1.Rolling Principle
Roll pressing is a process that utilizes a roll press machine (as shown in Figure 1,the roll press machine used in the industry consists of three core components: a pair of rollers, an unwinding device, and a rewinding device) to compress the thickness of the electrode (as shown in Figure 1). This compression increases the compaction density of the electrode coating, reduces the thickness of the electrode, and ultimately enhances the energy density of the battery.
2.Rolling Equipment
Loading Sequence: Loading Device - Correction Device - Edge Trimming Device - Tension Device - Preheater - Rolling Mills - Tension Device - Correction Device - Winding Device
3.Rolling Effect
1）Improving Electronic Conductivity：
When the compaction density of a porous electrode is low, there is poor contact between the
particles, resulting in a necessarily high contact resistance. As the compaction density increases,
the contact between the particles becomes tighter, leading to a decrease in contact resistance.
However, once the compaction density reaches a certain range where the contact between
particles has achieved an optimal value, further increasing the compaction density will not affect
the contact resistance.
From the graph, it can be observed that when the compaction density is less than3.8g/cm3, the
sheet resistance gradually decreases as the compaction density increases. Once the compaction
density exceeds 3.8g/cm3, further increasing the compaction density does not significantly affect
the sheet resistance, which remains relatively constant.
Note: The following figure illustrates the relationship between the compaction density and resistivity of an electrode made with the formula of LiCoO2: Super P:PVDF = 97.5%: 1.4%: 1.1% (materials sourced from Canrd: www.canrd.com).
2）Improving Electronic Conductivity
3）Improving Energy Density
From the figure below, we can observe that for the same model of battery, using different compaction density designs results in an increase in energy density by approximately 13% when comparing a higher compaction density to a lower one. This increase is accompanied by a similar gain in capacity (~13%) and a length extension of ~14%. When unpacking the battery electrode sheets, the difference becomes evident. Due to the fixed volume, by increasing the compaction density of both the positive and negative electrodes, the thickness of the electrode sheets decreases, allowing for more length and space to accommodate more active materials. This, in turn, releases more capacity and subsequently enhances the volumetric energy density of the battery.
4）Improving Adhesion
A comparison was made between the adhesive force of the rolled and unrolled positive electrode. From the data, it can be seen that the adhesive force of the rolled electrode has been improved. In addition, mainly looking at the outside of the electrode after the adhesive force test, the surface of the aluminum foil of the rolled electrode is darker, indicating that the bonding between the current collector and the particles has indeed been improved, which is conducive to the performance.
15.6N/m 12.3N/m Tensile tester
4.Effect of roll pressing
Ionic conductivity (electrode porosity)：
1）Ionic conductivity refers to the ability of a material to conductions. The stronger the ionic conductivity, the smaller the resistance of the material to ion conduction. For porous electrode materials, their ionic conductivity usually refers to the ability of lithium ions to transmit from the surface of the electrode into the underlying layer of the porous electrode (i.e., from the surface of the electrode to the current collector) after being immersed in the electrolyte.
2）As the compaction density increases, the porosity of the electrode gradually decreases (which inevitably leads to a decrease in its ionic conductivity). On the other hand, due to the increase in compaction density, the thickness of the electrode decreases, thereby shortening the ion transport path.
5.Rolling Effect
1）Battery rate performance：
Rate performance refers to the ability of a battery to charge and discharge under different
current densities. The rate performance of a battery is determined by various factors in a
complex system. When considering only the impact of compaction density on battery rate
performance while fixing other factors, the main influencing factors are: the better the electronic
conductivity and ionic conductivity, the better the battery's rate performance.
Taking lithium cobalt oxide as an example, when the compaction density of the electrode is
between 3.8~4.1g/cm3, the battery can maintain the best rate performance (rate performance
value = 2C discharge capacity / 0.2C discharge capacity, tested using a 2Ah soft-packed battery).
2）Electrolyte Retention：
During the entire cycling process of a battery, the internal resistance continuously increases,
and the electrolyte is continuously consumed (especially at the negative electrode, where
consumption is fastest due to the constant destruction and repair of the SEI layer). The cycle
retention rate continues to decline. With this background knowledge, we understand that the
compaction density of the positive and negative electrodes cannot be designed solely for energy
density. Instead, the compaction density must be reduced to ensure sufficient electrolyte
consumption during the long cycling process. This ensures that the internal resistance of the
battery does not increase sharply, leading to cycle decay. (Cyclic performance corresponding to
different compaction densities)
3）Integrity of Material Particle Structure
The rolling process involves passing the electrode sheet through a pair of rollers, with pressure
applied to the rollers and transmitted to the surface of the electrode sheet through
"line-to-surface contact". The contact area is very small(typically in the range of several square
centimeters). The pressure applied during the rolling process is enormous. Taking the rolling
process for lithium cobalt oxide electrode sheets as an example, the applied pressure is typically
at the level of 100 tons. As a result, the pressure borne by the electrode material during the
rolling process is extremely high: approximately 1,000,000,000 Pa, or 1,000 MPa, which is
equivalent to about 10,000 times the atmospheric pressure. Therefore, it is crucial to evaluate
the structural integrity of the material itself during the rolling process.
For scientific researchers developing practical electrode materials, it is crucial to consider the
material's resistance to compression, meaning that the material must maintain its structural
integrity under extreme pressure. Failure to do so would make it difficult to apply the material in
practical production settings.
Note: SEM images of mass-produced lithium cobalt oxide electrode sheets (with a formula of LiCoO2: Super P: PVDF = 97.5%: 1.4%: 1.1%) with a compaction density set at 4.25g/cm3 (materials sourced from Canrd: www.canrd.com)electrode sheets)
6.Canrd Brief Introduce
Canrd use high battery R&D technology(core members are from CATL) and strong Chinese supply chain to help many foreign companies with fast R&D. We provide lab materials, electrodes, custom dry cells, material evaluation, perfomance and test, coin/pouch/cylindrical cell equipment line, and other R&D services.
Email： contact@canrd.com Phone/Wechat/WhatsApp： +86 19867737979
Canrd Official Web Canrd Company Vedio Canrd Company profile
Website : www.canrud.com
7.
Q & A
The final factor, which tends to receive less attention, is the structural stability of materials. This is especially critical for many emerging silicon-carbon materials. For example, the yolk-shell structure designed by Cui Yi's group reserves significant space for internal silicon expansion. However, such materials cannot withstand standard compaction processes due to their structural instability, making them impractical for commercialization. Therefore, if you aim to develop application-oriented materials, the structural stability after compaction must be carefully considered. Without this, it will be challenging to implement these materials in practical production.
During the Q&A session, participants actively raised questions, and Dr. Ke provided detailed answers to each one.
"How can the calendering effect be assessed?"
Dr. Ke:
"Generally, it can be evaluated from several aspects. First, the appearance—check if the calendered electrode shines. Second, check if the thickness meets the target. Lastly, assess the flexibility of the electrode after calendering."
"Wouldn’t point-to-surface contact be better, using graphene?"
Dr. Ke:
"The resistance effect of graphene has not been effectively addressed yet. Additionally, most current graphene materials have multiple layers, and their conductivity is inferior to that of CNTs."
"Can you elaborate on edge trimming?"
Dr. Ke:
"Edge trimming primarily addresses the issue of current collector extension in the coated region during calendering while the uncoated region remains unaffected. The high pressure during calendering can lead to an elongation rate of over 1% for the current collector in coated areas, while the uncoated regions, not subjected to calendering, do not elongate. This can cause wrinkles at the junction between coated and uncoated regions."
"Does the anode extend as well?"
Dr. Ke:
"Both the cathode and anode extend, but the anode's extension is less significant."
"How can you tell if an electrode is over-calendered or under-calendered just by looking at the surface?"
Dr. Ke:
"A simple surface inspection can indicate over-calendering if the electrode appears very shiny and brittle."
"Previously, some active material fell off during calendering. Any tips for preventing this?"
Dr. Ke:
"This can happen if the rollers aren’t clean, the electrode isn’t properly dried, or the bonding strength is insufficient (issues with the current collector can also contribute)."
"Are there appropriate methods to measure roller gaps?"
Dr. Ke:
"Equipment often comes with roller gap testing devices."
"Can the cathode and anode use the same conductive additive?"
Dr. Ke:
"Yes, they can."
"Isn’t the anode more prone to sticking to the roller and wrinkling during over-calendering?"
Dr. Ke:
"Yes, the anode is more prone to roller sticking. This is mainly because the anode's bonding strength is weaker and insufficient drying can cause adhesive migration to the surface. For higher compaction, a two-step calendering process is recommended instead of applying excessive pressure in a single step."
"Are there images of properly calendered electrodes? Should they appear slightly shiny?"
Dr. Ke:
"The appearance of calendered materials varies depending on the material. The most reliable control method is to use compaction density as a metric."
"What does adhesive migration look like?"
Dr. Ke:
"This is an issue with coating. Adhesive migration isn’t visible to the naked eye but can be identified through elemental analysis of the surface and bottom layers of the coating (e.g., checking F content for cathodes). While not easily observable, surfaces with adhesive migration tend to feel sticky and are prone to roller sticking."
"How is adhesive strength tested?"
Dr. Ke:
"It’s tested using specialized tensile strength testing equipment, typically by measuring the peel strength."
"What dimensions of electrodes and adhesive areas are ideal for tensile strength testing? What are acceptable peel strengths for cathodes and anodes?"
Dr. Ke:
"Electrodes with a width of 30mm are commonly used, with adhesive areas based on internal production standards. Results vary based on testing conditions, so setting an internal benchmark is more practical."
"How should the two layers be separated for peel strength testing?"
Dr. Ke:
"The tensile testing machine can pull them apart. Choosing the right adhesive tape is also important."
"Does more electrolyte retention always mean better performance?"
Dr. Ke:
"Excessive electrolyte retention has downsides:
1.The battery becomes softer.
2.The appearance may suffer.
3.Production costs increase.
For lithium-sulfur batteries, excess electrolyte can also promote polysulfide dissolution and diffusion, degrading performance."
"So there’s an optimal range for electrolyte retention?"
Dr. Ke:
"Yes, the optimal range varies by system and process."
"If lab coatings are 100μm, is calendering still necessary?"
Dr. Ke:
"Calendering is recommended regardless. The purpose of calendering remains essential. If you don’t need those benefits, you can skip it, but it’s usually advised."
"What equipment is used to measure electrode conductivity after calendering?"
Dr. Ke:
"Four-point probe testing is used by applying the slurry on a non-conductive PET film and measuring its resistance."
"How is electrolyte retention measured in electrodes?"
Dr. Ke:
"By determining the porosity of the electrodes, which ultimately relates to controlling their compaction density."
"How is porosity measured?"
Dr. Ke:
"Two common methods:
1.Theoretical calculation.
2.Direct measurement using a mercury porosimeter, though this method is less common due to mercury toxicity. If feasible, experimental measurement is preferred, though theoretical calculations are sufficient for relative comparisons."
"Could particle size affect material density, leading to theoretical calculation errors?"
Dr. Ke:
"Yes, true material density assumes fully compacted materials. Porosity and secondary particle effects introduce errors, so this method is more suitable for relative comparisons."
"How is porosity calculated theoretically?"
Dr. Ke:
"Determine the electrode volume, weight, and material density (including conductive additives and binders). The remaining volume is the porosity."
"Can you explain the difference between roller gap and pressure control for calendering?"
Dr. Ke:
"The trend is toward 'large pressure, large gap,' ensuring minimum electrode thickness is pressure-controlled. This reduces the risk of over-calendering, as excessive pressure has a threshold. However, 'small pressure, small gap' can cause over-calendering if coating thickness varies, as the fixed gap doesn’t allow for deviations. Equipment for 'large gap, large pressure' tends to be more expensive."
"For lithium-sulfur batteries, is calendering necessary?"
Dr. Ke:
"Calendering is still required but at much lower pressures since the compaction density for lithium-sulfur is significantly lower."
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