The overall goal of this study is to demonstrate construction of a reliable three-electrode coin cell setup to investigate degradation phenomena in lithium-ion cells while simultaneously identifying the influence of the anode and cathode. This method can help answer key questions in the energy storage and scepter field, such what's the role of anode and cathode in the lithium-ion cells's degradation phenomena. The main advantage of this technique is that cell construction simplicity, along with the measurement reliability, of electrochemical properties.
The implications of this technique extend toward monitoring the electrochemical characteristics of anode and cathode separately during charge and discharge process due to the presence of the reference electrode. Construct the reference electrode with enameled copper wire. Cut a 120 millimeter-length wire for each three-electrode cell.
Place one end of wire about ten millimeters into a lab press and apply four megapascals of pressure. After removing the wire, cut it so the flattened section is about two millimeters long. Next, carefully use a scalpel to remove the outer insulation on the flattened end.
Put the wire aside and move on to make the slurry. Obtain a solution of 10%weight PVDF and NMP. Separately weigh out and add to the same mortar lithium titanate powder, synthetic graphite, and a conductive additive.
Mix and grind the powders until the mixture is homogenous. Next, transfer the powder mix to a 20 milliliter disposable mixing tube. Then, use a pipette to add NMP solution.
Continue by adding 16 six millimeter diameter silicate glass mixing balls before screwing on the cap. Use a high sheer mixing device to mix the slurry. After mixing, work with the slurry and the previously-prepared exposed wire.
By hand, dip the exposed copper tip into the mixed slurry. On removal, attach the wires to a base, cast end up, to dry. Dry the electrodes in a laboratory oven at 70 degrees Celsius for at least eight hours.
When all of the components and electrolyte are ready, put them in an argon glove box. Here are the case and cap, gasket, polypropylene separators, stainless steel spacer, and wave spring. Have the reference electrode bent into a spiral shape using pliers.
Inside the glove box, prepare two clean disks of lithium. Get one disk of lithium and a stainless steel spacer. Place the lithium disk at the center of the spacer and then press the two together firmly.
Check that the lithium sticks to the spacer. Now, work with the coin cell case and a small weigh boat and center the second lithium disk inside the case. Press it firmly and ensure that it sticks.
Place several drops of electrolyte on the lithium disk. Place several more drops around the edge of the lithium to fill the outside gap. Next, place a polypropylene separator on top of the wetted lithium disk.
When the separator is completely wetted, place the gasket in the cell with its lip facing upwards. Use a pair of plastic tweezers to gently place the reference electrode spiral in the cell's center. Add a few drops of electrolyte around the reference electrode.
Then, place a small square separator where the wire crosses over the gasket and case. Continue by placing a polypropylene separator on top of the reference electrode. Retrieve the lithium spacer disk and put it on top of the assembly, lithium side down.
Next, place the wave spring on top of the spacer. Then, use plastic tweezers to carefully place the cap on the entire assembly. Employ a crimper to crimp the coin cell to about five megapascals.
At this point, the cell can be removed from the glove box. Clean and dry the cell before going on. To seal it, use a toothpick to apply a small amount of non-conductive epoxy where the wire exits the coin cell.
For lithiation, set up the testing device. Place a small square of electrical tape across the top of the coin cell case and place it in the cell holder. Use an alligator clip to connect the reference electrode to the cell holder's positive connection.
Cycle the reference electrode several times at the appropriate voltage range. Take note of the reference voltage, which should occur during charging and discharging processes. Return the cell to the argon glove box equipped with the materials to make a working cell.
Place the cathode disk in the center of the cell case. Then, put several drops of the electrolyte onto the top and around the edge. Next, place a separator on top of the wetted electrode.
On top of that, place the gasket with the lip facing up. Put the assembly aside and get the lithiated preparation cell. Apply electrical tape on the case top and hold it with thin-nosed pliers.
Use cutting-end pliers to carefully yet firmly pry open the edge of the coin cell. When it is possible, separate the cell case and cap and carefully extract the lithiated reference electrode. With a pair of pliers, unbend the spiral of the reference electrode and straighten it.
Bring the working cell and the reference electrode together. Place the reference electrode's tip in the center of the working electrode. It will extend over the edge of the cell.
Re-bend the wire at the edge of the cell and place a small rectangular separator where the wire crosses the case. Next, position a separator on top of the reference electrode. Top this with the prepared anode disk.
Carefully place a one millimeter stainless steel spacer on the anode. Then, place the wave spring on the assembly. Now, fill the cell to the brim with electrolyte.
Use plastic tweezers to carefully place the cell cap on the assembly. Transfer the cell to a crimper and crimp the cell to five megapascals. After removing the cell from the argon box, clean and seal it.
Allow the completed cell to dry for at least one hour. Take the cell to an electrochemical measuring device. Connect the cell as in this schematic.
The positive power and positive sensor connect to the cathode. The negative power and sensor connect to the anode. The reference connects to the reference electrode.
Cycle the full cell at the desired C rate and simultaneously measure full cell cathode and anode potentials. Move the cell to the electrochemical impedance spectroscopy device. Connect the positive power and sensor to the cell's cathode and the negative power and sensor and reference sensor to the anode.
Keep the reference electrode disconnected. On the control panel, select an amplitude of ten millivolts. Select the frequency range of one megahertz to one millihertz.
Start collecting the impedance of the full cell. Here are the voltage measurements at constant current for an anode, using the scale on the right, and a cathode, and both a two-and three-electrode full cell using the scale on the left. The charging rate is 1/10th the calculated maximum.
The full cell voltages for the two-and three-electrode cells are the same, suggesting that the reference electrode does not modify behavior. Compare these with voltage measurements during discharge. During charging, the cathode potential increases with respect to lithium slash lithium plus as lithium moves from the cathode to the anode.
The potential at the anode decreases. The voltage measurements during discharge demonstrate the opposite occurs under those conditions. This is an example of the impedance spectrum collected for the three-electrode full cell as a function of the state of charge.
Here are additional spectra for the cathode and anode. The data are useful in finding individual contributions to the electrode impedance as the state of charge changes. Once mastered, this technique can be done in five days, including cycling and impedance tests, if it's performed properly.
While attempting this procedure, it's important to remember that the reference electrode wire is thin, and can break if not handled properly. Following this procedure, fast-charting and safety tests can be performed in order to answer questions like Is it possible to fast-chart a cell without inducing any degradation? And What's the role anode and cathode on failure events?
After its development, this technique paved the way for researchers in the field of energy storage to diagnose failure and improve the lifetime of lithium ion batteries. After watching this video, you should have a good understanding of how to build a stable and reliable three-electrode cell using typical components found in any electrochemical lab. Don't forget that working with lithium compounds, organic electrolytes, and electrical equipment can be extremely hazardous, and precautions, such as using personal protective equipment, should always be taken while performing this procedure.
