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Electrochemistry Communications 9 (2007) 31–34

Room temperature Na/S batteries with sulfur compositecathode materials

Jiulin Wang a,b,*, Jun Yang b, Yanna Nuli b, Rudolf Holze a,*

a Chemnitz University of Technology, Institute of Chemistry, Chemnitz D-09111, Germanyb Shanghai Jiao Tong University, Department of Chemical Engineering, Shanghai 200240, China

Received 24 July 2006; received in revised form 15 August 2006; accepted 16 August 2006Available online 18 September 2006

Abstract

Sodium/sulfur (Na/S) batteries were assembled with a sodium metal anode, liquid electrolyte and a sulfur composite cathode. Theirelectrochemical characteristics have been investigated at room temperature. Their charge/discharge curves indicate that sodium canreversibly react with sulfur at room temperature. The specific capacity of the sulfur composite cathode material in the first cycle wasinitially about 655 mA h g�1 and stayed at about 500 mA h g�1 up to the 18th cycle with about 100% charge/discharge efficiency.� 2006 Elsevier B.V. All rights reserved.

Keywords: Na/S battery; Na anode; Sulfur composite cathode; Room temperature battery

1. Introduction

The sodium/sulfur (Na/S) battery shows various signif-icant advantages: high energy density (theoretical specificenergy density of 760 W h kg�1), low cost material (abun-dant resources of sulfur and sodium in nature), low rateof self-discharge and high power density [1–3]. A typicalNa/S battery consists of sulfur at the positive electrodeand sodium at the negative electrode separated by a solidbeta alumina ceramic electrolyte. This type of Na/S batterymust be operated at about 300 �C to ensure sufficient Na+

conductivity in the electrolyte. At this operation tempera-ture, both sulfur and sodium electrodes are in the liquid(molten) state. Considering potentially extensive or evenexplosive reactions between liquid sulfur and liquidsodium, safety is an issue for the high temperature Na/Sbattery. Moreover, active cathode materials (sulfur and

1388-2481/$ - see front matter � 2006 Elsevier B.V. All rights reserved.

doi:10.1016/j.elecom.2006.08.029

* Corresponding authors. Address: Chemnitz University of Technology,Institute of Chemistry, Chemnitz D-09111, Germany. Tel.: +49 371 53121260x31509; fax: +49 371 531 21269.

E-mail addresses: jiulin.wang@chemie.tu-chemnitz.de (J. Wang),rudolf.holze@chemie.tu-chemnitz.de (R. Holze).

sodium polysulfide) are corrosive, this was considered asone of the major failure reasons for Na/S batteries [4].

Based on the successful development of lithium ion bat-teries, several groups begun to develop room temperaturesodium ion batteries which are promising substitutes forlithium ion batteries in various application areas [5–7]. Ina low temperature Na/S battery, the sulfur cathode willencounter the same problems as Li/S batteries: low utiliza-tion of active material, poor rechargeability and dissolutionof polysulfides into the electrolyte [8]. In previous papers[9,10], we reported on sulfur composite cathode materialswith sulfur embedded in a polymer matrix, which exhibitedgood electrochemical performances in lithium batteries. Inthis paper, sulfur composite materials were used as cathodesfor room-temperature Na/S battery with liquid electrolyte.

2. Experimental

2.1. Preparation and characterization of sulfur composite

materials

Sulfur composite materials were prepared as previouslyreported [9]. Typically, sublimed sulfur (with purity of99.99%) was thoroughly mixed with polyacrylonitrile

Fig. 1. TGA of sulfur composite material, sulfur and PAN.

Fig. 2. DSC of sulfur composite material, sulfur and PAN.

32 J. Wang et al. / Electrochemistry Communications 9 (2007) 31–34

(PAN, Aldrich). Ethanol was used as dispersant to improvethe mixing of sulfur and PAN. After drying, the mixturewas heated to 300 �C and dwelled for 6 h under Ar gas.A black, powdery material was obtained.

Scanning electron microscopy (SEM, JEOL JSM-6700F) and X-ray diffraction (XRD, D/max 2550 V) wereused to characterize the morphology and crystalline stateof composite materials, respectively. TGA and DSC testswere carried out with a TGA instrument Pyris 1 (Perkin–Elmer) at a heating rate of 10 �C min�1 under N2

protection.

2.2. Cell assembly and testing

Room temperature Na/S batteries were assembled simi-lar to Li/S batteries. NaClO4, ethylene carbonate (EC) anddimethyl carbonate (DMC, Aldrich) were used as received.EC and DMC were firstly mixed at a weight ratio of 2:1,and then NaClO4 was dissolved in solvents to get 1 MNaClO4 EC-DMC liquid electrolyte. To fabricate cath-odes, 70 wt% above-mentioned sulfur composite contain-ing 42 wt% sulfur was mixed with 20 wt% acetylene blackand 10 wt% polytetrafluoroethylene (PTFE), with ethanolserving as dispersant. The mixture was rolled into filmsapproximately 100 lm thin. A disk punched out of the film,with approximately 1.1 cm2 in area and 5 mg in weight, waspressed into a nickel foam. After drying at 100 �C, aCR2016-type coin battery was assembled in a glove bagfilled with Ar gas. A sodium metal sheet was used as anode.

Charge and discharge performances of the batteries wereinvestigated with LAND cycler (Wuhan, China) at a cur-rent density of 0.1 mA cm�2 between 0.8 and 2.5 V at roomtemperature.

3. Results and discussion

The results of SEM test showed an average particle sizeof the composite material of about 200 nm. In addition,there were some aggregated nano-particles [9]. XRD pat-terns indicated that the composites with sulfur contentbelow 50 wt% is amorphous [10]. Sulfur might be embed-ded, as nano-particles or even at the molecular level, insulfurized PAN matrix.

Fig. 1 shows the thermal properties of elemental sulfurand PAN, and the composite material with sulfur contentof about 45 wt%. Sulfur is easy to sublimate and almostall is evaporated at 280 �C. Under N2 gas protection,PAN shrunk and decomposed at 296 �C, and kept about68 wt% of its original mass at 350 �C. Compared to thestarting materials, the sulfur composite exhibits excellentthermal stability. The composite material with a sulfur con-tent of about 45 wt% slowly lost weight, but retained above90% of its original mass even at 400 �C. The result of DSCanalysis of the composite gave a horizontal line (as thegreen line shown in Fig. 2). Almost no endothermic andexothermic process occurred up to the temperature of400 �C, which indicates that the sulfur composite materials

possess excellent thermal stability. In a previous paper [9],solid state NMR (Nuclear Magnetic Resonance) was usedto characterize the structure of sulfur composite materials.The 13C NMR spectra suggested the formation of hetero-cyclic structures in PAN during reaction with sulfur. Thusit is possible that heterocyclic structures in sulfurized PANrestrained the evaporation of sulfur which was embeddedin the polymer matrix. It is also possible that sulfur mightreact with PAN and graft to the chains of sulfurized PANat high temperature.

The charge and discharge profiles of Na/S battery withcomposite cathode material containing 45 wt% sulfur areshown in Fig. 3. The Na/S battery exhibited an open-cir-cuit voltage (OCV) of 2.36 V at room temperature. Duringthe first discharge process, the specific capacity was654.8 mA h g�1 based on the composite cathode material.The average voltage based on half of specific capacitywas 1.33 V. Considering the 45 wt% sulfur content in thecomposite material, the calculated discharge capacity of

Fig. 4. The cycle characteristics of Na/S battery.

J. Wang et al. / Electrochemistry Communications 9 (2007) 31–34 33

pure sulfur was 1455 mA h g�1. The charge and dischargereactions of Na/S battery can be written as:2Na + xS M Na2Sx. When x is equal to 1, the theoreticalcapacity of the sulfur cathode is 1672 mA h g�1. Accord-ingly, 87% of sulfur in the composite material took partin the reaction with sodium to form Na2S during the firstdischarge process, assuming that the sulfurized PANmatrix stayed inert during the charge/discharge process.In a Na/b-Al2O3/S battery, Na2S2 solid will jam the ionicchannel of b-Al2O3 electrolyte and discharge process willbe ended before the formation of Na2S2. As a result, thespecific capacity of sulfur is actually less than 836 mA h g�1

for a Na/b-Al2O3/S battery. This problem did not exist inthe Na/S cells described here and sulfur could be reducedto S2� (Na2S).

As shown in Fig. 3, the specific capacities were about520 mA h g�1 during the following two cycles, which indi-cates that 69% of sulfur remained in reversible reactionwith sodium. There was no obvious voltage plateau inthe cycle profiles. Average charge and discharge voltagewas 1.8 and 1.4 V, respectively. Fig. 4 exhibits the recharge-ability of Na/S battery. The specific capacity of the sulfurcomposite cathode decreased slowly with the progressivecycles and retained about 500 mA h g�1 after 18 cycles. Itwas remarkable that the charge and discharge efficiencyof the battery was close to 100% except for the first dis-charge process, which indicated that the sulfur in the com-posite exhibited good electrochemical reversibility via thereaction, 2Na + xS M Na2Sx at room temperature.

After opening the battery in an argon-filled glove bag, itwas found that thick black moss, so-called ‘‘sodium den-drite’’, covered the surface of the sodium anode. Sulfurcomposite cathodes could be still well charged and dis-charged with a fresh sodium anode. A similar phenomenonwas observed in previously reported Li/S batteries [10].This phenomenon indicates that the dendrite is also a crit-ical problem for room temperature Na/S batteries. In view

Fig. 3. The charge/discharge profiles of Na/S battery at roomtemperature.

of the fact that the melting point of sodium is 98 �C and thesulfur composite materials keep stable up to 300 �C, we aredeveloping Na/S batteries with cross-linked PEO and newtypes of polymer electrolytes, which are expected to over-come the ‘‘sodium dendrite’’ problem in a operational tem-perature range between 110 and 150 �C [11,12]. Moreover,Na+ ionic liquid could be expected to perform the similarfunction [13].

4. Conclusions

Charge and discharge performances of Na/S batterieswere reported in this paper. Na/S batteries were assembledwith a sodium metal anode, liquid electrolyte and a sulfurcomposite cathode. The charge/discharge curves of Na/Sbatteries indicated that Na could reversibly react with sul-fur embedded in the sulfurized polymer matrix at roomtemperature and the reactions of Na/S battery could bewritten as: 2Na + xS M Na2Sx. During the first dischargeprocess, the specific capacity was 654.8 mA h g�1 basedon the composite cathode materials and the calculated dis-charge capacity of pure sulfur was 1455 mA h g�1. Thereversible specific capacity of sulfur composite cathodewas about 500 mA h g�1 and kept stable in the followingcycles with about 100% charge/discharge efficiencies. Aver-age charge and discharge voltage was 1.8 and 1.4 V, respec-tively. Similar to lithium batteries, dendrite was also acritical problem for room temperature Na/S batteries.

Acknowledgement

We are grateful to the Alexander von Humboldt Foun-dation for a research fellowship for one of us (J.L.W.).

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