How much electrolyte should be used in a cell?
Keep the cell “thirsty”. Just add the amount of electrolyte required to fully soak the porous components of the electrode stack – separator and electrodes, not much more. 100 µl is a good starting point and will work well with most LiB configurations. A large excess of electrolyte may cause contamination of the cell bottom and the reference pin. As a consequence, the reed contact of the insulation sleeve may lose contact, which in turn may result in scatter of the half cell voltages VxR.
We consider glass fiber Whatman (GF/A 260 µm thick) as best for general purpose use because of its superior wettability. Next best is the 2-layer-separator FS-5P made of a PP fiber layer (180 µm) and a highly porous UHMW-PE microporous membrane (40 µm). The FS-5P separator is glass-free, but only second-best in terms of wettability, reproducibility and impedance artefacts. Technical polyolefine (Celgard-like) separators are possible, but often show wettability issues. Don’t use them for single-electrode impedance measurements.
Finger type reference, which separator?
Finger type reference electrodes should only be used with glass fiber separators. We have good experience with Whatman GF/A 260 µm thick. Celgard type separators are not working well. Gas bubbles easily get enclosed between the two separator layers especially along the finger and cause artefacts due to current inhomogeneities.
Re-use (PEEK) variant of insulation sleeve: Can the standard reed contacts be re-used?
Yes, possible. However, best results with new reed contacts. If re-using: Clean with detergent wash, rinse with water, dry with compressed air, don’t touch the contact area at the reed contact, don’t bend too much. A little ultrasonic cleaning may help, but be cautious. We have seen the polyimide coating peeling off in case of excessive ultrasonic treatment.
Is it possible to use insulation sleeves in aqueous media?
Both the single-use variant (made of PP) and the re-use variant (PEEK) of the insulation sleeve can be used with many aqueous electrolytes. Just make sure that all components – sleeve, reed contact and separator – are compatible with the used electrolyte. The least problems are with neutral and alkaline electrolytes (such as KOH), while strongly acidic electrolytes (such as H2SO4) are often not compatible.
Glass fiber (such as Whatman GF/A) or polypropylene fiber separator (such as Freudenberg FS2226) is compatible with many aqueous electrolytes. Note that the two-layer FS5P separator does not work because of the PE membrane layer; it cannot be wetted with aqueous electrolytes. For the same reason, Celgard-like separators do not work.
In most neutral electrolytes, activated carbon (either free-standing or coated on stainless steel 1.4404) works well as a pseudo reference material. Activated carbon is known to corrode in strongly alkaline electrolytes (such as potassium hydroxide) and so cannot be used here.
What is the right plunger height?
The proper plunger size depends on the thickness and compressibility of the lower electrode and the built-in separator. The size is given as a number at the bottom of the lower plunger. The thicker the lower electrode and the separator are, the larger the number must be. For calculation, please use our plunger size calculator.
We offer plunger sizes only in steps of 50 µm. We have found that in-between sizes do not systematically improve behaviour. The right plunger size is especially important for single electrode impedance measurements, while it is much less critical for full cell impedance measurements and constant current cycling. It is generally acceptable to be +-50 µm off the calculated value even for single-electrode impedance measurements.
We have visualized the different scenarios in a video.
Single electrode impedance measurements – How to minimize artefacts?
Single electrode EIS measurements are especially prone to artefacts due to current inhomogeneities inside the electrode stack and imperfect placement of the reference electrode. Such artefacts cannot be completely eliminated, but can be minimized by the following measures.
- Use insulation sleeves with thick (260 µm) glass fiber separator (rather than separators with only some 10 µm thickness)
- Use two additional 18 mm glass fiber separators, one below and the second above the built-in separator.
- Use the proper lower plunger size.
- Observe the different quality of the two half-cell impedances. The larger of the two impedances is relatively less distorted. Note that the ratio Z1/Z2 is a function of both SOC and frequency.
Disposal of lithium rings / insulation sleeves with lithium ring
The Li metal ring is actually a stainless steel ring coated with about 1 mg of lithium metal. Just 1 mg. After use, the ring (or the sleeve with built-in ring) can just be disposed of in a beaker and allowed to react in the ambient air.