Electrochemistry Communications 16 (2012) 22–25

Contents lists available at SciVerse ScienceDirect

Electrochemistry Communications

j ourna l homepage: www.e lsev ie r .com/ locate /e lecom

Electrochemical properties of room temperature sodium–air batteries withnon-aqueous electrolyte

Qian Sun a, Yin Yang a,b, Zheng-Wen Fu a,⁎a Shanghai Key Laboratory of Molecular Catalysts and Innovative Materials, Department of Chemistry & Laser Chemistry Institute, Fudan University, Shanghai 200433, Chinab Department of Material and Science, Fudan University, Shanghai 200433, China

⁎ Corresponding author. Tel.: +86 21 65642522; fax:E-mail address: zhengwen@sh163.net (Z.-W. Fu).

1388-2481/$ – see front matter © 2011 Elsevier B.V. Alldoi:10.1016/j.elecom.2011.12.019

a b s t r a c t

a r t i c l e i n f o

Article history:Received 20 December 2011Accepted 22 December 2011Available online 28 December 2011

Keywords:Sodium–air batteryCharge discharge measurement

A novel type of rechargeable sodium–air battery working at room temperature is constructed and examinedfor the first time. The typical gravimetric capacities of the air electrodes (diamond-like carbon thin films) are1884 mAh/g (565 μAh/cm2) at 1/10 C and 3600 mAh/g (1080 μAh/cm2) at 1/60 C, respectively, which are sig-nificantly superior to intercalation-based cathode materials for rechargeable sodium or lithium batteries everreported. The electrochemical reaction of the sodium–air battery is investigated. The high reversible capacityand relevant high output voltage (about 2.3 V) of the room temperature sodium–air battery make it a poten-tial alternative battery in the future.

© 2011 Elsevier B.V. All rights reserved.

1. Introduction

Over the last decades much effort has been devoted to develop re-chargeable metal–air batteries due to their high theoretical energydensities and low costs in response to the rapidly growing demandsof advanced batteries for increasingly dependence on portable elec-tronic technology. Metal–air batteries with various metal anodes(such as Li [1,2], Na [3], Zn [4], Al [5], Mg [6] and Fe [7]) have beenwidely investigated, while a novel system of Si–air battery wasreported recently [8]. Among these batteries, sodium–oxygen couplehas a specific high energy of 1683 Wh/kg [9], and there are abundantsodium sources in both the earth's crust (2.3%) and oceans (1.1%) forthe feasible popularization of future rechargeable sodium–air batte-ries (SABs). Thus, it should be of great interest to develop Na–air bat-teries. Nevertheless, one available study about sodium–air batterieswas reported by Peled et al. [3], in which they made an attempt todemonstrate the feasibility of running a liquid sodium oxygen cellwith polymer electrolytes at above 100 °C. However, liquid sodiumis well known for its highly corrosive characteristic, while high oper-ating temperatures are inconvenience for practical use. Here, we willreport the electrochemical behavior of a non-aqueous sodium–air cellat room temperature based on diamond-like carbon (DLC) thin filmas air electrode.

+86 21 65102777.

rights reserved.

2. Experimental

The details of the preparation of the DLC thin film electrode havebeen described in our recent work [10]. For electrochemical measure-ment, a conventional two-electrode cell was constructed in the driedair filled glove box with the DLC thin film as the cathode and onesheet of high-purity sodium foil as the anode, respectively. Themodel cell consisted of an H shape glass tube to separate positiveand negative electrodes as well as two rubber plugs for sealing (asshown in the inset of Fig. 1(a)). The electrolyte was 1 M NaPF6 (Alfa-Aesar) non-aqueous solution in ethylene carbonate (EC) and dimethylcarbonate (DMC) with a volume ratio of 1:1 (Merck). Weight of thinfilm was examined by electrobalance (BP 211D, Sartorius). The preci-sion of the weight was ±0.01 mg. Charge–discharge measurementswere performed at room temperature with a Land BT 1–40 batterytest system. Fourier transform infrared (FTIR) spectra were recordedon a Nicolet Nexus-470 spectrometer. High resolution transmissionelectron microscopy (HR-TEM) and selected area electron diffraction(SAED) measurements were carried out on a JEOL 2010 TEM.

3. Result and discussion

The DLC air electrode/Na cell was tested under the dried air ambientand the typical voltage profiles are shown in Fig. 1(a). The initial open-circuit voltage (OCV) of the cell is 2.98 V. One voltage pseudoplateaufrom 2.38 V to 2.04 V is obtained in the first discharging processes,while two pseudoplateaus around 2.36 V and 2.0 V are observed inthe subsequent discharges. The gravimetric capacities were calculatedbased on the weight of the DLC thin film. The DLC thin film electrode

Fig. 1. (a) Galvanostatic cycling profiles of the DLC thin film/Na cell. The cell was cycledat discharge and charge rates of 0.1 C. The inset shows the composition of the cell;(b) discharge curves of DLC thin film/Na cell at different current rates.

23Q. Sun et al. / Electrochemistry Communications 16 (2012) 22–25

delivers an initial discharge capacity of 1884 mAh/g and the maximumvalue of the discharge capacity of 2070 mAh/g is achieved in the 3rdcycle. The discharge capacity of the 20th cycle is 1058 mAh/g, corre-sponding to 56.2% the initial discharge capacity. For the initial chargeprocess, the charging voltage profiles show a hump at the early stageand then increase steadily. The appearance of the hump is related tothe sluggish diffusion and reaction kinetics of the charging processesfor the thin film air electrodes. The charging of the DLC thin film elec-trode starts at 3.5 V after the hump region. The average voltage isabout 3.9 V and its charge capacity is 1970 mAh/g (591 μAh/cm2). Thecharge capacities gradually decrease during the cycling. The shapes ofthe discharge and charge curves are similar as those of Li–air cellbased on DLC air electrode [10],but the plateau voltage is about 0.3 Vlower comparing with the latter due to the difference between theredox potentials of Na+/Na (−2.714 V vs. SHE) and Li+/Li (−3.040 Vvs. SHE). In addition, no discharge/charge behavior of the cell was ob-served in argon ambient (not shown here). These results strongly sup-port that DLC/Na cell is a Na/air cell. As in the Li–air battery reportedpreviously [10], the source of oxygen in the presented sodium–air bat-tery here also comes from the dissolved O2 in the electrolyte.

The rate performance of the sodium–air battery was also investi-gated and the results are shown in Fig. 1(b). The discharging capaci-ties decrease with larger discharge current densities and are foundto be 3600 mAh/g (1080 μAh/cm2), 2523 mAh/g (757 μAh/cm2),1056 mAh/g (317 μAh/cm2) and 180 mAh/g (54 μAh/cm2) at 1/60 C,1/20 C, 1/6 C and 3 C, respectively. The discharge capacity at the cur-rent rate of 1/6 C is about 9% of that at 1/60 C, indicating a bad rate

performance. The increasing polarization at the larger discharge cur-rent densities can also be found.

Based on Gibbs free energy data and the thermodynamic equationΔG=−nFE, the theoretical voltages for possible reactions in Na/O2

cell with different products are calculated in Eqs. (1)–(3), respectively.

Naþ þ e þ O2→NaO2ΔG

0 ¼ −218:4kJ=molE0 ¼ 2:263V ð1Þ

2Naþ þ 2e þ O2→Na2O2ΔG

0 ¼ −449:6kJ=molE0 ¼ 2:330V ð2Þ

4Naþ þ 4e þ O2→2Na2OΔG

0 ¼ −751:0kJ=molE0 ¼ 1:946V ð3Þ

The electrode potentials corresponding to the formation of NaO2

and Na2O2 have very close values, which are close to with the plateauvoltages observed in the discharge curves. To determine the structuraland composition modification of air electrodes induced by Na uptake/removal, ex situ TEM, SAED, and FTIR measurements were performedupon DLC thin film electrode at various states of the cell cycled between2.0 and 4.5 V at a constant current of 1/60 C. HRTEMpicture of the initialDLC thin film is shown in Fig. 2(a), where some short moiré stripes areobserved. SAED spectra corresponding to the region show two weakrings (Fig. 2(b)), which can be attributed to (220) and (110) diffractionsof face-centered cubic structure of diamond carbon (JCPDS card no.89–3441). These results indicate that the as-deposited DLC thin filmsconsist of some diamond-like crystalline phases dispersed into amor-phous carbon matrix. When the cell is discharged to 1.5 V, some hazystrips appear in the TEM image of the DLC electrode (Fig. 2(c)). The cor-responding SAED pattern (Fig. 2(d)) exhibits some clear concentricrings and some bright spots, indicating the polycrystalline nature ofthe discharged electrode. Most of the measured d-spacings derivedfrom the SAED pattern can be well indexed to the diffractions ofNa2O2 (JCPDS card no. 74–0985), and another weak ring can be indexedto (220) diffraction of diamond carbon. It provides the strong evidencethat Na2O2 is generated and distributed in the DLC matrix during thedischarge process. The TEM image and the SAED pattern of the thinfilm electrode charged to 4.0 V are shown in Fig. 2(e)–(f), respectively.The diffuse “halo” rings, whose d-spacings (Fig. 2(f)) agree well withthose of diamond carbon, reappear with the absence of discrete spots,indicating the decomposition of Na2O2 during charging process.

For FTIR tests, DLC thin films were deposited on the double-sidedpolishing silicon. Comparing with the initial thin film (Fig. 3(a)),some new peaks corresponding to O–C=O in Na2CO3 or R–CO–O- ap-pear in the spectra of discharged electrode (Fig. 3(b)). After chargingto 4.0 V, the intensities of these peaks are weakened significantly(Fig. 3(c)). FTIR data suggest that amorphous Na2CO3 or NaOCOR isalso the discharge products when the electrode is discharged to1.5 V in the carbonate electrolyte. This is consistent with the previousstudies that argued that the reduction products of O2 in Li–air batte-ries could attack the carbonate solvents [11–13]. As a result, the dis-charge products contain carbonate species. After charging to 4.0 V,these discharge products are reversibly decomposed.

Combiningwith the ex situ TEM, SAED and FTIR results, it can be foundthat Na2O2, Na2CO3 and NaOCO-R are formed as the products of the dis-charge reaction. The electrochemical reactions at DLC thin filmwith sodi-umshouldbe similar to that of Li–air cells and canbedescribed as follows:

At cathode side during discharging:

2Naþ þ O2 þ 2e

−→Na2O2 ð4Þ

nNaþ þ O2 þ EC=DEC þ ne

−→Na2CO3 þ NaOCO� R þ side products

ð5Þ

Fig. 2. (a) TEM image and (b) SAED patterns of the as-deposited DLC thin film electrode; (c) TEM image and (d) SAED patterns of the DLC thin film electrode after discharging to1.5 V; (e) TEM image and (f) SAED patterns of DLC thin film electrode after charging to 4.0 V.

24 Q. Sun et al. / Electrochemistry Communications 16 (2012) 22–25

2Na2O2 þ 2CO2→2Na2CO3 þ O2 ð6ÞAt anode side during discharging:

2Na→2Naþ þ 2e

− ð7ÞAt cathode side during charging:

Na2O2→2Naþ þ O2 þ 2e

− ð8Þ

Na2CO3 þ NaOCO� R→nNaþ þ CO2 þ sideproducts þ ne

− ð9ÞAt anode side during charging:

2Naþ þ 2e

−→2Na ð10ÞDuring the discharge reaction, both Na ions and the dissolved oxy-

gen shouldmigrate through the liquid electrolyte and enrich at the sur-face of DLC thin film, then diffusion into the nanostructured DLC thin

Fig. 3. FTIR spectra of (a) the as-deposited DLC thin film electrode, (b) the electrodeafter discharging to 1.5 V and (c) the electrode after recharging to 4.0 V.

film. The dissolved oxygen in the electrolyte absorbedon the nanostruc-turedDLC thin films reactswithNa+ and formsNa2O2. The O2 reductionprocesses in non-aqueous sodium containing electrolyte are believed tobe a two-step reaction involving the electrochemical formation andchemical decomposing of NaO2 [14]. At the same time, similarly as thesituation of Li/air cell, the reduction products of O2 in the carbonate-based electrolyte might attack the carbonate solvents and results inthe Na2CO3 and other organic carbonate salt products at air electrode.Considering that the sodium–air cells are cycled in dried air, the partialsource of Na2CO3 may be also attributed to the chemical reaction be-tween the discharged product of Na2O2 and CO2 from the dried air.After charging reactions to 4.0 V, these discharge products are reversiblydecomposed. If compared with a liquid sodium oxygen cell, in whichthe discharge and charge voltage plateaus were found to be 1.75 Vand 3.0 V with the OCV close to 2.0 V [3], the present Na/air cell at theroom temperature exhibits utterly different electrochemical behaviorimplying completely different reaction routes with the existence of car-bonate solvents. Since the electrochemical reactions with carbonateelectrolytes in lithium-air batteries have been proved to be rather com-plicated [15,16], further clarification of the reaction mechanisms inSABs is still needed by using various in situ characterizations. In addi-tion, if compared with liquid sodium, the poor cycling properties ofsolid sodium electrode heremay lead to poor cycling behavior, dendrit-ic growth, and concomitant safety problems [3]. It may be beneficial toimprove the cyclic performance of SABs by using sodium-ion conduc-tive inorganic thin film (such as NASICON) or polymer coatings onsolid sodium electrodes.

4. Conclusion

In this work, our results have initially demonstrated the possibilityof Na–air cell operating at room temperature. The relevant largegravimetric capacities of air electrodes are superior to those ofintercalation-based cathode materials for rechargeable sodium orlithium ion batteries (below 300 mAh/g), indicating the potentialsof SABs as possible alternatives to lithium- or sodium-ion batteries.Crystallized Na2O2 and amorphous carbonate salts are found in thedischarge products but vanish in the charged electrode. Thus, thecomplex and poorly characterized reaction routes in the SABs,

25Q. Sun et al. / Electrochemistry Communications 16 (2012) 22–25

which involve not only sodium ion and O2 species but also carbonatesolvent molecules, need to be further clarified in the future.

Acknowledgements

This work was financially supported by the 973 Program(No.2011CB933300) of China and Science & Technology Commissionof Shanghai Municipality (08DZ2270500 and 11JC1400500).

References

[1] E.L. Littaucer, K.C. Tsai, Journal of the Electrochemical Society 124 (1997) 850.[2] K.M. Abraham, Z. Jiang, Journal of the Electrochemical Society 143 (1996) 1.[3] E. Peled, D. Golodnitsky, H. Mazor, M. Goor, S. Avshalomov, Journal of Power

Sources 196 (2011) 6853.[4] C. Chakkaravarthy, H.V.K. Udupa, Journal of Power Sources 10 (1983) 197.[5] S. Yang, H. Knickle, Journal of Power Sources 112 (2002) 162.

[6] Y. Ma, N. Li, D. Li, M. Zhang, X. Huang, Journal of Power Sources 196 (2011) 2346.[7] A. Ito, L. Zhao, S. Okada, J.I. Yamaki, Journal of Power Sources 196 (2011) 8154.[8] G. Cohn, D. Starosvetsky, R. Hagiwara, D.D. Macdonald, Y. Ein-Eli, Electrochemistry

Communications 11 (2009) 1916.[9] C.X. Zu, H. Li, Energy & Environmental Science 4 (2011) 2614.

[10] Y. Yang, Q. Sun, Y.S. Li, H. Li, Z.W. Fu, Journal of the Electrochemical Society 158(2011) B1211.

[11] J. Xiao, J. Hu, D. Wang, D. Hu, W. Xu, G.L. Graff, Z. Nie, J. Liu, J.-G. Zhang, Journal ofPower Sources 196 (2011) 5674.

[12] W. Xu, V.V. Viswanathan, D. Wang, S.A. Towne, J. Xiao, Z. Nie, D. Hu, J.G. Zhang,Journal of Power Sources 196 (2011) 3894.

[13] A.K. Thapa, K. Saimen, T. Ishihara, Electrochemical and Solid-State Letters 13(2010) A165.

[14] C.O. Laoire, S. Mukerjee, K.M. Abraham, E.J. Plichta, M.A. Hendrickson, Journal ofPhysical Chemistry C 113 (2009) 20127.

[15] S.A. Freunberger, Y.H. Chen, Z.Q. Peng, J.M. Griffin, L.J. Hardwick, F. Barde, P.Novak, P.G. Bruce, Journal of the American Chemical Society 133 (2011) 8040.

[16] C.O. Laoire, S. Mukerjee, K.M. Abraham, E.J. Plichta, M.A. Hendrickson, Journal ofPhysical Chemistry C 114 (2010) 9178.