1. INTRODUCTION:
In the 21st century, electricity demand is rapidly increasing worldwide. However, about 68% of electrical energy is supplied from fossil fuels and the electricity generated by burning fossil fuels coproduces CO2 emission. As a result, environmental problems arise, and therefore, there is a great demand for the use of renewable energy and a corresponding need for an energy storage device1,2. As representative energy storage devices, Lithium-ion batteries and Electric double layer capacitor (EDLC) are well-known, and research on performance improvement is being carried out continuously.
In case of lithium ion batteries, many efforts have been made to improve the short life span of the battery from the initial stage of the development. Much research has been progressed on the overall degradation of the battery such as deterioration of the electrolyte and active material, and corrosion of the electrodes3,4.
However, in case of EDLC, most studies on degradation have concentrated only on the deterioration between the electrolyte and the activated carbon5,6, and little attention has been paid to the study for deterioration between the electrolyte and the aluminum as the current collector for EDLC.
The energy density of the EDLC is defined as in Eq. (1).
1
E = -- CV^2 (1)
2
Where V is the cell voltage and C is the capacitance7-11.
As shown in above equation, since the energy density (E) increases in proportion to the square of the voltage (V), increasing the operating voltage of the cell is an effective way to increase the energy density12.
However, it is not only well known in EDLC industry that as the voltage gets higher, there is more deterioration from the reaction between the electrolyte and the activated carbon13, also serious corrosion occurs in the Al current collector due to the reaction with the electrolyte at the voltage limit and higher. Therefore, studies on improving energy density by raising the typical EDLC operating voltage of 2.5-2.7V must be accompanied by studying the aluminum corrosion in EDLC.
Furthermore, in order to study the corrosion of Al current collector in EDLC, it is necessary to remove the coated active material from the aluminum. If a significant influence is made on the Al current collector during the removing process, accurate analysis of the deterioration of the current collector, that is, the corrosion behavior of aluminum becomes impossible. Therefore, only the active material should be completely removed without affecting the Al current collector.
To remove the electrode active material in the EDLC industry, physical polishing, ultrasonic cleaning and/or heat treatment on the current collector has been used singly or in combination. However the influence of these pre-treatments on the Al current collector have not been studied and reported yet.
In this work, we studied the influenceof pre-treatment methods carbon electrode on Al current collector. Also we proposed the most effectivepre-treatment method to correctly analyze the deterioration of the current collector in the EDLC.
2. MATERIALS AND METHODS:
An Al current collector before coating with an active material (CS 200, henceforth called raw current collector) and acarbon electrode for EDLC that has been coated with an active material on a Al current collector and then completed a calendering process (henceforth called carbon electrode), were obtained from Korea JCC. The carbon electrodes were used to make the EDLC cells,and these cells (3.1V, 360F) before and after the life-cycle were obtained from Samwha Electric Co., Ltd. Images of the Al current collector, carbon electrode, and EDLC cell are shown in Figure 1.
Figure 1.Imageof materials used in experiments: (a) Al current collector, (b) carbon electrode, and (c) EDLC cell.
The Acetonitrile (ACN, from Enchem) solution containing 1M Tetraethylammoniumterrafluoroborate (TEABF4) was used as an electrolyte. Ethyl alcohol (99.5 %, from Daejung) was used as purchased.
The surfaces of the Al current collectors were analyzed by Scanning Electron Microscopy (SEM, S-3000N, from Hitachi) and the chemical composition analysis was performed by Energy Dispersive Spectrometer (EDS, HIT S-3000N, from Hitachi) and X-ray Photoelectron Spectroscopy (XPS, PHI Quantera-Ⅱ, from Ulvac-PHI).
In order to remove the active material of the electrode, three different methods were used. Physical Polishing was conducted by grinder, Ultrasonic Treatment was carried out by sonication in an ultrasonic cleaning machine (Power Sonic 420, from Adpore) with electrode soaked in a beaker containing ethanol, and Heat Treatment was conducted by heating the electrode in an electric muffle furnace (C-FMD, from Chang Shin Science), then, is washed with ethyl alcohol.
3. RESULTS AND DISCUSSION:
3.1.XPS and EDS analysis:
Figure 2 show the results of XPS analysis of each Al current collector in which the active material coated on the EDLC electrode was removed by three methods: (a) Physical Polishing, (b) Ultrasonic Treatment for 17min, and (c) Heat Treatment at 300 ℃ for 60 minutes.
Figure 2. XPS spectra of aluminum current collector: (a) Physical Polishing, (b) Ultrasonic Treatment, and (c) Heat Treatment.
As a result of the XPS analysis, the chemical composition detected on the surface of each Al current collector after removal of the active material, were found to be identical. Therefore, we can say all three electrode active material treatment methods do not change any chemical composition to the surface of the Al current collector, which has gone through deterioration with the electrolyte.
However, the Al current collector from which the electrode active material was removed by physical polishing revealed the smaller F, O and N amount compared to other methods in the XPS results patterns. Consequently, it was confirmed that the physical polishing method has an effect of reducing the amount of the chemical component on the surface of the Al current collector changed by the deterioration with the electrolyte (Table 1).
Table1.Intensity of XPS spectra: (a) Physical Polishing, (b) Ultrasonic Treatment, and (c) Heat Treatment(Unit: C/s).
|
Element |
(a) |
(b) |
(c) |
|
O1s |
12,457.65 |
21,946.44 |
20,448.61 |
|
N1s |
430.31 |
532.31 |
545.73 |
|
F1s |
2,749.12 |
6,877.11 |
15,114.19 |
|
C1s |
6,428.82 |
7,533.52 |
7,079.83 |
|
Al2p |
3,621.46 |
3,098.11 |
3,248.60 |
Figure 3 and Table 2 show the results of the EDS analysis of each Al current collector which is obtained by three deferent methods: (a) Physical Polishing, (b) Ultrasonic Treatment, and (c) Heat Treatment.
Table2.EDS results of the Al K shell, F K shell, O K shell, and C K shell: (a) Physical Polishing, (b) Ultrasonic Treatment, and (c) Heat Treatment (Unit: wt.%).
|
Element |
(a) |
(b) |
(c) |
|
C K |
4.00 |
13.59 |
0.00 |
|
O K |
3.14 |
8.98 |
4.63 |
|
F K |
1.94 |
2.68 |
0.76 |
|
Al K |
90.93 |
74.75 |
94.62 |
(a)
(b)
(c)
Figure 3. EDS spectra of the Al K shell, F K shell, O K shell, and C K shell: (a) Physical Polishing, (b) Ultrasonic Treatment, and (c) Heat Treatment.
The remaining carbon was 13.59 wt. % in the ultrasonic treated Al current collector, while the remaining carbon in the heat-treated Al current collector was almost 0.00 wt. %.
3.2.SEM analysis:
3.2.1.Ultrasonic Processing:
An electrode from the EDLC cell which has completed the life-cycle test (henceforth called cycle tested electrode), was cut in 5 × 3 (cm) size and heated in an electric muffle furnace at 300 ℃ for 10 minutes, then, washed with deionized water to remove the active material.
The Al current collector which the active material is removed, was soaked in a beaker containing ethanol and was sonicated at an interval of 15 seconds from 0 second to 60 seconds in an ultrasonic cleaning machine, to observe the surface changes of the Al current collector.
Figure 4 shows the SEM images of the surface observation on the same position of an Al current collector by increasing the ultrasonic treatment time. The Al current collector was obtained after the active material was removed from the cycle tested electrode by heat treatment.
(a)
(b)
Figure
4. The surface SEM images of aluminum current collector at different ultrasonic
treatment time (
300): (a) 0 second, (b) 60 seconds.
The surface of the Al current collector at ultrasonic treatment time of 60 seconds (Fig. 4 (b)) was found to be almost unchanged from the ultrasonic treatment time was 0 second (Fig. 4 (a)). This shows the ultrasonic cleaning within 60 seconds does not affect the pit of the Al current collector.
3.2.2.Heat Treatment:
Effect on raw current collector:
The raw current collector is heated at the electric muffle furnace at 300 ℃ for 10 minutes. By analyzing the surface of the current collector, the effects of the heat treatment on the Al current collector was observed. Then, second raw current collector was immersed in a beaker containing electrolyte (1M TEABF4 / ACN) for 10 seconds before the heat treatment in the same condition. The surface was analyzed to investigate the effects of the electrolyte during the heat treatment on the Al current collector.
Figure 5 shows SEM images of the surfaces of the raw current collect collector and the raw current collector containing the electrolyte after the heat treatment, respectively.
(a)
(b)
Figure 5. The surface SEM images of the raw current collector after heat treatment for 10 minutes at 300 ° C (×500): (a) The raw current collector containing no electrolyte, (b) The raw current collector containing electrolyte.
No changes were observed on the surface of the raw current collector subjected to only the heat treatment, but severe corrosion was observed on the surface of the raw current collector containing the electrolyte.
In order to investigate how residue of the cleaning solution, which is used to clean the electrolyte in the EDLC electrode, affect the Al current collector during the heat treatment, raw current collectors cleaned with deionized water and ethanol respectively were placed in the electric muffle furnace for 10 minutes at 300 ℃. Then, the surface changes were observed.
SEM images of the raw current collector surface after heat treatment, cleaned with demonized water and ethanol respectively, are shown in Figure 6. No deterioration was observed on the surfaces of the raw current collectors washed with each cleaning solution. In conclusion, there was no effect of cleaning solution on the surface of the raw current collector.
(a)
(b)
Figure 6. The surface SEM images of the raw current collector after heat treatment (×300): (a) Wash with ethanol, (b) Wash with demonized water.
Effect on electrode:
(1) Electrode of an EDLC containing electrolyte before the life-cycle test (henceforth called new EDLC electrode), which has not deteriorated was subjected to heat treatment at 300 ℃ for 10 minutes to remove the active material coated on the Al current collector. Investigation by analyzing the surface of this current collector, effects of the electrolyte on the Al current collector coated with an active material during the heat treatment is made.
Figure 7 shows the surface SEM images of the Al current collectors after active material removal from the heat treated new EDLC electrode. No corrosion was observed in the SEM image.
Figure 7.The surface SEM image of the aluminum current collector after active material removal from the heat treated new EDLC electrode(×300).
(2) The new EDLC electrode was then washed with deionized water and ethanol respectively and heat treated at the same condition to remove the active material. Surfaces of these two current collectors are analyzed to study the effects of deionized water and ethanol on the Al current collector coated with active material during the heat treatment.
Figure 8 shows SEM images of the surfaces of the Al current collectors after active material removal from the new EDLC electrode which was washed with deionized water and ethanol respectively then heat treated.
(a)
(b)
Figure 8.The surface SEM images of the aluminum current collector with Heat Treatment after cleaning with different cleaning solution(×500): (a) Washed with Ethyl alcohol, (b) Washed with deionized water.
There was no new corrosion on the surface of the Al current collector which used ethyl alcohol and heat treatment for removal of active material (Fig. 8 (a)). However, the Al current collector washed with deionized water before heat treatment to remove the active material displayed severe corrosion progression (Fig. 8 (b)). This can be explained by the fact that the water contained in the electrolyte greatly influenced the corrosion reaction of the aluminum.
3.2.3.Physical Polishing
Removal of the coated active material on the carbon electrode was conducted through heat treatment at 300 ℃ for 10 minutes in the electric muffle furnace. Active material from a new EDLC electrode was removed by physical method using a grinder. Surface of the Al current collectors obtained by heat treatment and polishing treatment were analyzed respectively.
The surface SEM images comparing these two Al current collector are shown in Figure 9.
(a)
(b)
Figure 9.The surface SEM images of the aluminum current collector after Heat Treatment and Physical Polishing (×300): (a) After Heat Treatment, (b) After Physical Polishing.
As seen in Figure 9(b), the physical polishing process was observed to severely damage the surface of the Al current collector.
4. CONCLUSION:
As a result of analyzing the effect of electrode active material removal methods on Al current collector, ultrasonic treatment within 60 seconds and heat treatment method which does not include water have little effect on the chemical composition or pit change of the Al current collector. The physical polishing and heat treatment when moisture is present however, was experimentally confirmed to be unsuitable as an electrode active material removal method to study the deterioration of the Al current collector due to damages on the surface during the process.
The ideal method to remove the active material from an electrode of an EDLC would be by performing ultrasonic treatment for less than 45 seconds after heat treatment at 300 ° C for 10 minutes on the electrode which has been sufficiently cleaned with ethanol avoiding water.
As seen in the preceding section, severe corrosion was observed on the surface of the raw current collector containing the electrolyte after the Heat Treatment (refer to Fig. 5(b)). However, there was no corrosion observed on the surface of the Al current collector from the new EDLC electrode containing the electrolyte with the active material (refer to Fig. 7).
This means that the influence of the electrolyte on the Al current collector depends on the presence or absence of the active material. Therefore, further studies on the corrosion mechanism of the Al current collector in the EDLC in more depth are needed.
5. ACKNOWLEDGMENT:
This research was financially supported by Hansung University.
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Received on 20.02.2018 Modified on 22.03.2018
Accepted on 16.06.2018 © RJPT All right reserved
Research J. Pharm. and Tech 2018; 11(10): 4575-4580.
DOI: 10.5958/0974-360X.2018.00837.5