Ultrafine β-AgVO₃ nanoribbons derived from α-AgVO₃ nanorods by water evaporation method and its application for lithium ion batteries

Abstract

Monoclinic β-AgVO₃ nanoribbons with thickness of 10–20 nm, width of 80–100 nm and length of several hundred micrometers have been successfully prepared by a water evaporation method at 100 °C for 6 h without using any template and organic surfactant. A possible evolution mechanism from α-AgVO₃ nanorods to β-AgVO₃ nanoribbons is proposed. The AgVO₃ nanostructures are demonstrated as promising cathode materials in lithium ion batteries. The as-prepared β-AgVO₃ nanoribbons show improved electrochemical performance in comparison with α-AgVO₃ nanorods due to the ultrafine morphology and thermodynamic stable crystal structure yielding a discharge capacity of 355 mA h g⁻¹ at a current density of 30 mA g⁻¹. It can still remain 32% of its initial discharge capacity after 20 cycles.

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1. Introduction

Silver vanadium oxides have been widely investigated as cathode materials for primary and rechargeable lithium batteries due to their high gravimetric/volumetric energy densities and high power.^1,2 A number of silver vanadium oxides with different structures, such as AgVO₃, Ag₂V₄O₁₁, Ag_0.33V₂O₅ etc., have attracted recent attention due to their interesting electrochemical performance and other properties.^3–7 In these different structures, AgVO₃ is supposed to have better electrochemical performance as it has a higher Ag : V molar ratio.^6,8 The AgVO₃ type oxides have three crystallographic forms, including α-, β- and γ-AgVO₃. Generally, the γ-AgVO₃ phase forms at high temperatures, while α-AgVO₃ is a metastable phase, which can be irreversibly transformed into the stable β-AgVO₃ phase at around 200 °C.⁹ Both of α- and β-AgVO₃ have the electrochemical activity according to the reported literatures.

One-dimensional (1-D) electrode materials have been attracted considerable attention in lithium ion batteries due to their ultrafine structure, which benefit for the electrochemical kinetics and is capable of the structural transformation during the charge–discharge process.^10–13 Some recent works on preparing 1-D β-AgVO₃ nanostructures and their potential application in lithium ion batteries have been reported.^14–19 Chen and co-workers¹⁴ reported a hydrothermal synthesis of α-AgVO₃ nanorods and β-AgVO₃ nanowires respectively at 180 °C, and the β-AgVO₃ nanowires presented a discharge capacity of 302.1 mA h g⁻¹ at a constant current of 0.01 mA when used as the cathode materials in lithium ion batteries. Zhou and co-workers¹⁶ reported the synthesis of Ag/β-AgVO₃ nanobelts with a width of about 100 nm and a length of several micrometers using a hydrothermal method, and the Ag/β-AgVO₃ nanobelts exhibited specific discharge capacity of 285 mA h g⁻¹ at the current density of 20 mA g⁻¹.

To the best of our knowledge, the main method for preparing the 1-D structured β-AgVO₃ can be summarized as the hydrothermal reactions which usually need the relatively high temperature and produce high pressure.^5,6,13,14 Therefore, exploration of novel methods for controllably preparing the 1-D structured β-AgVO₃ in a more tender condition and studies of their formation mechanisms are of great importance.

In this work, we report a novel method for preparing the β-AgVO₃ nanoribbons by water-evaporation and without using any template or organic surfactant. A “splitting-reassemble” formation mechanism from α-AgVO₃ nanorods to β-AgVO₃ nanoribbons is investigated and proposed. The β-AgVO₃ nanoribbons show enhanced electrochemical performances compared to α-AgVO₃ nanorods when they are used as the cathode materials in lithium-ion batteries.

2. Experiment section

All of the chemicals were of analytical grade and used as raw materials without further purification.

2.1 Preparation of β-AgVO₃ nanoribbons via a water evaporation method

A typical procedure to synthesize β-AgVO₃ was performed as follows. 0.093 g of NH₄VO₃ was dissolved into 35 mL of deionized water at 80 °C form a clear solution. 0.135 g AgNO₃ and 35 mL of deionized water at 80 °C were put in a 150 mL beaker under magnetic stirring. 35 mL of NH₄VO₃ aqueous solution (0.023 M) and 35 mL of AgNO₃ aqueous solution (0.023 M) were prepared at 80 °C, respectively. The NH₄VO₃ solution was then added dropwise into the AgNO₃ solution under magnetic stirring. An orange precipitates formed immediately. The beaker was kept in a fan-forced oven at 100 °C for 6 h. As time passed, water, as the solvent in the solution, was gradually evaporated and became less and less and finally the dry products deposited on the bottom of the beaker. The product was collected and washed with distilled water and ethanol for several times, and then dried in vacuum at 60 °C overnight for further characterization.

2.2 Characterization

The as-prepared samples were characterized by XRD in a Japan Rigaku D/max-γB X-ray diffractometer with a Cu Kα radiation source (λ = 1.5418 Å) operated at 40 kV and 80 mA. Field-emission scanning electron microscopy (FESEM) measurement was taken on a FEI Sirion-200 scanning electron microscope operating at an accelerating voltage of 10 kV. Transmission electron microscopic (TEM) images and the high resolution transmission electron microscopic (HRTEM) images were taken on a Philips CM 20 FEG and a Hitachi H-800 transmission electron microscope performed at an accelerating voltage of 200 kV.

2.3 Electrochemical characterization

Before battery testing, the β-AgVO₃ nanoribbons was heated at 300 °C in air atmosphere for 5 h in order to remove the trapped and adsorbed moisture (see ESI†).

The electrochemical properties of the as-prepared samples were characterized by coin-type cells (CR2032) with lithium disks as counter electrodes. A composite electrode was prepared by mixing the β-AgVO₃ powder, carbon black, and polyvinylidene fluoride (PVDF) in weight ratio of 70 : 20 : 10 and dissolved in n-methyl pyrrolidinone (NMP) solvent. The obtained slurry was then cast onto aluminum foil with the slurry thickness controlled by a blade coater. After the evaporation of the solvent by a mild heating, the resulted electrode film was subsequently pressed and punched into a circular disc with a diameter of 11 mm. Then the disks were further dried in vacuum at 120 °C for 5 h. Finally, coin-type cells (CR2032) were assembled in an Ar-filled dry glove box. The liquid electrolyte used was 1 M LiPF₆ in a 1 : 1 mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC), and the separator was Celgard 2400 micro-porous polypropylene membrane. Galvanostatical charge and discharge was measured on a multi-channel battery tester (Neware Battery Test System, Shenzhen Neware Electronic Co., China) between 1.5 and 3.5 V at room temperature. Cyclic voltammetry was performed in electrochemical work station (CHI604C, Shanghai Chenhua Instruments Co. Ltd.).

3. Results and discussion

Fig. 1 presents the crystal structure and morphology of the products prepared in a beaker by water evaporation method. Fig. 1(a) shows the X-ray diffraction (XRD) pattern of the as-prepared sample by water evaporation method. All the diffraction peaks can be indexed to the monoclinic β-AgVO₃ (JCPDS no. 29-1154). The lower magnification FESEM image in Fig. 1b and c shows that this production is composed of a large quantity of 1D nanostructured materials with the length of at least several hundred micrometers. The higher magnification FESEM image is shown in Fig. 1d. It can be seen that this one-dimensional nanostructured β-AgVO₃ presents nanoribbon structure with the thickness and width of about 10-20 nm and 80–100 nm, respectively.

Fig. 1 (a) XRD pattern of the β-AgVO₃ nanoribbons prepared in a beaker by water evaporation method at 100 °C for 6 h, and (b–d) FESEM images of β-AgVO₃ nanoribbons.

Further insight into the morphology and microstructure of the β-AgVO₃ nanoribbon has been gained with the help of TEM and HRTEM. Fig. 2(a) presents the TEM image of a single β-AgVO₃ nanoribbon. It can be seen that the width of this nanoribbon is about 100 nm and some particles with the diameter of 20–40 nm are attached on the surface of the nanoribbons. HRTEM image in Fig. 2b shows the clear crystal lattices of these particles in Fig. 2a (white frame I). The lattice interplanar spacings of 0.20 nm is corresponding to the (200) plane of cubic silver. This kind of formation of silver nanoparticles can be attributed to the electron beam irradiation in the process of TEM or HRTEM characterization. Based on the recent reports,^20–22 it can be seen that the existence of silver in the active material may promote the conductivity of vanadium oxides,²¹ which could present better electrochemical performance. HRTEM image of nanoribbons indicated with the white frame II in Fig. 2(a) clearly demonstrate its single crystal nature.^23,24

Fig. 2 (a) TEM image of the β-AgVO₃ nanoribbons, (b) HRTEM images of the nanoribbon indicated with the white frame I in (a).

Fig. 3 shows XRD patterns of samples prepared with different reaction durations. Fig. 3a is the XRD pattern of the sample prepared at 80 °C by mixing the NH₄VO₃ solution and AgNO₃ solution without evaporation process can be well indexed to α-AgVO₃ with the monoclinic crystal structure (JCPDS no. 89-4396). No impurities can be found. When an evaporation process is carried out, the α-AgVO₃ gradually evolute into β-AgVO₃ as shown in Fig. 3b–d. With the evaporation duration is increasing, the diffraction peaks of immediate sample only can be indexed to two kinds of phases and the α-AgVO₃ diffraction peaks become weaker, while the β-AgVO₃ diffraction peaks (as signed by*) become stronger. When the reaction time increased to 6 h, pure β-AgVO₃ is obtained (as shown in Fig. 3(e)). It indicates that the α-AgVO₃ can be directly converted into the β-AgVO₃, which can be ascribed to the more thermodynamic stability of β-AgVO₃ than that of α-AgVO₃.^9,23

Fig. 3 XRD patterns of the samples prepared with different reaction durations: (a) 0 min, (b) 30 min, (c) 1 h, (d) 4 h, and (e) 6 h.

Fig. 4 gives the samples morphology evolution in the water evaporation process. Fig. 4(a) shows the pure α-AgVO₃ obtained by mixing the NH₄VO₃ and AgNO₃ aqueous solution at 80 °C, it can be observed that the α-AgVO₃ sample consisted entirely of dispersive nanorods. From the Fig. S1 (ESI†), it can be seen that the diameter and the length of these α-AgVO₃ nanorods are determined to be 500 nm and dozens of micrometers. Fig. 4(b) gives the FESEM images of the immediate sample prepared at 2 h. It can be seen that the tiny crystal whiskers are divided from the α-AgVO₃ nanorods, and the diameter and the length of these tiny crystal whiskers are about 10–20 nm and dozens of nanometers, respectively. The driving force of this splitting process could be attributed to the inner stress, which could be arised from the phase transformation from the α-AgVO₃ to β-AgVO₃. With the reaction time increasing, these tiny crystal whiskers continue to divide from the nanorods, and some of them tend to agglomerate due to its high surface energy, and reassemble into a novel 1-D structure during the water evaporation, as shown in Fig. 4(c). Fig. 4(d) presents FESEM images of the final sample prepared at 6 h, it clearly can be seen that this β-AgVO₃ displays a nanoribbon structure. This “splitting-reassemble” process from α-AgVO₃ nanorods to β-AgVO₃ nanoribbons is different from the formation mechanism of the sample by hydrothermal reactions.^13,14 Bao and co-workers¹³ proposed that the β-AgVO₃ obtained by hydrothermal method has a strong tendency for splitting due to its strenuous reaction condition (e.g. high temperature and pressure), while the reaction condition of this water-evaporation method is tender compared with the hydrothermal conditions, so the driving force of this method can not hold the direct transformation from the nanorods to nanoribbons. In addition, the solution decreasing with the reaction time passed could limit the Ostwald-ripening process, which usually happens in hydrothermal reactions,^13,14 and this analysis could be proved by the increased dimension of the sample compared with the sample by hydrothermal reactions. Finally, this “splitting-reassemble” process is similar to the formation mechanism of long Cu_2−xSe nanowires bundles in our previous work.²⁵

Fig. 4 FESEM images of the samples prepared with different reaction durations: (a) 0 min, (b) 2 h, (c) 3 h, and (d) 6 h.

For further understanding above phase and morphology evolution mechanism in this solidification process, the relationship between evaporation time and weight ratio is presented in Fig. S2a.† When the water evaporation process is carried out, the weight ratio gradually become less and less. In the first four hours, the rate of water evaporation is almost constant. While the α-AgVO₃ nanorods are gradually transformed to the β-AgVO₃ nanoribbons from the XRD and FESEM analysis as shown in Fig. 3 and 4. When the evaporation time reach to 4 h, the weight ratio reduces to 20 wt%. It means that the part of the transformation of the phase and morphology occurs in the water as the solvent. When the evaporation time is closed to 6 h, the orange product deposits on the bottom of the beaker as shown in Fig. S2b,† and the α-AgVO₃ nanorods was entirely changed into β-AgVO₃ nanoribbons, which is consistent with the XRD pattern of samples prepared with different reaction durations as shown in Fig. 3. Based on the above analysis, it can be concluded that this transformation could take place in the whole solidification process. However, this kind of solidification process can provide a tender condition, which is good for forming the ultrafine nanostructure.

Before battery testing, the β-AgVO₃ nanoribbons was heated at 300 °C in air atmosphere for 5 h in order to remove the trapped and adsorbed moisture. The XRD pattern and the TEM images are given in Fig. S3 (ESI†). It can be seen that the XRD pattern of sample after calcination can still be indexed to a monoclinic β-AgVO₃ (JCPDS no. 29-1154) and the morphology still remain its nanoribbon structure.

Galvanostatic discharge–charge profiles for β-AgVO₃ nanoribbons recorded at a current density of 30 mA g⁻¹ showed two plateaus, which locate at 3.10 V and 2.25 V, and a sloping potential ranging at 2.2–1.5 V in the initial discharge cycle (as shown in Fig. 5(a)), which are consistent with the observation by Chen and co-workers.¹⁴ In the next charge process, this charge curve only displays two sloping potential ranging at 1.8–2.5 V and 3.0–3.5 V. In the subsequent charge–discharge processes, there is no obvious plateau or potential sloping occurred. The first discharge capacity is about 355 mA h g⁻¹, which is a significant value for silver vanadium oxides in the light of previous reports.^3,14,15

Fig. 5 (a) The first three charge–discharge profiles of β-AgVO₃ nanoribbons at a current density of 30 mA g⁻¹ of the voltage ranging from 3.5 to 1.5 V, (b) cyclic voltammetry curves at a scan rate of 0.1 mV s⁻¹ in the voltage ranging from 4 V to 1.5 V, and (c) the first three charge–discharge profiles of α-AgVO₃ nanorods at a current density of 30 mA g⁻¹ in the voltage ranging from 3.5 to 1.5 V.

For further investigating the mechanism of charge–discharge process, cyclic voltammetry was used, as shown in Fig. 5(b). In the first cycle, three cathodic peaks located at 2.95 V, 2.09 V and 1.89 V are observed in reduction process, which could be corresponding to the two plateaus and one potential sloping in the first discharge profiles, and three anodic peaks located at 2.12 V, 2.71 V and 3.50 V, are observed in the oxidation process. In the subsequent cycles, there is no cathodic or anodic peaks, which are consistent with the analysis of charge–discharge profiles.

Fig. 5(c) gives the charge–discharge profiles of α-AgVO₃ nanorods at a current density of 30 mA g⁻¹. In the first discharge process, the profiles only presents one plateau located at 2.85 V, which is consistent with the observation by Chen and co-workers,¹⁴ while there is no plateau in the subsequent cycles. Meanwhile, the initial capacity of the α-AgVO₃ nanorods is about 257 mA h g⁻¹ and less than that of the β-AgVO₃ nanoribbons.

Fig. 6 compares the cycling performance of β-AgVO₃ nanoribbons and α-AgVO₃ nanorods at a current density of 30 mA g⁻¹. It can be seen that the initial discharge capacity of the β-AgVO₃ nanoribbons (355 mA h g⁻¹) is much larger than that of the α-AgVO₃ nanorods (257 mA h g⁻¹). After 20 cycles, the discharge capacity of the β-AgVO₃ nanoribbons can still remain 32% compared with its initial discharge capacity, while the α-AgVO₃ nanorods only remains 38 mA h g⁻¹, which is 15% of the initial capacity. Therefore, these results can easily lead to the conclusion that the electrochemical performance of β-AgVO₃ nanoribbons is much better than that of the α-AgVO₃ nanorods. The difference of these two kinds of AgVO₃ initial discharge capacity can be attributed to ultrafine structure of the β-AgVO₃ nanoribbons, which can effectively shorten the lithium ion diffusion path, promote the rate performance during the charge–discharge process and ultimately increase its discharge capacity. Meanwhile, ultrafine structure could be capable of the structural change during the charge–discharge process.^10,11 In addition, the difference of the cycling performance can be also attributed to thermodynamic stability of the α-AgVO₃ phase, which could result in the structural change during the charge–discharge process.

Fig. 6 Cycling performance of (a) β-AgVO₃ nanoribbons and (b)α-AgVO₃ nanorods at a current density of 30 mA g⁻¹.

4. Conclusion

In conclusion, we have developed a novel method for preparing the ultrafine nanoribbons of monoclinic β-AgVO₃ with the thickness of 10–20 nm, the width of 80–100 nm and the length of several hundred micrometers via a water evaporation method and without using any template and organic surfactant. Different from the mechanism of ripening-splitting in the hydrothermal reactions, the formation process could contain splitting and reassembling stages from α-AgVO₃ nanorods to β-AgVO₃ nanoribbons due to its tender condition. Compared to the α-AgVO₃ nanorods, the ultrafine structure and thermodynamic stability of the β-AgVO₃ nanoribbons decide on the enhanced electrochemical performance by the charge–discharge test results.

Acknowledgements

This work has been supported by the National High Technology Research and Development Program of China (863 Program) (no. 2012AA110407).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra12472c

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