How To Make Coin Cells

/ Battery Research, Library

The article talks about how to make high-quality coin cells. The performance of new materials in lithium-ion batteries is usually evaluated with hand-made half coin cells with the new material as the positive electrode and a piece of lithium chip as the negative. Half coin cells are easy to make and can give reproducible data. A full cell in the form of coin cells or pouch cells would more accurately predict the performance of active materials in real lithium-ion batteries.

Active Materials and Binder:

Current Collectors

  • Aluminum foil (cathode)
  • Copper foil (anode)
  • Lithium foil

Coin Cell Type Battery Parts:

Equipment:

  • Mortar and pestle
  • Spoon
  • Pipette
  • Electronic Balance
  • Wipes
  • Alcohol
  • Hotplate magnetic stirrer
  • Blade & tape/doctor blade/notched bar
  • Glass slide
  • Cotton swab
  • Vacuum oven
  • Calender
  • Punches:
    • 15mm cathode
    • 19mm anode
    • 20mm separator
  • Micrometer
  • Weighing paper
  • Insulated tweezers
  • Glovebox
  • Nonconductive surface (sheets of paper/Plexiglas)
  • Pipette
  • Crimper
  • Marker Pen

Preparation:

  • Clean a mortar and pestle. Dry it in the oven.
  • Keep all materials and coin cell parts as dry as possible. If not, dry them in the oven (with vacuum). The coin cell parts should be put in a glovebox before use.

Coin Cell Assembly Procedure:

  1. Weigh the amount of active material. The electrode formulation was 96% active material, 2% PVDF binder, and 2% Super S carbon black conducting diluent by weight. This ratio is the one most close to that used in commercial lithium-ion batteries. In some labs, the ratio of 85% active material, 7.5% CB, and 7.5% PVDF is used. For the anode, the electrode formulation is 96% active material, 2% CMC/SBR binder and 2% Super S carbon black conducting diluent by weight. It works better to dissolve the binder in the solvent before use.
  2. Once all the powders are weighted into the mortar, use a pipette to drop the NMP into it. Grind the mixture until a viscous slurry forms. Grind for at least half an hour to ensure that the active material is fully mixed with the carbon black and binder.
  3. Casting the slurry with a doctor blade, or a notched bar. For this method, take a glass slide and drop some NMP on it. Place the current collector (copper foil for low voltage electrodes, or anodes, and aluminum for higher voltage electrodes, or cathodes) and with a cotton swab flatten and clean the foil. Make sure to remove any bubbles from underneath the foil. Place the tape along the sides, making a channel narrower than the width of the blade. Two layers of tape will give films of the desired thickness, but if you need thicker or thinner films you can add or remove layers of tape. Remember that the thickness of the wet film will be greater than that of the dry film.
  4. Pour the slurry on the foil, preferably close to the top, making a tooth shape. Quickly take the blade and drag it along. Pass it again if you are not pleased with the quality of the surface. Place it in a forced-air oven at 60~80 degree Celsius. Once it is dried for at least 6 hours, it can be taken out.
  5. Calendering is optional. Calendering will reduce the porosity of the film and improves the tap density. To do this, get the calender to the appropriate thickness, and feed your film through. The film will bounce back a little after calendering; make sure to take this into account when setting the thickness.
  6. Punch you electrodes out of the strip. To minimize edge defects, sandwich your strip between two weight papers. Make sure that the diameter of the working electrode is smaller than that of your counter electrode (lithium in our half cells), and both are smaller than the separator. For a 2032 coin cell (meaning 20 mm in diameter, 3.2 in height), the separator will be the full 20mm in diameter. Lithium foil already cut in disks will be 15.6 mm in diameter, so we can use the 12~15mm punch for our working electrode.
  7. Transfer the small disks into a vacuum oven and try overnight at 120 degree Celsius. Cool it down naturally before taking them out.
  8. Record the weight of each disk and put them in individual plastic pages. Mark the bag to keep track of which weights how much.
  9. Introduce the electrodes in the glovebox. Remember to flush the chamber at least twice, and place gloves on top of the glovebox gloves to avoid contamination. Keep the electrodes in the glovebox for hours. It helps to dry the electrodes since it may absorb some moisture during the weighing.
  10. To assemble a large number of coin cells quickly, align all the parts on top of a non-conductive surface (to avoid accidental shorting of the cell). Press the lithium disks to the stainless steel spacers. Label each coin cell with the corresponding one from the records. Place the o-ring on the smaller cap and press. Place the spring on top and then the stainless steel and lithium, with the lithium on top. Place the separator on top as centered as possible, and drop the desired amount of electrolyte. Using a fixed volume pipette will speed the process up. Place your electrodes on top, with the cast film facing the lithium. Try to center it as much as possible with the lithium to avoid uneven current densities. Place the larger cap on top.
  11. For this particular crimper, the large cap goes on bottom, so flip the coin cell and center it in the crevice. Yank down hard and take it out. Make sure that the edges are curved, otherwise the seal is not good. Once out, I recommend cleaning them with a paper and properly labeling them, to ensure your coin cells clearly match your notes.
  12. Now your coin cell is ready to test.
Diagram of Full Coin Cell
Diagram of Full Cell with (a) Two Celgard separators or (b) One BMF Separator. For Half Cell Substitue the Netagive Electrode with Lithium Chip

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