Synthesizing a Gel Polymer Electrolyte for Supercapacitors, Assembling a Supercapacitor Using a Coin Cell, and Measuring Gel Electrolyte Performance

Summary

We present a protocol to test the electrochemical and physical properties of a supercapacitor gel polymer electrolyte using a coin cell.

Abstract

Supercapacitors (SC) have attracted attention as energy storage devices due to their high density and long cycle performance. SCs used in devices operating in stretchable systems require stretchable electrolytes. Gel polymer electrolytes (GPEs) are an ideal replacement for liquid electrolytes. Polyvinyl alcohol (PVA) and polyvinylidene fluoride-co-hexafluoropropylene (PVDF-HFP) have been widely applied as a polymer-matrix-based electrolytes for supercapacitors because of their low cost, chemically stable, wide operating temperature range, and high ionic conductivities. Herein, we describe the procedures for (1) synthesizing a gel polymer electrolyte with PVA and PVDF-HFP, (2) measuring the electrochemical stability of the gel polymer electrolytes by cyclic voltammetry (CV), (3) measuring the ionic conductivity of the gel polymer electrolytes by electrochemical impedance spectroscopy (EIS), (4) assembling symmetric coin cells using activated carbon (AC) electrodes with the PVA- and PVDF-HFP-based gel polymer electrolytes, and (5) evaluating the electrochemical performance using galvanostatic charge-discharge analysis (GCD) and CV at 25 °C. Additionally, we describe the challenges and insights gained from these experiments.

Introduction

Flexible SCs have grown rapidly in recent years for the fabrication of electronics with stretchable displays and wearable energy devices. Flexible SCs typically consist of flexible electrodes¹, separators², and the electrolyte³ in a flexible assembly. Therefore, GPEs are the most effective structure owing to their flexibility⁴, separator-free nature, relatively high ionic conductivity⁵, and thin-film forming ability⁶.

To prepare the polymer matrices of GPEs, materials such as polymethylmethacrylate (PMMA), PVDF-HFP, and PVA have been developed in recent years. PVA and PVDF-HFP have especially been widely applied as polymer-matrix-based electrolytes for SCs due to their low cost, chemically stable, wide operating temperature range, and high ionic conductivities at room temperature (RT).

Herein, we describe a synthetic method for two representative polymer-matrix materials-PVA⁷ and PVDF-HFP-and the electrochemical characterization of the polymer-matrix material-based gel electrolyte. In summary, we illustrate the general synthesis, material processing methods, and performance evaluation methods employed to fabricate stretchable SCs.

For application in flexible SCs, polymer electrolytes should exhibit the following properties: (1) high ionic conductivity at ambient temperature, (2) high chemical and electrochemical stability, (3) good mechanical properties of dimensional stability, and (4) sufficient thin film processability. These features were confirmed using EIS, CV, and tensile tests. The EIS and CV measurements were conducted using a coin cell. First, the ionic conductivity of the polymer-matrix-based electrolyte was estimated according to the equation using impedance. Second, the chemical and electrochemical stabilities of the polymer-matrix-based electrolyte were estimated by the CV and GCD tests. The stabilities of the polymer-matrix-based electrolytes were demonstrated by controlling the voltage range tested by the CV. Third, the mechanical properties of the polymer-matrix-based electrolytes were evaluated by conducting tensile tests.

A coin cell was fabricated using PVA- and PVDF-HFP polymer-matrix-based electrolytes with AC symmetric cells. The supercapacitor performances of the two different coin cell supercapacitors were evaluated at 25 °C. Because this work mainly involves PVA- and PVDF-HFP polymer-matrix-based electrolytes, the remainder of this paper focuses on these electrolytes. The detailed procedures of these experiments, difficulties in execution, and insights gained from these experiments are described as below.

Protocol

1. Synthesis of PVA- and PVDF-HFP polymer-matrix-based electrolytes

NOTE: When handling methanol, it is best to avoid direct exposure as much as possible.

1. PVA polymer-matrix-based electrolyte synthesis
   1. Dissolve PVA (1 g) (Mw 146,000-186,000)in double-distilled water (10 mL) in a water bath at 90 °C and stir at 500 rpm until a clear solution is obtained. Then, add H₃PO₄ (1 mL) to the hot solution with constant stirring at RT for 24 h.
   2. Pour the stretchable polymer electrolyte into a glass Petri dish and dry it overnight in a vacuum oven at 40 °C.
      NOTE: The thickness of the as-formed GPE should be approximately 1 mm.
   3. Peel off the dried films from the molds and cut them into 19 mm specimens for further testing.
2. PVDF-HFP polymer-matrix-based electrolyte synthesis
   1. Prepare a gel polymer electrolyte using the solution-casting method. First, dissolve PVDF-HFP (MW 400,000) (3 g) in dimethylformamide (DMF, 15 mL) with a lidded container at RT and stir at 500 rpm for 3 h until a homogeneous, low-viscosity solution is formed.
   2. Add diglycidyl ether of bisphenol-A (DEBA; 1 g), poly (ethylene glycol) diglycidyl ether (PEGDE; 3 g), and diamino-poly (propylene oxide) (DPPO; 8 g) to the solution prepared in step 1.2.1 and stir at 500 rpm constantly at ambient temperature for 6 h.
   3. Pour the resultant mixture into a round polytetrafluoroethylene plate or plastic Petri dish and heat it in a vacuum oven at 80 °C for 24 h to evaporate the DMF solution and to afford the desired GPE.
   4. Cool the resultant GPE to RT, wash it thrice with methanol using a centrifuge at 12,329 × g for 5 min to remove the unreacted monomer, and dry under a vacuum for 12 h at 60 °C.
      NOTE: The thickness of the as-formed gel polymer electrolyte is approximately 100 µm. The resultant gel polymer electrolyte showed excellent mechanical properties when the weight ratio of PEGDE:DEBA:DPPO was optimized to 3:1:8 and the content of PVDF-HFP was optimized to 20 wt%.
   5. Prepare the synthesized GPEs by immersing the porous membranes in a liquid electrolyte (1 M LiPF₆ in EC/DMC = 1/1, v/v) for 24 h in an argon-filled glove box.
      ​NOTE: The liquid electrolyte uptake after soaking for 24 h was approximately 350 wt%.

2. Characterization of the GPEs

1. Fourier transform infrared (FTIR) spectroscopy
   ​NOTE: We recommend using a slid-on attenuated total reflection (ATR) accessory with the FTIR spectrometer for collecting spatially resolved FTIR spectra (high spatial resolution of ~10 µm²) of the interaction between the polymer-matrix-based electrolytes.
   1. Select a sample with appropriate dimensions for the FTIR microscope to ensure high-quality spectra using the ATR-FTIR accessory.
   2. Calibrate the FTIR and take the same sample measurements in the range of 500-4500 cm^-1 at a 5 cm^-1 resolution. This process includes cooling the detector and allowing sufficient time for stabilization.
   3. Collect an appropriate background spectrum to subtract from the sample spectrum.
   4. Depending on the appropriate objective, select the area of interest and focus on the same area for analysis.
   5. After determining the area of interest, attach the ATR accessory to the FTIR microscope objective. Lower the ATR accessory until it contacts the sample closely, and then collect the sample spectrum.
   6. Perform data processing after collecting the FTIR spectra.
2. X-ray diffraction (XRD)
   1. Mill the sample powder using an agate mortar. Then, deposit the powder on the sample holder of the X-ray diffractometer to fill the hole until it overflows, and press to form a uniform, smooth surface. The instrumental parameters of the XRD analysis are described in references^8,9.
   2. Before measuring the film-type XRD patterns of the sample, keep the polymer-matrix-based electrolyte as flat as possible in the holder. The instrumental parameters of the XRD analysis were the same as those described in step 2.2.1.

3. Preparation of the composite AC electrode

1. Prepare a powdered composite electrode by mixing AC, conductive carbon, and PTFE binder in a mass ratio of 8:1:1 using a mortar until it becomes a dough. Add a drop of isopropanol (IPA; 0.1-0.2 mL) to the dough, and spread the mixture repeatedly to thoroughly mix it.
2. Roll the dough using a roller to achieve the desired thickness (~100 mm), and construct AC electrodes with radius of 14 mm.
3. Dry the AC electrode in an oven at 80 °C for 24 h to completely evaporate the IPA.

4. Coin cell preparation and testing

1. Heat 15 mL of H₃PO₄-PVA at 80 °C and immerse the AC electrodes in this solution for 10 min. After the process, dry the electrodes in a hood for 4 h to evaporate the water.
2. Press the two AC electrodes face-to-face with the polymer electrolyte placed in between to form a sandwich structure.
3. Similarly, to prepare the coin cell containing the PVDF-HFP gel, assemble the AC symmetric cell using the electrolyte soaked in step 4.1
   NOTE: Figure 3 shows a schematic of the coin-cell assembly.
4. To prepare the coin cells for testing, close the 2032 coin cell with a cell cap and crimp two or three times using a manual crimping machine.

5. EIS, CV, and GCD testing methods for the PVA and PVDF-HFP GPEs

NOTE: The potentiostats consist of a working sensor (WS), a working electrode (WE), a reference electrode (RE), and a counter electrode (CE).

1. Before testing the two-electrode system, combine the WS line with the WE line, which is working as WE, and the RE line with the CE line, which is working as CE.
2. Then, insert the coin cell into the holder used for the electrochemical test and connect the WE line and CE line on both sides.
   NOTE: All tests were conducted using the coin cells that were prepared.
3. EIS test
   NOTE: The 'Rest Time' step is necessary to stabilize the cell before the EIS test. Smart Management 6 program is used for setting the sequence and measuring the electrochemical result.
   1. Run the program and set the EIS measurement experiment sequence file.
   2. Click on the Experiment option to generate a new file, and then click on the Add button to generate the first step.
   3. Then set the Rest Time parameters of the sequence file. Set the Control tab as Constant. Set the Type, Mode, and Range in the Configuration tab as PSTAT, Timer Stop, and Auto, respectively.
   4. Conduct complex impedance measurements using an EIS system in the frequency range 100 kHz-0.01 Hz.
   5. Click on the Add button to generate the next step.
   6. Click on the Control button and set it as EIS; for the Configuration, set the Type, Mode, and Range as PSTAT, LOG, and AUTO, respectively.
   7. Conduct the EIS at 100 kHz-0.01 Hz. For this, set the Initial (Hz) and Middle (Hz) as the same value, 100 x 10³, and the Final (Hz) value as 1 x 10^-2. Set the Bias (V) values as 400 x 10^-3. Then, click on the Ref button and set it as Eref.
   8. The resulting signal must exhibit a linear response to the applied signal. Therefore, set the amplitude (Vrms) to 10 x 10^-3.
   9. Set the Density and Iteration as 10 and 1, respectively, for this experiment.
   10. Click on the Save As button to save the file for EIS testing.
   11. Click on Apply to CH, and run the file for EIS testing to obtain results.
4. CV test
   NOTE: In this case, the operating voltage depends on the solvent used to prepare the GPE.
   1. Run the program to generate the sequence file.
   2. Click on Experiment to generate a new file, and then click on the Add button to generate the first step.
   3. Set the parameters of the sequence file: Control as SWEEP, Type, Mode, and Range in Configuration as PSTAT, CYCLIC, and AUTO, respectively, Ref as Eref, Initial (V) and Middle (V) as 0.0, and the Final (V) as 800 x 10^-3.
   4. Conduct CV at scan rates of 5, 10, 20, 50, and 100 mV/s. For this, create five identical steps, and set the scan rate (V/s) to 5 x 10^-3, 10 x 10^-3, 20 x 10^-3, 50 x 10^-3, and 100 x 10^-3 for the aforementioned scan rates taken in order. Set the other parameter values as the same as those in step 5.4.3.
      1. In each scan rate, set the Quiet Time(s) value as 0 and Segments as 21. The formula "2n+1" (n is the number of desired cycles) was used to determine the value of the Segments. For the Cut Off condition, Item was set as Step End and Go Next as Next.
      2. In the Misc. setting, set the Item Value as Time(s) and OP as >=. The Delta Value expresses the conditions for data collection. To collect nearly 300 data points at each scan rate, set the Delta Value as 0.9375, 0.5, 0.25, 0.125, and 0.0625.
      3. For the cycling stability test, set the scan rate (V/s) to 100 x 10^-3 and set Segments as 2001 for the 1000 cycle test. Set the other parameter values as the same as those in step 5.4.4.2.
   5. To save the sequence file for the CV test, click on the Save As button.
   6. Click on Apply to CH and run the sequence file of the CV test to obtain the results.
5. GCD test
   1. Run the program as mentioned in step 5.3.1 and create a new file for the GCD test.
   2. Click on Experiment to generate a new file, and then click on the Add button to generate the first step.
   3. Set the parameters of the sequence file. Set Control as Constant. Set the Type, Mode, and Range in Configuration as GSTAT, Normal, and Auto, respectively. The GCD test starts with a charge.
   4. Set Ref. as Zero. The current(A) value depends on the current density and electrode weight. A current density of 1 mA/g was selected for the GCD test.
      1. For the Cut Off condition, click on Item and set it as Voltage. Set OP as >=, Delta Value as 800 x 10^-3, and Go Next as Next. For the Misc. setting, set the Item as Time(s), OP as >=, and Delta Value as 1.
   5. Click on the Add button to create the next step (Discharge step).
      NOTE: This step is set the same as the charge step, but the current direction is different.
      1. For the discharge, the value of the current is the same as the charging flow, but the current direction is the opposite. Set the value of Current(A) to be the same as step 5.5.4, which is a charing step.
      2. For the Cut Off condition, set the Item as Voltage, OP as <=, Delta Value as 0, and Go Next as Next. For the Misc. setting, set the Item as Time(s), OP as >=, and Delta Value as 1.
   6. Click on the Add button to create the next step (Loop step).
      1. Set the Control as Loop, and for Configuration, set the Type as Cycle and Iteration as 21.
      2. For Condition-1 of the Cut Off condition, set the Item at List 1 as Loop Next. For each current density, set Go Next as SETP-2 for 1 mA/g.
   7. Click on the Save As button to save the sequence file of the GCD test.
   8. Click on Apply to CH, and run the sequence file of the GCD test to obtain the results.

6. Stretchable gel testing

1. Prepare rectangular shape gel films with dimensions of 1 cm × 10 cm, and repeat the following steps at least twice with different samples to obtain a reasonable value.
2. Fix the prepared sample between two grips of the tensile testing machine. In this study the gap was set as 5 cm.
3. Set the desired gap value by adjusting the button to lower the grip on the top.
4. Run the program to generate the sequence file.
   1. Choose the test method. Here, the stress-strain test was selected.
   2. Next, choose the number of trials, and apply the selected number. Then, check the test conditions, and set the stretching rate to 50 mm/min.
   3. Save the file and apply it to the program. Then, click on the Start button.

7. Stretchable gel deformation test

1. Prepare rectangular shaped gel films with dimensions of 1 cm × 10 cm, and repeat the test twice.
2. Fix the prepared sample between two grips of the tensile testing machine with a gap of 5 cm.
3. Run the program to generate the sequence file.
   1. Choose the test method. Here, select the stress-strain test.
   2. Select the number of trials and apply the selected number. Then, check the test conditions, set the stretching rate to 50mm/min, and the displacement to 10 mm. Repeat this procedure 10 times.
   3. Save the file and apply it to the program. Then, click on the Start button.

Results

PVA was widely applied as a polymer-matrix-based electrolyte for SCs because it is biodegradable, inexpensive, chemically stable and non-toxic, has a wide operating temperature range, and has a transparent-film forming capability^10,11. PVA enhances ionic conductivity due to its hydroxyl groups which absorb water¹². In this study, we prepared the PVA-based gel electrolyte by mixing H₃PO₄/H₂O, which served as...

Discussion

Our approach for developing stretchable SCs involved the synthesis of GPEs and their subsequent evaluation in prototypical coin cells. In particular, the PVA- and PVDF-HFP-based GPEs were tested in coin cells with symmetric AC electrodes or SUS plates. The critical steps in this approach include 1) preventing bubble generation during the preparation of GPEs, 2) developing a cell assembly procedure that accords with a working supercapacitor, and 3) setting an appropriate experimental range.

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Materials

Name	Company	Catalog Number	Comments
1 M LiPF6 in EC/DMC=1/1, v/v	Sigma aldrich	746738	Electrolyte for pvdf-hfp polymer based gel electrolyte
Activated carbon	Sigma aldrich	902470	Active material
Ag/AgCl electrode	BASi	RE-5B	Reference electrode
Carbon black	Sigma aldrich	699632	Conductive material
Diamino-poly (propylene oxide) (DPPO)	Sigma aldrich	80506-64-5	corss linking material for pvdf-hfp polymer based gel electrolyte
Diglycidyl ether of bisphenol-A (DEBA)	Sigma aldrich	106100-55-4	corss linking material for pvdf-hfp polymer based gel electrolyte
Dimethylformamide (DMF)	Samchun	D0551
Electrode pressing machine	Rotech	MP200
Extractor	WonA Tech		Convert program (raw data to Excel )
Isopropanol(IPA)	Samchun	I0346	Solvent to melt the binder
Phosphoric acid	Samchun	00P4277
poly (ethylene glycol) diglycidyl ether (PEGDE)	Sigma aldrich	475696	corss linking material for pvdf-hfp polymer based gel electrolyte
Polytetrafluoroethylene(PTFE)	Sigma aldrich	430935	Binder
polyvinyl alcohol (PVA)	Sigma aldrich	9002-89-5
Polyvinylidene fluoride-co-hexafluoropropylene (PVDF-HFP)	Sigma aldrich	427160
Potentiostat	WonA Tech	Zive SP1
Pt electrode	BASi	MW-018122017	Counter electrode
Smart management 6(SM6)	WonA Tech		Program of setting sequence and measuring electrochemical result
Sulfuric acid	Samshun	S1423	Electrolyte
Tensile testing machine	Nanotech	NA-50K	tensile testing machine
Zman	WonA Tech		EIS program