Electrochemistry Communications 13 (2011) 1462–1464

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Electrochemistry Communications

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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 Shanghai Key Laboratory of Molecular Catalysts and Innovative Materials, Department of Chemistry & Laser Chemistry, Fudan University, Shanghai 200433, Chinab Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China

⁎ Corresponding author. Tel.: +86 21 65642522; fax:E-mail address: zwfu@fudan.edu.cn (Z.-W. Fu).

1388-2481/$ – see front matter © 2011 Elsevier B.V. Alldoi:10.1016/j.elecom.2011.09.020

a b s t r a c t

a r t i c l e i n f o

Article history:Received 20 August 2011Received in revised form 24 September 2011Accepted 24 September 2011Available online 1 October 2011

Keywords:Sb2O4

Sodium ion batteriesMagnetron sputteringAnode

The electrochemical behavior of magnetron sputtered Sb2O4 thin film as anode materials for rechargeable so-dium ion batteries was investigated for the first time. Sb2O4 thin film electrodes exhibited a large reversiblecapacity of 896 mAh g−1. The reversible conversion reactions involving both alloying/dealloying and oxida-tion/reduction processes of antimony were revealed during the electrochemical reaction of Sb2O4 film elec-trode with sodium. The high reversible capacity and good cyclibility of Sb2O4 electrode made it become apromising anode material for future rechargeable sodium ion batteries.

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1. Introduction

Lithium ion batteries (LIBs) have been used widely in consumerelectronic devices since the invention of Sony company in 1990 [1].The potential markets for electrical vehicles and smart grids could ac-celerate the demanding on lithium resource to a large scale. This takesserious concern on sustainable development of lithium-containedbatteries [2–3]. It has been realized that rechargeable sodium ion bat-teries (SIBs) could be one of potential candidates to replace Li-ionbatteries at least partially [4]. Currently, many efforts have beenpaid to find suitable anode and cathode materials for SIBs. Similarlyas LIBs, carbon based materials are found being capable to store sodi-um reversibly for SIBs through intercalation mechanism [5–8]. Theobserved Na-storage capacities are below 350 mAh g−1. It has beencalculated that many materials could show high capacities for Na-storage through conversion reaction mechanism [9]. However, onlyvery few materials, such as NiCo2O4[10], FeS2[11] and Ni3S2[12]),have been investigated. It is found that the final products from the so-dium reduction of transition metal oxides and sulfides consisted of amixture of transition metal and Li2O or Li2S, and nanosized transitionmetal formed after the initial cycle could drive the reversible decom-position and formation of Li2O or Li2S. The reversible capacities ofNiCo2O4, FeS2, and Ni3S2 were found to be about 200 mAh g−1,450 mAh g−1, 342 mAh g−1, respectively. These works providedsome information on sodium electrochemistry of metal oxides or sul-fides and the possibility using them as storage sodium materials forthe application and development of SIBs. Here, an attempt to extend

the investigation of electrochemical properties of other metal oxidessuch as antimony oxide with sodium was made. To our knowledge,there is no available report on the sodium electrochemistry of anti-mony oxide.

Previously, Xue et al. have investigated the electrochemical con-version reaction of Sb2O3 thin film anode for LIBs [13]. It shows re-versible Li-storage behaviors through both alloy reaction andconversion reaction. It is curious to know whether antimony oxidecould also store sodium reversibly. In this study, Sb2O4 thin film hasbeen prepared and investigated in sodium batteries.

2. Experimental

Sb2O4 thin films were synthesized by a reactive magnetron sput-tering (r.f.) method. The details of the sputtering system have beendescribed previously [14]. The distance between the stainless steelsubstrate and the Sb target was about 10 cm in a vacuum chamber.Before the deposition, the chamber was evacuated below 5×10−4 Pausing a turbo-molecular pump and a mechanical pump. During the de-position, the oxygen gas (pure 99.99%) was continuously purged intothe chamber through a needle valve while its pressure was controlledat 3.5 Pa. The power of radio frequencywas set at 50W. The depositiontime was about 40 minutes after 5 minutes pre-sputtering. The thick-ness of the as-deposited Sb2O4 thin film was measured to be about280–340 nm by using a surface-roughness detector with stylus (TencorAlpha-Step 200). The weight of Sb2O4 layer was estimated by subtract-ing the original substrateweight from totalweight of the deposited thinfilm. The weight of the Sb2O4 layer was about 0.21±0.01 mgcm−2 ex-amined by an electrobalance (BP 211D, Sartorius).

For the electrochemical measurements, the cells were constructedby using the film as a working electrode and two lithium sheets as a

Fig. 1. (a) Galvanostatic curves of Sb2O4/Na cells at the current rates of 1/70 C; (b) thecorresponding specific capacities of the cells at the current rates of 1/70 C and 1/10 C asa function of cycle numbers; and (c) cyclic voltammograms for Sb2O4/Na thin film elec-trodes of the first five cycles at 0.1 mVs−1.

1463Q. Sun et al. / Electrochemistry Communications 13 (2011) 1462–1464

counter electrode and a reference electrode, respectively. The electro-lyte was consisted of 1 M NaClO4 (Aldrich) in a nonaqueous solutionof ethylene carbonate (EC) and dimethyl carbonate (DMC) with a vol-ume ratio of 1:1 (Merck). The cells were assembled in an Ar filledglove box. Galavanostatic cycling measurements were carried outwith a Land CT2001A battery testing system. Cyclic voltammogram(CV) tests were performed on a CHI660A electrochemical workingstation (CHI instruments, TN).

The crystal structure of the thin film electrode was characterizedby a Rigata/max-C diffratometer using Cu-Kα radiation and a trans-mission electron microscope (JEOL 2010 TEM). For the ex situ mea-surements, the tested electrochemical cells at different states weredisassembled in an Ar-filled glove box. The electrodes were takenout and rinsed in anhydrous dimethyl carbonate (DMC) to remove re-sidual 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 cyclesat a current rate of about 1/70 C. It can be seen for the dischargingprofiles that there are one slope from 0.9 V to 0.5 V, a short plateauaround 0.4 V and a long plateau from 0.17 V to 0.01 V. The chargingvoltage profiles are nearly reversible. These results suggest that theelectrochemical reactions of Sb2O4 with sodium occur at least threesteps. The initial and second discharge capacities of the electrode at1/70 C are 1120 mAh g−1 and 896 mAh g−1, respectively. The theo-retical capacity of Sb2O4 is 1227 mAh g−1 presuming that Sb2O4 isconverted into 4Na2O and 2Na3Sb after a fully reduction. The ob-served capacities are still lower than the calculated value, but theyare the largest reversible capacities compared to reported values upto now [5–8,10–12].

It is noticed that the voltage polarization between the dischargingand charging is about 0.7–1.0 V. This is not good for practical applica-tion. The capacity retention curves at 1/70 C and 1/10 C are shown inFig. 1(b). Although the Sb2O4 /Na cell shows a larger capacity loss be-tween the first two cycles, the cyclic performance is not bad and thedischarge capacity is remained at 724 mAh g−1 at 1/70 C after the20th cycles. However, the rate performance is not very good, perhapsdue to large volume variation.

CV curves of the thin film electrode between 0 V and 3.5 V at ascan rate of 0.1 mV s−1 are shown in Fig. 1(c). Three cathodic currentpeaks at 0.75 V, 0.43 V, and around 0.1 V are observed in the first re-duction process. The intensities of the former peak are graduallyweakened in the subsequent cycle, while the intensities and shapesof the latter two well maintain in all subsequent cycles. During thefirst oxidation process, there are corresponding three anodic peaksat 0.81 V, 0.88 V and 1.22 V. The subsequent reduction and oxidationpeaks can be kept in the same position during the cycles. The peak at1.22 V is substituted by single peak at 1.19 V in the 2nd and 3 rd cy-cles and two peaks at 1.12 V and 1.25 V in the 4th and 5th cycles. Itsintensity gradually decreases with the subsequent cycles. These re-sults are in a good agreement with the galvanostatic cycling profilesand indicate a reversible and complex electrochemical reactionmechanism of Sb2O4 electrode versus sodium.

In order to determine the Na-storage mechanism in Sb2O4, ex situXRD measurements were performed. The Sb2O4 thin film cells werekept at various states during galvanostatic discharging between 0.01and 3.5 V at a constant current of 2 μA cm−2. XRD patterns of thinfilm electrodes for the as-deposited, after the first discharging to0.5 V and 0.01 V, after the first charging to 1.2 V and 3.5 V areshown in Fig. 2(a)–(e), respectively. In the XRD pattern of the as-deposited thin film (Fig. 2(a)), two diffraction peaks at 2θ=43.7°and 50.8° are attributed to the stainless steel substrate, other diffrac-tion peaks could be assigned well to the orthorhombic structure ofSb2O4 with Pna21 space group (JCPDS card no.71-0143). When the

cell is discharged to 0.5 V, two diffraction peaks from metal Sb canbe found in the XRD pattern (Fig. 2(b)). However, no any other dif-fraction peaks except two peaks of stainless steel can be observed inthe XRD patterns of thin film after discharging to 0.01 V (Fig. 2(c)).During the charging process, two weak XRD diffraction peaks frommetal Sb appear again after charging to 1.2 V but vanish after charg-ing to 3.5 V.

To further clarifying the composition and structure of the reactedthin films, the ex situ TEM and SAED measurements were also per-formed. The ex situ TEM and SAED patterns of Sb2O4 thin film elec-trodes after the discharging to 0.01 V and after the charging to 3.5 Vare shown in Fig. 2(f–h), respectively. It was found that the full dis-charged product was not stable under electron beam focusing andmelted quickly. Only relevant low resolution TEM image (Fig. 2(f))

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 filmelectrode 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 to0.01 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)).

1464 Q. Sun et al. / Electrochemistry Communications 13 (2011) 1462–1464

can be achieved under small beam current. SAED patterns in this re-gion show several clear rings made up of discrete spots (shown inFig. 2(g)). All d-spacings derived from the SAED pattern could beassigned to two phases of Na3Sb and Na2O, indicating the alloying re-actions process of metal Sb with Na under the voltage range from0.5 V to 0.01 V (Fig. 2(c)). After charging to 3.5 V, the high resolutionTEM image is obtained and shown in Fig. 2(h), where stripes can beseen clearly in this image. After measuring the d-space, the grainscould be attributed to (112) of Sb2O4. The SAED pattern in this regionis shown in Fig. 2(i). All d-spacings of the rings can be well indexed toSb2O4. The results suggest the formation of Sb2O4 after the charging to3.5 V.

Based on XRD, TEM and SAED results mentioned above, the elec-trochemical reaction mechanism of Sb2O4 with sodium in the firstcycle can be expressed as following:

Sb2O4 þ 8Naþ þ 8e↔2Sbþ 4Na2O ð1Þ

2Sb þ 6Naþ þ 6e↔2Na3Sb ð2Þ

These reactions are similar with the electrochemical reactions ofantimony oxide Sb2O3 with lithium. The first step involves the revers-ible conversion of Sb2O4 into nanosized Sb and Na2O; and then followsthe alloy reaction. Two couples of reduction peaks at 0.75 V and 0.43 Vand oxidation peaks at 0.88 V and 1.22 in the first CV curve can beassigned to the reduction and oxidation of metal antimony. One cou-ple of reduction and oxidation peaks at 0.1 V and 0.81 V in the firstCV curve corresponds to the alloying/dealloying reaction of Sb withsodium as proposed in Eq. (2). The gradual decrease of the intensityof the reduction peak at 0.75 V and oxidation peak at 1.22 in the first5th CV curves with cycles indicates that the capacity fading of Sb2O4

after the second cycle should mainly be due to the part of irreversibleconversion reaction between Sb2O4 and metal Sb. Interestingly, Sb2O4

was found to be electrochemically inactive with lithium [15]. We havealso investigated the electrochemical reaction of other transitionmetal oxides such as FeO, CoO, and NiO with sodium. They did notshow significant electrochemical activity. However, it is well known

that they show very high Li-storage capacities [9,16]. It is quite inter-esting that the same material shows very different Li-storage andNa-storage activities, which needs comprehensive investigations.

4. Conclusions

Sb2O4 thin film has been fabricated by magnetron sputtering. Itshows a large reversible capacity of 896 mAh g−1 and reasonable cy-clic performance. The sodium storage in Sb2O4 occurs through con-version reaction and alloy reaction mainly, as clarified by XRD, SAEDand TEM investigations.

Acknowledgements

This work was financially supported by Science & Technology Com-mission of Shanghai Municipality (08DZ2270500 and 09JC1401300)and 973 Program (No. 2011CB933300) of China.

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