CHEMSUSCHEM COMMUNICATIONS DOI: 10.1002/cssc.201402463
Thin-Film Silicon for Flexible Metal–Air Batteries Ahmed Garamoun, Markus B. Schubert, and Jrgen H. Werner*[a] Due to its high energy density, theoretical studies propose silicon as a promising candidate material for metal–air batteries. Herein, for the first time, experimental results detail the use of n-type doped amorphous silicon and silicon carbide as fuel in Si–air batteries. Thin-film silicon is particularly interesting for flexible and rolled batteries with high specific energies. Our Si– air batteries exhibit a specific capacity of 269 Ah kg1 and an average cell voltage of 0.85 V at a discharge current density of 7.9 mA cm2, corresponding to a specific energy of 229 Wh kg1. Favorably in terms of safety, low concentrated alkaline solution serves as electrolyte. Discharging of the Si–air cells continues as long as there is silicon available for oxidation.
High energy densities of primary and secondary batteries are particularly important for mobile applications. Nowadays, state-of-the-art Li-ion secondary batteries provide specific energies of E 150 Wh kg1, with values of E 250 Wh kg1 projected by 2025.[1, 2] Primary metal–air batteries enable even higher specific energies, up to a theoretical values of E = 11 680 Wh kg1 for Li–air systems.[3] Theoretical values of E = 1370 Wh kg1 for Zn–air[4] and of E = 8100 Wh kg1 for Al–air[5] have been reported. Commercial Zn–air batteries present specific energies of E = (350–470) Wh kg1. Such Zn–air batteries are low-cost and ecologically friendly, but their applications are limited because of their low specific energy.[6] Al–air batteries promise higher specific energies, but practical applications suffer from high self-discharge rates.[7] In 2009, Cohn et al. introduced primary silicon–air batteries as a new type of “metal”–air batteries, featuring a theoretical specific energy E = 8470 Wh kg1.[8] First experimental results indicated excellent long-term stability at a voltage of (1.0–1.2) V by using heavily doped crystalline silicon wafers as fuel for oxidation, and a specially synthesized EMI·2.3 HF·F electrolyte.[8, 9] Zhong et al. reported an alkaline-based system with a specific capacity of 1206 Ah kg1, corresponding to a specific energy E = 1085 Wh kg1, at a discharge current density jd = 100 mA cm2 from a Si wafer in a 0.6 m KOH solution. That study employed metal-assisted chemical etching needed for nanostructuring of the silicon wafer surface, otherwise the discharge was self-limiting to a duration of 400 s.[10] This short Communication aims at realizing the potential of the high theoretical specific energy of silicon as a fuel for Si– air batteries. At the same time we avoid the restrictions due to the use of rigid silicon wafers. Instead, we investigate amor[a] A. Garamoun, Dr. M. B. Schubert, Prof. Dr. J. H. Werner Institut fr Photovoltaik, Universitt Stuttgart Pfaffenwaldring 47, 70569 Stuttgart (Germany) Fax: (+ 49) 711-685 67138 E-mail: [email protected]
2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
phous silicon (a-Si) or microcrystalline silicon (mc-Si) thin films, gas-phase deposited on ultrathin flexible foil substrates. Thus we open the door to flexible and rollable Si–air batteries with high specific energy and volumetric energy density. These are the first experimental results of Si–air batteries based on a-Si thin films. First, a-Si–air battery cells on rigid glass as well as on flexible stainless steel foils exhibited a specific capacity up to 269 Ah kg1. Figure 1 a shows the structure of the battery cell.
Figure 1. a) Scheme of experimental silicon–air battery cell. The cell is made of stainless steel. b) a-Si layer on conductive glass substrate of 2.7 cm diameter after full discharge; the thin film of silicon is fully dissolved during discharging.
In the case of glass substrates, the electrical contact to the negative a-Si electrode on the left-hand side was provided by a thin conductive oxide (TCO) layer, which is part of the commercial Asahi U-type substrate. The Asahi-U TCO substrate mainly consists of tin oxide and exhibits a textured surface, which improves the adhesion of the a-Si electrode. In the case of flexible stainless-steel foils, the metal directly contacts the aSi electrode. A low-concentration alkaline solution of potassium hydroxide (KOH) or sodium hydroxide (NaOH) served as electrolyte. The right-hand side of Figure 1 a shows a commercially available air electrode, serving as positive electrode of the battery. The air electrode comprises a polytetrafluoroethylene (PTFE) powder and carbon-black-loaded structure, with manganese dioxide as catalyst and pressed onto a nickel 200 mesh (strands per inch). A PTFE microporous layer faces the air side of the electrode, and a separator is attached to the electrolyte side. The complete air electrode was supplied by Electric Fuel, Inc. The active layers of our Si–air batteries were grown by plasma-enhanced chemical vapor deposition (PECVD) at a deposition temperature Tdep = 170 8C from silane (SiH4), methane (CH4), and phosphine (2 % PH3 in SiH4) as feedstock gases. A deposition rate of 10 nm min1 yielded 500 nm thick n-type aChemSusChem 2014, 7, 3272 – 3274
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www.chemsuschem.org
Si and amorphous silicon carbide (a-SiC) layers on the TCO contacts. The active area of the battery cells was 5.7 cm2, defined by the diameter of rubber seals as indicated in Figure 1 a. The discharging experiments analyzed batteries with different electrolyte concentrations and discharge current densities, and were carried out using a Keithley 2400 Series Sourcemeter in bias current mode.[12] In contrast to silicon-wafer-based Si-air batteries with an alkaline electrolyte, our amorphous silicon electrodes fully dissolve during discharging of the battery cell, without any surface modification prior to battery operation. The wafer-based porous silicon electrodes form a closed SiO2 layer at the surface, which stops the battery discharge after a few minutes.[10] Two competing electrochemical reactions take place in the Si–air battery cell: i) the battery discharge reaction, driving the electrical current flow between the electrodes; and ii) self-discharge. The discharge process proceeds according to Anode : Si þ 4 OH ! SiðOHÞ4 þ 4 e
ð1Þ
Cathode : O2 þ 2 H2 O þ 4 e ! 4 OH
ð2Þ
while the overall self-discharge in alkaline electrolytes is described as[13] Si þ 2 OH þ 2 H2 O ! SiO2 ðOHÞ2 2 þ 2 H2
ð3Þ
Similar to wafer-based Si–air cells, both reaction pathways finally dissolve the silicon anode in the electrolyte. In contrast to plain crystalline Si anodes,[10] no insoluble SiO2 forms at the surface of our thin film electrodes. Figure 1 b presents a photograph of the a-Si electrode after a full discharging process, indicating that most of the a-Si layer dissolved during the discharge. Figure 2 shows discharge curves of the a-Si and a-SiC anodes with different electrolytes, electrolyte concentrations, and discharge current densities. In order to etch off any native oxide, all electrode samples underwent etching in a 1 % HF solution before battery assembly. Table 1 gives detailed experimental data for discharge curves A–G displayed in Figure 2. The specific capacity for the different discharge experiments of Figure 2 is calculated by assuming the mass of the 500 nm a-Si or a-SiC films to be 105 mg cm2. This mass of the deposited silicon films derives from an a-Si or a-SiC density of 2.1 g cm3.[14] A specific capaci-
Figure 2. Discharge curves of thin-film Si–air batteries with anodes made of a-Si (battery cells A to E) and a-SiC (batteries F, G). The discharge current density jd increases from A to E. Table 1 presents details on the variation of the alkaline electrolytes, electrode materials, and the resulting specific capacities.
ty of 269 Ah kg1 is reached with a-SiC, 0.01 m KOH electrolyte concentration, and jd = 7.9 mA cm2. Comparing samples B and D of Table 1 shows very similar results for KOH and NaOH electrolytes. The performance of anode C on flexible steel foil compares well with the corresponding rigid anode B. As evident from Equations (1) to (3), only part of the silicon is used as battery fuel. Quantitative evaluation of Equations (1) and (2) yields an a-Si consumption rate of 0.17 nm min1 for jd = 7.9 mA cm2, while the total experimental silicon consumption amounts to 2.95 nm min1 for battery B of Table 1, indicating that only 5.5 at % of the a-Si anode serves as battery fuel in our experiments. This finding is in agreement with a separate experiment that completely dissolved a similar a-Si film in 0.01 m KOH, and thereby yielded a corrosion rate of 2.6 0.1 nm min1. Hence, self-discharge clearly limits the specific capacity of our first thin-film Si-air batteries. The batteries F and G of Table 1 exhibit the largest specific capacity, which we attribute to lower self-discharge. Quantitative evaluation yields a 7 at % silicon usage of the a-SiC anodes F and G, in contrast to the 5.5 at % yield of the a-Si anodes A to E. The most promising features of silicon thin-film electrodes arise from the use of ultrathin flexible substrates and low-temperature deposition. Figure 3 a shows an example of a n-type a-Si anode deposited on a 100 mm thick flexible stainless steel
Table 1. Parameters and performances of thin-film Si–air batteries at varying discharge current densities (jd). The anodes of the batteries A, B, D–G are deposited on conducting Asahi-U, while battery C uses stainless steel (ss) as substrate and electrical contact. Battery
Anode material (n-type)
Source gas flow (SiH4/PH3/CH4) [sccm]
Anode resistivity [W cm]
jd [mA cm2]
Electrolyte
Electrolyte concentration [mol L1]
Specific capacity [Ah kg1]
A B C D E F G
a-Si a-Si a-Si a-Si a-Si a-SiC a-SiC
3:3:0 3:3:0 3:3:0 3:3:0 3:3:0 3:3:1 3:3:3
440 440 440 440 440 610 3125
1.6 7.9 7.9 7.9 31.4 7.9 7.9
KOH KOH KOH NaOH KOH KOH KOH
0.01 0.01 0.01 0.01 0.01 0.01 0.01
77 209 219 203 150 269 262
2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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www.chemsuschem.org Acknowledgements The authors gratefully acknowledge fruitful discussions with N. Ohmer (Max Planck Institute for Solid State Research, Stuttgart, Germany). We owe special thanks to our colleagues S. Vollmer and L. Beisel for their skillful support of the experiments.
Figure 3. a) Thin-film a-Si layer on a flexible sheet of stainless steel. Its discharge process is shown in Figure 2 as process “C/ss”. b) Schematic of a rolled silicon–air battery. The battery consists of thin film of silicon on a flexible substrate, an electrolyte as gel or paste, an air electrode, and a foam layer to allow air to diffuse to an air cathode.
foil. Discharging this film with jd = 7.9 mA cm2 in presence of 0.01 m KOH electrolyte yields trace C in Figure 2, with a specific capacity of 219 Ah kg1. Figure 3 b sketches a rolled Si–air battery with a silicon thin-film anode deposited by web-coating. The battery cell is completed by a gel or paste electrolyte, an air electrode, and a foam layer enabling diffusion of air to the cathode. Moreover, with the help of patterning during deposition[15] or post-deposition structuring of the silicon thin films, single cells are connected in series and form a high-voltage battery stack. Amorphous silicon and silicon carbide films demonstrate high potential as fuel for silicon–air batteries. Our first experiments demonstrate a specific capacity of 269 Ah kg1 at an operating voltage of 0.85 V and jd = 7.9 mA cm2. At present, the performance of these silicon thin-film batteries suffers from high self-discharge rates. Ongoing work will reduce the selfdischarge and enhance specific capacity by using electrolytes such as EMI·2.3 HF·F, room-temperature ionic liquids (RTILs), and further improvement of the silicon anodes. Thin film Si–air batteries easily integrate with flexible electronic devices or micro-electromechanical systems, and offer the potential of high specific capacity and energy density.
2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Keywords: amorphous materials electrochemistry · silicon · thin films
·
batteries
·
[1] S. J. Gerssen-Gondelach, A. P. C. Faaij, J. Power Sources 2012, 212, 111 – 129. [2] J. Chen, Materials 2013, 6, 156 – 183. [3] G. Girishkumar, B. McCloskey, A. C. Luntz, S. Swanson, W. Wilcke, J. Phys. Chem. Lett. 2010, 1, 2193 – 2203. [4] C. J. Lan, T. S. Chin, P. H. Lin, T. P. Perng, J. New Mater. Electrochem. Syst. 2006, 9, 27 – 32. [5] C. S. Li, W. Q. Ji, J. Chen, Z. L. Tao, Chem. Mater. 2007, 19, 5812 – 5814. [6] V. Neburchilov, H. J. Wang, J. J. Martin, W. Qu, J. Power Sources 2010, 195, 1271 – 1291. [7] A. A. Mohamad, Corros. Sci. 2008, 50, 3475 – 3479. [8] G. Cohn, D. Starosvetsky, R. Hagiwara, D. D. Macdonald, Y. Ein-Eli, Electrochem. Commun. 2009, 11, 1916 – 1918. [9] G. Cohn, Y. Ein-Eli, J. Power Sources 2010, 195, 4963 – 4970. [10] X. Zhong, H. Zhang, Y. Liu, J. Bai, L. Liao, Y. Huang, X. Duan, ChemSusChem 2012, 5, 177 – 180. [11] http://www.electric-fuel.com/airelectrode/ (accessed September 2014). [12] http://www.keithley.com/products/dcac/voltagesource/broadpurpose/ ?mn = 2400/ (URL seems out of date, please check). [13] X. G. Zhang, Electrochemistry of Silicon and its Oxide, Springer Science & Business Media, 2001, pp. 294 – 298. [14] T. Haage, U. I. Schmidt, H. Fath, P. Hess, B. Schrçder, H. Oechsner, J. Appl. Phys. 1994, 76, 4894. [15] R. Merz, J. Kistner, M. A. Bouattour, M. B. Schubert, Phys. Status Solidi A 2010, 207, 686. Received: May 26, 2014 Published online on September 22, 2014
ChemSusChem 2014, 7, 3272 – 3274
3274
Thin-Film Silicon for Flexible Metal–Air Batteries Ahmed Garamoun, Markus B. Schubert, and Jrgen H. Werner*[a] Due to its high energy density, theoretical studies propose silicon as a promising candidate material for metal–air batteries. Herein, for the first time, experimental results detail the use of n-type doped amorphous silicon and silicon carbide as fuel in Si–air batteries. Thin-film silicon is particularly interesting for flexible and rolled batteries with high specific energies. Our Si– air batteries exhibit a specific capacity of 269 Ah kg1 and an average cell voltage of 0.85 V at a discharge current density of 7.9 mA cm2, corresponding to a specific energy of 229 Wh kg1. Favorably in terms of safety, low concentrated alkaline solution serves as electrolyte. Discharging of the Si–air cells continues as long as there is silicon available for oxidation.
High energy densities of primary and secondary batteries are particularly important for mobile applications. Nowadays, state-of-the-art Li-ion secondary batteries provide specific energies of E 150 Wh kg1, with values of E 250 Wh kg1 projected by 2025.[1, 2] Primary metal–air batteries enable even higher specific energies, up to a theoretical values of E = 11 680 Wh kg1 for Li–air systems.[3] Theoretical values of E = 1370 Wh kg1 for Zn–air[4] and of E = 8100 Wh kg1 for Al–air[5] have been reported. Commercial Zn–air batteries present specific energies of E = (350–470) Wh kg1. Such Zn–air batteries are low-cost and ecologically friendly, but their applications are limited because of their low specific energy.[6] Al–air batteries promise higher specific energies, but practical applications suffer from high self-discharge rates.[7] In 2009, Cohn et al. introduced primary silicon–air batteries as a new type of “metal”–air batteries, featuring a theoretical specific energy E = 8470 Wh kg1.[8] First experimental results indicated excellent long-term stability at a voltage of (1.0–1.2) V by using heavily doped crystalline silicon wafers as fuel for oxidation, and a specially synthesized EMI·2.3 HF·F electrolyte.[8, 9] Zhong et al. reported an alkaline-based system with a specific capacity of 1206 Ah kg1, corresponding to a specific energy E = 1085 Wh kg1, at a discharge current density jd = 100 mA cm2 from a Si wafer in a 0.6 m KOH solution. That study employed metal-assisted chemical etching needed for nanostructuring of the silicon wafer surface, otherwise the discharge was self-limiting to a duration of 400 s.[10] This short Communication aims at realizing the potential of the high theoretical specific energy of silicon as a fuel for Si– air batteries. At the same time we avoid the restrictions due to the use of rigid silicon wafers. Instead, we investigate amor[a] A. Garamoun, Dr. M. B. Schubert, Prof. Dr. J. H. Werner Institut fr Photovoltaik, Universitt Stuttgart Pfaffenwaldring 47, 70569 Stuttgart (Germany) Fax: (+ 49) 711-685 67138 E-mail: [email protected]
2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
phous silicon (a-Si) or microcrystalline silicon (mc-Si) thin films, gas-phase deposited on ultrathin flexible foil substrates. Thus we open the door to flexible and rollable Si–air batteries with high specific energy and volumetric energy density. These are the first experimental results of Si–air batteries based on a-Si thin films. First, a-Si–air battery cells on rigid glass as well as on flexible stainless steel foils exhibited a specific capacity up to 269 Ah kg1. Figure 1 a shows the structure of the battery cell.
Figure 1. a) Scheme of experimental silicon–air battery cell. The cell is made of stainless steel. b) a-Si layer on conductive glass substrate of 2.7 cm diameter after full discharge; the thin film of silicon is fully dissolved during discharging.
In the case of glass substrates, the electrical contact to the negative a-Si electrode on the left-hand side was provided by a thin conductive oxide (TCO) layer, which is part of the commercial Asahi U-type substrate. The Asahi-U TCO substrate mainly consists of tin oxide and exhibits a textured surface, which improves the adhesion of the a-Si electrode. In the case of flexible stainless-steel foils, the metal directly contacts the aSi electrode. A low-concentration alkaline solution of potassium hydroxide (KOH) or sodium hydroxide (NaOH) served as electrolyte. The right-hand side of Figure 1 a shows a commercially available air electrode, serving as positive electrode of the battery. The air electrode comprises a polytetrafluoroethylene (PTFE) powder and carbon-black-loaded structure, with manganese dioxide as catalyst and pressed onto a nickel 200 mesh (strands per inch). A PTFE microporous layer faces the air side of the electrode, and a separator is attached to the electrolyte side. The complete air electrode was supplied by Electric Fuel, Inc. The active layers of our Si–air batteries were grown by plasma-enhanced chemical vapor deposition (PECVD) at a deposition temperature Tdep = 170 8C from silane (SiH4), methane (CH4), and phosphine (2 % PH3 in SiH4) as feedstock gases. A deposition rate of 10 nm min1 yielded 500 nm thick n-type aChemSusChem 2014, 7, 3272 – 3274
3272
CHEMSUSCHEM COMMUNICATIONS
www.chemsuschem.org
Si and amorphous silicon carbide (a-SiC) layers on the TCO contacts. The active area of the battery cells was 5.7 cm2, defined by the diameter of rubber seals as indicated in Figure 1 a. The discharging experiments analyzed batteries with different electrolyte concentrations and discharge current densities, and were carried out using a Keithley 2400 Series Sourcemeter in bias current mode.[12] In contrast to silicon-wafer-based Si-air batteries with an alkaline electrolyte, our amorphous silicon electrodes fully dissolve during discharging of the battery cell, without any surface modification prior to battery operation. The wafer-based porous silicon electrodes form a closed SiO2 layer at the surface, which stops the battery discharge after a few minutes.[10] Two competing electrochemical reactions take place in the Si–air battery cell: i) the battery discharge reaction, driving the electrical current flow between the electrodes; and ii) self-discharge. The discharge process proceeds according to Anode : Si þ 4 OH ! SiðOHÞ4 þ 4 e
ð1Þ
Cathode : O2 þ 2 H2 O þ 4 e ! 4 OH
ð2Þ
while the overall self-discharge in alkaline electrolytes is described as[13] Si þ 2 OH þ 2 H2 O ! SiO2 ðOHÞ2 2 þ 2 H2
ð3Þ
Similar to wafer-based Si–air cells, both reaction pathways finally dissolve the silicon anode in the electrolyte. In contrast to plain crystalline Si anodes,[10] no insoluble SiO2 forms at the surface of our thin film electrodes. Figure 1 b presents a photograph of the a-Si electrode after a full discharging process, indicating that most of the a-Si layer dissolved during the discharge. Figure 2 shows discharge curves of the a-Si and a-SiC anodes with different electrolytes, electrolyte concentrations, and discharge current densities. In order to etch off any native oxide, all electrode samples underwent etching in a 1 % HF solution before battery assembly. Table 1 gives detailed experimental data for discharge curves A–G displayed in Figure 2. The specific capacity for the different discharge experiments of Figure 2 is calculated by assuming the mass of the 500 nm a-Si or a-SiC films to be 105 mg cm2. This mass of the deposited silicon films derives from an a-Si or a-SiC density of 2.1 g cm3.[14] A specific capaci-
Figure 2. Discharge curves of thin-film Si–air batteries with anodes made of a-Si (battery cells A to E) and a-SiC (batteries F, G). The discharge current density jd increases from A to E. Table 1 presents details on the variation of the alkaline electrolytes, electrode materials, and the resulting specific capacities.
ty of 269 Ah kg1 is reached with a-SiC, 0.01 m KOH electrolyte concentration, and jd = 7.9 mA cm2. Comparing samples B and D of Table 1 shows very similar results for KOH and NaOH electrolytes. The performance of anode C on flexible steel foil compares well with the corresponding rigid anode B. As evident from Equations (1) to (3), only part of the silicon is used as battery fuel. Quantitative evaluation of Equations (1) and (2) yields an a-Si consumption rate of 0.17 nm min1 for jd = 7.9 mA cm2, while the total experimental silicon consumption amounts to 2.95 nm min1 for battery B of Table 1, indicating that only 5.5 at % of the a-Si anode serves as battery fuel in our experiments. This finding is in agreement with a separate experiment that completely dissolved a similar a-Si film in 0.01 m KOH, and thereby yielded a corrosion rate of 2.6 0.1 nm min1. Hence, self-discharge clearly limits the specific capacity of our first thin-film Si-air batteries. The batteries F and G of Table 1 exhibit the largest specific capacity, which we attribute to lower self-discharge. Quantitative evaluation yields a 7 at % silicon usage of the a-SiC anodes F and G, in contrast to the 5.5 at % yield of the a-Si anodes A to E. The most promising features of silicon thin-film electrodes arise from the use of ultrathin flexible substrates and low-temperature deposition. Figure 3 a shows an example of a n-type a-Si anode deposited on a 100 mm thick flexible stainless steel
Table 1. Parameters and performances of thin-film Si–air batteries at varying discharge current densities (jd). The anodes of the batteries A, B, D–G are deposited on conducting Asahi-U, while battery C uses stainless steel (ss) as substrate and electrical contact. Battery
Anode material (n-type)
Source gas flow (SiH4/PH3/CH4) [sccm]
Anode resistivity [W cm]
jd [mA cm2]
Electrolyte
Electrolyte concentration [mol L1]
Specific capacity [Ah kg1]
A B C D E F G
a-Si a-Si a-Si a-Si a-Si a-SiC a-SiC
3:3:0 3:3:0 3:3:0 3:3:0 3:3:0 3:3:1 3:3:3
440 440 440 440 440 610 3125
1.6 7.9 7.9 7.9 31.4 7.9 7.9
KOH KOH KOH NaOH KOH KOH KOH
0.01 0.01 0.01 0.01 0.01 0.01 0.01
77 209 219 203 150 269 262
2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemSusChem 2014, 7, 3272 – 3274
3273
CHEMSUSCHEM COMMUNICATIONS
www.chemsuschem.org Acknowledgements The authors gratefully acknowledge fruitful discussions with N. Ohmer (Max Planck Institute for Solid State Research, Stuttgart, Germany). We owe special thanks to our colleagues S. Vollmer and L. Beisel for their skillful support of the experiments.
Figure 3. a) Thin-film a-Si layer on a flexible sheet of stainless steel. Its discharge process is shown in Figure 2 as process “C/ss”. b) Schematic of a rolled silicon–air battery. The battery consists of thin film of silicon on a flexible substrate, an electrolyte as gel or paste, an air electrode, and a foam layer to allow air to diffuse to an air cathode.
foil. Discharging this film with jd = 7.9 mA cm2 in presence of 0.01 m KOH electrolyte yields trace C in Figure 2, with a specific capacity of 219 Ah kg1. Figure 3 b sketches a rolled Si–air battery with a silicon thin-film anode deposited by web-coating. The battery cell is completed by a gel or paste electrolyte, an air electrode, and a foam layer enabling diffusion of air to the cathode. Moreover, with the help of patterning during deposition[15] or post-deposition structuring of the silicon thin films, single cells are connected in series and form a high-voltage battery stack. Amorphous silicon and silicon carbide films demonstrate high potential as fuel for silicon–air batteries. Our first experiments demonstrate a specific capacity of 269 Ah kg1 at an operating voltage of 0.85 V and jd = 7.9 mA cm2. At present, the performance of these silicon thin-film batteries suffers from high self-discharge rates. Ongoing work will reduce the selfdischarge and enhance specific capacity by using electrolytes such as EMI·2.3 HF·F, room-temperature ionic liquids (RTILs), and further improvement of the silicon anodes. Thin film Si–air batteries easily integrate with flexible electronic devices or micro-electromechanical systems, and offer the potential of high specific capacity and energy density.
2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Keywords: amorphous materials electrochemistry · silicon · thin films
·
batteries
·
[1] S. J. Gerssen-Gondelach, A. P. C. Faaij, J. Power Sources 2012, 212, 111 – 129. [2] J. Chen, Materials 2013, 6, 156 – 183. [3] G. Girishkumar, B. McCloskey, A. C. Luntz, S. Swanson, W. Wilcke, J. Phys. Chem. Lett. 2010, 1, 2193 – 2203. [4] C. J. Lan, T. S. Chin, P. H. Lin, T. P. Perng, J. New Mater. Electrochem. Syst. 2006, 9, 27 – 32. [5] C. S. Li, W. Q. Ji, J. Chen, Z. L. Tao, Chem. Mater. 2007, 19, 5812 – 5814. [6] V. Neburchilov, H. J. Wang, J. J. Martin, W. Qu, J. Power Sources 2010, 195, 1271 – 1291. [7] A. A. Mohamad, Corros. Sci. 2008, 50, 3475 – 3479. [8] G. Cohn, D. Starosvetsky, R. Hagiwara, D. D. Macdonald, Y. Ein-Eli, Electrochem. Commun. 2009, 11, 1916 – 1918. [9] G. Cohn, Y. Ein-Eli, J. Power Sources 2010, 195, 4963 – 4970. [10] X. Zhong, H. Zhang, Y. Liu, J. Bai, L. Liao, Y. Huang, X. Duan, ChemSusChem 2012, 5, 177 – 180. [11] http://www.electric-fuel.com/airelectrode/ (accessed September 2014). [12] http://www.keithley.com/products/dcac/voltagesource/broadpurpose/ ?mn = 2400/ (URL seems out of date, please check). [13] X. G. Zhang, Electrochemistry of Silicon and its Oxide, Springer Science & Business Media, 2001, pp. 294 – 298. [14] T. Haage, U. I. Schmidt, H. Fath, P. Hess, B. Schrçder, H. Oechsner, J. Appl. Phys. 1994, 76, 4894. [15] R. Merz, J. Kistner, M. A. Bouattour, M. B. Schubert, Phys. Status Solidi A 2010, 207, 686. Received: May 26, 2014 Published online on September 22, 2014
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