Calendering

In the calendering process the dried anode and cathode are run separately between rollers to compress the active layer. This brings the thickness and porosity to a given dimension.

Get the calendering correct and it will improve: volumetric energy density, cyclic stability, rate capability of the cells and also enhance the structural stability of the active material.

The Importance of Electrode Density

The primary purpose of calendering is to maximize the amount of active material within the limited volume of the cell casing and to minimize the electrode surface area to reduce unwanted side reactions within the cell which are shorten the cycle life. Excessive electrode surface area can increase the likelihood of undesirable electrochemical side reactions, making surface area optimization through calendering essential for cell safety.

In the calendering process we have to note the impact on the electrodes:

• Aluminium electrodes typically elongate by around 1% after calendaring.
• Copper electrodes don’t elongate during the calendaring.

Cell Manufacturing Process is broken down into 3 sections:

1. Electrode Manufacturing
2. Cell Assembly
3. Cell Finishing

In total there are 14 steps in the cell assembly process.

Electrode Parameter Changes After Calendaring

During the calendaring process, plastic deformation occurs, causing the electrode length to increase slightly while slightly reducing the coating thickness. This means:

• The coating pattern may expand, and
• The material loading per unit area may decrease.

Therefore, precise calculation of coating patterns and loading levels before trial production is essential to minimize deviations in the final product.

Asymmetric Calendering

Using calender rolls with unequal speeds or diameters to process materials, providing superior control over sheet curvature, thickness, and surface properties.

Modelling

Meyer et al [2] looked at the calendering process and the compaction of a graphite anode and NMC cathode. They found that on the basis of the measured porosity reduction the compaction behavior of the line load was describable by an exponential model equation.

Quality Control

Get the parameters right and the calendering process improves the cell, get the parameters wrong and the cell quality will be questionable. Thus it is really important to make the right measurements and control the process. These are typically uniformity in:

• thickness
• density
• surface smoothness

We also have to remember that this is more complex than the simple 2D sectional image:

• Deformation and shear is very different at the centre to the edge as the edge material is free to flow.
• The centre of rolled product is compressed by the sides and will push back from the rollers. If the reduction is too great the material will build up unevenly behind the rollers. Mixtures can separate and you can get shear planes.

Roll Temperature

Hot versus Cold Roll Calendering

• Hot roll calendering provides higher energy density and adhesion by increasing compaction and removing moisture
• Cold roll calendering is better for thermally sensitive materials, eg Lithium Sulfur

Calender Roll Temperature versus Chemistry

So what is the optimum calender roll temperature versus chemistry? We looked at a number of papers to offer an initial roll temperature and reasoning behind it. This is not extensive and as we have seen there are a number of other aspects that need to be considered such as energy use, LCA and capital investment.

NMC, LFP, Sodium Ion and Lithium Sulfur are all considered.

References

1. Mozaffar Abdollahifar, Heather Cavers, Sören Scheffler, Alexander Diener, Mark Lippke,and Arno Kwade, Insights into Influencing Electrode Calendering on theBattery Performance, Advanced Energy Materials, Volume 13, Issue 40, October 27, 2023
2. Chris Meyer, Henrike Bockholt, Wolfgang Haselrieder, Arno Kwade, Characterization of the calendering process for compaction of electrodes for lithium-ion batteries, Journal of Materials Processing Technology, Volume 249, 2017, Pages 172-178

See also: Coating, Drying, Slitting