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High capacity Sb2O4 thin film electrodes for rechargeable sodium battery
Electrochemistry Communications 13 (2011) 1462–1464
Contents lists available at SciVerse ScienceDirect
Electrochemistry Communications journal homepage: www.elsevier.com/locate/elecom
High capacity Sb2O4 thin film electrodes for rechargeable sodium battery Qian Sun a, Qin-Qi Ren a, Hong Li b, Zheng-Wen Fu a,⁎ a b
Shanghai Key Laboratory of Molecular Catalysts and Innovative Materials, Department of Chemistry & Laser Chemistry, Fudan University, Shanghai 200433, China Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China
a r t i c l e
i n f o
Article history: Received 20 August 2011 Received in revised form 24 September 2011 Accepted 24 September 2011 Available online 1 October 2011 Keywords: Sb2O4 Sodium ion batteries Magnetron sputtering Anode
a b s t r a c t The electrochemical behavior of magnetron sputtered Sb2O4 thin film as anode materials for rechargeable sodium ion batteries was investigated for the first time. Sb2O4 thin film electrodes exhibited a large reversible capacity of 896 mAh g− 1. The reversible conversion reactions involving both alloying/dealloying and oxidation/reduction processes of antimony were revealed during the electrochemical reaction of Sb2O4 film electrode with sodium. The high reversible capacity and good cyclibility of Sb2O4 electrode made it become a promising anode material for future rechargeable sodium ion batteries. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Lithium ion batteries (LIBs) have been used widely in consumer electronic devices since the invention of Sony company in 1990 [1]. The potential markets for electrical vehicles and smart grids could accelerate the demanding on lithium resource to a large scale. This takes serious concern on sustainable development of lithium-contained batteries [2–3]. It has been realized that rechargeable sodium ion batteries (SIBs) could be one of potential candidates to replace Li-ion batteries at least partially [4]. Currently, many efforts have been paid to find suitable anode and cathode materials for SIBs. Similarly as LIBs, carbon based materials are found being capable to store sodium reversibly for SIBs through intercalation mechanism [5–8]. The observed Na-storage capacities are below 350 mAh g − 1. It has been calculated that many materials could show high capacities for Nastorage through conversion reaction mechanism [9]. However, only very few materials, such as NiCo2O4[10], FeS2[11] and Ni3S2[12]), have been investigated. It is found that the final products from the sodium reduction of transition metal oxides and sulfides consisted of a mixture of transition metal and Li2O or Li2S, and nanosized transition metal formed after the initial cycle could drive the reversible decomposition and formation of Li2O or Li2S. The reversible capacities of NiCo2O4, FeS2, and Ni3S2 were found to be about 200 mAh g − 1, 450 mAh g − 1, 342 mAh g − 1, respectively. These works provided some information on sodium electrochemistry of metal oxides or sulfides and the possibility using them as storage sodium materials for the application and development of SIBs. Here, an attempt to extend
⁎ Corresponding author. Tel.: + 86 21 65642522; fax: + 86 21 65102777. E-mail address: [email protected] (Z.-W. Fu). 1388-2481/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2011.09.020
the investigation of electrochemical properties of other metal oxides such as antimony oxide with sodium was made. To our knowledge, there is no available report on the sodium electrochemistry of antimony oxide. Previously, Xue et al. have investigated the electrochemical conversion reaction of Sb2O3 thin film anode for LIBs [13]. It shows reversible Li-storage behaviors through both alloy reaction and conversion reaction. It is curious to know whether antimony oxide could also store sodium reversibly. In this study, Sb2O4 thin film has been prepared and investigated in sodium batteries. 2. Experimental Sb2O4 thin films were synthesized by a reactive magnetron sputtering (r.f.) method. The details of the sputtering system have been described previously [14]. The distance between the stainless steel substrate and the Sb target was about 10 cm in a vacuum chamber. Before the deposition, the chamber was evacuated below 5 × 10− 4 Pa using a turbo-molecular pump and a mechanical pump. During the deposition, the oxygen gas (pure 99.99%) was continuously purged into the chamber through a needle valve while its pressure was controlled at 3.5 Pa. The power of radio frequency was set at 50 W. The deposition time was about 40 minutes after 5 minutes pre-sputtering. The thickness of the as-deposited Sb2O4 thin film was measured to be about 280–340 nm by using a surface-roughness detector with stylus (Tencor Alpha-Step 200). The weight of Sb2O4 layer was estimated by subtracting the original substrate weight from total weight of the deposited thin film. The weight of the Sb2O4 layer was about 0.21 ± 0.01 mgcm− 2 examined by an electrobalance (BP 211D, Sartorius). For the electrochemical measurements, the cells were constructed by using the film as a working electrode and two lithium sheets as a
Q. Sun et al. / Electrochemistry Communications 13 (2011) 1462–1464
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counter electrode and a reference electrode, respectively. The electrolyte was consisted of 1 M NaClO4 (Aldrich) in a nonaqueous solution of ethylene carbonate (EC) and dimethyl carbonate (DMC) with a volume ratio of 1:1 (Merck). The cells were assembled in an Ar filled glove box. Galavanostatic cycling measurements were carried out with a Land CT2001A battery testing system. Cyclic voltammogram (CV) tests were performed on a CHI660A electrochemical working station (CHI instruments, TN). The crystal structure of the thin film electrode was characterized by a Rigata/max-C diffratometer using Cu-Kα radiation and a transmission electron microscope (JEOL 2010 TEM). For the ex situ measurements, the tested electrochemical cells at different states were disassembled in an Ar-filled glove box. The electrodes were taken out and rinsed in anhydrous dimethyl carbonate (DMC) to remove residual salts. 3. Result and Discussion The open circuit voltage of the Sb2O4 thin film/Na cell is 2.54 V. Fig. 1(a) shows the voltage profiles of the cell at the first three cycles at a current rate of about 1/70 C. It can be seen for the discharging profiles that there are one slope from 0.9 V to 0.5 V, a short plateau around 0.4 V and a long plateau from 0.17 V to 0.01 V. The charging voltage profiles are nearly reversible. These results suggest that the electrochemical reactions of Sb2O4 with sodium occur at least three steps. The initial and second discharge capacities of the electrode at 1/70 C are 1120 mAh g − 1 and 896 mAh g − 1, respectively. The theoretical capacity of Sb2O4 is 1227 mAh g − 1 presuming that Sb2O4 is converted into 4Na2O and 2Na3Sb after a fully reduction. The observed capacities are still lower than the calculated value, but they are the largest reversible capacities compared to reported values up to now [5–8,10–12]. It is noticed that the voltage polarization between the discharging and charging is about 0.7–1.0 V. This is not good for practical application. The capacity retention curves at 1/70 C and 1/10 C are shown in Fig. 1(b). Although the Sb2O4 /Na cell shows a larger capacity loss between the first two cycles, the cyclic performance is not bad and the discharge capacity is remained at 724 mAh g − 1 at 1/70 C after the 20th cycles. However, the rate performance is not very good, perhaps due to large volume variation. CV curves of the thin film electrode between 0 V and 3.5 V at a scan rate of 0.1 mV s − 1 are shown in Fig. 1(c). Three cathodic current peaks at 0.75 V, 0.43 V, and around 0.1 V are observed in the first reduction process. The intensities of the former peak are gradually weakened in the subsequent cycle, while the intensities and shapes of the latter two well maintain in all subsequent cycles. During the first oxidation process, there are corresponding three anodic peaks at 0.81 V, 0.88 V and 1.22 V. The subsequent reduction and oxidation peaks can be kept in the same position during the cycles. The peak at 1.22 V is substituted by single peak at 1.19 V in the 2nd and 3 rd cycles and two peaks at 1.12 V and 1.25 V in the 4th and 5th cycles. Its intensity gradually decreases with the subsequent cycles. These results are in a good agreement with the galvanostatic cycling profiles and indicate a reversible and complex electrochemical reaction mechanism of Sb2O4 electrode versus sodium. In order to determine the Na-storage mechanism in Sb2O4, ex situ XRD measurements were performed. The Sb2O4 thin film cells were kept at various states during galvanostatic discharging between 0.01 and 3.5 V at a constant current of 2 μA cm − 2. XRD patterns of thin film electrodes for the as-deposited, after the first discharging to 0.5 V and 0.01 V, after the first charging to 1.2 V and 3.5 V are shown in Fig. 2(a)–(e), respectively. In the XRD pattern of the asdeposited thin film (Fig. 2(a)), two diffraction peaks at 2θ = 43.7° and 50.8° are attributed to the stainless steel substrate, other diffraction peaks could be assigned well to the orthorhombic structure of Sb2O4 with Pna21 space group (JCPDS card no.71-0143). When the
Fig. 1. (a) Galvanostatic curves of Sb2O4/Na cells at the current rates of 1/70 C; (b) the corresponding specific capacities of the cells at the current rates of 1/70 C and 1/10 C as a function of cycle numbers; and (c) cyclic voltammograms for Sb2O4/Na thin film electrodes of the first five cycles at 0.1 mVs− 1.
cell is discharged to 0.5 V, two diffraction peaks from metal Sb can be found in the XRD pattern (Fig. 2(b)). However, no any other diffraction peaks except two peaks of stainless steel can be observed in the XRD patterns of thin film after discharging to 0.01 V (Fig. 2(c)). During the charging process, two weak XRD diffraction peaks from metal Sb appear again after charging to 1.2 V but vanish after charging to 3.5 V. To further clarifying the composition and structure of the reacted thin films, the ex situ TEM and SAED measurements were also performed. The ex situ TEM and SAED patterns of Sb2O4 thin film electrodes after the discharging to 0.01 V and after the charging to 3.5 V are shown in Fig. 2(f–h), respectively. It was found that the full discharged product was not stable under electron beam focusing and melted quickly. Only relevant low resolution TEM image (Fig. 2(f))
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Q. Sun et al. / Electrochemistry Communications 13 (2011) 1462–1464
Fig. 2. XRD patterns of (a) as-deposited film electrode; (b) the film electrode after the first discharging to 0.5 V; (c) the film electrode after the first discharging to 0.01 V;(d) the film electrode after the first charging to 1.2 V; and (e) the film electrode after the first charging to 3.5 V, and ex situ TEM images of the thin film electrodes (f) after the first discharging to 0.01 V and (h) after the first charging to 3.5 V and their corresponding SAED patterns (g), (i), respectively. Major diffraction circles are labelled with their respective hkl notation. (A: Antimony oxide (Sb2O4); M: Metal antimony (Sb); S: Sodium antimony alloy (Na3Sb); O: Sodium oxide (Na2O)).
can be achieved under small beam current. SAED patterns in this region show several clear rings made up of discrete spots (shown in Fig. 2(g)). All d-spacings derived from the SAED pattern could be assigned to two phases of Na3Sb and Na2O, indicating the alloying reactions process of metal Sb with Na under the voltage range from 0.5 V to 0.01 V (Fig. 2(c)). After charging to 3.5 V, the high resolution TEM image is obtained and shown in Fig. 2(h), where stripes can be seen clearly in this image. After measuring the d-space, the grains could be attributed to (112) of Sb2O4. The SAED pattern in this region is shown in Fig. 2(i). All d-spacings of the rings can be well indexed to Sb2O4. The results suggest the formation of Sb2O4 after the charging to 3.5 V. Based on XRD, TEM and SAED results mentioned above, the electrochemical reaction mechanism of Sb2O4 with sodium in the first cycle can be expressed as following: þ
ð1Þ
2Sb þ 6Na þ 6e↔2Na3 Sb
ð2Þ
Sb2 O4 þ 8Na þ 8e↔2Sb þ 4Na2 O
þ
These reactions are similar with the electrochemical reactions of antimony oxide Sb2O3 with lithium. The first step involves the reversible conversion of Sb2O4 into nanosized Sb and Na2O; and then follows the alloy reaction. Two couples of reduction peaks at 0.75 V and 0.43 V and oxidation peaks at 0.88 V and 1.22 in the first CV curve can be assigned to the reduction and oxidation of metal antimony. One couple of reduction and oxidation peaks at 0.1 V and 0.81 V in the first CV curve corresponds to the alloying/dealloying reaction of Sb with sodium as proposed in Eq. (2). The gradual decrease of the intensity of the reduction peak at 0.75 V and oxidation peak at 1.22 in the first 5th CV curves with cycles indicates that the capacity fading of Sb2O4 after the second cycle should mainly be due to the part of irreversible conversion reaction between Sb2O4 and metal Sb. Interestingly, Sb2O4 was found to be electrochemically inactive with lithium [15]. We have also investigated the electrochemical reaction of other transition metal oxides such as FeO, CoO, and NiO with sodium. They did not show significant electrochemical activity. However, it is well known
that they show very high Li-storage capacities [9,16]. It is quite interesting that the same material shows very different Li-storage and Na-storage activities, which needs comprehensive investigations. 4. Conclusions Sb2O4 thin film has been fabricated by magnetron sputtering. It shows a large reversible capacity of 896 mAh g − 1 and reasonable cyclic performance. The sodium storage in Sb2O4 occurs through conversion reaction and alloy reaction mainly, as clarified by XRD, SAED and TEM investigations. Acknowledgements This work was financially supported by Science & Technology Commission of Shanghai Municipality (08DZ2270500 and 09JC1401300) and 973 Program (No. 2011CB933300) of China. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16]
T. Nagaura, K. Tozawa, Prog. Batteries Solar Cells 9 (1990) 209. M. Armand, J.M. Tarascon, Nature 451 (2008) 652. M.R. Palacín, Chemical Society Reviews 38 (2009) 2565. X.C. Lu, G.G. Xia, J.P. Lemmon, Z.G. Yang, Journal of Power Sources 195 (2010) 2431. D.A. Stevens, J.R. Dahn, Journal of the Electrochemical Society 147 (2000) 1271. R. Alcantara, J.M. Jimenez-Mateos, P. Lavela, J.L. Tirado, Electrochemistry Communications 3 (2001) 639. D.A. Stevens, J.R. Dahn, Journal of the Electrochemical Society 148 (2001) A803. R. Alcantara, P. Lavela, G.F. Ortiz, J.L. Tirado, Electrochemical and Solid-State Letters 8 (2005) A222. C.X. Zu, H. Li, Energy Environmental Science 4 (2011) 2614. R. Alcantara, M. Jaraba, P. Lavela, J.L. Tirado, Chemistry of Materials 14 (2002) 2847. T.B. Kim, J.W. Choi, H.S. Ryu, G.B. Cho, K.W. Kim, J.H. Ahn, K.K. Cho, H.J. Ahn, Journal of Power Sources 174 (2007) 1275. J.S. Kim, H.J. Ahn, H.S. Ryu, D.J. Kim, G.B. Cho, K.W. Kim, T.H. Nam, J.H. Ahn, Journal of Power Sources 178 (2008) 852. M.Z. Xue, Z.W. Fu, Electrochemistry Communications 8 (2006) 1250. W.Y. Liu, Z.W. Fu, Q.Z. Qin, Thin Solid Film 515 (2007) 4045. D. Larcher, A.S. Prakash, L. Laffont, M. Womes, J.C. Jumas, J. Olivier-Fourcade, M.S. Hedge, J.M. Tarascon, Journal of the Electrochemical Society 153 (2006) A1778. P. Poizot, S. Grugeon, L. Dupont, J.M. Tarascon, Nature 407 (2000) 496.
Contents lists available at SciVerse ScienceDirect
Electrochemistry Communications journal homepage: www.elsevier.com/locate/elecom
High capacity Sb2O4 thin film electrodes for rechargeable sodium battery Qian Sun a, Qin-Qi Ren a, Hong Li b, Zheng-Wen Fu a,⁎ a b
Shanghai Key Laboratory of Molecular Catalysts and Innovative Materials, Department of Chemistry & Laser Chemistry, Fudan University, Shanghai 200433, China Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China
a r t i c l e
i n f o
Article history: Received 20 August 2011 Received in revised form 24 September 2011 Accepted 24 September 2011 Available online 1 October 2011 Keywords: Sb2O4 Sodium ion batteries Magnetron sputtering Anode
a b s t r a c t The electrochemical behavior of magnetron sputtered Sb2O4 thin film as anode materials for rechargeable sodium ion batteries was investigated for the first time. Sb2O4 thin film electrodes exhibited a large reversible capacity of 896 mAh g− 1. The reversible conversion reactions involving both alloying/dealloying and oxidation/reduction processes of antimony were revealed during the electrochemical reaction of Sb2O4 film electrode with sodium. The high reversible capacity and good cyclibility of Sb2O4 electrode made it become a promising anode material for future rechargeable sodium ion batteries. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Lithium ion batteries (LIBs) have been used widely in consumer electronic devices since the invention of Sony company in 1990 [1]. The potential markets for electrical vehicles and smart grids could accelerate the demanding on lithium resource to a large scale. This takes serious concern on sustainable development of lithium-contained batteries [2–3]. It has been realized that rechargeable sodium ion batteries (SIBs) could be one of potential candidates to replace Li-ion batteries at least partially [4]. Currently, many efforts have been paid to find suitable anode and cathode materials for SIBs. Similarly as LIBs, carbon based materials are found being capable to store sodium reversibly for SIBs through intercalation mechanism [5–8]. The observed Na-storage capacities are below 350 mAh g − 1. It has been calculated that many materials could show high capacities for Nastorage through conversion reaction mechanism [9]. However, only very few materials, such as NiCo2O4[10], FeS2[11] and Ni3S2[12]), have been investigated. It is found that the final products from the sodium reduction of transition metal oxides and sulfides consisted of a mixture of transition metal and Li2O or Li2S, and nanosized transition metal formed after the initial cycle could drive the reversible decomposition and formation of Li2O or Li2S. The reversible capacities of NiCo2O4, FeS2, and Ni3S2 were found to be about 200 mAh g − 1, 450 mAh g − 1, 342 mAh g − 1, respectively. These works provided some information on sodium electrochemistry of metal oxides or sulfides and the possibility using them as storage sodium materials for the application and development of SIBs. Here, an attempt to extend
⁎ Corresponding author. Tel.: + 86 21 65642522; fax: + 86 21 65102777. E-mail address: [email protected] (Z.-W. Fu). 1388-2481/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2011.09.020
the investigation of electrochemical properties of other metal oxides such as antimony oxide with sodium was made. To our knowledge, there is no available report on the sodium electrochemistry of antimony oxide. Previously, Xue et al. have investigated the electrochemical conversion reaction of Sb2O3 thin film anode for LIBs [13]. It shows reversible Li-storage behaviors through both alloy reaction and conversion reaction. It is curious to know whether antimony oxide could also store sodium reversibly. In this study, Sb2O4 thin film has been prepared and investigated in sodium batteries. 2. Experimental Sb2O4 thin films were synthesized by a reactive magnetron sputtering (r.f.) method. The details of the sputtering system have been described previously [14]. The distance between the stainless steel substrate and the Sb target was about 10 cm in a vacuum chamber. Before the deposition, the chamber was evacuated below 5 × 10− 4 Pa using a turbo-molecular pump and a mechanical pump. During the deposition, the oxygen gas (pure 99.99%) was continuously purged into the chamber through a needle valve while its pressure was controlled at 3.5 Pa. The power of radio frequency was set at 50 W. The deposition time was about 40 minutes after 5 minutes pre-sputtering. The thickness of the as-deposited Sb2O4 thin film was measured to be about 280–340 nm by using a surface-roughness detector with stylus (Tencor Alpha-Step 200). The weight of Sb2O4 layer was estimated by subtracting the original substrate weight from total weight of the deposited thin film. The weight of the Sb2O4 layer was about 0.21 ± 0.01 mgcm− 2 examined by an electrobalance (BP 211D, Sartorius). For the electrochemical measurements, the cells were constructed by using the film as a working electrode and two lithium sheets as a
Q. Sun et al. / Electrochemistry Communications 13 (2011) 1462–1464
1463
counter electrode and a reference electrode, respectively. The electrolyte was consisted of 1 M NaClO4 (Aldrich) in a nonaqueous solution of ethylene carbonate (EC) and dimethyl carbonate (DMC) with a volume ratio of 1:1 (Merck). The cells were assembled in an Ar filled glove box. Galavanostatic cycling measurements were carried out with a Land CT2001A battery testing system. Cyclic voltammogram (CV) tests were performed on a CHI660A electrochemical working station (CHI instruments, TN). The crystal structure of the thin film electrode was characterized by a Rigata/max-C diffratometer using Cu-Kα radiation and a transmission electron microscope (JEOL 2010 TEM). For the ex situ measurements, the tested electrochemical cells at different states were disassembled in an Ar-filled glove box. The electrodes were taken out and rinsed in anhydrous dimethyl carbonate (DMC) to remove residual salts. 3. Result and Discussion The open circuit voltage of the Sb2O4 thin film/Na cell is 2.54 V. Fig. 1(a) shows the voltage profiles of the cell at the first three cycles at a current rate of about 1/70 C. It can be seen for the discharging profiles that there are one slope from 0.9 V to 0.5 V, a short plateau around 0.4 V and a long plateau from 0.17 V to 0.01 V. The charging voltage profiles are nearly reversible. These results suggest that the electrochemical reactions of Sb2O4 with sodium occur at least three steps. The initial and second discharge capacities of the electrode at 1/70 C are 1120 mAh g − 1 and 896 mAh g − 1, respectively. The theoretical capacity of Sb2O4 is 1227 mAh g − 1 presuming that Sb2O4 is converted into 4Na2O and 2Na3Sb after a fully reduction. The observed capacities are still lower than the calculated value, but they are the largest reversible capacities compared to reported values up to now [5–8,10–12]. It is noticed that the voltage polarization between the discharging and charging is about 0.7–1.0 V. This is not good for practical application. The capacity retention curves at 1/70 C and 1/10 C are shown in Fig. 1(b). Although the Sb2O4 /Na cell shows a larger capacity loss between the first two cycles, the cyclic performance is not bad and the discharge capacity is remained at 724 mAh g − 1 at 1/70 C after the 20th cycles. However, the rate performance is not very good, perhaps due to large volume variation. CV curves of the thin film electrode between 0 V and 3.5 V at a scan rate of 0.1 mV s − 1 are shown in Fig. 1(c). Three cathodic current peaks at 0.75 V, 0.43 V, and around 0.1 V are observed in the first reduction process. The intensities of the former peak are gradually weakened in the subsequent cycle, while the intensities and shapes of the latter two well maintain in all subsequent cycles. During the first oxidation process, there are corresponding three anodic peaks at 0.81 V, 0.88 V and 1.22 V. The subsequent reduction and oxidation peaks can be kept in the same position during the cycles. The peak at 1.22 V is substituted by single peak at 1.19 V in the 2nd and 3 rd cycles and two peaks at 1.12 V and 1.25 V in the 4th and 5th cycles. Its intensity gradually decreases with the subsequent cycles. These results are in a good agreement with the galvanostatic cycling profiles and indicate a reversible and complex electrochemical reaction mechanism of Sb2O4 electrode versus sodium. In order to determine the Na-storage mechanism in Sb2O4, ex situ XRD measurements were performed. The Sb2O4 thin film cells were kept at various states during galvanostatic discharging between 0.01 and 3.5 V at a constant current of 2 μA cm − 2. XRD patterns of thin film electrodes for the as-deposited, after the first discharging to 0.5 V and 0.01 V, after the first charging to 1.2 V and 3.5 V are shown in Fig. 2(a)–(e), respectively. In the XRD pattern of the asdeposited thin film (Fig. 2(a)), two diffraction peaks at 2θ = 43.7° and 50.8° are attributed to the stainless steel substrate, other diffraction peaks could be assigned well to the orthorhombic structure of Sb2O4 with Pna21 space group (JCPDS card no.71-0143). When the
Fig. 1. (a) Galvanostatic curves of Sb2O4/Na cells at the current rates of 1/70 C; (b) the corresponding specific capacities of the cells at the current rates of 1/70 C and 1/10 C as a function of cycle numbers; and (c) cyclic voltammograms for Sb2O4/Na thin film electrodes of the first five cycles at 0.1 mVs− 1.
cell is discharged to 0.5 V, two diffraction peaks from metal Sb can be found in the XRD pattern (Fig. 2(b)). However, no any other diffraction peaks except two peaks of stainless steel can be observed in the XRD patterns of thin film after discharging to 0.01 V (Fig. 2(c)). During the charging process, two weak XRD diffraction peaks from metal Sb appear again after charging to 1.2 V but vanish after charging to 3.5 V. To further clarifying the composition and structure of the reacted thin films, the ex situ TEM and SAED measurements were also performed. The ex situ TEM and SAED patterns of Sb2O4 thin film electrodes after the discharging to 0.01 V and after the charging to 3.5 V are shown in Fig. 2(f–h), respectively. It was found that the full discharged product was not stable under electron beam focusing and melted quickly. Only relevant low resolution TEM image (Fig. 2(f))
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Q. Sun et al. / Electrochemistry Communications 13 (2011) 1462–1464
Fig. 2. XRD patterns of (a) as-deposited film electrode; (b) the film electrode after the first discharging to 0.5 V; (c) the film electrode after the first discharging to 0.01 V;(d) the film electrode after the first charging to 1.2 V; and (e) the film electrode after the first charging to 3.5 V, and ex situ TEM images of the thin film electrodes (f) after the first discharging to 0.01 V and (h) after the first charging to 3.5 V and their corresponding SAED patterns (g), (i), respectively. Major diffraction circles are labelled with their respective hkl notation. (A: Antimony oxide (Sb2O4); M: Metal antimony (Sb); S: Sodium antimony alloy (Na3Sb); O: Sodium oxide (Na2O)).
can be achieved under small beam current. SAED patterns in this region show several clear rings made up of discrete spots (shown in Fig. 2(g)). All d-spacings derived from the SAED pattern could be assigned to two phases of Na3Sb and Na2O, indicating the alloying reactions process of metal Sb with Na under the voltage range from 0.5 V to 0.01 V (Fig. 2(c)). After charging to 3.5 V, the high resolution TEM image is obtained and shown in Fig. 2(h), where stripes can be seen clearly in this image. After measuring the d-space, the grains could be attributed to (112) of Sb2O4. The SAED pattern in this region is shown in Fig. 2(i). All d-spacings of the rings can be well indexed to Sb2O4. The results suggest the formation of Sb2O4 after the charging to 3.5 V. Based on XRD, TEM and SAED results mentioned above, the electrochemical reaction mechanism of Sb2O4 with sodium in the first cycle can be expressed as following: þ
ð1Þ
2Sb þ 6Na þ 6e↔2Na3 Sb
ð2Þ
Sb2 O4 þ 8Na þ 8e↔2Sb þ 4Na2 O
þ
These reactions are similar with the electrochemical reactions of antimony oxide Sb2O3 with lithium. The first step involves the reversible conversion of Sb2O4 into nanosized Sb and Na2O; and then follows the alloy reaction. Two couples of reduction peaks at 0.75 V and 0.43 V and oxidation peaks at 0.88 V and 1.22 in the first CV curve can be assigned to the reduction and oxidation of metal antimony. One couple of reduction and oxidation peaks at 0.1 V and 0.81 V in the first CV curve corresponds to the alloying/dealloying reaction of Sb with sodium as proposed in Eq. (2). The gradual decrease of the intensity of the reduction peak at 0.75 V and oxidation peak at 1.22 in the first 5th CV curves with cycles indicates that the capacity fading of Sb2O4 after the second cycle should mainly be due to the part of irreversible conversion reaction between Sb2O4 and metal Sb. Interestingly, Sb2O4 was found to be electrochemically inactive with lithium [15]. We have also investigated the electrochemical reaction of other transition metal oxides such as FeO, CoO, and NiO with sodium. They did not show significant electrochemical activity. However, it is well known
that they show very high Li-storage capacities [9,16]. It is quite interesting that the same material shows very different Li-storage and Na-storage activities, which needs comprehensive investigations. 4. Conclusions Sb2O4 thin film has been fabricated by magnetron sputtering. It shows a large reversible capacity of 896 mAh g − 1 and reasonable cyclic performance. The sodium storage in Sb2O4 occurs through conversion reaction and alloy reaction mainly, as clarified by XRD, SAED and TEM investigations. Acknowledgements This work was financially supported by Science & Technology Commission of Shanghai Municipality (08DZ2270500 and 09JC1401300) and 973 Program (No. 2011CB933300) of China. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16]
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