The overall goal of this experimental demonstration is to provide a step-by-step procedural guideline for lithium-ion battery electrode preparation and coin cell assembly, along with electrochemical analysis and characterization that can be performed in an academic lab setting. This demonstration highlights the importance of electroprocessing on the performance of lithium-ion batteries and its electrochemical attributes. One of the key highlights is that we're trying to emphasize the importance of evaporation in the drying step for lithium battery electrode processing and fabrication.
We hope that this demonstration will provide a step-by-step guideline to students and alike who are attempting to initiate the program in electrochemical engineering, and especially in the ADR family storage. To begin this procedure, cut a 4.5 inch by 12 inch sheet of 15-micrometer thick aluminum foil, using a paper cutter. Spray acetone on the surface of a clean plastic board, and then place the foil sheet onto the board.
Next, spray a generous amount of acetone on the surface of the foil, and begin to scrub the entire surface with small semicircle motions, using a scotch pad. Spray additional acetone on the surface, and wipe down the residue with a paper towel. Rinse the etched aluminum sheet with deionized water on the casting side.
Flip the sheet and repeat the same procedure in the bright side, and again in the dull side. After rinsing with isopropyl alcohol, place the cleaned aluminum sheet in between two paper towels. Then, place the paper towels and cleaned aluminum sheet between two flat plates and compress, using a heavy object, for approximately 20 minutes.
Following this, place 0.875 grams of lithium manganese cobalt oxide, or NMC, and 0.25 grams of carbon black into a agate mortar and pestle. Lightly mix the materials together without grinding. After a mixture starts to form, mill by hand in the mortar and pestle for three to five minutes, until a uniform powder is visibly observed.
Using a piece of white paper, transfer the mixed power into a disposable mixing tube. Then, add 16 glass balls and 5.5 milliliters of 1-methyl-2-pyrrolidone, or NMP, to the powder. Place the disposable tube onto a tube drive station and lock into place.
Turn the drive on and slowly increase to the maximum speed, allowing the contents to mix for 15 minutes. When mixing is complete, add 1.25 grams of a 10%polyvinylidene difluoride in NMP solution directly to the tube. After placing the tube back onto the drive, allow the contents to mix for eight minutes.
Next, apply a layer of isopropyl alcohol to the surface of the film applicator and place the dried aluminum substrate, shiny side down, onto the surface. Press out the excess isopropyl alcohol with a folded paper towel until all the wrinkles and solvent are removed. After removing the mixing tube from the tube drive, pour the slurry onto the surface of the substrate in a two-to three-inch line, approximately one inch from the top of the substrate.
Then, set the casting speed to 20 millimeters per second and activate the casting arm of the film applicator. Once casting is complete, lift the cast electrode from the surface of the film applicator using a thin piece of cardboard to ensure no wrinkles form on the sheet. After allowing the electrode sheet to dry for 16 hours at room temperature, dry it at 70 degrees Celsius for approximately three hours in an oven.
At this point, place the dried electrode sheet onto the previously cleaned sheet of aluminum metal. Gently place a 1/2 inch hole punch onto a region of the sheet with a uniform surface. Slowly apply pressure to the punch, and roll the pressure around the edges of the punch, to ensure a clean cut.
Remove the cut electrode from the sheet with clean plastic tweezers, and place it into a labeled vial with the electrode surface facing out. Then, further dry the electrodes in a vacuum oven at 120 degrees Celsius, at minus 0.1 megaPascal, for 12 hours to remove any remaining moisture. After removing the electrodes from the oven, weigh them within 0.0001 grams.
In a glove box, place a coin cell case into a small weigh boat and place the cathode into the center of the coin cell case. Apply one to two drops of electrolyte to the center of the electrode, and apply one drop on opposite sides of the rim of the case. Place a single 3/4th inch separator onto the surface of the electrode.
Then, place the gasket into the case, with the flat side facing down and the lip side facing up. Apply two to three drops of electrolyte to the center of the cell and place the prepared counter electrode onto the center, with the lithium side facing down. Place the wave spring on top of the centered counter electrode.
Following this, fill the cell to the brim with electrolyte, until it forms a curved, convex meniscus that covers most of the wave spring surface. Carefully place the coin cell cap on top of the cell, utilizing the tweezers to hold the cap centered vertically over the cell. Press down on the cap until it sets into the lip of the gasket.
Transfer the cell to the crimper, and ensure that the cell is centered in the groove of the crimping die. Crimp the cell to a pressure of approximately 6.2 megaPascal, and release. Clean off any spilled electrolyte.
When finished, remove the cell from the glove box. After removing the cell from the glove box, clean up any excess electrolyte and label the cell. At this point, connect the clean cell to the battery cycler.
Ensure that the terminals are correctly connected by measuring the open circuit potential. With a measured electrode mass of 0.0090 grams an aluminum disk mass of 0.0054 grams, and a rated capacity of 155 milliAmp hours per gram, determine the desired current. Next, set the schedule on the cycler to charge and discharge the cell between the upper and lower voltage levels of 4.2 volts and 2.8 volts, respectively.
Cycle the cell four times at a rate of C over 10. Then, charge the cell once at C over 10. Following the fifth C over 10 charge, perform EIS on the cell after resting for one hour.
Then, place the cell back on the cycler and discharge at C over 10. After performing another EIS analysis and placing the cell back onto the cycler, cycle the cell five times at rates of C over five, C, two C, five C, and 10 C, followed by 100 one C cycles. Determine the specific capacity of the cells at each C rate, by dividing the capacity in milliAmp hours by the mass of active material present in the cathode.
Calculate the capacity retention by dividing the average specific capacity of the last five one C cycles by the average specific capacity of the first five one C cycles. A properly cast electrode sheet should appear uniform and adhere to the current collector. Flaking of the sheet is caused by poor etching or too little NMP.
And too much NMP can cause a higher degree of porosity. Lastly, a non-uniform surface can be caused by material pooling during drying. When the cell is not properly sealed, atmospheric exposure will cause swelling of the lithium, which causes the cell to pop open.
SEM imaging of the electrode surface reveals the complexity of the cathode. The large particles are the active material, and the remaining material is polyvinylidene difluoride and carbon black. Representative cycling results for a sheet that was dried too fast and one that was properly dried are shown here.
The specific energy of the cells can be determined as the area under the discharge curve. The slope of the tail in the EIS data indicates resistance due to diffusion, and the semicircle represents the number of resistances due to charge transfer resistances. The quickly dried sheet has a larger radius, indicating higher charge transfer resistance.
The casting thickness, slurry viscosity and composition, and the degree of calendaring all have a direct impact on the porosity and thickness of an electrode sheet. A thinner sheet allows for shorter diffusion distances. And the porosity can be optimized to allow for more efficient transfer.
Once mastered, the method can be accomplished within 24 hours if performed properly. While attempting this procedure, it's imperative to wear safety gloves and maintain clean electrode sheets, followed by final cell assembly in the glove box. Following this procedure with the assembled coin cell, electrochemical test, such as charge-discharge cycling, cycling voltometry, and electrochemical impedance spectrometry can be performed routinely to categorize cell behavior and performance characteristics under different cycling and environmental conditions.
This experimental exploration highlights the importance of electroprocessing on the cell performance and electrochemical attributes. After watching through demonstration video, you should have a good understanding of the essential steps in making electrode and coin cell assembly, followed by the electrochemical test for lithium-ion batteries in the lab. Don't forget that the electrolyte can easily react with oxygen and water, and hence, the coin cell assembly must be done in the glove box.
