Electrode vacuum drying and residual moisture: what to specify

Water left in electrodes and separators reacts with the electrolyte. Post-drying and the dry room exist to keep it out.

By BatteryEquip Editorial Team · 2026-11-04

Electrodes leave the coating oven dry to the touch, yet they still hold water. Some is adsorbed from the air during calendering and slitting, some stays bound in the active material and binder. In the finished cell that water reacts with the conducting salt in the electrolyte, usually LiPF6, and forms hydrofluoric acid and gas. The result is lost capacity, swelling and shorter life. So most cell makers dry electrodes again, under vacuum, just before assembly, and keep them in very dry air afterwards.

Where post-drying sits in the line

The process chain described by Kwade and co-authors runs from mixing and coating through calendering and slitting to cell assembly. Post-drying comes after slitting or after cutting, when the electrode is in its final form, and before winding or stacking. It is done on whole coils or on stacked sheets in batch vacuum ovens, or on assembled cells, depending on the format.

The coating oven and the vacuum oven do different jobs. The coating oven removes the solvent, NMP for most cathodes and water for most anodes, at high rates. The vacuum oven removes the last traces of water slowly, without changing the electrode structure.

What controls drying

Huttner and co-authors (2021) studied vacuum post-drying of electrode coils and designed a short, two-phase procedure that lowered residual moisture in anode and cathode coils, including longer and wider electrodes, without damaging the microstructure. Their work shows the levers a buyer should ask about: temperature, pressure, the time at each stage and how far water has to travel out of a tightly wound coil. Heat reaches the inside of a coil slowly under vacuum, because there is little gas to carry it, so some procedures alternate heating at a moderate pressure with deeper vacuum.

Separators and binders limit the temperature. A separator that shrinks or a binder that softens can be damaged by aggressive drying, so the right temperature depends on the materials, and the cell maker should set it.

Moisture targets and how to measure them

Residual moisture is usually stated in parts per million by weight and measured by Karl Fischer titration on samples. Targets vary with chemistry and cell maker; anodes and cathodes often have different limits, and nickel-rich cathodes are especially sensitive. Ask the supplier to show results on your materials, measured by your method, rather than quoting a typical figure.

The dry room

After drying, the electrodes must not pick water up again. Assembly therefore takes place in a dry room with very low humidity, expressed as a dew point. A TU Braunschweig Battery LabFactory project aims to predict post-drying results from dry-room conditions, which shows how closely the two are linked. A vacuum oven that unloads into a dry room or a glovebox, sometimes through an interlocked door, protects the result.

What to specify for the oven

State the load: coil dimensions and weight, sheet stacks or cells. Then ask for the chamber volume and internal dimensions, the temperature range and uniformity across a full load, the ultimate vacuum and the time to reach it, and whether heating is by shelves, by radiation or by hot nitrogen purges. Ask how the oven connects to the dry room, how many independent chambers there are and how a door is interlocked. Ask for the record each batch produces: temperature, pressure and time, linked to the coil or batch number.

Examples on BatteryEquip

For laboratory and pilot work, compare the AOT 210 L vacuum drying oven, the TOB DZF-6050 laboratory oven and the Gelon 25/50 L oven family. For production, the Xinyuren high-vacuum battery oven, the Times high-vacuum cell drying oven and the Supercom pouch-cell vacuum tunnel furnace show different load arrangements. Electrolyte filling should follow quickly, so read the vacuum fill equipment pages together with the drying ones. The coating width page and the acceptance testing page cover the steps before and after. Ask each maker for results on your own electrodes.

Sources

1. Huttner et al., Design of vacuum post-drying procedures for electrodes of lithium-ion batteries, Batteries & Supercaps 4 (2021) 1499-1515 · checked 2026-11-04
2. Kwade et al., Current status and challenges for automotive battery production technologies, Nature Energy 3 (2018) 290-300 · checked 2026-11-04
3. TU Braunschweig Battery LabFactory, project on electrode post-drying and dry room conditions · checked 2026-11-04