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Electrochemical properties of room temperature sodium–air batteries with non-aqueous electrolyte
Electrochemistry Communications 16 (2012) 22–25
Contents lists available at SciVerse ScienceDirect
Electrochemistry Communications journal homepage: www.elsevier.com/locate/elecom
Electrochemical properties of room temperature sodium–air batteries with non-aqueous electrolyte Qian Sun a, Yin Yang a, b, Zheng-Wen Fu a,⁎ a b
Shanghai Key Laboratory of Molecular Catalysts and Innovative Materials, Department of Chemistry & Laser Chemistry Institute, Fudan University, Shanghai 200433, China Department of Material and Science, Fudan University, Shanghai 200433, China
a r t i c l e
i n f o
Article history: Received 20 December 2011 Accepted 22 December 2011 Available online 28 December 2011 Keywords: Sodium–air battery Charge discharge measurement
a b s t r a c t A novel type of rechargeable sodium–air battery working at room temperature is constructed and examined for the first time. The typical gravimetric capacities of the air electrodes (diamond-like carbon thin films) are 1884 mAh/g (565 μAh/cm2) at 1/10 C and 3600 mAh/g (1080 μAh/cm 2) at 1/60 C, respectively, which are significantly superior to intercalation-based cathode materials for rechargeable sodium or lithium batteries ever reported. The electrochemical reaction of the sodium–air battery is investigated. The high reversible capacity and relevant high output voltage (about 2.3 V) of the room temperature sodium–air battery make it a potential alternative battery in the future. © 2011 Elsevier B.V. All rights reserved.
1. Introduction
2. Experimental
Over the last decades much effort has been devoted to develop rechargeable metal–air batteries due to their high theoretical energy densities and low costs in response to the rapidly growing demands of advanced batteries for increasingly dependence on portable electronic 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 been widely investigated, while a novel system of Si–air battery was reported recently [8]. Among these batteries, sodium–oxygen couple has a specific high energy of 1683 Wh/kg [9], and there are abundant sodium sources in both the earth's crust (2.3%) and oceans (1.1%) for the feasible popularization of future rechargeable sodium–air batteries (SABs). Thus, it should be of great interest to develop Na–air batteries. Nevertheless, one available study about sodium–air batteries was reported by Peled et al. [3], in which they made an attempt to demonstrate the feasibility of running a liquid sodium oxygen cell with polymer electrolytes at above 100 °C. However, liquid sodium is well known for its highly corrosive characteristic, while high operating temperatures are inconvenience for practical use. Here, we will report the electrochemical behavior of a non-aqueous sodium–air cell at room temperature based on diamond-like carbon (DLC) thin film as air electrode.
The details of the preparation of the DLC thin film electrode have been described in our recent work [10]. For electrochemical measurement, a conventional two-electrode cell was constructed in the dried air filled glove box with the DLC thin film as the cathode and one sheet of high-purity sodium foil as the anode, respectively. The model cell consisted of an H shape glass tube to separate positive and negative electrodes as well as two rubber plugs for sealing (as shown in the inset of Fig. 1(a)). The electrolyte was 1 M NaPF6 (AlfaAesar) non-aqueous solution in ethylene carbonate (EC) and dimethyl carbonate (DMC) with a volume ratio of 1:1 (Merck). Weight of thin film was examined by electrobalance (BP 211D, Sartorius). The precision of the weight was ±0.01 mg. Charge–discharge measurements were performed at room temperature with a Land BT 1–40 battery test system. Fourier transform infrared (FTIR) spectra were recorded on a Nicolet Nexus-470 spectrometer. High resolution transmission electron microscopy (HR-TEM) and selected area electron diffraction (SAED) measurements were carried out on a JEOL 2010 TEM.
⁎ 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.2011.12.019
3. Result and discussion The DLC air electrode/Na cell was tested under the dried air ambient and the typical voltage profiles are shown in Fig. 1(a). The initial opencircuit voltage (OCV) of the cell is 2.98 V. One voltage pseudoplateau from 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 in the subsequent discharges. The gravimetric capacities were calculated based on the weight of the DLC thin film. The DLC thin film electrode
Q. Sun et al. / Electrochemistry Communications 16 (2012) 22–25
23
performance. The increasing polarization at the larger discharge current densities can also be found. Based on Gibbs free energy data and the thermodynamic equation ΔG = −nFE, the theoretical voltages for possible reactions in Na/O2 cell with different products are calculated in Eqs. (1)–(3), respectively. þ
0
0
Na þ e þ O2 →NaO2 ΔG ¼ −218:4kJ=molE ¼ 2:263V
þ
0
0
ð2Þ
þ
0
0
ð3Þ
2Na þ 2e þ O2 →Na2 O2 ΔG ¼ −449:6kJ=molE ¼ 2:330V
4Na þ 4e þ O2 →2Na2 OΔG ¼ −751:0kJ=molE ¼ 1:946V
Fig. 1. (a) Galvanostatic cycling profiles of the DLC thin film/Na cell. The cell was cycled at 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.
delivers an initial discharge capacity of 1884 mAh/g and the maximum value of the discharge capacity of 2070 mAh/g is achieved in the 3rd cycle. The discharge capacity of the 20th cycle is 1058 mAh/g, corresponding to 56.2% the initial discharge capacity. For the initial charge process, the charging voltage profiles show a hump at the early stage and then increase steadily. The appearance of the hump is related to the sluggish diffusion and reaction kinetics of the charging processes for the thin film air electrodes. The charging of the DLC thin film electrode starts at 3.5 V after the hump region. The average voltage is about 3.9 V and its charge capacity is 1970 mAh/g (591 μAh/cm2). The charge capacities gradually decrease during the cycling. The shapes of the discharge and charge curves are similar as those of Li–air cell based on DLC air electrode [10],but the plateau voltage is about 0.3 V lower comparing with the latter due to the difference between the redox potentials of Na+/Na (−2.714 V vs. SHE) and Li+/Li (−3.040 V vs. SHE). In addition, no discharge/charge behavior of the cell was observed in argon ambient (not shown here). These results strongly support that DLC/Na cell is a Na/air cell. As in the Li–air battery reported previously [10], the source of oxygen in the presented sodium–air battery here also comes from the dissolved O2 in the electrolyte. The rate performance of the sodium–air battery was also investigated and the results are shown in Fig. 1(b). The discharging capacities decrease with larger discharge current densities and are found to be 3600 mAh/g (1080 μAh/cm 2), 2523 mAh/g (757 μAh/cm 2), 1056 mAh/g (317 μAh/cm 2) and 180 mAh/g (54 μAh/cm 2) at 1/60 C, 1/20 C, 1/6 C and 3 C, respectively. The discharge capacity at the current rate of 1/6 C is about 9% of that at 1/60 C, indicating a bad rate
ð1Þ
The electrode potentials corresponding to the formation of NaO2 and Na2O2 have very close values, which are close to with the plateau voltages observed in the discharge curves. To determine the structural and composition modification of air electrodes induced by Na uptake/ removal, ex situ TEM, SAED, and FTIR measurements were performed upon DLC thin film electrode at various states of the cell cycled between 2.0 and 4.5 V at a constant current of 1/60 C. HRTEM picture of the initial DLC thin film is shown in Fig. 2(a), where some short moiré stripes are observed. SAED spectra corresponding to the region show two weak rings (Fig. 2(b)), which can be attributed to (220) and (110) diffractions of face-centered cubic structure of diamond carbon (JCPDS card no. 89–3441). These results indicate that the as-deposited DLC thin films consist of some diamond-like crystalline phases dispersed into amorphous carbon matrix. When the cell is discharged to 1.5 V, some hazy strips appear in the TEM image of the DLC electrode (Fig. 2(c)). The corresponding SAED pattern (Fig. 2(d)) exhibits some clear concentric rings and some bright spots, indicating the polycrystalline nature of the discharged electrode. Most of the measured d-spacings derived from the SAED pattern can be well indexed to the diffractions of Na2O2 (JCPDS card no. 74–0985), and another weak ring can be indexed to (220) diffraction of diamond carbon. It provides the strong evidence that Na2O2 is generated and distributed in the DLC matrix during the discharge process. The TEM image and the SAED pattern of the thin film 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 with those 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-sided polishing silicon. Comparing with the initial thin film (Fig. 3(a)), some new peaks corresponding to O–C=O in Na2CO3 or R–CO–O - appear in the spectra of discharged electrode (Fig. 3(b)). After charging to 4.0 V, the intensities of these peaks are weakened significantly (Fig. 3(c)). FTIR data suggest that amorphous Na2CO3 or NaOCOR is also the discharge products when the electrode is discharged to 1.5 V in the carbonate electrolyte. This is consistent with the previous studies that argued that the reduction products of O2 in Li–air batteries could attack the carbonate solvents [11–13]. As a result, the discharge products contain carbonate species. After charging to 4.0 V, these discharge products are reversibly decomposed. Combining with the ex situ TEM, SAED and FTIR results, it can be found that Na2O2, Na2CO3 and NaOCO-R are formed as the products of the discharge reaction. The electrochemical reactions at DLC thin film with sodium should be similar to that of Li–air cells and can be described as follows: At cathode side during discharging: þ
−
2Na þ O2 þ 2e →Na2 O2
þ
ð4Þ
−
nNa þ O2 þ EC=DEC þ ne →Na2 CO3 þ NaOCO R þ side products ð5Þ
24
Q. Sun et al. / Electrochemistry Communications 16 (2012) 22–25
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 to 1.5 V; (e) TEM image and (f) SAED patterns of DLC thin film electrode after charging to 4.0 V.
2Na2 O2 þ 2CO2 →2Na2 CO3 þ O2
ð6Þ
At anode side during discharging: þ
−
2Na→2Na þ 2e
ð7Þ
At cathode side during charging: þ
Na2 O2 →2Na þ O2 þ 2e
−
ð8Þ
þ
−
Na2 CO3 þ NaOCO R→nNa þ CO2 þ sideproducts þ ne
ð9Þ
At anode side during charging: þ
−
2Na þ 2e →2Na
ð10Þ
During the discharge reaction, both Na ions and the dissolved oxygen should migrate through the liquid electrolyte and enrich at the surface of DLC thin film, then diffusion into the nanostructured DLC thin
film. The dissolved oxygen in the electrolyte absorbed on the nanostructured DLC thin films reacts with Na+ and forms Na2O2. The O2 reduction processes in non-aqueous sodium containing electrolyte are believed to be a two-step reaction involving the electrochemical formation and chemical decomposing of NaO2 [14]. At the same time, similarly as the situation of Li/air cell, the reduction products of O2 in the carbonatebased electrolyte might attack the carbonate solvents and results in the Na2CO3 and other organic carbonate salt products at air electrode. Considering that the sodium–air cells are cycled in dried air, the partial source of Na2CO3 may be also attributed to the chemical reaction between the discharged product of Na2O2 and CO2 from the dried air. After charging reactions to 4.0 V, these discharge products are reversibly decomposed. If compared with a liquid sodium oxygen cell, in which the discharge and charge voltage plateaus were found to be 1.75 V and 3.0 V with the OCV close to 2.0 V [3], the present Na/air cell at the room temperature exhibits utterly different electrochemical behavior implying completely different reaction routes with the existence of carbonate solvents. Since the electrochemical reactions with carbonate electrolytes in lithium-air batteries have been proved to be rather complicated [15,16], further clarification of the reaction mechanisms in SABs is still needed by using various in situ characterizations. In addition, if compared with liquid sodium, the poor cycling properties of solid sodium electrode here may lead to poor cycling behavior, dendritic growth, and concomitant safety problems [3]. It may be beneficial to improve the cyclic performance of SABs by using sodium-ion conductive inorganic thin film (such as NASICON) or polymer coatings on solid sodium electrodes. 4. Conclusion
Fig. 3. FTIR spectra of (a) the as-deposited DLC thin film electrode, (b) the electrode after discharging to 1.5 V and (c) the electrode after recharging to 4.0 V.
In this work, our results have initially demonstrated the possibility of Na–air cell operating at room temperature. The relevant large gravimetric capacities of air electrodes are superior to those of intercalation-based cathode materials for rechargeable sodium or lithium ion batteries (below 300 mAh/g), indicating the potentials of SABs as possible alternatives to lithium- or sodium-ion batteries. Crystallized Na2O2 and amorphous carbonate salts are found in the discharge products but vanish in the charged electrode. Thus, the complex and poorly characterized reaction routes in the SABs,
Q. Sun et al. / Electrochemistry Communications 16 (2012) 22–25
which involve not only sodium ion and O2 species but also carbonate solvent 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 Commission of 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.
25
[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 of Power 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 of Physical 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 of Physical Chemistry C 114 (2010) 9178.
Contents lists available at SciVerse ScienceDirect
Electrochemistry Communications journal homepage: www.elsevier.com/locate/elecom
Electrochemical properties of room temperature sodium–air batteries with non-aqueous electrolyte Qian Sun a, Yin Yang a, b, Zheng-Wen Fu a,⁎ a b
Shanghai Key Laboratory of Molecular Catalysts and Innovative Materials, Department of Chemistry & Laser Chemistry Institute, Fudan University, Shanghai 200433, China Department of Material and Science, Fudan University, Shanghai 200433, China
a r t i c l e
i n f o
Article history: Received 20 December 2011 Accepted 22 December 2011 Available online 28 December 2011 Keywords: Sodium–air battery Charge discharge measurement
a b s t r a c t A novel type of rechargeable sodium–air battery working at room temperature is constructed and examined for the first time. The typical gravimetric capacities of the air electrodes (diamond-like carbon thin films) are 1884 mAh/g (565 μAh/cm2) at 1/10 C and 3600 mAh/g (1080 μAh/cm 2) at 1/60 C, respectively, which are significantly superior to intercalation-based cathode materials for rechargeable sodium or lithium batteries ever reported. The electrochemical reaction of the sodium–air battery is investigated. The high reversible capacity and relevant high output voltage (about 2.3 V) of the room temperature sodium–air battery make it a potential alternative battery in the future. © 2011 Elsevier B.V. All rights reserved.
1. Introduction
2. Experimental
Over the last decades much effort has been devoted to develop rechargeable metal–air batteries due to their high theoretical energy densities and low costs in response to the rapidly growing demands of advanced batteries for increasingly dependence on portable electronic 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 been widely investigated, while a novel system of Si–air battery was reported recently [8]. Among these batteries, sodium–oxygen couple has a specific high energy of 1683 Wh/kg [9], and there are abundant sodium sources in both the earth's crust (2.3%) and oceans (1.1%) for the feasible popularization of future rechargeable sodium–air batteries (SABs). Thus, it should be of great interest to develop Na–air batteries. Nevertheless, one available study about sodium–air batteries was reported by Peled et al. [3], in which they made an attempt to demonstrate the feasibility of running a liquid sodium oxygen cell with polymer electrolytes at above 100 °C. However, liquid sodium is well known for its highly corrosive characteristic, while high operating temperatures are inconvenience for practical use. Here, we will report the electrochemical behavior of a non-aqueous sodium–air cell at room temperature based on diamond-like carbon (DLC) thin film as air electrode.
The details of the preparation of the DLC thin film electrode have been described in our recent work [10]. For electrochemical measurement, a conventional two-electrode cell was constructed in the dried air filled glove box with the DLC thin film as the cathode and one sheet of high-purity sodium foil as the anode, respectively. The model cell consisted of an H shape glass tube to separate positive and negative electrodes as well as two rubber plugs for sealing (as shown in the inset of Fig. 1(a)). The electrolyte was 1 M NaPF6 (AlfaAesar) non-aqueous solution in ethylene carbonate (EC) and dimethyl carbonate (DMC) with a volume ratio of 1:1 (Merck). Weight of thin film was examined by electrobalance (BP 211D, Sartorius). The precision of the weight was ±0.01 mg. Charge–discharge measurements were performed at room temperature with a Land BT 1–40 battery test system. Fourier transform infrared (FTIR) spectra were recorded on a Nicolet Nexus-470 spectrometer. High resolution transmission electron microscopy (HR-TEM) and selected area electron diffraction (SAED) measurements were carried out on a JEOL 2010 TEM.
⁎ 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.2011.12.019
3. Result and discussion The DLC air electrode/Na cell was tested under the dried air ambient and the typical voltage profiles are shown in Fig. 1(a). The initial opencircuit voltage (OCV) of the cell is 2.98 V. One voltage pseudoplateau from 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 in the subsequent discharges. The gravimetric capacities were calculated based on the weight of the DLC thin film. The DLC thin film electrode
Q. Sun et al. / Electrochemistry Communications 16 (2012) 22–25
23
performance. The increasing polarization at the larger discharge current densities can also be found. Based on Gibbs free energy data and the thermodynamic equation ΔG = −nFE, the theoretical voltages for possible reactions in Na/O2 cell with different products are calculated in Eqs. (1)–(3), respectively. þ
0
0
Na þ e þ O2 →NaO2 ΔG ¼ −218:4kJ=molE ¼ 2:263V
þ
0
0
ð2Þ
þ
0
0
ð3Þ
2Na þ 2e þ O2 →Na2 O2 ΔG ¼ −449:6kJ=molE ¼ 2:330V
4Na þ 4e þ O2 →2Na2 OΔG ¼ −751:0kJ=molE ¼ 1:946V
Fig. 1. (a) Galvanostatic cycling profiles of the DLC thin film/Na cell. The cell was cycled at 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.
delivers an initial discharge capacity of 1884 mAh/g and the maximum value of the discharge capacity of 2070 mAh/g is achieved in the 3rd cycle. The discharge capacity of the 20th cycle is 1058 mAh/g, corresponding to 56.2% the initial discharge capacity. For the initial charge process, the charging voltage profiles show a hump at the early stage and then increase steadily. The appearance of the hump is related to the sluggish diffusion and reaction kinetics of the charging processes for the thin film air electrodes. The charging of the DLC thin film electrode starts at 3.5 V after the hump region. The average voltage is about 3.9 V and its charge capacity is 1970 mAh/g (591 μAh/cm2). The charge capacities gradually decrease during the cycling. The shapes of the discharge and charge curves are similar as those of Li–air cell based on DLC air electrode [10],but the plateau voltage is about 0.3 V lower comparing with the latter due to the difference between the redox potentials of Na+/Na (−2.714 V vs. SHE) and Li+/Li (−3.040 V vs. SHE). In addition, no discharge/charge behavior of the cell was observed in argon ambient (not shown here). These results strongly support that DLC/Na cell is a Na/air cell. As in the Li–air battery reported previously [10], the source of oxygen in the presented sodium–air battery here also comes from the dissolved O2 in the electrolyte. The rate performance of the sodium–air battery was also investigated and the results are shown in Fig. 1(b). The discharging capacities decrease with larger discharge current densities and are found to be 3600 mAh/g (1080 μAh/cm 2), 2523 mAh/g (757 μAh/cm 2), 1056 mAh/g (317 μAh/cm 2) and 180 mAh/g (54 μAh/cm 2) at 1/60 C, 1/20 C, 1/6 C and 3 C, respectively. The discharge capacity at the current rate of 1/6 C is about 9% of that at 1/60 C, indicating a bad rate
ð1Þ
The electrode potentials corresponding to the formation of NaO2 and Na2O2 have very close values, which are close to with the plateau voltages observed in the discharge curves. To determine the structural and composition modification of air electrodes induced by Na uptake/ removal, ex situ TEM, SAED, and FTIR measurements were performed upon DLC thin film electrode at various states of the cell cycled between 2.0 and 4.5 V at a constant current of 1/60 C. HRTEM picture of the initial DLC thin film is shown in Fig. 2(a), where some short moiré stripes are observed. SAED spectra corresponding to the region show two weak rings (Fig. 2(b)), which can be attributed to (220) and (110) diffractions of face-centered cubic structure of diamond carbon (JCPDS card no. 89–3441). These results indicate that the as-deposited DLC thin films consist of some diamond-like crystalline phases dispersed into amorphous carbon matrix. When the cell is discharged to 1.5 V, some hazy strips appear in the TEM image of the DLC electrode (Fig. 2(c)). The corresponding SAED pattern (Fig. 2(d)) exhibits some clear concentric rings and some bright spots, indicating the polycrystalline nature of the discharged electrode. Most of the measured d-spacings derived from the SAED pattern can be well indexed to the diffractions of Na2O2 (JCPDS card no. 74–0985), and another weak ring can be indexed to (220) diffraction of diamond carbon. It provides the strong evidence that Na2O2 is generated and distributed in the DLC matrix during the discharge process. The TEM image and the SAED pattern of the thin film 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 with those 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-sided polishing silicon. Comparing with the initial thin film (Fig. 3(a)), some new peaks corresponding to O–C=O in Na2CO3 or R–CO–O - appear in the spectra of discharged electrode (Fig. 3(b)). After charging to 4.0 V, the intensities of these peaks are weakened significantly (Fig. 3(c)). FTIR data suggest that amorphous Na2CO3 or NaOCOR is also the discharge products when the electrode is discharged to 1.5 V in the carbonate electrolyte. This is consistent with the previous studies that argued that the reduction products of O2 in Li–air batteries could attack the carbonate solvents [11–13]. As a result, the discharge products contain carbonate species. After charging to 4.0 V, these discharge products are reversibly decomposed. Combining with the ex situ TEM, SAED and FTIR results, it can be found that Na2O2, Na2CO3 and NaOCO-R are formed as the products of the discharge reaction. The electrochemical reactions at DLC thin film with sodium should be similar to that of Li–air cells and can be described as follows: At cathode side during discharging: þ
−
2Na þ O2 þ 2e →Na2 O2
þ
ð4Þ
−
nNa þ O2 þ EC=DEC þ ne →Na2 CO3 þ NaOCO R þ side products ð5Þ
24
Q. Sun et al. / Electrochemistry Communications 16 (2012) 22–25
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 to 1.5 V; (e) TEM image and (f) SAED patterns of DLC thin film electrode after charging to 4.0 V.
2Na2 O2 þ 2CO2 →2Na2 CO3 þ O2
ð6Þ
At anode side during discharging: þ
−
2Na→2Na þ 2e
ð7Þ
At cathode side during charging: þ
Na2 O2 →2Na þ O2 þ 2e
−
ð8Þ
þ
−
Na2 CO3 þ NaOCO R→nNa þ CO2 þ sideproducts þ ne
ð9Þ
At anode side during charging: þ
−
2Na þ 2e →2Na
ð10Þ
During the discharge reaction, both Na ions and the dissolved oxygen should migrate through the liquid electrolyte and enrich at the surface of DLC thin film, then diffusion into the nanostructured DLC thin
film. The dissolved oxygen in the electrolyte absorbed on the nanostructured DLC thin films reacts with Na+ and forms Na2O2. The O2 reduction processes in non-aqueous sodium containing electrolyte are believed to be a two-step reaction involving the electrochemical formation and chemical decomposing of NaO2 [14]. At the same time, similarly as the situation of Li/air cell, the reduction products of O2 in the carbonatebased electrolyte might attack the carbonate solvents and results in the Na2CO3 and other organic carbonate salt products at air electrode. Considering that the sodium–air cells are cycled in dried air, the partial source of Na2CO3 may be also attributed to the chemical reaction between the discharged product of Na2O2 and CO2 from the dried air. After charging reactions to 4.0 V, these discharge products are reversibly decomposed. If compared with a liquid sodium oxygen cell, in which the discharge and charge voltage plateaus were found to be 1.75 V and 3.0 V with the OCV close to 2.0 V [3], the present Na/air cell at the room temperature exhibits utterly different electrochemical behavior implying completely different reaction routes with the existence of carbonate solvents. Since the electrochemical reactions with carbonate electrolytes in lithium-air batteries have been proved to be rather complicated [15,16], further clarification of the reaction mechanisms in SABs is still needed by using various in situ characterizations. In addition, if compared with liquid sodium, the poor cycling properties of solid sodium electrode here may lead to poor cycling behavior, dendritic growth, and concomitant safety problems [3]. It may be beneficial to improve the cyclic performance of SABs by using sodium-ion conductive inorganic thin film (such as NASICON) or polymer coatings on solid sodium electrodes. 4. Conclusion
Fig. 3. FTIR spectra of (a) the as-deposited DLC thin film electrode, (b) the electrode after discharging to 1.5 V and (c) the electrode after recharging to 4.0 V.
In this work, our results have initially demonstrated the possibility of Na–air cell operating at room temperature. The relevant large gravimetric capacities of air electrodes are superior to those of intercalation-based cathode materials for rechargeable sodium or lithium ion batteries (below 300 mAh/g), indicating the potentials of SABs as possible alternatives to lithium- or sodium-ion batteries. Crystallized Na2O2 and amorphous carbonate salts are found in the discharge products but vanish in the charged electrode. Thus, the complex and poorly characterized reaction routes in the SABs,
Q. Sun et al. / Electrochemistry Communications 16 (2012) 22–25
which involve not only sodium ion and O2 species but also carbonate solvent 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 Commission of 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.
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