Coin Cell Assembly Mistakes That Look Like Chemistry Problems

When a new material cycles badly, it's natural to blame the chemistry. But coin cells are small and unforgiving, and many "material problems" start at the assembly bench. Before you rework a synthesis, rule out these common assembly mistakes.

The telltale sign: scatter between cells

If cells built from the same electrode sheet disagree with each other, suspect assembly before chemistry. A material problem tends to show up the same way in every cell. An assembly problem shows up in some cells and not others. Build at least three cells per condition so you can tell the difference.

Electrode alignment and sizing

A misaligned electrode leaves part of the cathode facing no anode. In full cells, the anode should overhang the cathode on all sides. Otherwise lithium can plate at the edges, and you'll see capacity loss or sudden shorts. Center the electrodes carefully and check the punched edges for burrs, which can pierce the separator.

Crimping pressure

Too little pressure and the cell can leak or slowly dry out, so impedance creeps up over time. Too much pressure can deform the case and damage the stack inside. Use the same crimper setting for every cell, and check sealed cells for bulging or electrolyte at the edge.

Stack height: spacers and springs

The internal stack needs steady, even pressure. If the stack is too loose, contact is poor and resistance is high, which shows up as noisy or low capacity. If it's too tight, you risk crushing the separator and causing soft shorts. Choose spacer thickness and spring type to match your electrode thickness, and keep that combination the same across a test series.

Electrolyte volume

Too little electrolyte leaves the electrodes partly dry and causes early fading. Too much leaks during crimping and makes cells inconsistent. Measure a fixed volume with a pipette for every cell rather than adding "a few drops."

Moisture and contamination

Water reacts with common lithium salts and hurts performance. Dry your electrodes, separators, and cell parts before they go into the glovebox, and watch your glovebox moisture and oxygen levels. Stray particles on a case or spacer can cause self-discharge that looks like a side reaction.

Case material at higher voltages

Standard stainless steel cases can corrode on the cathode side at higher voltages. That shows up as extra capacity or poor Coulombic efficiency that isn't coming from your material. If you're testing at high cutoff voltages, consider a more corrosion-resistant case option. Our [cell components guide](LINK TO GUIDE 01) covers case choices.

Give cells time to wet

Let freshly assembled cells rest before cycling so the electrolyte can fully soak into the electrodes and separator. Testing too soon can make the first cycles look worse than they are.

Before you blame the material, check:

• At least three cells per condition, and how much they scatter
• Electrodes centered, anode larger than cathode, no burrs
• Same crimper setting for every cell
• Spacer and spring matched to your electrode thickness
• A fixed, measured electrolyte volume
• Parts dried, glovebox levels in range
• A case suited to your voltage window
• A rest period before the first cycle

Quick check: 3 questions

1. Cells made from the same electrode sheet give very different results. What should you suspect first?
A) The material   B) Assembly   C) The cycler

2. In a full cell, which electrode should be larger?
A) The cathode   B) The anode   C) It doesn't matter

3. At a high cutoff voltage, you see extra capacity and poor Coulombic efficiency. What could be causing it besides your material?
A) Case corrosion on the cathode side   B) Too much spring pressure   C) The separator color

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