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Conducting and interface characterization of carbonate-type organic electrolytes containing EMImBF4 as an additive against activated carbon electrode

  • Kim, Mingyeong (Department of Chemical and Biochemical Engineering, Pusan National University) ;
  • Kim, Kyungmin (Department of Chemical and Biochemical Engineering, Pusan National University) ;
  • Kim, Seok (Department of Chemical and Biochemical Engineering, Pusan National University)
  • Received : 2014.10.03
  • Accepted : 2014.11.12
  • Published : 2015.01.31
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Abstract

Carbonate-type organic electrolytes were prepared using propylene carbonate (PC) and dimethyl carbonate (DMC) as a solvent, quaternary ammonium salts, and by adding different contents of 1-ethyl-3-methyl imidazolium tetrafluoroborate ($EMImBF_4$). Cyclic voltammetry and linear sweep voltammetry were performed to analyze conducting behaviors. The surface characterizations were analyzed by scanning electron microscopy method and X-ray photoelectron spectroscopy. From the experimental results, increasing the $EMImBF_4$ content increased the ionic conductivity and reduced bulk resistance and interfacial resistance. In particular, after adding 15 vol% $EMImBF_4$ in 0.2 M $SBPBF_4$ PC/DMC electrolyte, the organic electrolyte showed superior capacitance and interfacial resistance. However, when $EMImBF_4$ content exceeded 15 vol%, the capacitance was saturated and the voltage range decreased.

Keywords

1. Introduction

Today, electric double layer capacitors (EDLCs), also called supercapacitors, play an important role in electrical energy storage systems, as well as in the power sources of controlled emission, electric or hybrid vehicles [1-3]. EDLCs are energy storage devices that collect energy between the electrode and electrolyte interface, and EDLCs have high power density and long cycle life [4-8]. However, their energy density is much lower than those of rechargeable batteries. Some studies have investigated methods to improve the energy density, including the use of non-aqueous solvents or ionic liquids (ILs) [9].

For the EDLCs electrolyte, an organic electrolyte, generally a solid quaternary ammonium salt dissolved in either acetonitrile (AN) or propylene carbonate (PC), has been used for EDLCs with voltages higher than 2 V [10]. On the other hand, in the case of salts, the most favorable electrolyte salt is tetraethyl ammonium tetrafluoroborate (TEABF4) [11-13]. It has good electrochemical stability but demonstrates a limited solubility in solvents. Spiro-1,1'-bipyrolidinium tetrafluoroborate (SBP-BF4), a spiro-type quaternary ammonium salt, has a low viscosity and a high solubility in various solvents. Furthermore, it shows excellent electrochemical stability and a high conductivity [14].

Recently, ILs have attracted much interest as ion conductive materials owing to their unusual properties, such as a wide voltage range, non-volatility, and non-flammability [15-19]. However, pure ILs have a viscosity higher than that of non-aqueous solvent, limiting their ionic diffusion. Increasing that ion conductivity and reducing the viscosity is important because improved ion mobility in the electrolyte would allow faster charge/discharge and better performance for the ILs based EDLCs [17-21].

Various salts have been reported for EDLCs, such as asymmetric ammonium and pyrrolidinium [18-20]. In addition, 1-ethyl-3methylimidazolium (EMIm) has been intensely studied because of its low viscosity and high conductivity [21-30].

In this paper, we prepared carbonate-type solvent as solvent, spiro-1,1'-bipyrolidinium tetrafluoroborate (SBP-BF4) as salt, and 1-ethyl-3-methyl imidazolium tetrafluoroborate (EMImBF4) as additive. In the present study, it was expected that EMImBF4 additive would increase the ion conductivity of the organic electrolyte. From this viewpoint, the effects of EMImBF4 additive concentrations on the structural and electrochemical behavior of the organic electrolyte were studied.

 

2. Experimental

2.1. Materials

SBP-BF4 (98%) was purchased from SK Chemical Co. SBP-BF4 was stored under glove box with dry Ar-atmosphere before use. N-Methyl-2-pyrrolidone (99%) was purchased from Junsei Chemical Co., Ltd. PC (99.7%) was acquired from Aldrich. Dimethylcarbonate (DMC, 99%) was acquired from Alfa Aesar. Carbon blacks (99%) were also purchased from Alfa Aesar. EMImBF4 and sodium carboxymethyl cellulose (CMC, average molecular weights of 1 × 104 [Aldrich] 1.5 × 105) were acquired from Aldrich.

2.2. Preparation of electrolytes and electrode

The electrolytes were prepared by dissolving the salts in PC, DMC. EMImBF4 was used without further purification. We used 0.2 M SBP-BF4 in PC/DMC (vol ratio = 1:1) solution, x vol% (x = 0, 10, 15, 20) EMImBF4 as electrolytes. The fabrication of the working electrodes was carried out as follows. First, activated carbon (MSP-20) and carbon blacks (super-p) were used as the active material and conductive agent in the electrode, respectively. And CMC and styrene-butadiene rubber (SBR) were used as binders. Slurries of 85:5:6:4 w/w/w/w active materials (MSP-20): carbon blacks (Super-P, Alfa Aesar): CMC (Aldrich): SBR were mixed with NMP solvent. Then the resulting slurry was coated on nickel foam substrate (1 × 1 cm) with a spatula, which was followed by drying at 100℃ for 12 h in a vacuum oven.

2.3. Characterization of electrolytes

All electrochemical tests were done in a three electrode system. A nickel foam coated with slurry, a platinum wire and Ag/Ag+ served as working, counter and reference electrodes, respectively. Ionic conductivity was investigated by using a conductivity meter (HORIBA ES-51). Electrochemical measurements were performed by an Iviumstat (Ivium Technologies, Netherlands). Cyclic voltammetry (CV) tests were analyzed between 0 and 1 V at different scan rates of 5 or 100 mV s-1. Also, the bulk resistance and interfacial resistance measurements of the various electrolytes were carried out by means of AC impedance spectroscopy over the frequency range from 120 kHz to 256 Hz. Linear sweep voltammetry was also investigated by using the above Ivium stat. The morphology of the electrode cycled in the prepared electrolytes was investigated by scanning electron microscopy (SEM, Hitachi S3500N). The surface characterization of electrodes cycled in the prepared electrolytes was investigated using energy dispersive X-ray spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS).

 

3. Results and Discussion

3.1. Ionic conductivity of electrolytes

Fig. 1 displays the effect of EMImBF4 content on the changes of the room temperature ionic conductivity of the organic electrolytes. The ionic conductivity increased sharply with EMImBF4 IL initial content. That means that charge carriers, which affect the ionic conductivity, were increased in the electrolytes because ions of EMImBF4 were added to the electrolytes. However, when EMImBF4 content exceeded 15 vol%, the ionic conductivity was not sharply changed because the viscosity of the electrolytes was increased. An increase in the ionic conductivity, as shown by the decreased semi-circle size and beginning plot, was achieved by adding EMImBF4 IL. The maximum ionic conductivity was achieved at a content of 20 vol% (72 mS/cm).

Fig. 1.Conductivity changes as a function of the EMImBF4 contents as vol% in propylene carbonate/dimethyl carbonate electrolyte with 0.2 M SBP-BF4.

3.2. Electrochemical property of electrolytes

Fig. 2 shows the impedance plots of the organic electrolytes as a function of EMImBF4 contents. The beginning point of the impedance plot, bulk resistance (meaning the resistance of the electrolytes), was decreased by increasing EMImBF4 content. But, for EMImBF4 amounts over 15 vol%, bulk resistance was not changed. It is suggested that this is probably related to a combination of increased viscosity and ions in the electrolytes. A decrease of the interfacial resistance between electrode and electrolyte, as shown by the decreased semi-circle sizes in the high frequency region, was achieved by adding EMImBF4. The minimum interfacial resistance achieved was 5.5 Ohm/cm at a content of 15 vol% (Table 1). When the EMImBF4 IL was increased over 15 vol%, the interfacial resistance barely changed from the minimum value. Instead, the interfacial resistance slightly increased. It can be concluded that the addition of the optimum content of EMImBF4 IL provided the most favorable environment for charge adsorption and desorption and, thereby, resulted in the highest capacitance. The charge transfer mobility at the interface between the electrode and the electrolyte was enhanced in comparison with those of the pristine electrolyte by adding EMImBF4, as a result of the beneficial features of the EMImBF4.

Fig. 2.Impedance spectra of organic electrolytes containing different contents of EMImBF4.

PC: propylene carbonate, DMC: dimethyl carbonate .

The electrochemical properties of the organic electrolytes were analyzed using CV curves. CV curves of the organic electrolytes are shown in Fig. 3. The calculated specific capacitance values of electrolytes are shown in Fig. 4. The capacitance of the organic electrolytes was compared to those of pristine 0.2 M SBP-BF4 PC/DMC electrolyte. The capacitance was largely improved by the addition of EMImBF4 IL. The EMImBF4 had a positive effect on the charge adsorption at the interface between the electrode and the electrolyte, so interfacial resistance decreased the positive effect on the charge desorption. However, enhanced capacitance was almost not changed for EMImBF4 amounts over 15 vol%. As a result, EMImBF4 enhanced the charge transfer mobility, resulting in increased capacitance, however, the capacitance for EMImBF4 amounts exceeding 15 vol% was almost not changed because the interfacial reaction between the electrode and electrolyte did not occur. It is suggested that when the ions were saturated, double layer capacitance which involved in capacitance existence in electrolyte.

Fig. 3.Cyclic voltammograms at 5 mV s-1 of organic electrolytes containing different contents in propylene carbonate/dimethyl carbonate electrolyte with 0.2 M SBP-BF4.

Fig. 4.Capacitance changes at various scan rates (5, 10, 30, 50, 100 mV s-1) of organic electrolytes containing different contents of EMImBF4 in propylene carbonate/dimethyl carbonate electrolyte with 0.2 M SBP-BF4.

Fig. 4 shows the variation in the specific capacitance of the as-prepared organic electrolytes. It can be seen that the specific capacitance decreased with the increase of scan rates from 5 mV s-1 to 100 mV s-1. At a high scan rate, the diffusion of electrolyte ions was limited due to the time constraint and only the outer active surface was utilized for charge storage or accumulation. The maximum specific capacitance of the organic electrolytes was 180 F g-1 at 5 mV s-1 and 10 F g-1 at 100 mV s-1 at the 15 vol%.

Fig. 5 shows the linear sweep voltammetry curve of the organic electrolytes. As shown here, the operating potential was increased by adding EMImBF4. This was due to the unique features of ILs, with their wide operating potential.

Fig. 5.Linear sweep of voltammograms in the anodic and cathodic regions.

3.3. Structural properties of electrodes surface

The surface morphology of the activated carbon electrodes cycled in the electrolytes with various contents of EMImBF4 and without EMImBF4 was examined by SEM. Fig. 6 shows SEM images of electrodes cycled in (a) no EMImBF4 electrolyte, (b) 10 vol% EMImBF4, (c) 15 vol%, and (d) 20 vol% EMImBF4 containing electrolyte. The SEM images were obtained after 500 charge-discharge cycles, and show that the surface morphology of the electrodes was slightly changed by EMimBF4.

Fig. 6.Scanning electron microscopy images of activated carbon electrodes after 500 charge-discharge cycles in different electrolyte solutions: (a) 0.2 M SBP-BF4 in propylene carbonate/dimethyl carbonate (PC/DMC), (b) 0.2 M SBP-BF4 in PC/DMC/10 vol% EMImBF4, (c) 0.2 M SBP-BF4 in PC/DMC/15 vol% EMImBF4, (d) 0.2 M SBP-BF4 in PC/DMC/20 vol% EMImBF4.

The surface morphology of the electrode cycled in the electrolytes without EMImBF4 looks a bit rough. This originates from the internal high strain during ion insertion into and extraction from the particles [17]. However, in the SEM images of electrodes cycled with added EMImBF4 electrolytes, they become gradually rather smooth, because the weaker polarization of the electrodes caused by the conductive solid electrolyte interphase (SEI) film [18].

By increasing the EMimBF4 additive content from 10 to 20 vol%, the film morphology was changed. The surface of the electrode was changed and the electric double layer became thicker after increasing the EMImBF4 additive content.

In order to check the composition of the film at the electrode surface, EDS was measured and the results are shown in Fig. 7. The resulting values of atomic percent are summarized in Table 2. Fluorine is formed by the decomposition of salts. The electrolyte with 10 vol% EMImBF4 has a higher number of fluorine elements than the electrolyte with free EMImBF4. However, fluorine atomic percent in the 10 vol% EMImBF4 electrolyte was lower than the electrolyte with EMImBF4. This means that the electrolyte without EMimBF4 decomposed effectively and the decomposition reaction of salts was restricted by the addition of EMImBF4. Also, it is thought that O elements combined with C elements, thereby greatly increasing the resistance of SEI. By adding EMImBF4, O atomic percent was decreased, so it is expected that the interfacial resistance of SEI was reduced.

Fig. 7.Energy dispersive X-ray spectra of activated carbon electrodes after 500 charge-discharge cycles in different electrolyte solutions: (a) 0.2 M SBP-BF4 in propylene carbonate/dimethyl carbonate (PC/DMC), (b) 0.2 M SBP-BF4 in PC/DMC/10 vol% EMImBF4, (c) 0.2 M SBP-BF4 in PC/DMC/15 vol% EMImBF4, (d) 0.2 M SBP-BF4 in PC/DMC/20 vol% EMImBF4.

PC: propylene carbonate, DMC: dimethyl carbonate

In order to identify differences in the deposited film component, the electrode surface was analyzed by XPS, and the results are shown in Figs. 8 and 9. The C 1s spectra of the surface films in all electrolyte solutions show four peaks at 283.3, 285, 285, and 285.5 eV. These peaks can be attributed to the C 1s binding energies of graphite-C, C-C, C=C, and CF, respectively.

Fig. 8.X-ray photoelectron spectra of activated carbon electrodes after 500 charge-discharge cycles in different electrolyte solutions: (a) 0.2 M SBP-BF4 in propylene carbonate/dimethyl carbonate (PC/DMC), (b) 0.2 M SBP-BF4 in PC/DMC/10 vol% EMImBF4, (c) 0.2 M SBP-BF4 in PC/DMC/15 vol% EMImBF4, (d) 0.2 M SBP-BF4 in PC/DMC/20 vol% EMImBF4.

Fig. 9.X-ray photoelectron spectra of activated carbon electrodes after 500 charge-discharge cycles in different electrolyte solutions: (a) 0.2 M SBP-BF4 in propylene carbonate/dimethyl carbonate (PC/DMC), (b) 0.2 M SBP-BF4 in PC/DMC/10 vol% EMImBF4, (c) 0.2 M SBP-BF4 in PC/DMC/15 vol% EMImBF4, (d) 0.2 M SBP-BF4 in PC/DMC/20 vol% EMImBF4.

The thickness of the SEI layer can be identified indirectly through the configuration ratio of graphite-C and C-C. If the SEI film is thick, the configuration ratio of graphite-C to C-C has low values because it’s difficult for X-rays to pass. Therefore, in the case of 15 vol% EMImBF4, it can be inferred that the thickness of the SEI film is thinnest, because the configuration ratio of graphite-C and C-C was increased. The observed thickness trend was 15 vol% EMImBF4 > 20 vol% > 10 vol% > 0 vol%. These results correspond to interfacial resistance, and means that the electrolyte containing 20 vol% EMImBF4 formed a thicker film, and the electrochemical properties of the electrolyte decreased.

In the XPS survey spectra of 0 vol% added EMImBF4, 1375.0 (C=C) at 285 eV and 270.0 (CF) at 285.5 eV, corresponding to C1s respectively, can be observed. By adding the EMImBF4, the double-bond of unsaturated carbon (C=C) decreased. When the content of EMImBF4 is 15% or higher, the intensity value becomes larger. In contrast, CF peak intensity tended to increase. And, the C-F peak increased when the EMImBF4 reacted with the carbon double bond (C=C). These results correspond to the configuration ratio of F. At 0 vol% EMImBF4, ionic-C-F peak was the smallest. This is because fluorine, which has large electronegativity, chemically reacted with a double bond of carbon. From the these results, at 15 vol% EMImBF4, the thinnest SEI film having the highest electrochemical properties could be confirmed.

 

4. Conclusions

The use of IL EMImBF4 represents an interesting strategy for the realization of high voltage and high performance EDLCs. In this work, we showed that the conductivity of electrolyte at 15 vol% EMImBF4 content in PC/DMC displayed the best value among various EMImBF4 contents. This allowed the realization of EDLCs with an operative voltage of 4.5 V. These high voltage EDLCs displayed higher energy and power than PC/DMC-based electrolytes. Moreover, thanks to the stability of this electrolyte, these EDLCs displayed good performance capacitance as well as ionic conductivity at an operating voltage of 4.5 V. By adding 0, 10, 15 vol% of EMImBF4, electrolyte resistance and electrode resistance were decreased. Also, the specific capacitance was increased. But, at EMImBF4 amounts over 15 vol%, electrochemical performances were saturated. From the XPS spectra, the thinnest SEI film was confirmed when 15 vol% EMIMBF4 was added. Finally, the electrolyte consisting of 15 vol% EMImBF4 in 0.2 M SBP-BF4 PC/DMC can be considered as effective candidate for application to high voltage capacitors.

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