Analysis of Calendering Pressure Effects on Electrode Compression Behavior and Resistivity

1. Background: Why Electrode Calendering Pressure Matters

The calendering process is a critical step in lithium-ion battery electrode manufacturing. By applying pressure between rollers—through predefined gap or force control—at optimal speed and temperature, this step ensures electrodes achieve their target compaction density. Calendering aims to increase volumetric capacity, improve electronic conductivity, and enhance overall electrochemical performance, making precise calendering pressure control essential for both cell energy density and cycle life.

In 2022, Zhang et al.^[1] conducted a systematic micro- and macro-scale study of electrode calendering by combining Discrete Element Method (DEM) simulation with experimental trials. Their work, supplemented by the Heckel equation for compression modeling, confirmed that electrode deformation involves particle crushing, secondary particle fusion, binder network compression, and current collector surface deformation. The study also established that improved electronic conductivity after calendering stems from two simultaneous effects: enhanced internal conductive pathways between active material particles and improved tightness of contact between the coating and the current collector surface.

Building on this foundational research, we used an IEST BER2500 electrode resistance analyzer to test the conductive properties of graphite anode electrodes calendered at four different pressures (yielding four different compaction densities). Simultaneously, the instrument’s compression fixture was used to analyze mechanical compression behavior. This combined approach enables rapid, quantitative evaluation of how calendering pressure affects both the electrical and mechanical state of the electrode sheet — without requiring full cell fabrication.

Figure 1. Schematic force-displacement curve of an electrode sheet during calendering — green: experimental results; gray: DEM simulation results. Shows progressive stages of particle rearrangement, elastic deformation, plastic deformation, and fracture^[1]

2. Experimental Equipment and Test Methods

2.1 Experimental Equipment

Tests were conducted using the IEST BER2500 Electrode Resistance Analyzer (Figure 2). This instrument applies pressures from 5 to 60 MPa on a 14 mm diameter electrode sample, simultaneously measuring resistance, resistivity, conductivity, and electrode thickness — enabling combined electrode compression testing and resistivity characterization in a single instrument run.

Figure 2. (a) BER2500 appearance; (b) BER2500 structural diagram — 14 mm probe, 5–60 MPa, simultaneous resistance and electrode compression measurement

2.2 Sample Preparation and Testing

2.2.1 Sample Fabrication

Four graphite anode electrode sheets were prepared with the same slurry formulation but calendered at increasing pressures, yielding four compaction density levels:

• Electrode 1: lowest calendering pressure → compaction density 1.35 g/cm³
• Electrode 2: medium-low calendering pressure → compaction density 1.5 g/cm³
• Electrode 3: medium-high calendering pressure → compaction density 1.6 g/cm³
• Electrode 4: highest calendering pressure → compaction density 1.65 g/cm³

Compaction density was calculated by the cutting-thickness-weighing method. Higher calendering pressure consistently yielded higher compaction density, as expected from the fundamental powder compression mechanism.

2.2.2 Testing Procedure

The BER2500 steady-state test mode was used with the following parameters: pressure range 5–60 MPa, 5 MPa steps, 15 s hold at each step, followed by stepwise unloading back to 5 MPa. At each pressure point, electrode thickness and resistance were recorded simultaneously. The initial 5 MPa point was used as the benchmark for normalized thickness deformation calculations (stress-strain curves). This loading-unloading protocol separates elastic (reversible) deformation from plastic and fracture (irreversible) deformation for each electrode.

3. Data Analysis and Discussion

3.1 Electrode Compression Behavior Under Applied Pressure

Figure 3 shows the stress-strain (compression performance) curves for the four electrode sheets under the 5–60 MPa loading-unloading cycle, and Table 1 summarizes the key deformation metrics.

Key finding: Maximum deformation, reversible deformation, and irreversible deformation all decrease as calendering pressure (and compaction density) increases — following the order Electrode 1 > 2 > 3 > 4. However, the rate of decrease slows at higher pressures, indicating diminishing returns from further calendering.

This trend reflects the powder compaction mechanisms within the electrode coating. During initial calendering at low pressure, powder particles rearrange and slide into denser packing — the dominant mechanism that closes inter-particle voids. At higher calendering pressure, particles undergo elastic then plastic deformation once their yield stress is exceeded. Electrodes that have already passed through these stages at high calendering pressure have less compressible structure remaining, so they show smaller total deformation and less elastic rebound (spring-back) when compressed again during testing.

Figure 3. Stress-strain compression curves for four electrode sheets (Electrodes 1–4, compaction densities 1.35, 1.5, 1.6, 1.65 g/cm³) — maximum, reversible, and irreversible deformation all decrease with increasing initial calendering pressure

3.2 Thickness and Resistivity Response to Applied Pressure

During lithium-ion battery electrode manufacturing, the calendering process primarily compresses electrode thickness while keeping areal mass nearly constant — reducing coating thickness, increasing compaction density, and improving particle-to- particle and particle-to-current-collector adhesion. During roll-pressing, particles form micro-indentations (“pits”) on the current collector surface, increasing the contact area and mechanical adhesion between coating and current collector.

Figures 4 and 5 show the thickness change and electrical conductivity curves for all four electrodes during the stepwise BER2500 pressure test:

• Thickness behavior: Electrodes 1 and 2 (lower calendering pressure, lower compaction density) start thicker and exhibit more significant thickness reduction under test pressure, as well as greater elastic rebound on unloading. Electrodes 3 and 4 (higher calendering pressure, ≥1.6 g/cm³) are denser, deform less under the same test pressure, and show less elastic rebound — because their structure is already highly consolidated.

• Resistivity behavior: The resistivity of Electrodes 1 and 2 changes more significantly with applied test pressure than Electrodes 3 and 4. This occurs because Electrodes 3 and 4 already have tightly consolidated inter-particle contacts and coating-to-current-collector interfaces from high-pressure calendering, leaving little room for further pressure-induced conductivity improvement during the test. Notably, the absolute resistivity values of the lower-pressure electrodes may appear lower under test pressure due to their larger thickness reduction — resistivity is geometry-dependent, and significant thickness compression can numerically lower apparent resistivity even if intrinsic conductivity changes are modest. This highlights that electrode resistivity assessment must account for the test geometry and report both thickness and resistance together.

Figure 4. Thickness vs pressure curves for four electrode sheets (5–60 MPa, loading and unloading) — Electrodes 1 and 2 show larger thickness reduction and elastic rebound; Electrodes 3 and 4 show stable, near-constant thickness

Figure 5. Electrical conductivity vs pressure for four electrode sheets — Electrodes 1 and 2 show larger conductivity variation with applied test pressure; Electrodes 3 and 4 show stable response, as pre-existing high-pressure calendering has already consolidated inter-particle and coating-current-collector contacts

4. Practical Guidance for Process Engineers on Calendering Pressure

Based on the measured behavior, the following process guidelines apply to graphite anode electrode calendering:

• Target compaction density by calendering pressure: Higher calendering pressure reliably delivers higher compaction density (1.35 → 1.65 g/cm³ in this study), which reduces subsequent compressibility and stabilizes electrode thickness under load. Compaction densities ≥1.6 g/cm³ for graphite anodes produce near-stable structures that are robust to downstream assembly pressure.

• Account for test geometry when evaluating electrode resistivity: Test geometry and transient thickness change during flat-plate compression can mask or reverse apparent conductivity trends between electrodes at different compaction densities. Always report both thickness and resistance together — not resistivity alone — to avoid misleading comparisons.

• Balance volumetric energy density against ion transport: Higher calendering pressure increases volumetric energy density by raising compaction density, but over-calendering collapses inter-particle pore connectivity, restricting electrolyte wetting and lithium-ion transport. The optimal calendering pressure must be co-optimized with slurry formulation and conductive-additive content for the target C-rate and cycle-life requirements.

5. Summary

This study demonstrates the effective use of the BER2500 electrode resistance analyzer to differentiate the compression and conductive properties of graphite anode electrodes produced under varying calendering pressures. The methodology clearly reveals how initial calendering pressure dictates an electrode’s mechanical resilience and its electrical resistivity response to subsequent compression testing.

The choice of optimal calendering pressure in actual production must be carefully tailored to the specific electrode formulation and cell design targets. The right balance maximizes battery volumetric energy density while ensuring robust mechanical integrity, efficient electronic conduction, and adequate porosity for ion transport — collectively determining cell performance and cycle life.

6. References

[1] Zhang J, Huang H, Sun J. Investigation on mechanical and microstructural evolution of lithium-ion battery electrode during the calendering process[J]. Powder Technology, 2022, 409: 117828.

[2] BG Westphal et al. Influence of high intensive dry mixing and calendering on relative electrode resistivity determined via an advanced two point approach. Journal of Energy Storage 2017, 11, 76–85

[3] Yang Shaobin, Liang Zheng. Principles and applications of lithium-ion battery manufacturing process[M]. Chemical Industry Press, 2020.

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