A silicon–air battery utilizing a composite polymer electrolyte

Electrochimica Acta 58 (2011) 161–164

Contents lists available at SciVerse ScienceDirect

Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta

A silicon–air battery utilizing a composite polymer electrolyte Gil Cohn a , Anna Altberg a , Digby D. Macdonald b , Yair Ein-Eli a,∗ a b

Department of Materials Engineering, Technion-Israel Institute of Technology, Haifa 32000, Israel Center for Electrochemical Science and Technology, Department of Materials Science and Engineering, Pennsylvania State University, University Park, PA 16802, USA

a r t i c l e

i n f o

Article history: Received 4 August 2011 Received in revised form 8 September 2011 Accepted 10 September 2011 Available online 17 September 2011 Keywords: Battery Metal air Ionic liquids Gel polymer electrolyte

a b s t r a c t Gel polymer electrolytes (GPEs) made of blends of ionic liquids and polymer elements, are of high interest as electrolytes in batteries. They possess the high mechanical stability and non-fluidity of solids, together with the thermal and electrochemical stabilities of ionic liquids. In the present work we studied GPE, comprised of a EMI·(HF)2.3 F ionic liquid and 2-hydroxyethyl methacrylate polymer. Morphological characteristics, thermal, electrical, and electrochemical studies confirmed the ability of the GPE to serve as an electrolyte in an all solid Si–air battery. The ionic transport number, determined via the Wagner polarization technique, was found to be close to unity, suggesting ionic conduction domination over an electrical conduction. GPE containing 70 mol% ionic liquid showed a discharge voltage of 0.6 V over more than 850 h, in a discharge current of 0.1 mA cm−2 . This work demonstrates the feasibility of using gel polymer electrolytes in silicon–air batteries that offer unprecedented specific energy for powering low power devices. © 2011 Elsevier Ltd. All rights reserved.

1. Introduction Silicon–air battery is a highly promising battery system, based on its high theoretical specific energy of 8470 Wh kg−1 and an energy density of 21,090 Wh L−1 , excluding O2 . Its capacity, 3.816 Ah kg−1 , is very close to that of the highly attractive, and recently very well-studied, lithium–air battery system, 3.86 Ah kg−1 . The Si–air battery, first reported in 2009 [1], has an operating voltage of 0.8–1.1 V, under loading of 0.3–0.01 mA cm−2 . Unlike the Li–air system, the Si–air battery does not exhibit many of the environmental and safety concerns that plague that system. Exposure of the Si anode to ambient atmosphere is not hazardous, the ionic liquid electrolyte is non-volatile and the discharge products are disposable or recyclable. Moreover, in our latest work, we showed that humid environment and water uptake by the electrolyte not only did not damage the battery, but rather improved it performance [2]. Deliberate addition of 15 vol.% of water to the EMI·(HF)2.3 F (EMI: 1-ethyl-3-methylimidazolium) ionic liquid electrolyte improved the cell capacity by more than 40%, from 53.4 mAh cm−2 (for water-free electrolyte) to 72.5 mAh cm−2 (for IL containing 15 vol.%). Therefore, the use of the hydrophilic ionic liquid contributes to improved performance of the cell. Gel polymer electrolytes (GPEs) have attracted much attention as electrolytes for many solid state electrochemical devices, such as chemical sensors [3], solar cells [4] and mainly lithium ion

∗ Corresponding author. Tel.: +972 4 8294588; fax: +972 4 8295677. E-mail address: [email protected] (Y. Ein-Eli). 0013-4686/$ – see front matter © 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2011.09.026

batteries [5,6]. GPEs are formed by incorporating a liquid electrolyte into a polymeric matrix; therefore, the conduction mechanism in polymer gels is similar to the liquid electrolyte, with the advantage of a solid structure. The GPEs are, thus, highly safe, shape flexible, mechanically stable, and display only a modest loss in ionic conductivity upon operation. The most common polymer matrices used for GPEs are poly(methyl methacrylate) (PMMA), poly(vinylidene fluoride) (PVdF), poly(ethylene oxide) (PEO), and polyacrylonitrile (PAN). GPEs for Li-ion electrolytes, containing the polymeric matrices mentioned above, show a typical ionic conductivity of the order of 1 mS cm−1 , at room temperature. Within the massive introduction of room temperature ionic liquids (RTILs) into electrochemical applications, the combination of these interesting liquids together with polymers, in order to form gel-like electrolyte, is under wide investigations [7–11]. In previous work, Tsuda et al. [12] explored the feasibility of various combinations of EMI·(HF)2.3 F:polymer composites, in terms of ionic conductivity, air stability and electrochemical window. The compounds, 2-hydroxyethyl methacrylate (HEMA), vinyl acetate, 1-vinyl imidazole, and methyl methacrylate were used as monomers. From all of the above, the only successive combination was identified to be the mixture of EMI·(HF)2.3 F and HEMA, which polymerized into a transparent gel. It was found that the conductivity of the GPE decreased dramatically with a decrease in RTIL molar content in the mixture; for 60 mol% RTIL, at room temperature, the conductivity was found to be 23 mS cm−1 , compared to 100 mS cm−1 for the neat RTIL. The apparent electrochemical window of the 50 mol% RTIL mixture was found to be 0.3 V wider than for the pure RTIL. No change was observed in the cyclic

162

G. Cohn et al. / Electrochimica Acta 58 (2011) 161–164 Table 1 Transport number of GPE in different compositions.

Fig. 1. Photograph of a EMI·(HF)2.3 F RTIL/HEMA polymer gel electrolyte.

voltammogram for the 50 mol% RTIL mixture after one week of exposure to air, indicating a stability of the mixture in contact with air. Even though there is no need to improve the performance of the Si–air cell in contact with ambient atmosphere, by incorporating a GPE as a moister barrier, the use of a gel electrolyte may eliminate the need to handle liquid electrolyte and will simplify technical issues in cell architecture. Here, we report on a successful research and development of a novel, all solid state, silicon–air battery employing a EMI·(HF)2.3 F:HEMA gel electrolyte. Various studies, including thermal analysis, ionic transport number, morphology investigations, and cell discharging were carried out in order to characterize the GPE and the new integrated battery. 2. Experimental Gel-polymer electrolyte was prepared according to methods described previously [12]. EMI·(HF)2.3 F RTIL (synthesized by R. Hagiwara, Kyoto University, Japan) and HEMA (Alfa Aesar, 97%), in the proper molar ratios, together with 0.1 wt% benzoyl peroxide (Fluka, >97%) as a initiator were stirred together for 2 h and then the mixture was polymerized at 80 ◦ C for 12 h. After cooling to room temperature, free standing polymer gel electrolyte films were obtained, as shown in Fig. 1. This procedure was followed for several molar fractions of ionic liquid, from 40 mol% to 70 mol%. The thermal behavior of a gel electrolyte was analyzed using thermo-gravimetric analysis (TGA, 2050 TGA, TA Instruments). Each sample was heated from room temperature to 600 ◦ C at a heating rate of 10 ◦ C min−1 in air. The morphology and structural property of the gel polymer electrolyte have been observed by scanning electron microscope (SEM, FEI Quanta 200). High resolution SEM (HRSEM, Hitachi S4700) was used to investigate the silicon electrode morphology. The ionic transport number, ti , was determined by a standard Wagner polarization technique [13]. This method consists of applying a square wave potential of 1 V on a symmetrical cell, comprised of GPE placed between two stainless steel (SS) blocking electrodes, while measuring the generated current as a function of time. The value of the initial current, i0 , is proportional to the total conductivity of the electrolyte. However, after long times the only current is a residual current, i∞ , due to electronic conduction. The ionic transport number is determined from the ratio: ti =

i0 − i∞ i0

(1)

mol% of EMI·(HF)2.3 F RTIL in GPE

Ionic transport number, ti

40 50 60 70

0.936 0.98 0.989 0.982

Si–air battery cell discharge evaluation were conducted with an Arbin BT2000 battery testing system. The cells were comprised of GPE electrolyte, with various ionic liquid content, silicon wafer anodes (As-doped 0.001–0.005  cm−1 1 0 0, University Wafer USA), and commercial air cathodes (Electric Fuel Ltd). The cells were kept under an open circuit condition for 8 h prior to any discharge, in order to allow a complete wetting of the porous carbon air electrode with the electrolyte. Wetting of the electrodes was attributed to “moisture” in the GPE, but future work will involve wetting the interfaces of the GPE with a small amount of RTIL to facilitate electrolytic contact between the GPE and the electrodes. The cells were operated under ambient air conditions at 25 ◦ C. Prior to cell discharge experiments, the silicon working electrode was immersed in 1:1 H2 SO4 /H2 O2 solution for 15 min in order to remove organic contamination, followed by native oxide removal in 20 vol.% HF solution for 10 s. The sample was rinsed in DI water after each step and finally dried under a N2 gas stream. 3. Results and discussion Gels of all compositions, from 40 mol% to 70 mol% were flexible, but yet mechanically strong and compressible without any structural damage or liquid phase dripping. Even after many hours of storage or many hours of active operation, no phase changes or drying were observed. Morphological study of the blend EMI·(HF)2.3 F RTIL and HEMA gel polymer electrolyte has been carried out using a low vacuum mode (∼1 Torr) SEM. The relevant micrographs, for 40 mol% and 60 mol% RTIL are shown in Fig. 2. The gels of these compositions, and other compositions, as well (not shown), possess a homogenous compact structure, without phase separation or particles aggregation. The thermal stability of the RTIL–HEMA polymer gels was analyzed by thermo-gravimmetry, from room temperature to 600 ◦ C at a heating rate of 10 ◦ C min−1 (Fig. 3). Below 100 ◦ C, a slight weight loss was observed, most likely due to desorption of absorbed water from the blend. Above this temperature, a slow weight loss was detected. This loss, according to Hagiwara et al. [14], is attributed to elimination of HF from the (HF)n F− anions, where n is 2 or 3 (Hx Fy − → Hx−1 Fy−1 − + HF; x = 2, y = 3 or x = 3, y = 4). Dramatic weight loss is observed at 270–300 ◦ C. This temperature range is known to induce both decomposition to volatile components of the ionic liquid and decomposition of the HEMA polymer [15]. This suggests that the mixture of the ionic liquid and HEMA components, and their polymerization, does not improve the thermal stability of the GPE blend. It is important to note that the thermal stability, up to 270 ◦ C, is perfectly satisfying from an applications point of view. The Wagner polarization technique was used in this study to determine the total ionic (cationic and anionic) transport number ti . This number represents the fraction of the current carried by all ions, from the total current, carried by ions and electrons. Fig. 4 shows a representative variation of the current as a function of time, while applying step voltage of 1 V over the SS|GPE|SS cell. As shown in Fig. 4, at extended times the GPE exhibits a very low, but yet non-zero, current due to electronic conduction. The value of ti was calculated using Eq. (1), for different RTIL mol% in the GPE, and the results are shown in Table 1. The values of ti are found to

G. Cohn et al. / Electrochimica Acta 58 (2011) 161–164

163

Fig. 4. Chronoamperograms using an applied voltage of 1 V across SS|EMI·(HF)2.3 F RTIL–HEMA (70 mol%)|SS cell.

Fig. 2. SEM image of a EMI·(HF)2.3 F RTIL/HEMA polymer gel electrolyte: (a) 40 mol%, (b) 70 mol% RTIL.

Fig. 3. TGA curves for EMI·(HF)2.3 F RTIL/HEMA polymer gel electrolyte in different compositions.

be above 0.93, with numbers much closer to 1 for RTIL molar fractions of 0.5 and above, due to the ionic liquid being the dominant species contributing to the ionic conduction. These values suggest that the total conductivity is governed ionically, mainly through the ionic liquid entrapped in the polymeric matrix. The ionic transport number calculated here is the sum of all ionic contributions, both cationic and anionic. The ionic species in our system are the anions and cations, composing the RTIL, and they are responsible for the ionic transport number. However, since the initiator, benzoyl peroxide is a redox agent [16,17], we can assume that the peroxide redox couple contributes, as well, to the conductivity in addition to the ions of the RTIL. Therefore, the total electrical conductivity is due to electronic transport via the polymerization initiator, i.e. benzoyl peroxide, in addition to the ionic transport via the RTIL constituents ions. Fig. 5 shows the discharge curves obtained from discharging Si–air batteries, utilizing GPE of 50–70 mol% ionic liquid, at a current density of 0.1 mA cm−2 , in an ambient atmosphere. It is shown that, with increasing RTIL concentration in the electrolyte (50 mol%, 60 mol%, 70 mol%), the operating potentials are 0.4, 0.5 and 0.6 V, at a discharge current of 0.1 mA cm−2 , respectively. These discharge potentials are significantly lower, by roughly 0.5 V than the discharge voltage of a cell employing neat RTIL as electrolyte. This can be attributed to the difference in ionic conductivity of the GPEs, and hence the IR potential drop across the cell, compared with the ionic conductivity of pure ionic liquid. As reported by Tsuda et al. [12], the conducting gel polymer electrolyte, EMI·(HF)2.3 F RTIL and HEMA system, shows a typical conductivity of the order of 10−2 S cm−1

Fig. 5. Galvanostatic discharge curves of Si–air cell, comprised of silicon wafer anode, air cathode and EMI·(HF)2.3 F RTIL–HEMA (70 mol% RTIL gel polymer electrolyte).

164

G. Cohn et al. / Electrochimica Acta 58 (2011) 161–164

cell discharge utilizing gel electrolyte, are an outcome of an actually higher discharge polarization and IR potential drop than expected. Consequently, we expect higher discharge voltage for GPE cell with enhanced contacts between the GPE and the silicon anode as well as with the air electrode. 4. Conclusions Free standing, mechanically stable, composite polymer gel electrolyte, based on EMI·(HF)2.3 F RTIL and HEMA polymer, were produced. Four different compositions, from 40 mol% to 70 mol% of RTIL, were investigated and characterized. The GPEs have a uniform structure, without any evidence of aggregation or phase separation. It was observed that all compositions are thermally stable up to a temperature of 270 ◦ C. The ionic transport number was found to increase with increase in RTIL content in the GPE and all electrolytes exhibited an ionic transport number close to unity, implying an ionic conduction as the charge transfer mechanism across the GPE. It was found that the GPEs have good compatibility with both the silicon anode and the air cathode. Battery cells, comprising silicon wafers, air electrodes and GPEs were evaluated and operated under a discharge current density of 0.1 mA cm−2 . The results show a relatively long discharge times, up to 850 h. However, the operating voltage is lower with respect to a cell discharge utilizing the ionic liquid alone, due to higher ionic conductivity of the pure RTIL. Nonetheless, the contact between the silicon anode and the air cathode was not intimate; thereby the surface area for reacted silicon (oxidation) is lower than expected. Further experiments will be conducted, in order to achieve better contact and adhesion between the GPE and the electrodes thereby overcoming technical and engineering obstacles. This work demonstrates the feasibility of using GPEs in silicon–air batteries that offer unprecedented specific energy for powering low power devices. Acknowledgments Fig. 6. Top view HRSEM images of the silicon wafer anode after discharge process, as indicated in the green curve ( symbol) in Fig. 5.

at room temperature, which is one order of magnitude lower than the neat RTIL conductivity. However, as the ratio of RTIL to HEMA increases in favor for the RTIL, the discharge voltage of the cell increases as well. This is due to the improved ionic conductivity of the “rich-IL” GPEs. It is important to mention that, due to the GPE preparation procedure and cell assembly, the contact between the Si anode and the GPE is inhomogeneous over the Si surface. Fig. 6a shows HRSEM image of a silicon wafer, in low magnification, after discharging in a current density of 0.1 mA cm−2 , utilizing 70 mol% RTIL in the GPE. One can see that the reaction area is nonuniform, but, rather, is confined only to selected (framed) areas, where contact between the Si anode and the GPE, presumably, was established. Fig. 6b shows a higher magnification micrograph, taken from the same silicon anode shown in Fig. 6a. As can be seen, the surface morphology of the silicon electrode is porous, with pores in the 100 nm size. This morphology is in agreement with the surface morphology obtained with pure RTIL [18]. As shown in Fig. 6a, the actual area available for electrochemical reaction and ion transport is lower than the nominal cell area of 0.5 cm2 , determined by cell engineering. As a result of smaller contact area between the gel and the electrodes, the real current density at discharge is higher than 0.1 mA cm−2 . Accordingly, the discharge voltages, recorded during

This work was supported by U.S.–Israel Binational Science Foundation (BSF), FP7 InnoEnergy Program, European Office of Aerospace Research and Development (EOARD), Grand Technion Energy Program (GTEP) and the Helmsley Charity Fund. References [1] G. Cohn, D. Starosvetsky, R. Hagiwara, D.D. Macdonald, Y. Ein-Eli, Electrochem. Commun. 11 (2009) 1916. [2] G. Cohn, D.D. Macdonald, Y. Ein-Eli, ChemSusChem 4 (2011) 1124. [3] L. Rotariu, L. Zamfir, C. Bala, Sens. Actuators B 150 (2010) 73. [4] Y. Wang, Solar Energy Mater. Sol. Cells 93 (2009) 1167. [5] J.W. Fergus, J. Power Sources 195 (2010) 4554. [6] S. Ahmad, Ionics 15 (2009) 309. [7] S. Ferrari, E. Quartarone, P. Mustarelli, A. Magistris, M. Fagnoni, S. Protti, C. Gerbaldi, A. Spinella, J. Power Sources 195 (2010) 559. [8] M. Egashira, H. Todo, N. Yoshimoto, M. Morita, J. Power Sources 178 (2008) 729. [9] D. Kumar, S.A. Hashmi, Solid State Ionics 181 (2010) 416. [10] C.C. Ho, J.W. Evans, P.K. Wright, J. Micromech. Microeng. 20 (2010) 104009. [11] A. Noda, M. Watanabe, Electrochim. Acta 45 (2000) 1265. [12] T. Tsuda, T. Nohira, Y. Nakamori, K. Matsumoto, R. Hagiwara, Y. Ito, Solid State Ionics 149 (2002) 295. [13] S.A. Hashmi, S. Chandra, Mater. Sci. Eng. B 34 (1995) 18. [14] R. Hagiwara, T. Hirashige, T. Tsuda, Y. Ito, J. Electrochem. Soc. 149 (2002) D1. [15] M. Fernández-Garcıˇıa, M.F. Torrado, G. Martıˇınez, M. Sánchez-Chaves, E.L. Madruga, Polymer 41 (2000) 8001. [16] I.D. Sideridou, D.S. Achilias, O. Karava, Macromolecules 39 (2006) 2072. [17] G. Mabilleau, C. Cincu, M.F. Baslé, D. Chappard, J. Raman Spectrosc. 39 (2008) 767. [18] G. Cohn, Y. Ein-Eli, J. Power Sources 195 (2010) 4963.