- Email: [email protected]
Novel sodium intercalated (NH4)2V6O16 platelets: High performance cathode materials for lithium-ion battery
Journal of Colloid and Interface Science 415 (2014) 85–88
Contents lists available at ScienceDirect
Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
Novel sodium intercalated (NH4)2V6O16 platelets: High performance cathode materials for lithium-ion battery Hailong Fei, Xiaomin Wu, Huan Li, Mingdeng Wei ⇑ Institute of Advanced Energy Materials, Fuzhou University, 523 Industry Road, Fuzhou 350002, PR China
a r t i c l e
i n f o
Article history: Received 13 August 2013 Accepted 14 October 2013 Available online 24 October 2013 Keywords: Vanadium bronze Intercalation Cathode Sodium-ion battery Raman Bi-cation Hydrothermal Oxalic acid Sodium nitrate Ammonium metavanadate
a b s t r a c t A simple and versatile method for preparation of novel sodium intercalated (NH4)2V6O16 is developed via a simple hydrothermal route. It is found that ammonium sodium vanadium bronze displays higher discharge capacity and better rate cyclic stability than ammonium vanadium bronze as lithium-ion battery cathode material because of smaller charge transfer resistance, which would favor superior discharge capacity and rate performance. Crown Copyright Ó 2013 Published by Elsevier Inc. All rights reserved.
1. Introduction With fossil fuels depletion, global warming and environmental pollution, increasing attention has been paid to develop and use renewable clean energy to lessen green house gas emissions. Lithium-ion batteries with high energy density, low cost and long cycle life are the ideal power source for electric vehicles, which has great significance for lessening greenhouse gas emissions and alleviating the energy crisis. But the energy density, security and cost of lithium-ion battery should be further enhanced [1]. Vanadium oxides and bronzes offer the advantages for being cheap, easy to synthesize, abundance in the earth and high-energy density, and thus has attracted much interest [2–10]. Vanadium oxide bronzes are a wide range of crystalline structures with mixed valence networks [VxOy] containing the V5+AV4+ or V4+AV3+ couples whose electrical balance of the ionocovalent network is directly linked to the amount of inserted or intercalated elements—alkalis, alkaline earths or metals [11]. Compared with vanadium oxides, the advantages of vanadium bronzes as lithium-ion battery electrode materials are as follows. Metal ions would stabilize the crystal lattice, increase the diffusion rate of lithium ion and increase electric conductivity. In addition, it is
possible to find new lithium-ion battery electrode materials with high capacity and good cyclic stability for the complicated crystalline structures of vanadium bronzes. Recently, various ammonium vanadium oxide nano-structures have been receiving a great deal of attention as lithium-ion battery cathode materials, for example, NH4V4O10 nano-belts [12,13], (NH4)2V6O16 nano-rods [14], NH4V3O80.2H2O flakes [15] and NH4V3O8 nano-rods [16]. Also, Na2V6O16xH2O nanowires were used as Li inserting material for lithium-ion battery [17]. The CV curves of all these single ammonium and sodium vanadium oxides are characteristic of multi-pairs of oxidation and reduction peaks. Furthermore, their discharge cures also display several discharge platforms, which resulted in limits to practical application. It would be very important to find novel crystalline and cheap sodium and ammonium vanadium bronze with high capacity and good cyclic stability and fewer discharge platforms as cathode materials for lithium-ion battery. However, few attentions were paid to prepare bi or multi-cations vanadium bronze, such as ammonium and sodium bi-cation vanadium bronze. Herein, we controllably fabricated sodium intercalated (NH4)2V6O16 via the intercalation of Na+. When they were tested as cathode materials for lithium-ion battery, the ammonium sodium vanadium bronze exhibited higher discharge capacity and better cyclic stability than ammonium vanadium bronze.
⇑ Corresponding author. Fax: +86 591 83753180. E-mail address: [email protected] (M. Wei). 0021-9797/$ - see front matter Crown Copyright Ó 2013 Published by Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.jcis.2013.10.025
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H. Fei et al. / Journal of Colloid and Interface Science 415 (2014) 85–88
2. Materials and methods Ammonium sodium vanadium bronze was prepared via a simple hydrothermal method. 1.28 g oxalic acid and 0.21 g NaNO3 was dissolved in 30 ml deionized water, and then 0.59 g ammonium metavanadate (NH4VO3) was added into the solution under stirring at room temperature for 2 h. After that the mixture was transferred into a 50-ml Teflon-lined stainless autoclave, sealed, kept at 200 °C for 90 h and cooled to room temperature. Ammonium vanadium bronze was prepared under the identical condition without NaNO3. The morphology of products was observed by Hitachi S-4800 field emission scanning electron microscope (SEM). X-ray diffraction (XRD) patterns were recorded on a diffractometer (Co Ka, PANalytical, X’Pert). X-ray photoelectron spectroscopy (XPS) measurements were performed with an Escalab 250 spectrometer. Thermal analysis measurements were performed using a Netzsch Sta449C analyzer. Energy dispersive spectroscopy (EDS) measurement is also performed with Hitachi S-4800 field emission scanning electron microscope. The as-synthesized materials were tested as the cathode materials for lithium-ion battery. The composite positive electrodes were consisted of the active material, conductive material (acetylene black) and binder (PVDF) in a weight ratio of 7/2/1. The Li metal was used as the counter electrode. The electrolyte was 1 M LiPF6 in a 1/1/1 (volume ratio) mixture of ethylene carbonate (EC), propylene carbonate (PC) and dimethyl carbonate (DMC). The cells were charged and discharged between a 2.0 and 3.4 V voltage limit at different discharge rate. A Land CT2001A battery tester was used to measure the electrode activities at room temperature. The electrochemical impedance measurements were carried out by Zahner IM6 using a 5 mV acoscillation amplitude which was applied over the frequency of 5 MHz to 0.1 Hz frequency ranges. The equivalent circuit was fitted by the Zman 2.0 software of the Zahner IM6 electrochemical workstation. The cells were discharged and charged for 1 cycle, and then were kept at the opencircuit condition for 2 h before performing impedance tests. Cyclic voltammetry (CV) experiments were performed with a Chi660c electrochemical workstation at a scan rate of 1 mV s1.
Fig. 1. SEM images of ammonium vanadium bronze (a) and ammonium sodium vanadium bronze (b).
3. Results and discussion SEM observations show that the ammonium vanadium bronze was micro-platelets as well as the ammonium sodium vanadium bronze prepared with NaNO3 in Fig. 1a and b, respectively. X-ray diffraction (XRD) pattern displays that both the ammonium vanadium bronze and ammonium sodium vanadium bronze have the same crystalline structure to HNaV6O164H2O (Fig. 2a and b). So both the ammonium vanadium bronze and ammonium sodium vanadium bronze may be expressed as (NH4)2V6O16 and (NH4)xNayV6O16, respectively. X-ray photoelectron spectroscopy (XPS) was performed to identify elemental composition of ammonium sodium vanadium bronze. The results show that the surface of ammonium sodium vanadium bronze is composed of N, Na, V and O. Fig. 3 shows that the binding energy of N1s is at 401.4 eV, which is ascribed to NHþ 4 [18]. The binding energy of Na1s is at 1071.4 eV ascribed to Na+. The XPS spectra of the O1s region for ammonium sodium vanadium bronze is composed of three peaks at 530.0, 531.4 and 532.9 eV, corresponding to O2, C@O and CAO(H), respectively [19]. The V2p3/2 peak is composed of two peaks at 517.4 and 515.9 eV corresponding to V (V) and V (IV), respectively [20]. The average oxidation number for V is +4.74 calculated from X-ray photoelectron Spectroscopy (XPS) data. TG curve (Fig. 4) shows
Fig. 2. Wide-angle powder XRD patterns of ammonium vanadium bronze (a) and ammonium sodium vanadium bronze (b).
that there is the weight loss of 2.18% between 280 and 400 °C, due to the release of NH3. EDS results show that the molar ratio of V to Na is 1:0.12. Based on the above analysis, the formula of ammonium sodium vanadium bronze can be expressed as (NH4)0.71Na0.72V6O16. Both the ammonium vanadium bronze and ammonium sodium vanadium bronze were tested as cathode material for lithium-ion battery at a current density of 750 mA g1. The first, second and final charge–discharge profiles of the ammonium sodium vanadium bronze and ammonium vanadium bronze/Li cell are shown in Fig. 5a and b, respectively. They have similar discharge curves,
H. Fei et al. / Journal of Colloid and Interface Science 415 (2014) 85–88
Fig. 3. XPS spectra of ammonium sodium vanadium bronze.
Fig. 4. TG curve of ammonium sodium vanadium bronze.
87
but difference in charge curves. The ammonium sodium vanadium bronze exhibits better charge ability than ammonium vanadium bronze for the coinciding of first and second charge curves. It is also found that ammonium sodium vanadium bronze exhibits higher discharge capacity than ammonium vanadium bronze. The maximum insertion capacity is 182 mA hg1 for ammonium sodium vanadium bronze, while 158 mA hg1 for ammonium vanadium bronze. Ammonium sodium vanadium bronze is also stable in multiple insertion/extraction processes at a current density of 100, 200, 300, 500, 750 and 1000 mA g1, and the capacity retention being 79.4% after 70 cycles. Cyclic voltammetry (CV) was used to study the lithium intercalation for ammonium and sodium ammonium vanadium bronze. The anodic high current is because the primary potential of CV is set up at 3.4 V, which is higher than the open circuit potential. So the scans are first from 3.4 to 2.0 V and then from 2.0 to 3.4 V for a cycle. The first CV curve of ammonium vanadium bronzes presents four catholic peaks at 2.49, 2.61, 2.77 and 3.0 V in Fig. 6a, while the catholic peaks at 2.61, 2.77 and 3.0 V become weakened and the peak at 2.49 V is almost the same for ammonium sodium vanadium bronze in Fig. 6b. So this is a facile way to weaken discharge reaction of ammonium vanadium bronze and get fewer discharge platforms via inserting sodium-ion to ammonium vanadium bronze. In other words, ammonium sodium vanadium bronze is potential candidate for cathode materials with fewer discharge platforms. The electrochemical impedance measurements were performed to elucidate the difference in electrochemical properties. The Nyquist plots for the two samples are one semicircle and a straight sloping line at low frequency in Fig. 7, which was fitted by the simplified equivalent circuit in the inset of Fig. 7. The equivalent electrical circuit consists of an active electrolyte resistance RS in series with the parallel combination of the double-layer capacitance C1 and an impedance of a faradaic reaction. In this model, the impedance of a faradaic reaction consists of an active charge transfer resistance Rct and a specific electrochemical element of diffusion W called Warburg element. The fitted values of Rs, Rct, C1 and W are 2.38, 22.11, 5 .00 106 and 0.04 X for ammonium sodium vanadium bronze, while 5.19, 30.54, 2.99 106 and
Fig. 5. The first, second and final charge–discharge profiles of the ammonium sodium vanadium bronzes/Li cell (a) and the ammonium vanadium bronzes/Li cell (b), the cyclic performance of ammonium sodium vanadium bronze (c) and its corresponding evolution of the reversible capacity for ammonium sodium vanadium bronzes cycled at a current density of 20, 100, 300, 500, 750 and 100 mA g1 using 2.0–3.4 V potential window (d).
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method. It exhibits better cyclic stability than ammonium vanadium bronze with same crystalline structure as cathode material for lithium-ion battery due to enhanced Li-ion transfer rate. It is an effective way to improve the electrochemical performance of ammonium vanadium bronze by Na+ intercalation. Furthermore, this facile method is significant to fabricate other new crystalline structured ammonium vanadium bronze. Our further attention will be focus on fabricating other new crystalline structured ammonium sodium vanadium bronzes, which would be applied to sodium-ion battery cathode. Acknowledgments
Fig. 6. Cyclic voltammograms of ammonium vanadium bronze (a) and ammonium sodium vanadium bronze (b).
The project was supported by National Science Foundation of China (Grants No. 51204058), the open project of Key Lab Adv Energy Mat Chem (Nankai University, KLAEMC-OP201201) and the funds (2010J05025 and JA12037). References
Fig. 7. Nyquist-diagram of ammonium sodium vanadium bronze (a) and ammonium vanadium bronze (b) after discharging and charging for 1 cycle (inset is the fitting impedance circuit model).
0.03 X for ammonium vanadium bronze, respectively. It can be found that ammonium sodium vanadium bronze have smaller charge transfer resistance than ammonium vanadium bronze, which would favor superior discharge capacity and rate performance [21]. 4. Conclusions Novel sodium ammonium vanadium bronze was fabricated from cheap reagent at a large scale via a facile template-free
[1] H.G. Jung, M.W. Jang, J. Hassoun, Y.K. Sun, B. Scrosati, Nat. Commun. 2 (2011) 511–515. [2] Y. Wang, K. Takahashi, K. Lee, G.Z. Cao, Adv. Funct. Mater. 16 (2006) 1133. [3] Y.X. Tang, X.H. Rui, Y.Y. Zhang, T.M. Lim, Z. Li Dong, H.H. Hng, X.D. Chen, Q.Y. Yan, Z. Chen, J. Mater. Chem. A 1 (2013) 82. [4] H.C. Pang, P. Cheng, H.B. Yang, J.L. Lu, C.X. Guo, G.L. Ning, C.M. Li, Chem. Commun. 49 (2013) 1536–1538. [5] X.W. Zhou, G.M. Wu, G.H. Gao, J.C. Wang, H.Y. Yang, J.D. Wu, J. Shen, B. Zhou, Z.H. Zhang, J. Phys. Chem. C 116 (2012) 21685–21692. [6] Y.Z. Wu, P.N. Zhu, X. Zhao, M.V. Reddy, S.J. Peng, B.V.R. Chowdari, S. Ramakrishna, J. Mater. Chem. A 1 (2013) 852–859. [7] A.Q. Pan, H.P. Wu, L. Yu, T. Zhu, X.W. Lou, ACS Appl. Mater. Inter. 4 (2012) 3874–3879. [8] H.P. Zhao, L.Y. Pan, S.Y. Xing, J. Luo, J.Q. Xu, J. Power Sources 222 (2013) 21–31. [9] J. Bao, M. Zhou, Y.Q. Zeng, L.F. Bai, X.D. Zhang, K. Xu, Y. Xie, J. Mater. Chem. A 1 (2013) 5423–5429. [10] W.D. Li, C.Y. Xu, X.L. Pan, Y.D. Huang, L. Zhen, J. Mater. Chem. A 1 (2013) 5361– 5369. [11] J.C. Trombe, O. Szajwaj, P. Salles, J. Solid State Chem. 180 (2007) 2102–2109. [12] H.Y. Wang, K.L. Huang, C.H. Huang, S.Q. Liu, Y. Ren, X.B. Huang, J. Power Sources 196 (2011) 5645–5650. [13] K.F. Zhang, G.Q. Zhang, X. Liu, Z.X. Su, J. Power Sources 157 (2006) 528–532. [14] H.K. Park, G. Kim, Solid State Ionics 181 (2010) 311–314. [15] H.Y. Wang, K.L. Huang, S.Q. Liu, C.H. Huang, W.J. Wang, Y. Ren, J. Power Sources 196 (2011) 788–792. [16] H.Y. Wang, Y. Ren, W.J. Wang, X.B. Huang, K.L. Huang, Y. Wang, S.Q. Liu, J. Power Sources 199 (2012) 315–321. [17] H.Y. Wang, W.J. Wang, Y. Ren, K.L. Huang, S.Q. Liu, J. Power Sources 199 (2012) 263–269. [18] G.P. Kostikova, D.V. Korol’kov, Y.P. Kostikov, Russ. Chem. Rev. 66 (1997) 281– 300. [19] G. Silversmit, D. Depla, H. Poelman, G.B. Marin, R. De Gryse, J. Electron Spectrosc. Relat. Phenom. 135 (2004) 167–175. [20] J. Mendialdua, R. Casanova, Y. Barbaux, J. Electron Spectrosc. Relat. Phenom. 71 (1995) 249–261. [21] T.L. Zhao, S. Chen, L. Li, X.F. Zhang, R.J. Chen, I. Belharouak, F. Wu, K. Amine, J. Power Sources 228 (2013) 206–213.
Contents lists available at ScienceDirect
Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
Novel sodium intercalated (NH4)2V6O16 platelets: High performance cathode materials for lithium-ion battery Hailong Fei, Xiaomin Wu, Huan Li, Mingdeng Wei ⇑ Institute of Advanced Energy Materials, Fuzhou University, 523 Industry Road, Fuzhou 350002, PR China
a r t i c l e
i n f o
Article history: Received 13 August 2013 Accepted 14 October 2013 Available online 24 October 2013 Keywords: Vanadium bronze Intercalation Cathode Sodium-ion battery Raman Bi-cation Hydrothermal Oxalic acid Sodium nitrate Ammonium metavanadate
a b s t r a c t A simple and versatile method for preparation of novel sodium intercalated (NH4)2V6O16 is developed via a simple hydrothermal route. It is found that ammonium sodium vanadium bronze displays higher discharge capacity and better rate cyclic stability than ammonium vanadium bronze as lithium-ion battery cathode material because of smaller charge transfer resistance, which would favor superior discharge capacity and rate performance. Crown Copyright Ó 2013 Published by Elsevier Inc. All rights reserved.
1. Introduction With fossil fuels depletion, global warming and environmental pollution, increasing attention has been paid to develop and use renewable clean energy to lessen green house gas emissions. Lithium-ion batteries with high energy density, low cost and long cycle life are the ideal power source for electric vehicles, which has great significance for lessening greenhouse gas emissions and alleviating the energy crisis. But the energy density, security and cost of lithium-ion battery should be further enhanced [1]. Vanadium oxides and bronzes offer the advantages for being cheap, easy to synthesize, abundance in the earth and high-energy density, and thus has attracted much interest [2–10]. Vanadium oxide bronzes are a wide range of crystalline structures with mixed valence networks [VxOy] containing the V5+AV4+ or V4+AV3+ couples whose electrical balance of the ionocovalent network is directly linked to the amount of inserted or intercalated elements—alkalis, alkaline earths or metals [11]. Compared with vanadium oxides, the advantages of vanadium bronzes as lithium-ion battery electrode materials are as follows. Metal ions would stabilize the crystal lattice, increase the diffusion rate of lithium ion and increase electric conductivity. In addition, it is
possible to find new lithium-ion battery electrode materials with high capacity and good cyclic stability for the complicated crystalline structures of vanadium bronzes. Recently, various ammonium vanadium oxide nano-structures have been receiving a great deal of attention as lithium-ion battery cathode materials, for example, NH4V4O10 nano-belts [12,13], (NH4)2V6O16 nano-rods [14], NH4V3O80.2H2O flakes [15] and NH4V3O8 nano-rods [16]. Also, Na2V6O16xH2O nanowires were used as Li inserting material for lithium-ion battery [17]. The CV curves of all these single ammonium and sodium vanadium oxides are characteristic of multi-pairs of oxidation and reduction peaks. Furthermore, their discharge cures also display several discharge platforms, which resulted in limits to practical application. It would be very important to find novel crystalline and cheap sodium and ammonium vanadium bronze with high capacity and good cyclic stability and fewer discharge platforms as cathode materials for lithium-ion battery. However, few attentions were paid to prepare bi or multi-cations vanadium bronze, such as ammonium and sodium bi-cation vanadium bronze. Herein, we controllably fabricated sodium intercalated (NH4)2V6O16 via the intercalation of Na+. When they were tested as cathode materials for lithium-ion battery, the ammonium sodium vanadium bronze exhibited higher discharge capacity and better cyclic stability than ammonium vanadium bronze.
⇑ Corresponding author. Fax: +86 591 83753180. E-mail address: [email protected] (M. Wei). 0021-9797/$ - see front matter Crown Copyright Ó 2013 Published by Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.jcis.2013.10.025
86
H. Fei et al. / Journal of Colloid and Interface Science 415 (2014) 85–88
2. Materials and methods Ammonium sodium vanadium bronze was prepared via a simple hydrothermal method. 1.28 g oxalic acid and 0.21 g NaNO3 was dissolved in 30 ml deionized water, and then 0.59 g ammonium metavanadate (NH4VO3) was added into the solution under stirring at room temperature for 2 h. After that the mixture was transferred into a 50-ml Teflon-lined stainless autoclave, sealed, kept at 200 °C for 90 h and cooled to room temperature. Ammonium vanadium bronze was prepared under the identical condition without NaNO3. The morphology of products was observed by Hitachi S-4800 field emission scanning electron microscope (SEM). X-ray diffraction (XRD) patterns were recorded on a diffractometer (Co Ka, PANalytical, X’Pert). X-ray photoelectron spectroscopy (XPS) measurements were performed with an Escalab 250 spectrometer. Thermal analysis measurements were performed using a Netzsch Sta449C analyzer. Energy dispersive spectroscopy (EDS) measurement is also performed with Hitachi S-4800 field emission scanning electron microscope. The as-synthesized materials were tested as the cathode materials for lithium-ion battery. The composite positive electrodes were consisted of the active material, conductive material (acetylene black) and binder (PVDF) in a weight ratio of 7/2/1. The Li metal was used as the counter electrode. The electrolyte was 1 M LiPF6 in a 1/1/1 (volume ratio) mixture of ethylene carbonate (EC), propylene carbonate (PC) and dimethyl carbonate (DMC). The cells were charged and discharged between a 2.0 and 3.4 V voltage limit at different discharge rate. A Land CT2001A battery tester was used to measure the electrode activities at room temperature. The electrochemical impedance measurements were carried out by Zahner IM6 using a 5 mV acoscillation amplitude which was applied over the frequency of 5 MHz to 0.1 Hz frequency ranges. The equivalent circuit was fitted by the Zman 2.0 software of the Zahner IM6 electrochemical workstation. The cells were discharged and charged for 1 cycle, and then were kept at the opencircuit condition for 2 h before performing impedance tests. Cyclic voltammetry (CV) experiments were performed with a Chi660c electrochemical workstation at a scan rate of 1 mV s1.
Fig. 1. SEM images of ammonium vanadium bronze (a) and ammonium sodium vanadium bronze (b).
3. Results and discussion SEM observations show that the ammonium vanadium bronze was micro-platelets as well as the ammonium sodium vanadium bronze prepared with NaNO3 in Fig. 1a and b, respectively. X-ray diffraction (XRD) pattern displays that both the ammonium vanadium bronze and ammonium sodium vanadium bronze have the same crystalline structure to HNaV6O164H2O (Fig. 2a and b). So both the ammonium vanadium bronze and ammonium sodium vanadium bronze may be expressed as (NH4)2V6O16 and (NH4)xNayV6O16, respectively. X-ray photoelectron spectroscopy (XPS) was performed to identify elemental composition of ammonium sodium vanadium bronze. The results show that the surface of ammonium sodium vanadium bronze is composed of N, Na, V and O. Fig. 3 shows that the binding energy of N1s is at 401.4 eV, which is ascribed to NHþ 4 [18]. The binding energy of Na1s is at 1071.4 eV ascribed to Na+. The XPS spectra of the O1s region for ammonium sodium vanadium bronze is composed of three peaks at 530.0, 531.4 and 532.9 eV, corresponding to O2, C@O and CAO(H), respectively [19]. The V2p3/2 peak is composed of two peaks at 517.4 and 515.9 eV corresponding to V (V) and V (IV), respectively [20]. The average oxidation number for V is +4.74 calculated from X-ray photoelectron Spectroscopy (XPS) data. TG curve (Fig. 4) shows
Fig. 2. Wide-angle powder XRD patterns of ammonium vanadium bronze (a) and ammonium sodium vanadium bronze (b).
that there is the weight loss of 2.18% between 280 and 400 °C, due to the release of NH3. EDS results show that the molar ratio of V to Na is 1:0.12. Based on the above analysis, the formula of ammonium sodium vanadium bronze can be expressed as (NH4)0.71Na0.72V6O16. Both the ammonium vanadium bronze and ammonium sodium vanadium bronze were tested as cathode material for lithium-ion battery at a current density of 750 mA g1. The first, second and final charge–discharge profiles of the ammonium sodium vanadium bronze and ammonium vanadium bronze/Li cell are shown in Fig. 5a and b, respectively. They have similar discharge curves,
H. Fei et al. / Journal of Colloid and Interface Science 415 (2014) 85–88
Fig. 3. XPS spectra of ammonium sodium vanadium bronze.
Fig. 4. TG curve of ammonium sodium vanadium bronze.
87
but difference in charge curves. The ammonium sodium vanadium bronze exhibits better charge ability than ammonium vanadium bronze for the coinciding of first and second charge curves. It is also found that ammonium sodium vanadium bronze exhibits higher discharge capacity than ammonium vanadium bronze. The maximum insertion capacity is 182 mA hg1 for ammonium sodium vanadium bronze, while 158 mA hg1 for ammonium vanadium bronze. Ammonium sodium vanadium bronze is also stable in multiple insertion/extraction processes at a current density of 100, 200, 300, 500, 750 and 1000 mA g1, and the capacity retention being 79.4% after 70 cycles. Cyclic voltammetry (CV) was used to study the lithium intercalation for ammonium and sodium ammonium vanadium bronze. The anodic high current is because the primary potential of CV is set up at 3.4 V, which is higher than the open circuit potential. So the scans are first from 3.4 to 2.0 V and then from 2.0 to 3.4 V for a cycle. The first CV curve of ammonium vanadium bronzes presents four catholic peaks at 2.49, 2.61, 2.77 and 3.0 V in Fig. 6a, while the catholic peaks at 2.61, 2.77 and 3.0 V become weakened and the peak at 2.49 V is almost the same for ammonium sodium vanadium bronze in Fig. 6b. So this is a facile way to weaken discharge reaction of ammonium vanadium bronze and get fewer discharge platforms via inserting sodium-ion to ammonium vanadium bronze. In other words, ammonium sodium vanadium bronze is potential candidate for cathode materials with fewer discharge platforms. The electrochemical impedance measurements were performed to elucidate the difference in electrochemical properties. The Nyquist plots for the two samples are one semicircle and a straight sloping line at low frequency in Fig. 7, which was fitted by the simplified equivalent circuit in the inset of Fig. 7. The equivalent electrical circuit consists of an active electrolyte resistance RS in series with the parallel combination of the double-layer capacitance C1 and an impedance of a faradaic reaction. In this model, the impedance of a faradaic reaction consists of an active charge transfer resistance Rct and a specific electrochemical element of diffusion W called Warburg element. The fitted values of Rs, Rct, C1 and W are 2.38, 22.11, 5 .00 106 and 0.04 X for ammonium sodium vanadium bronze, while 5.19, 30.54, 2.99 106 and
Fig. 5. The first, second and final charge–discharge profiles of the ammonium sodium vanadium bronzes/Li cell (a) and the ammonium vanadium bronzes/Li cell (b), the cyclic performance of ammonium sodium vanadium bronze (c) and its corresponding evolution of the reversible capacity for ammonium sodium vanadium bronzes cycled at a current density of 20, 100, 300, 500, 750 and 100 mA g1 using 2.0–3.4 V potential window (d).
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method. It exhibits better cyclic stability than ammonium vanadium bronze with same crystalline structure as cathode material for lithium-ion battery due to enhanced Li-ion transfer rate. It is an effective way to improve the electrochemical performance of ammonium vanadium bronze by Na+ intercalation. Furthermore, this facile method is significant to fabricate other new crystalline structured ammonium vanadium bronze. Our further attention will be focus on fabricating other new crystalline structured ammonium sodium vanadium bronzes, which would be applied to sodium-ion battery cathode. Acknowledgments
Fig. 6. Cyclic voltammograms of ammonium vanadium bronze (a) and ammonium sodium vanadium bronze (b).
The project was supported by National Science Foundation of China (Grants No. 51204058), the open project of Key Lab Adv Energy Mat Chem (Nankai University, KLAEMC-OP201201) and the funds (2010J05025 and JA12037). References
Fig. 7. Nyquist-diagram of ammonium sodium vanadium bronze (a) and ammonium vanadium bronze (b) after discharging and charging for 1 cycle (inset is the fitting impedance circuit model).
0.03 X for ammonium vanadium bronze, respectively. It can be found that ammonium sodium vanadium bronze have smaller charge transfer resistance than ammonium vanadium bronze, which would favor superior discharge capacity and rate performance [21]. 4. Conclusions Novel sodium ammonium vanadium bronze was fabricated from cheap reagent at a large scale via a facile template-free
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