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Solid State Sciences 12 (2010) 397–403

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Solid State Sciences

journal homepage: www.elsevier .com/locate/ssscie

A novel anode material of antimony nitride for rechargeable lithium batteries

Qian Sun, Wen-Jing Li, Zheng-Wen Fu*

Department of Chemistry & Laser Chemistry Institute, Shanghai Key laboratory of Molecular Catalysts and Innovative Materials, Fudan University, Shanghai, 200433, PR China

a r t i c l e i n f o

Article history:Received 10 September 2009Received in revised form7 November 2009Accepted 3 December 2009Available online 11 December 2009

Keywords:Sb3NAnodeElectrochemical reactionRechargeable lithium batteries

* Corresponding author. Tel.: þ86 21 65642522.E-mail address: zhengwen@sh163.net (Z.-W. Fu).

1293-2558/$ – see front matter � 2009 Elsevier Masdoi:10.1016/j.solidstatesciences.2009.12.003

a b s t r a c t

Antimony nitride thin film has been successfully fabricated by magnetron sputtering method and itselectrochemistry with lithium was investigated for the first time. The reversible discharge capacity ofSb3N/Li cells cycled between 0.3 V and 3.0 V was found above 600 mAh/g. By using transmission electronmicroscopy and selected area electron diffraction measurements, the conversion reaction of Sb3N intoLi3Sb and Li3N was revealed during the lithium electrochemical reaction of Sb3N thin film electrode. Thehigh reversible capacity and the good cycleability made Sb3N one of promising anode materials for futurerechargeable lithium batteries.

� 2009 Elsevier Masson SAS. All rights reserved.

1. Introduction

In recent years, the rapid development of the technetronic hasbeen promoting the demands for higher store densities of lithiumfor rechargeable lithium-ion batteries as power sources. Manyefforts have been devoted to investigate antimony and itscompounds as anode materials for rechargeable lithium-ionbatteries instead of carbonaceous and graphite electrode toincrease the specific energy of lithium-ion batteries. For example,some Sb-based intermetallic materials such as antimony basedalloys such as such as MnSb [1], FeSb2 [2], CoSb3 [3,4], NiSb2 [5],Cu2Sb [6], Zn4Sb3 [7], Ag3Sb [8], InSb [9], SnSb [10,11] show excel-lent electrochemical behaviors but their performances are notsatisfied. Part of metal in antimony based alloys could not alloywith lithium (such as Mn, Fe, Co, Ni and Cu) [1–6] and serve as anappropriate matrix. Because the presence of these ‘‘inactive’’elements could alleviate the large volume change during thealloying process and make the cycle performance well, but theyalso reduce the gravimetric capacities of the materials. Anotherantimony based alloys have an ‘‘active’’ component, such as Zn, Ag,Sn and In [7–11]. Contrary to the former ones, these compoundshave high gravimetric capacities but poor cycleability because ofthe large volume change in the alloying process of Sn, Ag, etc.Besides the intermetallic compounds, nanopartices of antimonywas encapsulated by pyrolytic polyacrilonitride [12] and electro-deposited on Sb2O3 [13] or deposited onto cellulose fibers [14]. In

son SAS. All rights reserved.

addition, Sb2O3 exhibited a promise anode material [15–17], andboth alloying/dealloying processes and oxidation/reductionprocesses of Sb were revealed during the lithium electrochemicalreaction of Sb2O3 with lithium. Although this material has highreversible capacity of about 794 mAh g�1 and good cycle perfor-mance, it has a large irreversible capacity loss (234 mAh g�1).Recently, we studied the electrochemical reaction of Sb2Se3 withlithium and a large reversible discharge capacity of 605.1 mAh g�1

of Sb2Se3/Li cell was achieved [18]. However, the investigationsabout antimony based anode materials have not been satisfactorilycarried out yet. More efforts must be made to develop other Sbcompounds in search of a suitable matrix that leads to high cycla-bility and further improvements of capacity. To our knowledge,there is no available report on the use of Sb3N as storing Li elec-trodes. In fact, there has been surge of interest in developingadvanced materials of twofold metal nitrides such as Sn3N4, Zn3N4,Cu3N, Ge3N4, Fe3N, Co3N and Ni3N as a class of attractive lithium-ion storage materials due to the low and flat potentials of metalnitrides close to that of lithium metal with high reversibility andlarge reversible capacities [19–25].

In previous reports, antimony nitride was fabricated by themethod of discharge through the mixture of nitrogen and antimonyvapor [26], flash heating antimony metal in the presence of N2 [27],microwave discharge through SbCl5 in N2 and He or antimonymetal in N2 [28] and laser ablation under NH3 or N2 in argon [29].And its thin film was fabricated by plasma chemical vapor depo-sition [30]. In this study, we attempted to synthesis antimonynitrides by radio frequency (RF) sputtering for the first time and toexamine its electrochemical behavior mechanism with lithiumbased on the evidence provided by galvanostatic cycling and cyclic

Fig. 1. (a) Sb 3d and (b) N 1s XPS spectra of the as-deposited antimony nitride thinfilm.

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voltammetry (CV). X-ray photoelectrospectra (XPS), in situ spec-troelectrochemical characterization, high-resolution transmissionelectron microscopy (HR-TEM) and selected area electron diffrac-tion (SAED) measurements are used to characterize Sb3N thin film.The motivation of this work is to explore the possibility of usingSb3N as anode material for lithium-ion batteries and to elucidate itselectrochemical reaction mechanism with lithium.

2. Experimental

Antimony nitride thin films were directly deposited on the glasssubstrate by R.F. reactive sputtering metal Sb target. The sputteringchamber was evacuated below 5�10�4 Pa with a turbo-molecularpump and a mechanical pump. The metal Sb targets were pressedfrom the powders of metal Sb (99.99%). The pressure of N2 ambientgas was controlled at 1.5 Pa by a needle valve during deposition.The R.F. power was about 50 W and the distance between substrateand target was 7 cm. The deposition time was fixed about half anhour after pre-sputtering the target for 10 min to remove targetcontamination. The thickness of the as-deposited thin film wasmeasured to be about 400 nm by a profilometer (Tencor Alpha-Step200). Weight of thin film was directly obtained by subtracting theoriginal substrate weight from total weight of the substrate anddeposited thin film onto its surface, which were examined byelectrobalance (BP 211D, Sartorius).

The structure and the degree of crystallization of antimonynitride thin films were characterized by transmission electronmicroscopy (TEM) and selected area electron diffraction (SAED)(JEOL 2010 TEM). The chemical composition of the as-depositedthin film and its lithiated and delithiated products were investi-gated by X-ray photoelectron spectroscopy (XPS). XPS measure-ments were performed on a Perkin–Elmer PHI 5000C ECSA systemwith monochromatic AlKa (1486.6 eV) irradiation.

For the electrochemical measurements, the cells were con-structed by using the as-deposited thin films as a working electrodeand two lithium sheets as a counter electrode and a referenceelectrode, respectively. The electrolyte consisted of 1 M LiPF6 ina nonaqueous solution of ethylene carbonate (EC) and dimethylcarbonate (DMC) with a volume ratio of 1:1 (Merck). The cells wereassembled in an Ar-filled glove box. Galavanostatic cyclingmeasurements were carried out with a Land CT2001A batterytesting system. The cyclic voltammogram (CV) tests were per-formed with a scanning rate of 1 mV S�1 on a CHI660A electro-chemical working station (CHI instruments, TN). The ac impedancespectra of the cells were recorded by a CHI 660a electrochemicalworking station at the room temperature in a frequency range of0.01 Hz to 100 KHz. The amplitude of applied alternating voltagewas 5 mV.

For in situ spectroelectrochemical measurement, antimonynitride thin film deposited on an Au-coated (w10 nm) glass wasused as a working electrode. The electrochemical cell consisted ofone pair of glass windows with an H form in which the counter andreference electrode were placed. A couple of quartz windows weresealed in another tube with the working electrode; the detectedlight beam with a diameter of about 4 mm can pass through thequartz window and be focused on Sb3N thin film electrode surface.In situ absorbance spectra of the electrochemical cell includingquartz windows, electrolyte, and working electrode were measuredby a BTC100E thermoelectric cooled linear charge-coupled device(CCD) spectrophotometer (B & W TEK, inc., DE).

In order to gain insight into the reaction mechanism of anti-mony nitride with lithium, ex situ SEM, XPS, TEM, SAED measure-ments were carried out. For the ex situ measurements, to avoid theexposure to oxygen or water, the model cells at different stagesincluding the as-deposited, discharging to 0.3 V and charging to

3.0 V were dismantled in an Ar-filled glove box and the electrodeswere rinsed in anhydrous, dimethyl carbonate (DMC) to eliminateresidual salts. For TEM and SAED measurements, the active mate-rials were scratched from the stainless steel substrate. The loosepowders were then mixed with ethanol to prepare slurry, out ofwhich one drop was taken, and deposited on a copper grid, and thegrids were rapidly transferred into the chambers for cleanliness.

3. Result and discussion

The R.F. reactive magnetron sputtering technique is very effec-tive in the preparing thin film of metal nitrides, while our primaryexperimental results suggested that the composition of thedeposited thin film was found to be dependent on the sputteringcondition such as N2 pressure and sputtering fluence. In order toconfirm the structure and composition of the sputtered thin film,XRD and XPS measurements were used to examine the thin films.No peak besides the diffraction peaks of stainless steel wasobserved in the XRD pattern, indicating that the as-deposited thinfilm in our special experimental condition has an amorphousstructure. Sb 3d and N 1s XPS spectra of the as-deposited thin filmare presented in Fig. 1, respectively. The electron binding energiespeaked at 530.2 eV and 539.6 eV were assigned to Sb3d3/2 and Sb3d5/2. N 1s peak was observed at the binging energy 401.8 eV

Fig. 3. First three cyclic voltammograms for the as-deposited Sb3N thin film.

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(Fig. 1(b)), indicating that antimony nitride is formed by nitridationof metal Sb during the sputtering process. From the integratedintensities of XPS peaks and the values of atomic sensitivity factor(N: 0.38; Sb: 3.55) [31], the atomic Sb and N concentration radio isestimated to be 3:1. This result indicates that the as-deposited thinfilm mainly consist of Sb3N.

Fig. 2(a) shows the charge–discharge curves of Sb3N thin filmdeposited on stainless steel substrates. The cycling profiles of Sb3N/Li cell cycled between 0.3 V and 3.5 V under a current density of0.02 mA/cm2. The open circuit voltage (OCV) is 2.7 V. There arethree smooth discharge voltage plateaus appearing around 1.45 V,1.25 V, 0.8 V in the initial discharge process and two plateausaround 1.1 V, 2.5 V in the initial charge process. In addition, there isone couple of small discharge voltage at 1.7 V and charge voltage at2.5 V in galvanostatic cycling profiles. The initial discharge capacityof the as-deposited thin film is found to be 748 mAh/g, corre-sponding to around 10.6 Li per Sb3N. The second discharge capacityis about 629 mAh/g, corresponding to 8.9 Li per Sb3N. The specificcapacities of Sb3N thin film as a function of the cycle number areshown in Fig. 2(b). For comparison, the cycle performance of Sb thinfilm prepared by R.F. sputtering deposition is also shown. It can beseen that the discharge capacities of Sb3N increase from the secondcycle and the maximum value can achieve 825 mAh/g,

Fig. 2. Galvanostatic cycling curves of the as-deposited sb3N thin film; (b) thedischarge capacities of the as-deposited Sb3N and Sb thin films as a function of cyclenumber.

corresponding to 11.6 Li per Sb3N. If comparing with that of Sb thinfilm, it is very clear that Sb3N can improve the reversible capacityand cycle stability of Sb with Li.

Fig. 3 shows the first three cyclic voltammograms for the as-deposited Sb3N thin film electrode between 0.3 V and 3.0 Vmeasured at a scan rate of 1 mV/s. There are three cathode peaks at

Fig. 4. Three-dimensional platform of selected in situ transmission spectra obtained(a) during first discharging of Li/Sb3N cell to 0.3 V and (b) during first charging ofLi/Sb3N cell to 3.0 V.

Fig. 5. (a) TEM and (b) corresponding SAED pattern of the as-deposited Sb3N thin film; (c) TEM and (d) corresponding SAED pattern of Sb3N thin film after discharging to 0.3 V; (e)TEM and (f) corresponding SAED pattern of Sb3N thin film after charging to 3.0 V.

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1.05 V, 0.81 V, 0.77 V in the first discharge process. The cathodicpeak at 1.05 V becomes very weak and moves to 1.3 V in thesubsequent cycles. This peak could be attributed to an irreversibleprocess. Other cathodic peaks in subsequent cycles remain the sameas the first. Two anodic current peaks at 1.1 and 1.3 V are observed inthe charge process. They remain the same position between the firstcycle and subsequent cycles, indicating the well cycle stability. Onecouple of small but clear discharge voltage at 1.7 V and chargevoltage at 2.5 V in galvanostatic cycling profiles could not beobserved in the CV curves at the scan rate of 1.0 mV/s due to slowkinetics. The two profiles indicate that the electrochemical reaction

of Sb3N with lithium involves multi-steps. By comparing with CVcurves of the as-deposited Sb3N thin film electrodes and the curvesof Sb2O3 and Sb thin film electrodes reported before [16], the coupleof anodic peak around 0.77, 0.81 V and cathodic peaks at 1.1 V can beattributed to the alloy reaction between metal Sb and Li. The anodicpeak at 1.05 V and the cathodic at 1.3 V can be assigned to thedecomposition and formation of Sb3N. The reactive potentials arelower than that of the decomposition and formation of Sb2O3 (about1.41 and 1.48 V) [16].

The optical character of thin film is related to its compositionand structure. In order to further confirm the electrochemical

Table 1d-spacings (Å) derived from SAED analysis of the samples and JCPDS standards areshown for references.

Discharging to 0.3 V

Li3Sb (P63-mmc) (65-3515)

TW Reference

4.158 4.163 Li3Sb (002)1.87 1.854 Li3Sb (104)1.433 1.4457 Li3Sb (211)a¼ 4.61� 0.1 a¼ 4.71c¼ 8.39� 0.08 c¼ 8.33

a-Li3N(P6-mmm) (76-0822) b-Li3N(P63-mmc) (46-1201)

TW Reference TW Reference

3.877 3.877 a-Li3N(002) 3.06 3.08 b-Li3N(100)2.488 2.445 a-Li3N(101) 1.588 1.587 b-Li3N(004)a¼ 3.64 a¼ 3.53 a¼ 3.57c¼ 3. 87 c¼ 6.35 c¼ 6.35

TW: This Work.

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behavior of Sb3N with lithium, the difference of optical absorptioncharacteristics during the electrochemical reaction was recorded.Fig. 4(a) and (b) show the three-dimensional graph of in situabsorbance spectra collect at various potential in the initialdischarge and charge processes, which can be observed at 0.3 V and3.0 V. The color of the as-deposition Sb3N thin film is light yellow.After the initial discharge, the thin film became grey, while aftercharge process, the color returned to the yellow. The changes ofcolor and transmission spectra of thin film during the electro-chemical process electrode strongly verify the lithiation and deli-thiation reaction processes.

To obtain further insight on the electrochemical behavior featureof Sb3N as electrode material for lithium-ion batteries, the ex situTEM and SAED measurements were also used to define thecomposition and structure of the lithiated and delithiated thin film.Fig. 5 show the ex situ TEM and SAED spectra of the as-depositionSb3N, the lithated and delithiated Sb3N thin film, respectively. Asshown in Fig. 5(a), no obvious crystalline particles can be detected ina high–magnification, and the observed particles are stackeddensely but high disorderly. The SAED pattern in this phase shown inFig. 5(b) exhibits several very diffuse rings. These results supportthat the as-deposited Sb3N thin film is amorphous. After dischargingto 0.3 V, most crystalline particles with several nanometers in sizewere observed as shown in Fig. 5(c). The clear rings consisting ofdiscrete spots were observed in Fig. 5(d), indicating the

Fig. 6. N 1s XPS spectra of Sb3N thin film of (a) the as-deposited; (b) after the firstdischarging to 0.3 V and (c) after the first charging to 3.0 V.

polycrystalline nature of the lithiated thin film, which can beindexed to two phases of Li3Sb and Li3N (Table 1). When Sb3N thinfilm electrode was charged to 3.0 V, the non-structure of TEM image(Fig. 5(e)) and typical rings characteristics appeared in SAED pattern(Fig. 5(e)) strongly suggest that the lithiated Sb3N film is amorphous.

Fig. 6 shows the N 1s peak of the lithated and delithiated Sb3Nthin film. The data of the as-deposition Sb3N is included forcomparison. It is interesting to note that N 1s XPS spectrum of thethin film after discharging to 0.3 V, was distinctly different fromthat of the as-deposited thin film. There was dividedly peaked at400.2 eV and 401.7 eV in the lithiated thin film, while N 1s spectraof delithiated Sb3N thin film had only one peak at 401.8 eV. The splitof the N 1s spectra of lithiated Sb3N thin film implies differentchemical position of N. The peak at 400.2 eV can be attributed toLi3N [24]. This result is consistent with SAED data. When chargingthe cell to 3.0 V, N 1s XPS spectrum has same peak as that of the as-deposited Sb3N thin film, suggesting the charged product maymatch the initial thin film. Unfortunately, there is not a measurablechemical shift for binding energy Sb 3d XPS spectra for furtherdischarging to 0.3 V and charging to 3.0 V (no shown here) forproviding more information.

Based on the ex situ TEM, SAED and XPS results, the electro-chemical reaction mechanism of Sb3N with lithium might beexpressed as follows:

Sb3N D 3LiD D 3e 4 3Sb D Li3N (1)

3Sb D 9LiD D 9e 4 3Li3Sb (2)

The first step reaction is similar to the oxidation and reductionof metal nanoparticles proposed by Tarascon et al. [32]. Nano-metallic Sb might have the similar electrochemical driven as other3d metal of Ni, Co and Fe to enhance their electrochemical activitytowards the composition/decomposition of Li3N [24,25]. The latteris similar to the classical Li-alloying, in which Li ion would reactwith metallic Sb to form antimony lithium alloy during furtherdischarging. According to this reaction, the theoretical capacity forSb3N could be calculated to be 848 mAh/g. This value was close tothe maximum reversible capacity of 825 mAh/g. When Li ions weredriven and incorporated into the amorphous Sb3N, interaction ofSb–N bond will be reduced and nano-sized metal Sb and Li3N willbe formed. Nano-sized metal Sb then alloy with lithium. Thefeature of the multi-steps’ reactions can be revealed by CV curves inwhich multi-reduction peaks were observed. During the electro-chemical charging reaction, firstly, Li3Sb was first decomposed tolithium and metal Sb, at the meantime Li3N could be an ‘‘inactive’’matrix-glue holding small antimony particles to cushion thevolume change of the Li–Sb alloys, which obviously elevate thecycleability of the material, then the nano-sized metal and Li3N wasconverted into Sb3N under the electrochemical potential, whichcould be confirmed by N 1s XPS spectra.

Although the irreversibility between the first two cycles is about16%, afterward the cycling, the discharge capacity graduallyincreases. The discharge capacity of the 50th cycle is 825 mAh/g,more than that of the initial one. Apparently, Sb3N exhibits morelarge reversible reaction capacity of Sb with Li. The original of theincrease of the discharge capacity is still unclear, and might involvethe surface morphology change of thin film electrode. The SEMimages of the as-deposited, lithiated and delithiated thin film arepresented in Fig. 7. As shown in Fig. 7(a), the as-deposited Sb3N thinfilm with a quit smooth, dense surface texture is composed of verysmall particles that can’t be distinguished by SEM. The film elec-trode consisting of particles with the size less than several tens nmshould be responsible for its well electrochemical performance.After discharging to 0.3 V, the surface is composed by particles with

Fig. 7. Scanning electron microscopic images for Sb3N thin film electrode of (a) the as-deposited; (b) after the first discharging to 0.3 V and (c) after the first charging to 3.0 V.

Fig. 8. Impedance spectra for the as-deposited Sb3N thin film and thin film electrodeafter the discharging to 0.01 after 25th cycles.

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the same size. When charging to 3.0 V, the size of the grainsobviously reduced and there are some cracks could be observed onthe surface. SEM micrograph provides direct images of the surfaceof the thin film electrodes and surface reactions. The apparition ofcracks on cycled electrode should be associated to the formation ofnew solid electrolyte interphase (SEI) film inside the crack. Fig. 8shows the impedance spectra of Sb3N/Li cell for the as-depositedSb3N thin film and thin film electrode after the discharging to

0.01 V after 25th cycles. The shape broadening of semi-circle andthe resistance increase of the cell after cycling suggest the forma-tion of passivation film (SEI) on the surface, which could be relatedwith the new surface exposed to the electrolyte inside the cracksduring the cycle process. The cracks in the surface of the thin filmelectrode might provide larger the contacting area between theelectrode and liquid electrolyte during the cycle process andgenerate more smooth pathways for the Li ion reaction processes.The further explanation for the improved electrochemical perfor-mance of the amorphous Sb3N film electrode is rather complicated,more works should be done in the future.

4. Conclusions

In this paper, Sb3N thin film has been successful prepared by R.F.sputtering deposition. The electrochemical reaction mechanism ofSb3N with lithium was revealed by using ex situ TEM, SAED, SEMand XPS measurements. Galvanostatic cycling measurements andcyclic voltammetry (CV) were used to examine the electrochemicalbehavior of Sb3N/Li cell and this film shows a large reversibledischarge capacity of 629 mAh/g. They indicate the potential ofSb3N thin film electrode as energy storage materials for futurerechargeable lithium batteries.

Acknowledgements

This work was financially supported by Science & TechnologyCommission of Shanghai Municipality (08DZ2270500 and09JC1401300), the NSFC (Project No. 20773031), the ‘‘973’’ Projects(2007CB209702) and ‘‘863’’ Projects (2007AA03Z322).

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