Structure-controlled synthesis and electrochemical properties of NH4V3O8 as cathode material for Lithium ion batteries

Electrochimica Acta 212 (2016) 217–224

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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta

Structure-controlled synthesis and electrochemical properties of NH4V3O8 as cathode material for Lithium ion batteries Yayi Cheng, Jianfeng Huang* , Jiayin Li, Liyun Cao* , Zhanwei Xu, Jianpeng Wu, Shanshan Cao, Hailing Hu School of Materials Science & Engineering, Shaanxi University of Science and Technology, Xi’an 710021, China

A R T I C L E I N F O

Article history: Received 28 May 2016 Received in revised form 30 June 2016 Accepted 2 July 2016 Available online 4 July 2016 Keywords: NH4V3O8 Morphology Lithium ion battery Cathode

A B S T R A C T

NH4V3O8 flower, nanobelt, lath and sheet were synthesized using a facile microwave hydrothermal method. The formation mechanism of NH4V3O8 with various structures was proposed. As an cathode in Li-ion battery, the NH4V3O8 nanobelt with one-dimensional structure as well as nanosized morphology, presents excellent cycling stability and enhanced rate capability when comparing with other NH4V3O8 structures. Further study finds that the NH4V3O8 nanobelt could provide high Li ion diffusion, excellent structural stability and good reversibility during the charge/discharge process, indicating a strong connection between the morphology and the electrochemical performance of NH4V3O8 cathode. ã 2016 Elsevier Ltd. All rights reserved.

1. Introduction Nowadays, Lithium ion batteries (LIBs) have drawn considerable attention owning to high energy density, long life cycle and environmental benignity, which are widely used in the hybrid electric vehicles (HEVs) and portable electronic devices [1–3] like mobile phones, digital cameras and table personal computers [4]. At present, many kinds of vanadium oxides and their derivative compounds have been developed due to their easy synthesis, less expensive and good structural stability [5], such as LiV3O8 [6], NH4V3O8 [7,8], NH4V4O10 [9–11], NH4V3O8
0.2H2O [12], NH4V3O8
0.37H2O [13], NH4V3O8
0.42H2O [13], (NH4)0.6V2O5 [14] and (NH4)2V6O16 [15,16]. In comparison with LiV3O8, NH4V3O8, a layered trivanadate with larger interlayer spacing, is believed to be the more promising electrode material for LIBs. Since the larger interlayer spacing can improve the Li+ migration (electrode kinetics) within the lattice drastically, enhancing the electrode rate performance [5]. In addition, a network of N-H . . . O hydrogen bonds between the cationic and polyanionic layers increase the structural stability of NH4V3O8, which is beneficial for the longterm cycling stability of electrode materials [7]. Although the NH4V3O8 as electrode material for LIBs shows low working voltage

* Corresponding author.at: Tel.: +86 29 86168802; fax: +86 29 86168802. E-mail addresses: [email protected] (J. Huang), [email protected] (L. Cao) . http://dx.doi.org/10.1016/j.electacta.2016.07.008 0013-4686/ã 2016 Elsevier Ltd. All rights reserved.

(around 1.9 V), which is suitable as anode materials. The limited specific capacity make it more appropriate as the cathode materials for LIBs. Thereby the NH4V3O8 is considered as the cathode materials in our present work. However, the intrinsic low electronic conductivity in crystalline NH4V3O8 results in poor electrochemical performance, limiting the practical application of this cathode material largely. In order to obtain good electrochemical properties, numerous strategies have been made to improve performances of vanadate cathode. Generally, most of these strategies mainly involve the addition of anion doping or carbon, which greatly affect the electrochemical properties. For instance, Torardi et al. [17] have reported that fluoride doped ammonium trivanadate (NH4)0.9V3O7.9F0.1
0.9H2O has an initial discharge capacity of 409 mAh g1. Wang et al. [8] incorporated CNTs into NH4V3O8 flakes, exhibiting high discharge capacity of up to 359 mAh g1 at 30 mA g1 as well as excellent cycling stability. Wu et al. [6] prepared polypyrrole-coated LiV3O8nanorod that presents higher capacity and better rate capability compared to the pristine LiV3O8. These works did improve the electrochemical performance of the vanadium oxides to some extent, but the property could be further enhanced. Apart from carbon compositing and anion doping, micro/nanostructural controlling should be equally important to boost the NH4V3O8 cathode performance, since nanostructures are demonstrated to significantly affect the materials optical [18,19], physical [20] and electrochemical performance [21,22]. Currently, various structures of NH4V3O8 micro/nano-materials have been reported with

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enhanced electrochemical performance, such as NH4V3O8 nanorods [12], NH4V3O8 belts [7] and NH4V3O8 nanoflakes [8]. While these different structures of NH4V3O8 were prepared by hydrothermal route, which requires longer time than 24 h. Thereby, much more efforts on the nanostructural control with low cost in NH4V3O8 cathodes should be made to facilitate their practical applications. In our present work, we employed a facile microwave hydrothermal method to produce NH4V3O8 with controlled shape and morphology by simply controlling pH value, including flower, nanobelt, lath and sheet structures. The electrochemical performance of NH4V3O8 with various structures were compared and the results were very different. Among them, the NH4V3O8 nanobelt showed remarkable rate capacity and cycling stability, which was reported in our previous work [23]. The present work indicates morphology and particle size affects the electrochemical performance of NH4V3O8 cathode strongly. Fig. 1. XRD patterns of (a) NF, (b) NB, (c) NL and (d) NS.

2. Experimental 2.1. Sample preparation The NH4V3O8 with structures of flower (labelled as NF), nanobelt (labelled as NB), lath (labelled as NL) and sheet (labelled as NS) were synthesized by a facile microwave hydrothermal method. In a typical procedure, 1.16 g NH4VO3 was dissolved in 100 mL deionized water under ambient temperature with magnetic stirring to form a transparent solution, then the pH value of the solution was adjusted by the dilute hydrochloric acid (2 mol L1) to the set value (pH = 2, 3, 4 and 5). After that, the mixed solution was transferred into a 100 mL Teflon-lined autoclave, sealed and maintained at 150  180  C for 1 h in a MDS-10 microwave hydrothermal system. After cooling down naturally, the resulting precipitate was washed with deionized water for three times, then dried in a vacuum oven at 60  C overnight to obtain the final samples. The corresponding samples which synthesized by adjusting pH value and microwave hydrothermal reaction temperature were denoted as NF (pH = 2, 180  C), NB (pH = 3, 150  C), NL (pH = 4, 180  C) and NS (pH = 5, 180  C), respectively. 2.2. Materials characterization The phase purity and crystalline structure of the obtained samples were identified by powder X-ray diffractometer (XRD, Rigaku, D/max-2200) with Cu Ka radiation. The size, morphology and structure of the samples were characterized by scanning electron microscope (SEM, Hitachi, S-4800), transmission electron microscopy (TEM, Tecnai G2F20S) and high-resolution transmission electron microscopy (HRTEM). 2.3. Electrochemical tests Electrochemical measurements were carried out with twoelectrode cells (CR2032) assembled in argon-filled glove box (Mbraun, Germany) using lithium metal as the counter and reference electrode. To fabricate the working electrode, the synthesized NH4V3O8 powder, polyvinylidene fluoride (PVDF) binder and acetylene black were mixed together at a weight ratio of 8:1:1 in N-Methyl pyrrolidone and pasted on aluminum foil. The area of the electrode was about 2 cm2 and the mass of NH4V3O8 on electrode was 2.0–2.2 mg cm2. The electrolyte solution was 1 M LiPF6 dissolved in a mixture of ethylene carbonate (EC), ethyl methyl carbonate (EMC) and dimethyl carbonate (DMC) (1:1:1 in volume). The charge-discharge performance of the electrodes was performed on multichannel battery testing system (Neware, 5 V

2 mA) at desired current densities in a voltage range of 1.5 V to 4.0 V vs. Li/Li+. Cyclic voltammetry (CV) was operated using electrochemical station (Chenhua, CHI 660E) with a scan rate of 0.1 mV s1. Electrochemical impedance spectroscopy (EIS) was recorded by electrochemical station under the open-circuit condition over the frequency range from 100 kHz to 10 mHz with an amplitude of 5 mV. In order to investigate the electrochemical reaction of NH4V3O8, the cycled cells were disassembled and washed with dimethyl carbonate (DMC) several times. For the ex situ XRD analysis, the electrode slice were sealed with Kapton tape in an Ar-filled glovebox without exposure to air. 3. Results and discussion The XRD patterns of the as-prepared NF, NB, NL, and NS are shown in Fig. 1. All diffraction peaks could be indexed to monoclinic NH4V3O8 phase with lattice parameters of a = 4.9898 Å, b = 8.4249 Å, c = 7.8367 Å and b = 96.455 , which belongs to the space group P21/m (11) according to the JCPDS file No. 88-1473. While the pattern of NF synthesized at pH = 2 shows a small impurity peak at 2u of 9.5 , which can be attributed to the NH4V4O10 phase (space group C*/* (12), JCPDS No. 31-0075). The lattice parameters of NH4V4O10 are a = 11.73 Å, b = 3.67 Å, c = 9.81 Å and b = 101.13 . This suggests the NF sample is composed of NH4V3O8 and NH4V4O10. When pH value is controlled at 3 to 5, all the samples display high purity of NH4V3O8 without any impurity phase. And the intensity of (001) peak becomes weaker with the lower of pH value (higher concentration of H+). Since the H+ are preferentially adsorbed on the positive V3O8 (001) surface, they prevent contact between the new growth unit and the (001) crystal surface, which leads to suppressed growth of the (001) facets. This demonstrates that the phase structure of the products is strongly dependent on the acidity of the solution, which is in accordance with the previous report [24,25]. In addition, the relative intensity of diffraction peak (001) implies the degree of crystallinity. While the preferential ordering of crystal is disadvantageous to the Li+ insertion and extraction since it would lead to a long Li+ diffusion path [12,13]. Therefore, the intensity of diffraction peak (001) of NH4V3O8 will also affect the electrochemical property. Such a phenomenon is also seen for LiV3O8 [26]. Fig. 2 shows the SEM images of the prepared NF, NB, NL and NS, respectively. In Fig. 2a, the NH4V3O8 synthesized at pH = 2 (NF) presents flower-like morphology and the particle size is about 4 mm. From the close-up image, the NF is woven of a large quantity of nanobelts with width of 30–40 nm. Fig. 2b shows the dispersive NH4V3O8 nanobelts (NB) prepared at pH = 3, the width of the nanobelt is 40–60 nm. Some bent-belts with the length of tens of

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Fig. 2. SEM images of (a) NF, (b) NB, (c) NL and (d) NS. Inset of (a), (b) and (c) show corresponding SEM images at high magnification.

micrometers are also been observed. This bending arises from the formation of a twin boundary at the (100) planes [20]. When the pH value is adjusted to 4 (Fig. 2c, NL sample), a large number of irregularly shaped laths with a width of about 200 nm are obtained. In Fig. 2d, the NH4V3O8 prepared at pH value of 5 (NS) shows irregular sheet morphology with the thickness around 100 nm. Therefore, concluding from the above structural changes indicates the concentration of H+ in solution have strong effects on the growth of NH4V3O8 crystal nucleus. Detailed crystal structure and morphology of the NF, NB and NS were further confirmed by TEM. Fig. 3a shows an overview of flower-like NH4V3O8. It is found that the flower structure is constructed of numerous nanobelts, and the lattice spacing of the NH4V3O8 (002) crystal plane with 0.389 nm are well indexed. This could also be supported by FFT pattern (the inset lower right in Fig. 3a). Fig. 3b displays the homogeneously dispersed nanobelt structure of NH4V3O8, the calculated space of lattice fringes in the HRTEM image is about 0.779 nm, which corresponds to the interplanar spacing of (001) plane in NH4V3O8. Fig. 3c shows twodimensional sheet structure. From the HRTEM image, the NH4V3O8 (103) plane could be detected. All of the above information are in good agreement with XRD and SEM results. Based on the above results, we propose a formation mechanism of NH4V3O8 with different structures, as shown in Fig. 4. Naturally grown monoclinic NH4V3O8 crystallite presents a sheet-like microstructure. When dilute hydrochloric acid is added, pH = 2, a large quantity of hydrogen ions suppress the growth of the (001) facets in NH4V3O8 crystallite, thus initiating oriented growth along another direction to form NH4V3O8 nanobelt. These tenuous nanobelts tend to assemble together to form flower-like NH4V3O8 so that the surface energy could be further lowered. With the pH increasing to 3, the uniformly dispersed NH4V3O8 nanobelts are obtained. When the pH value is adjusted to 4, the NH4V3O8 crystal nucleus growth are partly prohibited by hydrogen ions to form

oriented growth. While other crystallites grow naturally, resulting in the mixture structures of the belt and sheet. After the Ostwald ripening process, NH4V3O8 displays lath structure. As the concentration of H+ further reducing (pH = 5), the growth of NH4V3O8 crystallite is barely affected by H+ and naturally grown to sheet structure. According to previous reports, the morphology, size and structure of material have an important effect on the transfer of Li+ and e in the electrode. Therefore, we infer that the NH4V3O8 with various strucures would reveal different lithium storage performance. Electrochemical reactions of the NH4V3O8 were initially evaluated by cyclic voltammetry (CV), which was obtained at a scan rate of 0.1 mV s1 in the range of 1.5-4.0 V. As shown in Fig. 5, the NF presents two obvious reduction peaks around 2.33 V, 2.71 V and two broadened but overlapped oxidation peaks about 3.1 V. The broadened peaks indicates that the electrochemical reaction of NF mainly derives from the surface reaction with Li+. For NB, NL and NS electrodes, there are three obvious pairs of reduction peaks locating at 1.62, 2.45 and 2.83 V, the corresponding oxidation peaks at 1.85, 2.65 and 3.18 V, respectively. This reveals multistep extraction and insertion process of Li+ in NH4V3O8 electrode material. According to the previous investigation on NH4V3O8 electrode [11,27], the electrochemical reaction of Li+ in NH4V3O8 is a stepwise de-/intercalation process. Nevertheless, the explicit lithium reaction mechanism remains unclear because of a complex configuration of the crystal structure. Therefore, the electrochemical reaction of NH4V3O8 can be described simply as follow. NH4 V 3 O8 þ xLiþ þ xe Ð NH4 Lix V 3 O8

ð1Þ

First charge-discharge profiles of the NF, NB, NL and NS are shown in Fig. 6a. It can be seen that the NF presents very different charge-discharge potential plateaus from the NB, NL and NS. The NB, NL and NS electrode have three similar voltage plateaus, while the NF presents one long sloping region, which implies that they

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Fig. 3. TEM images of (a) NF, (b) NB and (c) NS (HRTEM images obtained in frame regions were inserted as upper insets).

[(Fig._4)TD$IG]

Fig. 4. Schematic illustration of the morphology change of the NH4V3O8 nano/micro- structures.

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Fig. 5. Cyclic voltammograms of the NF, NB, NL and NS in the initial cycle.

have different electrochemical mechanism. In Fig. 6b, the NB shows higher capacity and better cycling stability in comparison with the NF, NL and NS. The first discharge capacities are 328 mAh g1 for NB, and 308 mAh g1 for NF, 295 mAh g1 for NL, 267 mAh g1 for NS. After 20 cycles, the NB shows a capacity of about 315 mAh g1, displaying higher capacity retention of 96.3% than NH4V3O8 with other structure (NF: 82%, NL: 86%, NS: 94%). It demonstrates that the structure has great influence on the electrochemical performance. Rate capability of the NF, NB, NL and NS were investigated under different current densities and results were demonstrated in Fig. 6c. The NB electrode delivers a discharge capacity of 330, 276, 252 and 229 mAh g1 at the current rate of 15, 60, 120 and 240 mA g1, respectively, displaying excellent rate capability. Even when cycled at 300 mA g1, a discharge capacity of 208 mAh g1 is still maintained, which is the highest among other structure of NH4V3O8 (NF: 67 mAh g1, NL: 138 mAh g1, NS: 117 mAh g1).

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Further observation from the first charge-discharge curves of NB (Fig. 6d) finds that NH4V3O8 nanobelt shows similar chargedischarge potential plateaus. Except that voltage platform of the NH4V3O8 nanobelt changes with the testing at different current density. This may be related to the polarization of the electrode. Electrochemical impedance spectroscopy (EIS) was carried out to further understand the different electrochemical performance of the NF, NB, NL and NS. As shown in Fig. 7a, all the spectra present a similar curve with depressed semicircle in the high-frequency and a slope line in the low-frequency range. To analyze the EIS curves, the equivalent circuit mode was constructed as an inset shown in Fig. 7a. The Rs, Rct, Zw and CPE in the equivalent circuit mode represent resistance of electrolyte, charge transfer impedance, Warburg impedance and constant phase element, respectively. According to the fitted results of the EIS curves, it is found that the four samples show similar Rs but very different Rct value. The Rct of the NF and NB are much smaller than that of NL and NS, indicating the Li+ transfer at the interface between the electrolyte and the working electrode (NF or NB) are much easier. Lithium ion diffusion coefficient was calculated to further compare the difference of the NF, NB, NL and NS. According to the previous report, the low-frequency Warburg impendence corresponding to the diffusion of Li ions in the bulk of the electrode, was used to determine the Li ion diffusion coefficient [28]. The Warburg coefficient s W can be calculated based on the slope by Eq. (2) Zre ¼ Rs þ Rct þ s W v1=2

ð2Þ

Where Zre is the real part of the impedance; v is the angular frequency in the low frequency region. The plot of Zre vs. v1/2 for all samples is shown in Fig. 7b. It can be obtained the Warburg coefficient sw of the NH4V3O8 with various structures. Then the lithium ion diffusion coefficient could be calculated from the formula as following [13,29]: DLiþ ¼

R2 T 2 2A2 n4 F 4 C O 2

s2

ð3Þ

[(Fig._6)TD$IG]

Fig. 6. The first charge-discharge curves (a) and the cycling performances (b) of the NF, NB, NL and NS tested at 15 mA g1; (c) rate capability of the NF, NB, NL and NS; (d) the first charge-discharge curves of the NB tested at various current density.

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Fig. 7. (a) Nyquist plots of electrochemical impedance spectroscopy (EIS) (inset is the equivalent circuit mode used to fit the impedance data) of the NF, NB, NL and NS after 3 charge/discharge cycles. (b) Graph of Z0 plotted against v1/2 of the NF, NB, NL and NS at low frequency region.

Table 1 The Warburg coefficient sw and Li+ diffusion coefficient of the NF, NB, NL and NS. Samples

sw (V cm2 s1/2)

Li+ diffusion coefficient (cm2 s1)

NF NB NL NS

18.87 8.76 21.51 31.45

7.99  1013 2.56  1012 6.15  1013 2.88  1013

Where R is gas constant (8.314472 J mol1 k1); T is test temperature (298.15 K); A is the total contact area; n is the number of the electrons transfer per mole; F is the Faraday constant (96500C mol1); C0 is the concentration of Li+ in electrodes; s is the Warburg coefficient. The obtained Li+ diffusion coefficient is shown in Table 1. Obviously, the Li+ diffusion coefficient of the NB is about one order of magnitude than that of NF, NL and NS, indicating the diffusion of Li+ in electrode is greatly affected by the structure. Therefore, the high Li+ diffusion coefficient of the NB is believed to be a main reason for enhanced cycling and rate properties.

Based on the performance results, it is found that the NB shows more excellent electrochemical property in comparison with the NF, NL and NS. Since the NB presents the structure with onedimensional and nano sized. The nano sized material can short the distance of electron transport and ion diffusion in the electrode. While for NF with numerous nanobelts woved presents poor cycling stability and low rate capacity, which may stem from its bad structural stability and low crystallinity. The NL and NS with micro-size demonstrate poor electrochemical performance as well, since micro-size is disadvantageous for the fast insertion/ extraction of Li+ in the bulk electrode. Combining the above discussion, it is concluded that controlling the morphology and particle size are important factors for synthesis NH4V3O8 cathode material with enhanced performance. To further observe the structure change before and after cycling, SEM images of four cycled samples were performed. As shown in Fig. 8a, the NF electrodes have lost their initial flower-like structure and present collapsed and agglomerated morphology, even pulverized into small particles. In Fig. 8b, the NB still exhibits original nanobelt structure and the belts are well interconnected to each other, which provides a highly conductive

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Fig. 8. SEM images of (a) NF, (b) NB, (c) NL and (d) NS electrodes after 2 cycles.

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Fig. 9. XRD patterns of NF (a) and NB (b) electrodes charged/discharged to different voltage states.

pathway for electron diffusion. In addition, the voids among the belts allow the access of lithium ions, boosting the rate performance of the NB. While the NL (Fig. 8c) and NS (Fig. 8d) are unable to maintain initial structure only after 2 cycles, in which the NH4V3O8 sheets have pulverized into small block and particles. Thereby, the NB have the best structural stability among NF, NL and NS electrodes, which leads to the improved cycling stability of the NB electrode. The ex situ XRD test was conducted to further understand the different Li+ insertion/extraction mechanism in the NF and NB electrode. As shown in Fig. 9a, the diffraction peak of NH4V3O8 couldn't be observed in the NF even for the fresh electrode, which results from the low crystallinity of the NF powder (can be seen in Fig. 1). Moreover, XRD patterns of the NF have no obvious difference when charged to different voltages, indicating that the NF is under the amorphous state all the time during the electrochemical reaction process. In Fig. 9b, the NB electrode presents a phenomenon that the diffraction peak (001) and (002) of NH4V3O8 shift without appearing other new peaks. The fresh NB electrode shows lower intensity of diffraction peak (001) than the as-prepared NB powder, which is probably attributed to the more smooth surface of electrode film. When charged to 4.0 V in the first cycle, there is no obvious shift of (001) diffraction peak, demonstrating the good structural stability of NB. Further discharged from 4.0 V to 1.5 V, its diffraction peak (001) shifts toward the low angle direction, revealing plenty of Li+ inserted into the interlayer space of the NH4V3O8. When charging again, the diffraction peak (001) shifts to the opposite direction, corresponding to the process of the extraction Li+. As observed, the XRD patterns of the NB charged to 4.0 V between the first and second cycle are very similar, indicating that the NB has good reversibility of Li ions insertion/extraction. Based on the SEM images of cycled samples and ex-situ XRD results, we find that the flower-like NH4V3O8 particle is very small and amorphous. This structure may provide more structural defects on the NF surface. Related report [30] has proved that the ions storage at defect sites will produce very different electrochemical reaction from intercalation of ions in the interlayer space, presenting a sloping region rather than voltage plateaus in charge/ discharge profiles. The report is very similar with our results (in Fig. 6a). Therefore, we think the electrochemical reactions of NF mainly derive from defect sites, while NB electrode attributes to the intercalation in the interlayer space. 4. Conclusions In summary, the NH4V3O8 materials with various structures are synthesized using a facile microwave hydrothermal method. Different NH4V3O8 structures with flower, nanobelt, lath and

sheet are obtained by controlling pH value during the hydrothermal reaction. Electrochemical results find that the NH4V3O8 nanobelt could provide best electrochemical performance among all of the structure in this work, presenting the first discharge capacity of 328 mAh g1, the capacity retention of 96% over 20 cycles and a superior rate capacity with 208 mAh g1 realized at a very high current density of 300 mA g1. This excellent electrochemical performance is attributed to the nanosized onedimensional structure of the NH4V3O8 nanobelt, which leads to high Li+ diffusion coefficient and a good structural stability during the electrochemical process. These results suggest the morphology and particle size are important factors for synthesis NH4V3O8 cathode material with enhanced performance. Acknowledgements We are grateful for the financial support provided by the National Key Technology R&D Program (No. 2013BAF09B02), National Natural Science foundation of China (No. 51472152), Innovation Team Assistance Foundation of Shaanxi Province (No. 2013KCT-06), 973 Special Preliminary Study Plan (No. 2014CB260411) and Graduate Innovation Fund of Shaanxi University of Science and Technology. References [1] A. Pan, J. Liu, J.G. Zhang, G. Cao, W. Xu, Z. Nie, X. Jie, D. Choi, B.W. Arey, C. Wang, S. Liang, Template free synthesis of LiV3O8 nanorods as a cathode material for high-rate secondary lithium batteries, J. Mater. Chem. 21 (2011) 1153–1161. [2] Z.K. Wang, J. Shu, Q.C. Zhu, B.Y. Cao, H. Chen, X.Y. Wu, B.M. Bartlett, K.X. Wang, J. S. Chen, Graphene-nanosheet-wrapped LiV3O8 nanocomposites as high performance cathode materials for rechargeable lithium-ion batteries, J. Power Sources 307 (2016) 426–434. [3] D. Sun, G. Jin, H. Wang, X. Huang, Y. Ren, J. Jiang, H. He, Y. Tang, LixV2O5/LiV3O8 nanoflakes with significantly improved electrochemical performance for Liion batteries, J. Mater. Chem. A 2 (2014) 8009–8016. [4] J. Huang, X. Qiao, Z. Xu, L. Cao, H. Ouyang, J. Li, R. Wang, V2O5 self-assembled nanosheets as high stable cathodes for Lithium-ion batteries, Electrochim. Acta 191 (2016) 158–164. [5] D. Fang, Y. Cao, R. Liu, W. Xu, S. Liu, Z. Luo, C. Liang, X. Liu, C. Xiong, Novel hierarchical three-dimensional ammonium vanadate nanowires electrodes for lithium ion battery, Appl. Surf. Sci. 360 (2016) 658–665. [6] L.L. Liu, X.J. Wang, Y.S. Zhu, C.L. Hu, Y.P. Wu, R. Holze, Polypyrrole-coated LiV3O8-nanocomposites with good electrochemical performance as anode material for aqueous rechargeable lithium batteries, J. Power Sources 224 (2013) 290–294. [7] A. Ottmann, G.S. Zakharova, B. Ehrstein, R. Klingeler, Electrochemical performance of single crystal belt-like NH4V3O8 as cathode material for lithium-ion batteries, Electrochim. Acta 174 (2015) 682–687. [8] H. Wang, K. Huang, Y. Ren, X. Huang, S. Liu, W. Wang, NH4V3O8/carbon nanotubes composite cathode material with high capacity and good rate capability, J. Power Sources 196 (2011) 9786–9791. [9] S. Sarkar, P.S. Veluri, S. Mitra, Morphology controlled synthesis of layered NH4V4O10 and the impact of binder on stable high rate electrochemical performance, Electrochim. Acta 132 (2014) 448–456.

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