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The preparation and properties of a novel electrolyte of electrochemical double layer capacitors based on LiPF6 and acetamide
Electrochimica Acta 58 (2011) 330–335
Contents lists available at SciVerse ScienceDirect
Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
The preparation and properties of a novel electrolyte of electrochemical double layer capacitors based on LiPF6 and acetamide Qi Li, Xiaoxi Zuo, Jiansheng Liu, Xin Xiao, Dong Shu, Junmin Nan ∗ School of Chemistry and Environment, Key Lab of Electrochemical Technology on Energy Storage and Power Generation in Guangdong Universities, South China Normal University, Guangzhou 510006, China
a r t i c l e
i n f o
Article history: Received 6 December 2010 Received in revised form 8 September 2011 Accepted 20 September 2011 Available online 1 October 2011 Keywords: Electrochemical double layer capacitor Liquid electrolyte Preparation LiPF6 Acetamide
a b s t r a c t A novel electrolyte applied in electrochemical double-layer capacitors (EDLCs) has been prepared based on lithium hexafluorophosphate (LiPF6 ) and acetamide and subsequently characterized by differential scanning calorimetry (DSC), thermal gravimetric analysis (TGA), electrochemical techniques and so on. The mixtures of LiPF6 and acetamide at the molar ratios of 1:4 to 1:6 exist as liquids below 25 ◦ C, which is attributed to the melting point depression of mixture and the coordination of the polar groups (C O and NH groups) of acetamide with Li+ and PF6 − ions. The strong interaction between LiPF6 and acetamide results in the rupture of the electrovalent bond of LiPF6 and the breakage of hydrogen bonds among the acetamide molecules, leading to the formation of a liquid electrolyte. The LiPF6 /acetamide electrolyte with a molar ratio of 1:5.5 exhibits a 5.2 V electrochemical window and suitable ionic conductivity at room temperature. In particular, the coin-type cells with carbon electrodes and LiPF6 /acetamide electrolyte possess high thermal stability and electrochemical properties, showing that the as-prepared LiPF6 /acetamide electrolyte is a promising candidate for EDLCs. © 2011 Elsevier Ltd. All rights reserved.
1. Introduction Electrochemical double-layer capacitors (EDLCs) are promising electrochemical charge-storage devices that accumulate electric charges at the interface of electrode (electronic conductor) and electrolyte (ionic conductor) to fill the gap existing between batteries and dielectric capacitors from the energy and power density viewpoints [1–3]. Besides the active materials used in two electrodes, the electrolyte with broad potential window is also crucial to the performances of EDLCs because the energy density of EDLCs is in direct proportion to the square of the cell voltage. Although organic electrolytes have relatively broad electrochemical windows in comparison with the acidic or basic aqueous electrolytes, their potential problems must also be emphasized due to their volatile, flammable, and toxic nature. Thus, the room-temperature ionic liquids or molten salts characterized by high ionic conductivity, large electrochemical windows, excellent thermal stability, nonvolatility, and nonflammability, have attracted enormous interest as electrolytes in EDLCs [4,5], especially for those simple, cheap, alternative complex systems [6]. McManis et al. [7] investigated the properties of ionic liquid electrolytes formed with amide and alkali-metal nitrates or
∗ Corresponding author. Tel.: +86 20 39310255; fax: +86 20 39310187. E-mail address: [email protected] (J. Nan). 0013-4686/$ – see front matter © 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2011.09.059
ammonium nitrates two decades ago. These systems possess high conductivity and large electrochemical windows, but they were unstable and prone to recrystallization. In the past ten years, the room-temperature ionic liquid or molten salt electrolytes based on LiX [X = N(SO2 CF3 )2 − , N(SO2 C2 F5 )2 − , CF3 SO3 − , ClO4 − , B(C2 O4 )2 − , I− ] and organic compounds with an acylamino group have been extensively investigated [6,8–19]. It was shown that most of these complex systems were liquid at room temperature in an appropriate mixing molar ratio range, except lithium bi(oxalate)borate/acetamide and LiI/acetamide complex systems [8,9]. Especially, the complex systems based on LiN(SO2 CF3 )2 and organic compounds with an acylamino group were considered to be promising electrolytes for electrochemical devices due to their excellent thermal stability, low kinetic viscosity at room temperature, high ionic conductivity, and broad electrochemical stability. Wu and co-workers [6,10,11,17,18] have conducted many studies on the performances of the complex systems based on LiN(SO2 CF3 )2 and organic compounds with an acylamino group, and investigated their application in EDLCs and lithium-ion batteries as well. Chen and co-workers [13–16] not only investigated the electrochemical performances of this type of electrolyte in lithium-ion batteries and but also explored their liquid-formation mechanism using spectroscopic techniques and quantum chemistry calculation. However, Song et al. [20] also reported that LiN(SO2 CF3 )2 was one of the most reactive salts, causing corrosion of the aluminum in battery electrolytes. Thus, the use of this salt must be cautioned and thoroughly considered before implementation, and the investigation of novel
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electrolytes matching the requirement of EDLCs development is still a hot topic. In fact, as an electrochemical charge-storage device used in room temperature, the preparation of room-temperature ionic liquid or molten salt electrolytes in EDLCs are also restricted by other factors. For example, the high oxidation capability of ClO4 − and the higher melting-point temperature of electrolyte with B(C2 O4 )2 − , will restrict the using of ClO4 − and B(C2 O4 )2 − in the electrolyte of EDLCs. Considering the popular use in lithium-ion batteries and the intrinsic properties of LiPF6 , especially for its high conductivity and voltage stability, and relatively little side effect to the environment, the preparation and properties of molten salt-like electrolytes based on LiPF6 and organic compounds with acylamino group including urea, acetamide, methylurea, 1,3-dimethylurea and 2,2,2-trifluoroacetamide have been investigated in our lab. A binary complex electrolyte with homogeneous liquid phase at room temperature was obtained when LiPF6 and acetamide were mixed in an appropriate molar ratio range. The acetamide precursor could decrease the melting point of the complex system due to its dipolar nature and ‘water-like’ physical properties [21]. In addition, the electrochemical performances of as-prepared electrolyte in the cell were also evaluated. In this paper, the preparation of LiPF6 /acetamide electrolyte and its application in a coin-type cell with carbon electrodes are presented. 2. Experimental 2.1. Preparation and characterization of LiPF6 /acetamide electrolytes LiPF6 (3M Inc., 99%) was dried under vacuum at 100 ◦ C for 24 h. Acetamide (Acros Inc., AP) was recrystallized with chloroform and then dried at 55 ◦ C for 24 h under vacuum. All the preparation of electrolytes and other nonaqueous operations were performed in a nitrogen-purified glove box, in which the moisture and oxygen content was maintained below 10 ppm and 20 ppm, respectively. The water contents in the complex electrolytes were determined by the Karl–Fischer titration method (DL37KF coulometer, Mettler Toledo). The thermal properties of the complex electrolytes were characterized on a differential scanning calorimeter (DSC; DSC200PC, NETZSCH) and thermal gravimetric analyzer (TGA; STA409PC, NETZSCH). For DSC testing, the aluminum pan containing approximately 10 mg of the sample was first cooled to approximately −100 ◦ C with liquid nitrogen and then heated to 100 ◦ C at a rate of 5 ◦ C min−1 . For TGA, the sample was heated from room temperature to 350 ◦ C at a rate of 5 ◦ C min−1 while continuous nitrogen flow around the sample was supplied during measurement to avoid exposing the hygroscopic samples to moisture. The infrared spectra of the samples were recorded on a Fourier transform infrared spectrometer (FTIR; IR200, Nicolet) between 4000 cm−1 and 400 cm−1 with a resolution of 2 cm−1 . The solid sample was mixed with dry KBr and pressed into a pellet, while a droplet of the liquid sample was spread on a dry CaF2 disk for the infrared (IR) spectroscopic measurements. The ionic conductivity measurement was carried out by using a conductance instrument (DDSJ-308A, Leici, China) in an electrochemical cell with Pt electrodes, and the cell constant was determined with a standard KCl solution (0.1 mol L−1 ) at 25 ◦ C. The kinetic viscosity measurement (C-FRFIV, Youlaibo, China) was carried out by using a standard measure method (GB/T 10247-2008). 2.2. Electrochemical performances of LiPF6 /acetamide electrolyte The electrochemical window of as-prepared electrolyte was measured by linear sweep voltammetry (LSV) on an
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electrochemical workstation (CHI650B, CH Instruments, Inc.) with glassy carbon (ϕ = 3 mm) as the working electrode at a scan rate of 0.5 mV s−1 at 25 ◦ C, lithium foil (99.9%) and platinum wire (ϕ = 0.1 mm) as the reference electrode and counter electrode, respectively. In addition, the performances of as-prepared electrolyte in cell were also evaluated using a coin-type cell with carbon electrodes. Circular-type carbon electrodes with a diameter of 15 mm and a thickness of 0.15 mm were prepared by coating the active materials onto aluminum current collector foil and subsequently drying and pressing operations. Carboxymethylcellulose (CMC) and styrene-butadiene rubber (SBR) were used as binders. The active layer composition was 85 wt% activated carbon (purchased from Senyuan Company, and has a BET surface area of 2188 m2 g−1 ), 2 wt% carbon black, 3 wt% CMC and 10 wt% SBR. Carbon/carbon coin-type cells were assembled with carbon electrodes and as-prepared electrolyte in the nitrogen-purified glove box. The two electrodes were separated with a 0.05 mm thick porous polymeric film separator that was provided by Tianci Company to constitute coin-type cell. The electrochemical performances of coin-type cell were evaluated by CV and electrochemical impedance spectroscopy (EIS) using the CHI650B electrochemical workstation and by galvanostatic cycling using a computer-controlled battery test system (Land, China).
3. Results and discussion 3.1. Preparation and characterization of LiPF6 /acetamide electrolytes A homogeneous liquid phase was obtained at room temperature when LiPF6 and acetamide were mixed in an appropriate molar ratio range. The surface of LiPF6 particles became wet upon contacting acetamide particles, and then some liquid drops appeared on the container wall immediately when the mixture of LiPF6 and acetamide with a molar ratio between 1:4 and 1:6 was mechanically stirred at room temperature. The transparent and homogeneous solutions were formed after 10 min of stirring. The even mixtures of LiPF6 /acetamide with other molar ratios could also be made after being heated to approximately 50 ◦ C and subsequently cooled to room temperature. Water content of less than 20 ppm in these solutions was determined by the Karl–Fischer titration method, showing liquid products were prepared through mixing LiPF6 and acetamide at room temperature. DSC was performed to explore the thermal properties of the mixtures of LiPF6 and acetamide. It was observed that all DSC diagrams of the mixtures have an endothermic peak at approximately −52 ◦ C. The typical DSC diagrams of LiPF6 /acetamide mixtures with different molar ratios are shown in Fig. 1. The DSC curves of LiPF6 /acetamide with molar ratios of 1:3.5 (curve a) and 1:6.0 (curve c) have an endothermic peak at −55.43 ◦ C and −50.43 ◦ C, respectively. In addition, both curve a and curve c have an additional endothermic peaks at 36.82 ◦ C and 14.74 ◦ C, which is related to the liquidus temperature (Tl ). Whereas, the DSC curve of LiPF6 /acetamide with a molar ratio of 1:5.5 (curve b) only exhibits one endothermic peak at −51.91 ◦ C, showing the eutectic temperature (Te ) to be −51.91 ◦ C. Te and Tl of LiPF6 /acetamide mixtures with different molar ratios determined from the DSC tests are summarized and presented in a preliminary phase diagram shown in Fig. 2. Furthermore, most of the DSC curves show that an exothermic peak, which is the devitrification temperature (Td ), existed in similar systems [6,11]. The phenomena observed during the sample preparation and the DSC results indicate that a eutectic system can be formed by mixing LiPF6 with acetamide. The eutectic temperature of LiPF6 /acetamide complex system at approximately −52 ◦ C is also remarkably lower than the melting
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Fig. 1. The DSC curves of LiPF6 /acetamide mixtures at molar ratios of (a) 1:3.5, (b) 1:5.5, and (c) 1:6. Te , Td and Tl are the eutectic temperature, devitrification temperature, and liquidus temperature, respectively.
Fig. 2. The liquid–solid phase diagram of LiPF6 /acetamide mixtures.
the IR spectra of LiPF6 /acetamide mixtures with different molar ratios between 1500 and 1800 cm−1 . The band at 1672 cm−1 is assigned to the C O stretching mode. After the introduction of LiPF6 , two obvious spectral changes for C O vibrations of acetamide are observed, while no obvious changes can be seen in the rest region. One change is the broadening of the band at 1672 cm−1 and its subsequent red-shift to 1666 cm−1 , the other one is a gradual enhancement of a weak band located at 1618 cm−1 and its red-shift to 1612 cm−1 as the molar ratio of LiPF6 increases. The possible reasons of the band broadening and the red-shift are all attributed to an intense interaction between LiPF6 and acetamide. Thereinto, the O atoms in the C O group of acetamide have a tendency to coordinate with the Li+ cations because O atoms are negatively charged and Li+ cations have high capability of adsorbing electron. The hydrogen bonding (N–H· · ·O) in acetamide is a central factor for acetamide existing in solid form at room temperature, while the interaction with LiPF6 weakens or even breaks the hydrogen bonding due to the competitive Li+ –O interaction in the concentrated complex. At the same time, the ionic bond in LiPF6 is a weakening process of reciprocity between LiPF6 and acetamide. The possible process of reciprocity between LiPF6 and acetamide is shown in Fig. 4. TGA was performed to explore the thermal stability of LiPF6 /acetamide mixtures. Fig. 5 shows the TG diagrams of the LiPF6 /acetamide mixtures with different molar ratios. A relatively large weight loss can be observed for LiPF6 from 70 ◦ C, which is ascribed to the formation of HF and other volatile resultants in LiPF6 due to the water adsorbed during the operation and the water left in LiPF6 . However, compared to individual LiPF6 and acetamide, these LiPF6 /acetamide complexes exhibit higher thermal stability, revealing the interaction of LiPF6 and acetamide and the formation of complexes. In particular, higher thermal stability than 150 ◦ C is obtained for all LiPF6 /acetamide complexes, showing the possibility of applying them in EDLCs as electrolytes at high temperature. The kinetic viscosity and ionic conductivity of the LiPF6 /acetamide complexes with molar ratios between 1:4 and 1:6 at different temperatures were evaluated. The temperaturedependent kinetic viscosity of the LiPF6 /acetamide complexes with different molar ratios is shown in Fig. 6. The LiPF6 /acetamide complexes exhibit a higher kinetic viscosity when more acetamide is contained in the mixtures at room temperature, i.e. 30 ◦ C. However, the kinetic viscosity decreases gradually as the temperature is raised from 30 ◦ C to 70 ◦ C, and the difference is synchronously reduced from 19.36 mm2 s−1 to 10.29 mm2 s−1 at 70 ◦ C. As shown in Fig. 7, corresponding with the increases in temperature, the ionic conductivity of LiPF6 /acetamide complexes with different molar ratios has a reverse direction in comparison with the kinetic viscosity, and the LiPF6 /acetamide complex with a molar ratio of 1:5.5 exhibits the highest conductivity. Based on these results, it is considered that LiPF6 /acetamide complexes will be of interest for EDLCs due to their low kinetic viscosity and high ionic conductivity, which is in favor of the rate capacity of EDLCs. Because the LiPF6 /acetamide complex with a molar ratio of 1:5.5 possesses a eutectic temperature and higher conductivity, its electrochemical properties were evaluated in the coin-type cell. 3.2. Electrochemical performances of LiPF6 /acetamide electrolyte
Fig. 3. The FT-IR spectra of LiPF6 /acetamide mixtures with molar ratio of (a) 1:3.5, (b) 1:4, (c) 1:4.5, (d) 1:5, (e) 1:5:5, (f) 1:6, and (g) 1:7, and the FT-IR spectrum of acetamide.
points of LiPF6 and acetamide, i.e. 200 ◦ C and 81 ◦ C. The low eutectic temperature is attributed to the melting point depression of mixture and the coordination of the polar groups (C O and NH groups) of acetamide with Li+ and PF6 − ions. The interaction between LiPF6 and acetamide was characterized by FTIR spectroscopy. Fig. 3 shows
The electrolyte in EDLCs with a wide potential window allows the charge–discharge operating at a high cell voltage, and thus, high energy densities can be obtained because it is in direct proportion to the square of cell voltage [2]. An inert glass carbon electrode was used to evaluate the electrochemical window of electrolyte. From the LSV curve of glass carbon electrode shown in Fig. 8, an electrochemical window of more than 5.2 V can be obtained for LiPF6 /acetamide electrolyte (molar ratio is 1:5.5), Furthermore, it was also shown that the LSV curves of glass carbon electrode
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Fig. 4. The possible process of reciprocity in the mixture of LiPF6 and acetamide.
Fig. 5. The TG curves of LiPF6 /acetamide complexes with different molar ratios.
Fig. 7. The temperature-dependent ionic conductivity of LiPF6 /acetamide complexes with different molar ratios.
Fig. 6. The temperature-dependent kinetic viscosity of LiPF6 /acetamide complexes with different molar ratios.
Fig. 8. The linear sweep voltammogram of glassy carbon electrode in LiPF6 /acetamide (molar ratio 1:5.5) electrolyte with Li reference electrode and platinum counter electrode; the scan rate = 0.5 mV s−1 .
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Fig. 9. The cyclic voltammograms of the coin-type cell with carbon electrodes and LiPF6 /acetamide (1:5.5) electrolyte at different potential ranges; the scan rate is 1 mV s−1 .
measured in the LiPF6 /acetamide electrolytes with other molar ratios are similar to the curve in Fig. 8, suggesting that LiPF6 /acetamide complex is a promising electrolyte with wide potential window. Fig. 9 shows the cyclic voltammograms of the carbon/carbon coin-type cell with LiPF6 /acetamide (1:5.5) electrolyte at different potential ranges at a potential sweep rate of 1 mV s−1 . As indicated in curves 1, 2, 3 and 4 for the potential ranges of 0–1 V, 0–1.5 V, 0–2 V and 0–2.25 V, respectively, the rectangular shape and mirror-image (symmetric) characteristics, indicating capacitive behavior, appear in the potential range of 2.25 V, for which is double that found in traditional aqueous electrolytes. In particular, the CV response current remained almost constant during the forward and backward scans but immediately changes the flow direction when the potential was switched to the reverse at the potential range within 0–2.25 V. Correspondingly, an oxidation current was observed when the cutoff charge voltage increased more than 2.25 V (curves 5 and 6). Therefore, the working voltage of the cell can reach 2.25 V, which is lower than the above-determined electrochemical window on glass carbon electrode. Similar observations have been reported previously [17,22]. The main reason is that the electrodes of the coin-type cell are composed of porous carbon material with various active functional groups on the surface layer, while the electrochemical window of electrolytes are usually determined on inert electrodes, such as glassy carbon and platinum, and with inert counter-electrode and lithium reference electrode as well. These results clearly indicate that ideal capacitive behavior and great reversibility can be achieved when the as-prepared LiPF6 /acetamide complex is used as electrolyte in the cell. The specific capacitance C of the coin-type cell was calculated using the following equation [2]: C=
Fig. 10. The coulomb efficiency vs. cycling numbers of the coin-type cell with carbon electrodes and LiPF6 /acetamide (1:5.5) electrolyte charge-discharged at 0.1 mA cm−2 .
capacity decay of cell is nearly zero after 5000 cycles at 0.1 mA cm−2 charge–discharge current, showing that excellent capacity retention can be obtained for the cell with the LiPF6 /acetamide (1:5.5) electrolyte. The effect of temperature on the performances of coin-type cell was also investigated using the electrochemical impedance spectroscopy technique. Fig. 11 shows the Nyquist plots of the coin-type cell with LiPF6 /acetamide (1:5.5) electrolyte between 10 mHz and 10 kHz at different temperatures. The analog circuit is also shown in Fig. 11 as an inset image, where Rs is the ohm resistance, Rf and Cf are the reaction resistance and capacitance, respectively, Cd is the double electric layer capacitance on the interface of carbon and electrolyte. At high frequencies, the ohm resistance including electrolytic and circuit resistances can be obtained, whereas the ion migration inside the porous active material of the electrodes can be deduced from the EIS curves when the frequency is decreased. At low frequencies, as represented by a nearly vertical line, the imaginary part of the impedance increases, showing its capacitive behavior [23]. The Nyquist plot at 30 ◦ C still has a sloping (approximately 45◦ ) linear region at a high-to-medium frequency, indicating that diffusion control is the limiting factor in the kinetics of the electrode process because of the large diffusion resistance [24]. The slope
Qm V
where Qm is the specific voltammetric charge (based on weight) integrated from the CV sweep, and V is the potential scanning range. The calculated specific capacitance of the coin-type cell is 87.1 F g−1 , which is a half of the single electrode capacity, in the potential range of 0–2.25 V at 1 mV s−1 . Fig. 10 shows the typical cycling performance of the coin-type cell using the LiPF6 /acetamide (1:5.5) electrolyte between 0 and 2.25 V at room temperature. At the first cycle, the low coulomb efficiency, i.e. 55%, is mainly attributed to the reaction of functional groups and formation of surface layer in activated carbon material, which can be validated by the experiment using glass carbon electrode as working electrode. After the formation of a stable surface layer on carbon material, the
Fig. 11. The Nyquist plots of the coin-type cell with carbon electrodes and LiPF6 /acetamide (1:5.5) electrolyte at (a) 30 ◦ C, (b) 40 ◦ C, (c) 50 ◦ C, (d) 60 ◦ C, and (e) 70 ◦ C. The inset image is the analog circuit of the electrochemical impedance spectroscopy.
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increases and the length of the 45◦ region decreases in the Nyquist plot as the temperature increases to 40 ◦ C. When the temperature increases to 70 ◦ C, the sloping linear region becomes hardly recognizable. The equivalent series resistance (ESR) is also a function of the temperature, which decreases from 38.9 at 30 ◦ C to 10.3 at 70 ◦ C. The decrease in the diffusion resistance and ESR with increasing temperature, beneficial for obtaining high-power density, can be attributed to the increasing ionic conductivity and decreasing kinetic viscosity of the electrolyte (see Figs. 6 and 7). Especially, as the temperature increases, the specific capacitance of the cell increases, while the equivalent series resistance decreases, showing a high energy density and power density can be obtained at high temperatures. In contrast, the performances of cell with conventional organic electrolytes will be severely deteriorated and may result in safety concerns at such high temperatures due to the application of volatile and flammable organic solvents. Thus, the asprepared LiPF6 /acetamide electrolyte is considered to be superior in comparison with the conventional aqueous or organic electrolytes, especially at high temperatures. 4. Conclusions A novel LiPF6 /acetamide electrolyte suitable for EDLCs has been prepared and characterized. The as-prepared LiPF6 /acetamide electrolytes have an eutectic temperature at approximately −52 ◦ C and exist as a liquid at room temperature between the molar ratios of 1:4 and 1:6, which is attributed to the melting point depression of mixture and the coordination of the polar groups (C O and NH groups) of acetamide with Li+ and PF6 − ions. Because of the interaction of the acetamide with Li+ cations and PF6 − anions, the electrovalent bond of LiPF6 and the hydrogen bonds among acetamide molecules become weak or even break, leading to the formation of a room-temperature molten salt. In addition, LiPF6 /acetamide electrolytes have an electrochemical window of higher than 5.2 V and possess high ionic conductivity and low kinetic viscosity at high temperature. This LiPF6 /acetamide electrolyte shows good compatibility in the coin-type cell with carbon electrodes, suggesting it is a promising electrolyte of EDLCs.
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Acknowledgements This study was financially supported by the cooperation project in industry, education and research of Guangdong Province and Ministry of Education of PR China (Grant No. 2009B090300389), the natural science foundation of Guangdong Province (Grant No. S2011010003416), and the Science and Technology Project of Huangpu District of Guangzhou. References [1] B.E. Conway, Electrochemical Supercapacitors: Scientific Fundamentals and Technological Applications, Kluwer Academic Publishers, New York, 1999. [2] J.K. Chang, M.T. Lee, C.W. Cheng, W.T. Tsai, M.J. Deng, I.W. Sun, Electrochem. Solid-State Lett. 12 (2009) A19. [3] A. Nishino, K. Naoi (Eds.), Technologies & Materials for EDLC, CMC, Tokyo, 1998. [4] H. Ohno, Electrochemical Aspects of Ionic Liquids, John Wiley & Sons, Hoboken, New Jersey, 2005. [5] J.F. Huang, H. Luo, C.D. Liang, I.W. Sun, G.A. Baker, S. Dai, J. Am. Chem. Soc. 127 (2005) 12784. [6] R.J. Chen, F. Wu, H.Y. Liang, L. Li, B. Xu, J. Electrochem. Soc. 152 (2005) A1979. [7] G.E. McManis, A.N. Fletcher, D.E. Bliss, J. Electroanal. Chem. 190 (1985) 171. [8] Z.X. Yu, H. Li, K.X. Li, D. Qin, M.H. Deng, D.G. Li, Y.H. Luo, Q.B. Meng, L.Q. Chen, Electrochim. Acta 55 (2010) 895. [9] B. Xie, L.F. Li, H. Li, L.Q. Chen, Solid State Ionics 180 (2009) 688. [10] B. Xu, F. Wu, R.J. Chen, G.P. Cao, S. Chen, Y.S. Yang, J. Power Sources 195 (2010) 2118. [11] R.J. Chen, F. Wu, L. Li, B. Xu, X.P. Qiu, S. Chen, J. Phys. Chem. C 111 (2007) 5184. [12] H.Y. Liang, H. Li, Z.X. Wang, F. Wu, L.Q. Chen, X.J. Huang, J. Phys. Chem. B 105 (2001) 9966. [13] Y.S. Hu, H. Li, X.J. Huang, L.Q. Chen, Electrochem. Commun. 6 (2004) 28. [14] Y.S. Hu, Z.X. Wang, H. Li, X.J. Huang, L.Q. Chen, Spectrochim. Acta A 61 (2005) 403. [15] Y.S. Hu, Z.X. Wang, X.J. Huang, L.Q. Chen, Solid State Ionics 175 (2004) 277. [16] Y.S. Hu, Z.X. Wang, H. Li, X.J. Huang, L.Q. Chen, Spectrochim. Acta A 61 (2005) 2009. [17] B. Xu, F. Wu, R.J. Chen, G.P. Cao, S. Chen, G.Q. Wang, Y.S. Yang, J. Power Sources 158 (2006) 773. [18] R.J. Chen, F. Wu, B. Xu, L. Li, X.P. Qiu, S. Chen, J. Electrochem. Soc. 154 (2007) A703. [19] R.J. Chen, F. Wu, Acta Phys. Chim. Sin. 21 (2005) 177. [20] S.W. Song, T.J. Richardson, G.R.V. Zhuang, T.M. Devine, J.W. Evans, Electrochim. Acta 49 (2004) 1483. [21] D.H. Kerridge, Chem. Soc. Rev. 17 (1988) 181. [22] A. Lewandowski, M. Galinski, J. Phys. Chem. Solids 65 (2004) 281. [23] R.D. Levie, Electrochim. Acta 8 (1963) 751. [24] K. Okajima, K. Ohta, M. Sudoh, Electrochim. Acta 50 (2005) 2227.
Contents lists available at SciVerse ScienceDirect
Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
The preparation and properties of a novel electrolyte of electrochemical double layer capacitors based on LiPF6 and acetamide Qi Li, Xiaoxi Zuo, Jiansheng Liu, Xin Xiao, Dong Shu, Junmin Nan ∗ School of Chemistry and Environment, Key Lab of Electrochemical Technology on Energy Storage and Power Generation in Guangdong Universities, South China Normal University, Guangzhou 510006, China
a r t i c l e
i n f o
Article history: Received 6 December 2010 Received in revised form 8 September 2011 Accepted 20 September 2011 Available online 1 October 2011 Keywords: Electrochemical double layer capacitor Liquid electrolyte Preparation LiPF6 Acetamide
a b s t r a c t A novel electrolyte applied in electrochemical double-layer capacitors (EDLCs) has been prepared based on lithium hexafluorophosphate (LiPF6 ) and acetamide and subsequently characterized by differential scanning calorimetry (DSC), thermal gravimetric analysis (TGA), electrochemical techniques and so on. The mixtures of LiPF6 and acetamide at the molar ratios of 1:4 to 1:6 exist as liquids below 25 ◦ C, which is attributed to the melting point depression of mixture and the coordination of the polar groups (C O and NH groups) of acetamide with Li+ and PF6 − ions. The strong interaction between LiPF6 and acetamide results in the rupture of the electrovalent bond of LiPF6 and the breakage of hydrogen bonds among the acetamide molecules, leading to the formation of a liquid electrolyte. The LiPF6 /acetamide electrolyte with a molar ratio of 1:5.5 exhibits a 5.2 V electrochemical window and suitable ionic conductivity at room temperature. In particular, the coin-type cells with carbon electrodes and LiPF6 /acetamide electrolyte possess high thermal stability and electrochemical properties, showing that the as-prepared LiPF6 /acetamide electrolyte is a promising candidate for EDLCs. © 2011 Elsevier Ltd. All rights reserved.
1. Introduction Electrochemical double-layer capacitors (EDLCs) are promising electrochemical charge-storage devices that accumulate electric charges at the interface of electrode (electronic conductor) and electrolyte (ionic conductor) to fill the gap existing between batteries and dielectric capacitors from the energy and power density viewpoints [1–3]. Besides the active materials used in two electrodes, the electrolyte with broad potential window is also crucial to the performances of EDLCs because the energy density of EDLCs is in direct proportion to the square of the cell voltage. Although organic electrolytes have relatively broad electrochemical windows in comparison with the acidic or basic aqueous electrolytes, their potential problems must also be emphasized due to their volatile, flammable, and toxic nature. Thus, the room-temperature ionic liquids or molten salts characterized by high ionic conductivity, large electrochemical windows, excellent thermal stability, nonvolatility, and nonflammability, have attracted enormous interest as electrolytes in EDLCs [4,5], especially for those simple, cheap, alternative complex systems [6]. McManis et al. [7] investigated the properties of ionic liquid electrolytes formed with amide and alkali-metal nitrates or
∗ Corresponding author. Tel.: +86 20 39310255; fax: +86 20 39310187. E-mail address: [email protected] (J. Nan). 0013-4686/$ – see front matter © 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2011.09.059
ammonium nitrates two decades ago. These systems possess high conductivity and large electrochemical windows, but they were unstable and prone to recrystallization. In the past ten years, the room-temperature ionic liquid or molten salt electrolytes based on LiX [X = N(SO2 CF3 )2 − , N(SO2 C2 F5 )2 − , CF3 SO3 − , ClO4 − , B(C2 O4 )2 − , I− ] and organic compounds with an acylamino group have been extensively investigated [6,8–19]. It was shown that most of these complex systems were liquid at room temperature in an appropriate mixing molar ratio range, except lithium bi(oxalate)borate/acetamide and LiI/acetamide complex systems [8,9]. Especially, the complex systems based on LiN(SO2 CF3 )2 and organic compounds with an acylamino group were considered to be promising electrolytes for electrochemical devices due to their excellent thermal stability, low kinetic viscosity at room temperature, high ionic conductivity, and broad electrochemical stability. Wu and co-workers [6,10,11,17,18] have conducted many studies on the performances of the complex systems based on LiN(SO2 CF3 )2 and organic compounds with an acylamino group, and investigated their application in EDLCs and lithium-ion batteries as well. Chen and co-workers [13–16] not only investigated the electrochemical performances of this type of electrolyte in lithium-ion batteries and but also explored their liquid-formation mechanism using spectroscopic techniques and quantum chemistry calculation. However, Song et al. [20] also reported that LiN(SO2 CF3 )2 was one of the most reactive salts, causing corrosion of the aluminum in battery electrolytes. Thus, the use of this salt must be cautioned and thoroughly considered before implementation, and the investigation of novel
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electrolytes matching the requirement of EDLCs development is still a hot topic. In fact, as an electrochemical charge-storage device used in room temperature, the preparation of room-temperature ionic liquid or molten salt electrolytes in EDLCs are also restricted by other factors. For example, the high oxidation capability of ClO4 − and the higher melting-point temperature of electrolyte with B(C2 O4 )2 − , will restrict the using of ClO4 − and B(C2 O4 )2 − in the electrolyte of EDLCs. Considering the popular use in lithium-ion batteries and the intrinsic properties of LiPF6 , especially for its high conductivity and voltage stability, and relatively little side effect to the environment, the preparation and properties of molten salt-like electrolytes based on LiPF6 and organic compounds with acylamino group including urea, acetamide, methylurea, 1,3-dimethylurea and 2,2,2-trifluoroacetamide have been investigated in our lab. A binary complex electrolyte with homogeneous liquid phase at room temperature was obtained when LiPF6 and acetamide were mixed in an appropriate molar ratio range. The acetamide precursor could decrease the melting point of the complex system due to its dipolar nature and ‘water-like’ physical properties [21]. In addition, the electrochemical performances of as-prepared electrolyte in the cell were also evaluated. In this paper, the preparation of LiPF6 /acetamide electrolyte and its application in a coin-type cell with carbon electrodes are presented. 2. Experimental 2.1. Preparation and characterization of LiPF6 /acetamide electrolytes LiPF6 (3M Inc., 99%) was dried under vacuum at 100 ◦ C for 24 h. Acetamide (Acros Inc., AP) was recrystallized with chloroform and then dried at 55 ◦ C for 24 h under vacuum. All the preparation of electrolytes and other nonaqueous operations were performed in a nitrogen-purified glove box, in which the moisture and oxygen content was maintained below 10 ppm and 20 ppm, respectively. The water contents in the complex electrolytes were determined by the Karl–Fischer titration method (DL37KF coulometer, Mettler Toledo). The thermal properties of the complex electrolytes were characterized on a differential scanning calorimeter (DSC; DSC200PC, NETZSCH) and thermal gravimetric analyzer (TGA; STA409PC, NETZSCH). For DSC testing, the aluminum pan containing approximately 10 mg of the sample was first cooled to approximately −100 ◦ C with liquid nitrogen and then heated to 100 ◦ C at a rate of 5 ◦ C min−1 . For TGA, the sample was heated from room temperature to 350 ◦ C at a rate of 5 ◦ C min−1 while continuous nitrogen flow around the sample was supplied during measurement to avoid exposing the hygroscopic samples to moisture. The infrared spectra of the samples were recorded on a Fourier transform infrared spectrometer (FTIR; IR200, Nicolet) between 4000 cm−1 and 400 cm−1 with a resolution of 2 cm−1 . The solid sample was mixed with dry KBr and pressed into a pellet, while a droplet of the liquid sample was spread on a dry CaF2 disk for the infrared (IR) spectroscopic measurements. The ionic conductivity measurement was carried out by using a conductance instrument (DDSJ-308A, Leici, China) in an electrochemical cell with Pt electrodes, and the cell constant was determined with a standard KCl solution (0.1 mol L−1 ) at 25 ◦ C. The kinetic viscosity measurement (C-FRFIV, Youlaibo, China) was carried out by using a standard measure method (GB/T 10247-2008). 2.2. Electrochemical performances of LiPF6 /acetamide electrolyte The electrochemical window of as-prepared electrolyte was measured by linear sweep voltammetry (LSV) on an
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electrochemical workstation (CHI650B, CH Instruments, Inc.) with glassy carbon (ϕ = 3 mm) as the working electrode at a scan rate of 0.5 mV s−1 at 25 ◦ C, lithium foil (99.9%) and platinum wire (ϕ = 0.1 mm) as the reference electrode and counter electrode, respectively. In addition, the performances of as-prepared electrolyte in cell were also evaluated using a coin-type cell with carbon electrodes. Circular-type carbon electrodes with a diameter of 15 mm and a thickness of 0.15 mm were prepared by coating the active materials onto aluminum current collector foil and subsequently drying and pressing operations. Carboxymethylcellulose (CMC) and styrene-butadiene rubber (SBR) were used as binders. The active layer composition was 85 wt% activated carbon (purchased from Senyuan Company, and has a BET surface area of 2188 m2 g−1 ), 2 wt% carbon black, 3 wt% CMC and 10 wt% SBR. Carbon/carbon coin-type cells were assembled with carbon electrodes and as-prepared electrolyte in the nitrogen-purified glove box. The two electrodes were separated with a 0.05 mm thick porous polymeric film separator that was provided by Tianci Company to constitute coin-type cell. The electrochemical performances of coin-type cell were evaluated by CV and electrochemical impedance spectroscopy (EIS) using the CHI650B electrochemical workstation and by galvanostatic cycling using a computer-controlled battery test system (Land, China).
3. Results and discussion 3.1. Preparation and characterization of LiPF6 /acetamide electrolytes A homogeneous liquid phase was obtained at room temperature when LiPF6 and acetamide were mixed in an appropriate molar ratio range. The surface of LiPF6 particles became wet upon contacting acetamide particles, and then some liquid drops appeared on the container wall immediately when the mixture of LiPF6 and acetamide with a molar ratio between 1:4 and 1:6 was mechanically stirred at room temperature. The transparent and homogeneous solutions were formed after 10 min of stirring. The even mixtures of LiPF6 /acetamide with other molar ratios could also be made after being heated to approximately 50 ◦ C and subsequently cooled to room temperature. Water content of less than 20 ppm in these solutions was determined by the Karl–Fischer titration method, showing liquid products were prepared through mixing LiPF6 and acetamide at room temperature. DSC was performed to explore the thermal properties of the mixtures of LiPF6 and acetamide. It was observed that all DSC diagrams of the mixtures have an endothermic peak at approximately −52 ◦ C. The typical DSC diagrams of LiPF6 /acetamide mixtures with different molar ratios are shown in Fig. 1. The DSC curves of LiPF6 /acetamide with molar ratios of 1:3.5 (curve a) and 1:6.0 (curve c) have an endothermic peak at −55.43 ◦ C and −50.43 ◦ C, respectively. In addition, both curve a and curve c have an additional endothermic peaks at 36.82 ◦ C and 14.74 ◦ C, which is related to the liquidus temperature (Tl ). Whereas, the DSC curve of LiPF6 /acetamide with a molar ratio of 1:5.5 (curve b) only exhibits one endothermic peak at −51.91 ◦ C, showing the eutectic temperature (Te ) to be −51.91 ◦ C. Te and Tl of LiPF6 /acetamide mixtures with different molar ratios determined from the DSC tests are summarized and presented in a preliminary phase diagram shown in Fig. 2. Furthermore, most of the DSC curves show that an exothermic peak, which is the devitrification temperature (Td ), existed in similar systems [6,11]. The phenomena observed during the sample preparation and the DSC results indicate that a eutectic system can be formed by mixing LiPF6 with acetamide. The eutectic temperature of LiPF6 /acetamide complex system at approximately −52 ◦ C is also remarkably lower than the melting
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Fig. 1. The DSC curves of LiPF6 /acetamide mixtures at molar ratios of (a) 1:3.5, (b) 1:5.5, and (c) 1:6. Te , Td and Tl are the eutectic temperature, devitrification temperature, and liquidus temperature, respectively.
Fig. 2. The liquid–solid phase diagram of LiPF6 /acetamide mixtures.
the IR spectra of LiPF6 /acetamide mixtures with different molar ratios between 1500 and 1800 cm−1 . The band at 1672 cm−1 is assigned to the C O stretching mode. After the introduction of LiPF6 , two obvious spectral changes for C O vibrations of acetamide are observed, while no obvious changes can be seen in the rest region. One change is the broadening of the band at 1672 cm−1 and its subsequent red-shift to 1666 cm−1 , the other one is a gradual enhancement of a weak band located at 1618 cm−1 and its red-shift to 1612 cm−1 as the molar ratio of LiPF6 increases. The possible reasons of the band broadening and the red-shift are all attributed to an intense interaction between LiPF6 and acetamide. Thereinto, the O atoms in the C O group of acetamide have a tendency to coordinate with the Li+ cations because O atoms are negatively charged and Li+ cations have high capability of adsorbing electron. The hydrogen bonding (N–H· · ·O) in acetamide is a central factor for acetamide existing in solid form at room temperature, while the interaction with LiPF6 weakens or even breaks the hydrogen bonding due to the competitive Li+ –O interaction in the concentrated complex. At the same time, the ionic bond in LiPF6 is a weakening process of reciprocity between LiPF6 and acetamide. The possible process of reciprocity between LiPF6 and acetamide is shown in Fig. 4. TGA was performed to explore the thermal stability of LiPF6 /acetamide mixtures. Fig. 5 shows the TG diagrams of the LiPF6 /acetamide mixtures with different molar ratios. A relatively large weight loss can be observed for LiPF6 from 70 ◦ C, which is ascribed to the formation of HF and other volatile resultants in LiPF6 due to the water adsorbed during the operation and the water left in LiPF6 . However, compared to individual LiPF6 and acetamide, these LiPF6 /acetamide complexes exhibit higher thermal stability, revealing the interaction of LiPF6 and acetamide and the formation of complexes. In particular, higher thermal stability than 150 ◦ C is obtained for all LiPF6 /acetamide complexes, showing the possibility of applying them in EDLCs as electrolytes at high temperature. The kinetic viscosity and ionic conductivity of the LiPF6 /acetamide complexes with molar ratios between 1:4 and 1:6 at different temperatures were evaluated. The temperaturedependent kinetic viscosity of the LiPF6 /acetamide complexes with different molar ratios is shown in Fig. 6. The LiPF6 /acetamide complexes exhibit a higher kinetic viscosity when more acetamide is contained in the mixtures at room temperature, i.e. 30 ◦ C. However, the kinetic viscosity decreases gradually as the temperature is raised from 30 ◦ C to 70 ◦ C, and the difference is synchronously reduced from 19.36 mm2 s−1 to 10.29 mm2 s−1 at 70 ◦ C. As shown in Fig. 7, corresponding with the increases in temperature, the ionic conductivity of LiPF6 /acetamide complexes with different molar ratios has a reverse direction in comparison with the kinetic viscosity, and the LiPF6 /acetamide complex with a molar ratio of 1:5.5 exhibits the highest conductivity. Based on these results, it is considered that LiPF6 /acetamide complexes will be of interest for EDLCs due to their low kinetic viscosity and high ionic conductivity, which is in favor of the rate capacity of EDLCs. Because the LiPF6 /acetamide complex with a molar ratio of 1:5.5 possesses a eutectic temperature and higher conductivity, its electrochemical properties were evaluated in the coin-type cell. 3.2. Electrochemical performances of LiPF6 /acetamide electrolyte
Fig. 3. The FT-IR spectra of LiPF6 /acetamide mixtures with molar ratio of (a) 1:3.5, (b) 1:4, (c) 1:4.5, (d) 1:5, (e) 1:5:5, (f) 1:6, and (g) 1:7, and the FT-IR spectrum of acetamide.
points of LiPF6 and acetamide, i.e. 200 ◦ C and 81 ◦ C. The low eutectic temperature is attributed to the melting point depression of mixture and the coordination of the polar groups (C O and NH groups) of acetamide with Li+ and PF6 − ions. The interaction between LiPF6 and acetamide was characterized by FTIR spectroscopy. Fig. 3 shows
The electrolyte in EDLCs with a wide potential window allows the charge–discharge operating at a high cell voltage, and thus, high energy densities can be obtained because it is in direct proportion to the square of cell voltage [2]. An inert glass carbon electrode was used to evaluate the electrochemical window of electrolyte. From the LSV curve of glass carbon electrode shown in Fig. 8, an electrochemical window of more than 5.2 V can be obtained for LiPF6 /acetamide electrolyte (molar ratio is 1:5.5), Furthermore, it was also shown that the LSV curves of glass carbon electrode
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Fig. 4. The possible process of reciprocity in the mixture of LiPF6 and acetamide.
Fig. 5. The TG curves of LiPF6 /acetamide complexes with different molar ratios.
Fig. 7. The temperature-dependent ionic conductivity of LiPF6 /acetamide complexes with different molar ratios.
Fig. 6. The temperature-dependent kinetic viscosity of LiPF6 /acetamide complexes with different molar ratios.
Fig. 8. The linear sweep voltammogram of glassy carbon electrode in LiPF6 /acetamide (molar ratio 1:5.5) electrolyte with Li reference electrode and platinum counter electrode; the scan rate = 0.5 mV s−1 .
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Fig. 9. The cyclic voltammograms of the coin-type cell with carbon electrodes and LiPF6 /acetamide (1:5.5) electrolyte at different potential ranges; the scan rate is 1 mV s−1 .
measured in the LiPF6 /acetamide electrolytes with other molar ratios are similar to the curve in Fig. 8, suggesting that LiPF6 /acetamide complex is a promising electrolyte with wide potential window. Fig. 9 shows the cyclic voltammograms of the carbon/carbon coin-type cell with LiPF6 /acetamide (1:5.5) electrolyte at different potential ranges at a potential sweep rate of 1 mV s−1 . As indicated in curves 1, 2, 3 and 4 for the potential ranges of 0–1 V, 0–1.5 V, 0–2 V and 0–2.25 V, respectively, the rectangular shape and mirror-image (symmetric) characteristics, indicating capacitive behavior, appear in the potential range of 2.25 V, for which is double that found in traditional aqueous electrolytes. In particular, the CV response current remained almost constant during the forward and backward scans but immediately changes the flow direction when the potential was switched to the reverse at the potential range within 0–2.25 V. Correspondingly, an oxidation current was observed when the cutoff charge voltage increased more than 2.25 V (curves 5 and 6). Therefore, the working voltage of the cell can reach 2.25 V, which is lower than the above-determined electrochemical window on glass carbon electrode. Similar observations have been reported previously [17,22]. The main reason is that the electrodes of the coin-type cell are composed of porous carbon material with various active functional groups on the surface layer, while the electrochemical window of electrolytes are usually determined on inert electrodes, such as glassy carbon and platinum, and with inert counter-electrode and lithium reference electrode as well. These results clearly indicate that ideal capacitive behavior and great reversibility can be achieved when the as-prepared LiPF6 /acetamide complex is used as electrolyte in the cell. The specific capacitance C of the coin-type cell was calculated using the following equation [2]: C=
Fig. 10. The coulomb efficiency vs. cycling numbers of the coin-type cell with carbon electrodes and LiPF6 /acetamide (1:5.5) electrolyte charge-discharged at 0.1 mA cm−2 .
capacity decay of cell is nearly zero after 5000 cycles at 0.1 mA cm−2 charge–discharge current, showing that excellent capacity retention can be obtained for the cell with the LiPF6 /acetamide (1:5.5) electrolyte. The effect of temperature on the performances of coin-type cell was also investigated using the electrochemical impedance spectroscopy technique. Fig. 11 shows the Nyquist plots of the coin-type cell with LiPF6 /acetamide (1:5.5) electrolyte between 10 mHz and 10 kHz at different temperatures. The analog circuit is also shown in Fig. 11 as an inset image, where Rs is the ohm resistance, Rf and Cf are the reaction resistance and capacitance, respectively, Cd is the double electric layer capacitance on the interface of carbon and electrolyte. At high frequencies, the ohm resistance including electrolytic and circuit resistances can be obtained, whereas the ion migration inside the porous active material of the electrodes can be deduced from the EIS curves when the frequency is decreased. At low frequencies, as represented by a nearly vertical line, the imaginary part of the impedance increases, showing its capacitive behavior [23]. The Nyquist plot at 30 ◦ C still has a sloping (approximately 45◦ ) linear region at a high-to-medium frequency, indicating that diffusion control is the limiting factor in the kinetics of the electrode process because of the large diffusion resistance [24]. The slope
Qm V
where Qm is the specific voltammetric charge (based on weight) integrated from the CV sweep, and V is the potential scanning range. The calculated specific capacitance of the coin-type cell is 87.1 F g−1 , which is a half of the single electrode capacity, in the potential range of 0–2.25 V at 1 mV s−1 . Fig. 10 shows the typical cycling performance of the coin-type cell using the LiPF6 /acetamide (1:5.5) electrolyte between 0 and 2.25 V at room temperature. At the first cycle, the low coulomb efficiency, i.e. 55%, is mainly attributed to the reaction of functional groups and formation of surface layer in activated carbon material, which can be validated by the experiment using glass carbon electrode as working electrode. After the formation of a stable surface layer on carbon material, the
Fig. 11. The Nyquist plots of the coin-type cell with carbon electrodes and LiPF6 /acetamide (1:5.5) electrolyte at (a) 30 ◦ C, (b) 40 ◦ C, (c) 50 ◦ C, (d) 60 ◦ C, and (e) 70 ◦ C. The inset image is the analog circuit of the electrochemical impedance spectroscopy.
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increases and the length of the 45◦ region decreases in the Nyquist plot as the temperature increases to 40 ◦ C. When the temperature increases to 70 ◦ C, the sloping linear region becomes hardly recognizable. The equivalent series resistance (ESR) is also a function of the temperature, which decreases from 38.9 at 30 ◦ C to 10.3 at 70 ◦ C. The decrease in the diffusion resistance and ESR with increasing temperature, beneficial for obtaining high-power density, can be attributed to the increasing ionic conductivity and decreasing kinetic viscosity of the electrolyte (see Figs. 6 and 7). Especially, as the temperature increases, the specific capacitance of the cell increases, while the equivalent series resistance decreases, showing a high energy density and power density can be obtained at high temperatures. In contrast, the performances of cell with conventional organic electrolytes will be severely deteriorated and may result in safety concerns at such high temperatures due to the application of volatile and flammable organic solvents. Thus, the asprepared LiPF6 /acetamide electrolyte is considered to be superior in comparison with the conventional aqueous or organic electrolytes, especially at high temperatures. 4. Conclusions A novel LiPF6 /acetamide electrolyte suitable for EDLCs has been prepared and characterized. The as-prepared LiPF6 /acetamide electrolytes have an eutectic temperature at approximately −52 ◦ C and exist as a liquid at room temperature between the molar ratios of 1:4 and 1:6, which is attributed to the melting point depression of mixture and the coordination of the polar groups (C O and NH groups) of acetamide with Li+ and PF6 − ions. Because of the interaction of the acetamide with Li+ cations and PF6 − anions, the electrovalent bond of LiPF6 and the hydrogen bonds among acetamide molecules become weak or even break, leading to the formation of a room-temperature molten salt. In addition, LiPF6 /acetamide electrolytes have an electrochemical window of higher than 5.2 V and possess high ionic conductivity and low kinetic viscosity at high temperature. This LiPF6 /acetamide electrolyte shows good compatibility in the coin-type cell with carbon electrodes, suggesting it is a promising electrolyte of EDLCs.
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Acknowledgements This study was financially supported by the cooperation project in industry, education and research of Guangdong Province and Ministry of Education of PR China (Grant No. 2009B090300389), the natural science foundation of Guangdong Province (Grant No. S2011010003416), and the Science and Technology Project of Huangpu District of Guangzhou. References [1] B.E. Conway, Electrochemical Supercapacitors: Scientific Fundamentals and Technological Applications, Kluwer Academic Publishers, New York, 1999. [2] J.K. Chang, M.T. Lee, C.W. Cheng, W.T. Tsai, M.J. Deng, I.W. Sun, Electrochem. Solid-State Lett. 12 (2009) A19. [3] A. Nishino, K. Naoi (Eds.), Technologies & Materials for EDLC, CMC, Tokyo, 1998. [4] H. Ohno, Electrochemical Aspects of Ionic Liquids, John Wiley & Sons, Hoboken, New Jersey, 2005. [5] J.F. Huang, H. Luo, C.D. Liang, I.W. Sun, G.A. Baker, S. Dai, J. Am. Chem. Soc. 127 (2005) 12784. [6] R.J. Chen, F. Wu, H.Y. Liang, L. Li, B. Xu, J. Electrochem. Soc. 152 (2005) A1979. [7] G.E. McManis, A.N. Fletcher, D.E. Bliss, J. Electroanal. Chem. 190 (1985) 171. [8] Z.X. Yu, H. Li, K.X. Li, D. Qin, M.H. Deng, D.G. Li, Y.H. Luo, Q.B. Meng, L.Q. Chen, Electrochim. Acta 55 (2010) 895. [9] B. Xie, L.F. Li, H. Li, L.Q. Chen, Solid State Ionics 180 (2009) 688. [10] B. Xu, F. Wu, R.J. Chen, G.P. Cao, S. Chen, Y.S. Yang, J. Power Sources 195 (2010) 2118. [11] R.J. Chen, F. Wu, L. Li, B. Xu, X.P. Qiu, S. Chen, J. Phys. Chem. C 111 (2007) 5184. [12] H.Y. Liang, H. Li, Z.X. Wang, F. Wu, L.Q. Chen, X.J. Huang, J. Phys. Chem. B 105 (2001) 9966. [13] Y.S. Hu, H. Li, X.J. Huang, L.Q. Chen, Electrochem. Commun. 6 (2004) 28. [14] Y.S. Hu, Z.X. Wang, H. Li, X.J. Huang, L.Q. Chen, Spectrochim. Acta A 61 (2005) 403. [15] Y.S. Hu, Z.X. Wang, X.J. Huang, L.Q. Chen, Solid State Ionics 175 (2004) 277. [16] Y.S. Hu, Z.X. Wang, H. Li, X.J. Huang, L.Q. Chen, Spectrochim. Acta A 61 (2005) 2009. [17] B. Xu, F. Wu, R.J. Chen, G.P. Cao, S. Chen, G.Q. Wang, Y.S. Yang, J. Power Sources 158 (2006) 773. [18] R.J. Chen, F. Wu, B. Xu, L. Li, X.P. Qiu, S. Chen, J. Electrochem. Soc. 154 (2007) A703. [19] R.J. Chen, F. Wu, Acta Phys. Chim. Sin. 21 (2005) 177. [20] S.W. Song, T.J. Richardson, G.R.V. Zhuang, T.M. Devine, J.W. Evans, Electrochim. Acta 49 (2004) 1483. [21] D.H. Kerridge, Chem. Soc. Rev. 17 (1988) 181. [22] A. Lewandowski, M. Galinski, J. Phys. Chem. Solids 65 (2004) 281. [23] R.D. Levie, Electrochim. Acta 8 (1963) 751. [24] K. Okajima, K. Ohta, M. Sudoh, Electrochim. Acta 50 (2005) 2227.