Fluoroethylene Carbonate Addition Effect on Electrochemical Properties of Mixed Carbonate-based Organic Electrolyte Solution for a Capacitor

Abstract

In this paper, organic solvent electrolytes were prepared by a mixture of propylene carbonate (PC), dimethyl carbonate (DMC), tetraethylammonium tetrafluoroborate ($TEABF_4$)s to evaluate the ionic properties of propylene carbonate (PC)/dimethyl carbonate (DMC) mixtures as solvents for a capacitor application, in view of improving the electrochemical performances. The bulk resistance and interfacial resistance of the mixture electrolytes were investigated using an AC impedance method. The morphology of carbon-based electrodes which were contained in different electrolytes was analyzed by scanning electron microscopy (SEM) method. From the experimental results, by increasing the FEC content, capacitance of electrodes was increased, and the interfacial resistance was decreased. In particular, by a content of 2 vol % FEC in 0.2 M $TEABF_4$ PC/DMC solvent, the electrolyte showed the superior capacitance. However, when FEC content exceeds 2 vol %, the capacitance was decreased and the interfacial resistance was increased.

Introduction

Electrochemical double layer capacitors (EDLCs), which are based on non-faradaic capacitance, are promising energy storage devices. Because of charge storage of EDLCs occurs entirely via electrostatic forces at the electrode/electrolyte interface, it displays a high power and an extremely high cycle life.1-6 Currently, many efforts have been spent on the development of EDLCs. Most of all, studies on electrolyte are proceeding briskly.7,8 An electrolyte is extremely important for EDLCs, as it determines the capacitance, internal resistance and temperature characteristic of EDLCs. Usually, electrolyte for EDLCs consists of either acetonitrile (AN) or propylene carbonate (PC) as solvent and quaternary ammonium salts as salt.9,10 Acetonitrile shows high ionic conductivity, low resistance and wide operating potential, but, it is harmful and flammable (Fp = 6 °C), thus acetonitrile based electrolyte has a problem. Propylene carbonate is rather safe when it is compared with AN due to its high flash point (Fp = 135 °C). However, its conductivity is quite low.11 Lithium ion rechargeable battery electrolytes were studied and idealized, but studies on the capacitor electrolytes is still far from an optimization and commercialization. In order to improve the performance of electrolyte, a lot of methods have been studied. Among them, the use of additives in electrolytes is the most economic and effective way. Recently, many study reported about the solid electrolyte interphase (SEI)/particle interface stability by fluoroethylene carbonate (FEC) based electrolytes to achieve stable cycling performance in lithium ion battery.12,13 But, the effect of FEC as additive on the performance of the EDLC electrolyte was still unclear. Here we try to investigate the addition effect of FEC into carbonate-based electrolytes which were contained against activated carbon electrodes. Fluoroethylene carbonate (FEC) has been studied as a SEI film-formation additive to improve cycle performance of lithium-ion battery at room temperature,14-17 but almost no literatures were focused on the influence of FEC in electrochemical double-layer capacitors (EDLCs). In this paper, FEC was introduced as a modifier to improve ionic property of mixture electrolytes. The purpose of this paper is to investigate the ion conducting properties of propylene carbonate (PC)/dimethyl carbonate (DMC) mixtures as solvents for EDLC application, in view of improving the electrochemical performances. The addition effect of FEC in carbonate based electrolytes was investigated to evaluate the capacitance and cycle performance of PC/DMC-based electrolytes.

 

Experimental

Materials. TEABF4 (99％) was purchased from Aldrich and TEABF4 was stored under glove box with dry Ar-atmosphere before use. Propylene carbonate (99.7％) was acquired from Aldrich. Dimethylcarbonate (99％) was acquired from Alfa Aesar. MSP-20 was acquired from Kuraray Chemical Co., Ltd. Carbon blacks (99％) were also purchased from Alfa Aesar. Fluoroethylene carbonate (FEC) was acquired from Aldrich. Sodium carboxymethyl cellulose (average molecular weights of 1 × 104 (Aldrich) 1.5 × 105) was acquired Aldrich. N-Methyl-2-pyrrolidone (NMP) (99％) was purchased from Junsei chemical Co. Ltd.

Preparation of Electrolytes and Electrode. The electrolytes were prepared by dissolving the salts in propylene carbonate (PC), dimethylcarbonate (DMC). Fluoroethylene carbonate (FEC) was used without a further purification. We used 0.2 M TEABF4/PC/DMC, × vol ％ ( × = 0, 2, 4, 5) FEC as electrolytes. The fabrication of working electrodes was carried out as follows. First, activated carbon (MSP-20) and super-P were used as the active material and conductive agent in the electrode, respectively. And CMC and styrenebutadiene rubber (SBR) were used as binder. Slurries of 85:5:6:4 (weight percent) active materials (MSP-20):carbon blacks (Super-P, Alfa Aesar):CMC (Aldrich):SBR were mixed with NMP solution as solvent. Then the resulting slurry was coated on nickel foam substrate (1 cm × 1 cm) with spatula, which was followed by drying at 100 °C for 24 h in a vacuum oven.

Characterization of Electrolytes. All electrochemical tests were done in three electrode system. A nickel foam coated with slurry, a platinum wire and Ag/Ag+ served as working, counter and reference electrode respectively. Electrochemical measurements were performed by an Iviumstat (Ivium Technologies, Netherlands). Cyclic voltammetry (CV) tests were analyzed between 0 V and 0.6 V at different scan rate of 5 or 10 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 at the several voltages (0 V, 1.5 V, 2 V, 2.5 V). The morphology of the electrode cycled in prepared electrolytes was investigated by scanning electron microscopy (SEM) (HITACHI S3500N). The surface characterization of electrodes cycled in prepared electrolytes investigated using energy dispersive x-ray spectroscopy (EDS).

 

Results and Discussion

Electrochemical Properties. The electrochemical performance of various electrolytes was analyzed using a CV curve. Figure 1 displays CV curves of various electrolytes. The specific capacitance of the electrode can be calculated according to the following equation:18

C is the specific capacitance based on the mass of electroactive materials (F g−1), I is the response current density (A cm−2), V is the potential scan rate (mV s−1), and m is the mass of the electroactive materials in the electrode (g).

Figure 1.Cyclic voltammetry curves at 5 mV s−1 scan rate of organic electrolytes containing a different content of FEC in PC/DMC electrolyte with 0.2 M TEABF4.

The specific capacitance of electrolytes was shown in Figure 1 and the calculated capacitance values based on above equation were shown in Table 1. The capacitance of various electrolytes was compared to those of pristine 0.2 M TEA BF4 in PC/DMC electrolyte. The capacitance was increased with a FEC addition. It is thought that FEC had a positive effect on the solid electrolyte interphase (SEI) at the interface between the electrode and the electrolyte. However, capacitance was decreased at FEC amounts over 2 vol ％. FEC maybe bring a decreased solvation of electrolyte ions,19 and the SEI film between the electrode and electrolyte was excessively formed or became thicker.

Figure 2 shows the impedance plots for the carbon in the electrode with different electrolyte compositions. The high-frequency part of the impedance plot, bulk resistance was not changed by increasing contents FEC. The diameter of the semi-circle in the high frequency region corresponds to the resistance that describes the impedance through the electrolyte and surface of electrode.20 In other words, the diameter of the semi-circle corresponds to charge transfer resistance at the interface of the SEI and active material surface of electrode.

A decrease in the interfacial resistance, as shown by the decreased semi-circle sizes in the high frequency region, was achieved by adding FEC. The minimum interfacial resistance was achieved at a content of 2 vol ％. When the FEC was increased over 2 vol ％, the interfacial resistance was increased. It could be concluded that the 2 vol ％ addition of the FEC content made the most proper thickness of SEI films for the charge adsorption-desorption and, thereby, achieved the highest capacitance and the lowest resistance.

Structural Properties of Electrodes Surface. Surface morphology of activated carbon electrodes cycled in the electrolytes with various contents FEC and without FEC was examined by SEM. Figure 3 shows SEM images of cycled in (a) without FEC electrolyte, cycled in (b) 2 vol ％, (c) 4 vol ％, and (d) 5 vol ％ FEC containing electrolyte. The SEM images observed after 30 charge-discharge cycles. SEM images show that the surface morphology of electrodes was changed by FEC. Surface morphology of electrode cycled in the electrolytes without FEC exists the cracks that formed from internal high strain during ion insertion into and extraction from the particles.17 However, images of electrodes cycled in containing FEC electrolytes show rather smooth and crack-repaired morphology, because the weaker polarization of electrode caused by conductive SEI film.21 The cracks will cause enhance polarization of electrode and poor electric contact between active particles and conductive materials or current collector.22 By increasing the FEC additive content from 2 to 5 vol ％, the film morphology was changed. The surface of electrode was changed and SEI-film was deposited thicker by increasing the FEC additive content. This implied that the additional SEI film had been grown on electrode surface. Furthermore, when FEC content exceeds 2 vol ％, SEI film on surface morphology of electrode was grown too much. This explained that excessively formed SEI film interrupts ion transport and charge absorption-desorption.

Table 1.Resistance and Capacitance of 0.2 M TEABF4 PC/DMC based electrolytes containing different contents FEC

Figure 2.Impedance spectra of organic electrolytes containing a different content of FEC.

Figure 3.SEM images of electrodes after 30 charge-discharge cycles in different electrolyte solutions: (a) 0.2 M TEABF4 in PC/ DMC, (b) 0.2 M TEABF4 in PC/DMC/2 vol ％ FEC, (c) 0.2 M TEABF4 in PC/DMC/4 vol ％ FEC, (d) 0.2 M TEABF4 in PC/ DMC/5 vol ％ FEC.

To further study about the influence of SEI film on electrode cycled in the electrolytes with various contents FEC and without FEC, EDS were determined. Figure 4 shows the EDS spectrum of the electrode after cycling in prepared electrolytes. Calculated atomic percent values were shown in Table 2. According to literature, FEC is decomposed into vinylene carbonate (VC) and HF.23 Decomposed VC is easily reduced and tends to be polymerized compare to the carbon atom, which has single bond by a double bond between the carbon atoms.23-26 Such polymerized SEI films prevent further decomposition of the electrolyte.27-29 As a result, the electrode of free FEC electrolyte was found to have the highest amount F− anion. It explains that the decomposition of salt was substantially occurred in the electrolyte involved TEABF4. In other words, it means that inorganic components, which were combined F− anion of disintegrated salt with other component, formed heavy SEI films. Also, it can be confirmed that F− anion increasing by increase the FEC contents. According to results on electrochemical analysis (capacitance, resistance, and capacitance retention), too much amount of F− anion can have a negative influence on electrochemical properties. Electrolyte con-taining FEC 2 vol ％ was formed the most stable SEI film and indicated the highest values on electrochemical properties.

Figure 4.EDS spectra of electrodes after 30 charge-discharge cycles in different electrolyte solutions: (a) 0.2 M TEABF4 in PC/ DMC, (b) 0.2 M TEABF4 in PC/DMC/2 vol ％ FEC, (c) 0.2 M TEABF4 in PC/DMC/4 vol ％ FEC, (d) 0.2 M TEABF4 in PC/ DMC/5 vol ％ FEC.

Table 2.Atomic percent of 0.2 M TEABF4 PC/DMC based electrolytes containing different FEC contents

Impedance and Cycle Performances. To further understand the effect of FEC in improving the electrochemical performance, EIS measurements under various voltages were determined and analyzed by equivalent circuit in Figure 5. Resistances obtained from the EIS results are shown in Table 3. The bulk resistances and interfacial resistances of electrolyte containing FEC 2 vol ％ were obtained low value at 1.5 V, 2 V, 2.5 V, and were compared with free FEC electrolyte. Particularly, bulk and interfacial resistance of electrolyte with FEC 2 vol ％ was lower than electrolyte without FEC at 2.5 V. It means that FEC addition effect was prominent at higher voltage, namely, FEC has wide operative potential range with no redox reaction or no big impedance change.

Figure 5.Impedance spectra of electrolyte without FEC and with 2 vol ％ FEC at 1.5 V (a), 2 V (b), and 2.5 V (c).

Table 3.Resistances of electrolytes at various applied voltages

Figure 6 shows the cycling performance and capacitance retention, respectively. Superior reversible capacities were observed for the electrolyte containing 2 vol ％ FEC in comparison to the electrolyte without FEC. The capacitance retention of the electrolyte at FEC 2 vol ％ increased by 15％ (from 80％ to 95％) after 200 cycles. FEC additive could bring an excellent electrode cycling stability. The electrolyte containing FEC can be attributed to forming more effective, compact and stable SEI film on the surface of electrode.

Figure 6.Capacitance retention curves of the electrolytes without FEC and with 2 vol ％ FEC.

 

Conclusion

In this work, we have evaluated the formation mechanisms of film at the surface of the electrode of EDLC when FEC is used as electrolyte additive. Capacitance, interfacial resistance, and morphology were investigated to analyze the addition effect of FEC additive into organic electrolytes. These results indicated that electrolyte with 2 vol ％ FEC was the optimized candidate. Then, electrolyte without FEC and with 2 vol ％ FEC was compared. The capacitance retention of the electrolyte at FEC 2 vol ％ was enhanced to the electrolyte without FEC by 15％ (from 80％ to 95％) after 200 cycles. Also, resistances of electrolyte at FEC 2 vol ％ was obtained as a lower values in all voltage. Especially, resistances of electrolyte at FEC 2 vol ％ was lower than electrolyte without FEC in the high voltage of 2.5 V. It means that FEC addition effect was prominent at high voltage, namely, FEC has wide operative potential. These results indicated that the FEC is beneficial to form a thinner, stable and conductive SEI film on the electrode surface. The results of this study enable a better understanding of the FEC effect as electrolyte additive to improve the properties of the SEI films at the surface of the electrodes.