthesized at 350 °C, a relatively high initial discharge capacity can be achieved, but it shows a poor cycling performance, as shown in parts b and c of Figure 6. The poor cycling stability was attributed to the large solid aggregates and low crystallinity.8,52 For the composites synthesized at 400
550 °C, the discharge capacities decrease with increasing the calcining temperature. The LiV3O8 compound synthesized at 550 °C only gives an initial 25512

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Figure 6. (a) The initial charge
discharge curves, (b) cycling performance at a current density of 50 mA g
1, and (c) cycling performance at a current density of 120 mA g
1 for the Li
V
O composites synthesized at different temperatures between 2.0 and 4.0 V. (d) The rate capacity of the Li
V
O composite synthesized at 400 °C at various charge
discharge current densities. The charge and discharge current densities are the same correspondingly.

capacity of 104.7 mAh g
1 at a current density of 120 mA g
1. This is because the higher calcining temperature will result in good crystallinity which leads to the compound have a slightly lower a value (Table S1 of Supporting Information),10
12,30 indicating a slightly smaller interlayer spacing in the structure for Li+ intercalation/deintercalation. Therefore, the anisotropic crystallinity would lead to a longer diffusion path for the lithium ions and depress electrochemical performance.8 Figure 6d shows the rate capability of the porous Li
V
O composite synthesized at 400 °C between 2.0 and 4.0 V. At a low current density of 20 mA g
1, the composite can deliver a high discharge capacity of 300.5 mAh g
1. With the increase of current density, the discharge capacity decreases regularly. However, even at a high current density of 480 mA g
1, it still can deliver a discharge capacity of 144.7 mAh g
1. When the current density returns to 20 mA g
1, the discharge capacity can be still recovered to 244.0 mAh g
1, revealing good electrochemical reversibility and structural stability. In the open literature, several potential windows were employed to test the Li+ intercalation/ deintercalation behavior such as 2.0
4.0,12 1.8
4.0,14,16 and 1.5
4.0 V10 or an even wider window of 1.5
4.5 V.25 A wider potential window will be easy to achieve a higher discharge capacity. For instance, Liu et al.25 prepared a porous LiV3O8 composite by combination of a freeze-drying method and posttreatment in air, which could deliver an initial discharge capacity

of 304 mAh g
1 at a current density of 50 mA g
1 in the potential range of 1.5
4.5 V. However, in the potential window of 2.0
4.5 V, the initial discharge capacity was about 255 mAh g
1. Here, we choose a relatively narrower potential range of 2.0
4.0 V. Sakunthala et al.12 also used the potential window of 2.0
4.0 V, and they found that the as-synthesized LiV3O8 rods could deliver a maximum discharge capacity of 191 mAh g
1 at a current density of 120 mA g
1. In the present work, the Li
V
O composite synthesized at 400 °C can deliver a discharge capacity of 265.7 and 237.0 mAh g
1 at a current density of 50 and 120 mA g
1, respectively, which are higher than some values in the published papers. The good electrochemical performance for the composite can be attributed to its porous structure, which is in favor of enhancing the contact between active material and electrolyte and improving the lithium ion diffusion and electron transfer across the Li
V
O/electrolyte interfaces. To further understand the Li+ intercalation/deintercalation behavior of the Li
V
O composites synthesized at different temperatures, CV curves of the first
fourth cycles are recorded at a scanning rate of 0.1 mV s
1 in the potential range of 2.0
4.0 V, and the results are shown in Figure 7. It is well-known that a sharp and well-defined CV peak generally suggests fast Li+ intercalation/deintercalation, whereas a broad peak implies a sluggish process.11,51 It is clearly seen that the Li
V
O composite synthesized at 350 °C shows highly broad anodic/cathodic peaks 25513

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Figure 7. CV curves of Li
V
O composites synthesized at different temperatures: (a) 350 °C, (b) 400 °C, and (c) 550 °C. (d) CV curves of the composite synthesized at 400 °C after charge
discharge for 90 and 160 cycles at a current density of 50 mA g
1. Scan rate: 0.1 mV s
1. Potential range: 2.0
4.0 V.

which can be attributed to its large solid aggregates and low crystallinity (Figure 7a). With increasing the calcining temperature to 400 °C, the composite presents well-defined peaks and small value of potential intervals, indicating the excellent reversibility of the Li+ extraction and insertion reactions (Figure 7b). It is apparent that the first CV curves are rather different from the rest, which may be ascribed to the activation of the electrodes or some structural modifications during the first CV scan.53
55 The following CV curves, however, remain almost the same as the second, which suggests that the Li+ intercalation/deintercalation becomes easier due to the full wetting of the electrode after the first charge
discharge. There are four anodic peaks around 2.50, 2.84, 3.45, and 3.65 V for the Li
V
O composite synthesized at 400 °C, and corresponding seven main cathodic peaks around 2.52, 2.62, 2.72, 2.80, 2.84, 3.41, and 3.62 V. For the pure LiV3O8 compound synthesized at 550 °C, only two anodic peaks around 2.84 and 3.68 V and corresponding four cathodic peaks around 2.52, 2.71, 2.79, and 3.61 V can be observed in the CV curves (Figure 7c). These redox peaks are related to the x value in Li1 +xV3O8, inducing several phase transformations between couples of Li1+xV3O8.52
58 Thus, the anodic peaks around 2.50 and 3.45 V and the cathodic peaks around 2.62, 2.84, and 3.41 V should belong to the other active phase Li0.3V2O5, which are very close to those data in previous reports.10
12,45,46 It is noticed that the active phase Li0.3V2O5 also shows good reversibility, as shown in Figure 7b. However, Sakunthala et al.12,59 found that the Li0.3V2O5 phase could be suppressed with increasing the cycle

number. Figure 7d shows the CV curves of the waferlike porous Li
V
O composite synthesized at 400 °C after charge
discharge for 90 and 160 cycles. After 90 cycles, the anodic peaks around 2.47 and 3.45 V and the cathodic peaks around 2.64, 2.86, and 3.42 V are still clearly reflected in the CV curve. However, after 160 cycles, the anodic peak at 3.45 V and the cathodic peaks at 2.86 and 3.42 V almost disappear. XRD patterns of the Li
V
O composite synthesized at 400 °C after 90 and 160 cycles are shown in parts a and b of Figure S3 of Supporting Information, respectively. Li0.3V2O5 phase in the electrode was found to be suppressed extremely during the electrochemical cycling, with a highly reduced relative intensity compared to the powder XRD pattern before cycling (Figure 2a). The ex situ XRD results correspond well with the studies of Sakunthala et al.12,59 and the CV analysis in the present work (Figure 7d). 3.3. Li Ion Kinetic Studies. EIS is a well-established method to identify the electrochemical reaction kinetics of electrode materials.12,60
62 Parts a and b of Figure 8 show the Nyquist plots for the waferlike porous Li
V
O composite synthesized at 400 °C at different charge and discharge states during the sixth cycle, respectively. The detailed information of Nyquist plots is shown in Figures S4 and S5 of Supporting Information. The Nyquist plots mainly consist of two partially overlapped semicircles in the high- and medium-frequency regions and a straight sloping line in the low-frequency region. Generally, the first highfrequency semicircle is related to the resistance of surfacepassivating layer. The second intermediate-frequency semicircle 25514

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Figure 9. Variations of (a) surface resistance and (b) charge-transfer resistance at different charge and discharge states during the sixth cycle calculated from fitting the Nyquist plots for the waferlike porous Li
V
O composite synthesized at 400 °C.

Figure 8. Three-dimensional Nyquist plots for the waferlike porous Li
V
O composite synthesized at 400 °C: (a) at different charge states and (b) at different discharge states during the sixth cycles (2.0
4.0 V). (c) Equivalent circuit model for porous Li
V
O electrode.

is indexed to the resistance of charge transfer on solid/electrolyte interfaces (SEIs), and the straight line results from the diffusion of Li+ within the electrode.60
63 Figure 8c presents an equivalent circuit to simulate the electrochemical impedance data. Rel represents the solution resistance; Rsl(i) and Csl(i) (i = 1, 2, and 3) stand for the Li+ migration resistance and capacity of surface layer, respectively; Rct and Cdl stand for the related chargetransfer resistance and double-layer capacitance, respectively; ZW represents the diffusion-controlled Warburg impedance in the low frequency. The evaluated impedance parameters according to the equivalent circuit as a function of electrode potential during the charge and discharge processes are presented in Tables S2 and S3 of Supporting Information, respectively. It can be found that the values of Rel are similar in the charge and discharge processes, but the values of Rel in the discharge process

Figure 10. Three-dimensional Nyquist plots measured for the waferlike porous Li
V
O composite synthesized at 400 °C after cycling (around 2.60 V after each cycle).

are higher than those in the charge process, indicating some differences in the two processes (Tables S2 and S3 of Supporting Information). Rsl and Rct show a similar variation as a function of potential in both the charge and discharge processes, as shown in parts a and b of Figure 9. It is clear that the values of Rsl and Rct exhibit a sudden decrease around 2.75 V during the charge 25515

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Figure 11. Variation of Rsf and Rct with cycle number calculated from fitting the Nyquist plots for the waferlike porous Li
V
O composite synthesized at 400 °C.

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discharge process, the as-produced SEI film may be partially dissolved or decomposed. Thus, the values of Rsl decrease obviously due to the high activation of the electrode surface. Since the charge transfer occurs at the interface, the Li ions have to pass through SEI film and a more resistive and thicker surface film would hinder the charge-transfer kinetics.61,62 Therefore, the nature of variation of Rct is similar to that of Rsl. Figure 10 shows the Nyquist plots of the waferlike porous Li
V
O composite synthesized at 400 °C after different number of cycles around 2.60 V. The shapes of the Nyquist plots for each cycle are similarly. The impedance spectra are fitted using the equivalent circuit model of Figure 8c, as shown in Figure 11. For the fresh cell, the electrode has a high value of impedance (Rct and Rsf). After five cycles, the value of Rct and Rsf decreases to 10.19 and 46.11 Ω, respectively. With increasing the cycle number, the impedance (Rct and Rsf) becomes changeless, indicating a stable surface film formed. At the 100th cycle, the value of Rct and Rsf is only increased to 30.91 and 76.42 Ω, respectively. It is wellknown that the lower increase of impedance during cycling means lower polarization, which, in turn, indicates good cycling behavior. GITT is considered to be a reliable method to determine the diffusion coefficient of Li ions (DLi+) with greater accuracy for compounds with varying composition or voltage, which has been extensively used to calculate the value of DLi+ in electrode

Figure 12. (a) The charge/discharge GITT curves of Li
V
O electrode synthesized at 400 °C as a function of time in the potential range of 2.0
4.0 V. (b) t vs E profile of Li
V
O electrode for a single GITT during charge process at 2.62 V with schematic representation of different profile parameters. (c) Linear behavior of the E vs τ1/2 relationship at charge process. (d) The calculated DLi+ from the GITT data for the Li
V
O compound as a function of potential during both charge and discharge processes. 25516

dx.doi.org/10.1021/jp2080176 |J. Phys. Chem. C 2011, 115, 25508–25518

The Journal of Physical Chemistry C

ARTICLE

materials.61,62 Figure 12a shows the GITT curves of the waferlike porous Li
V
O composite synthesized at 400 °C as a function of time in the potential range of 2.0
4.0 V. The cell was charged with a constant current flux of 10 mA g
1 for an interval of 10 min followed by an open circuit stand for 40 min to allow the cell voltage to relax to its steady-state value (Es). The cell voltage can be stabilized to a value after 40 min open-circuit stand after each current flux, as seen the two partial GITT curves as inserts in Figure 12a during both charge and discharge processes. Figure 12b simply illustrates a single step of GITT. On the basis of Fick’s second law of diffusion and after a series of assumptions and simplifications, the diffusion coefficient of Li ions can be obtained by the following equation64 !2
 4 mB Vm 2 ΔEs pffiffiffi DLi ¼ ðτ , L2 =DLi Þ π MB A τðdEτ =d τÞ

ð1Þ

by galvanostatic intermittent titration technique are in the region of 10
14 to 10
9 cm2 s
1 in the charge/discharge processes.

’ ASSOCIATED CONTENT

bS

Supporting Information. Figures depicting SEM images of the precursor materials, SEM images of he LiV3O8 compound synthesized at 550 °C, XRD patterns of the Li
V
O composite, Nyquist plots for the waferlike porous Li
V
O composite at different charge and discharge states and tables depicting refinement parameters for waferlike porous Li
V
O composites and evaluated impedance parameters of the charge and discharge processes. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

where Vm is the molar volume of the compounds, MB is the relative formula mass, mB is the active mass in electrode, A is the total contact area between the electrolyte and electrode, and L is the thickness of electrode. If E vs τ1/2 shows a straight line behavior over the entire time period of current flux (as shown in Figure 12c), eq 1 can be further simplified as64 DLi ¼



 4 mB Vm 2 ΔEs 2 πτ MB A ΔEτ

ð2Þ

On the basis of eq 2, the diffusion coefficients of Li ions calculated from the GITT curves as a function of potential during both charge and discharge processes are shown in Figure 12d. It is found that the log (DLi+) vs potential plots show several minima in the charge/discharge processes, corresponding to the potential plateaus of charge/discharge curve and the CV peaks (Figures 6a and 7). A similar minimum DLi+i+ is commonly observed around charge/discharge potential plateaus in cathode materials, which shows a two-phase transition region for strong attractive interactions between the intercalation elements and the host matrix or some order
disorder transition during cycling.61,62,65 The values of DLi+ are in the range from 10
14 to 10
9 cm2 s
1 in the charge/discharge processes. The above values in the whole charge/discharge processes obtained from GITT in our work are very close to those reported by Pan et al.10 (10
14 to 10
10 cm2 s
1, by GITT), Yang et al.66 (6.47  10
11, by EIS), Yang et al.67 (10
10 cm2 s
1, by EIS), and Wu et al.68 (10
12 to 10
7 cm2 s
1, by CV).

4. CONCLUSIONS In this work, we have successfully demonstrated a facile selfassembled synthesis to prepare waferlike porous Li
V
O composites by using a glycine-assisted solution route followed by a low-temperature reaction. The porous composites are composed of LiV3O8 and Li0.3V2O5 phases, and both of them are considered as active phases during the Li+ intercalation/deintercalation processes. The Li
V
O composite synthesized at 400 °C exhibited a high initial discharge capacity of 265.7 mAh g
1 at a current density of 50 mA g
1 and good capacity retention between 2.0 and 4.0 V. Electrochemical impedance data demonstrate the lower increase of impedance during cycling, indicating a good cycling behavior of the porous composite. In addiction, the diffusion coefficients of Li ions in this composite determined

*Phone: +86 571 87952856. Fax: +86 571 7952573. E-mail: tujp@ zju.edu.cn.

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