The global imperative to transition towards sustainable energy sources has placed electrochemical energy storage at the forefront of technological innovation. Among the various technologies, the li ion battery stands as a cornerstone due to its superior energy density, extended cycle life, and versatile applications ranging from portable electronics to electric vehicles and grid-scale storage. The performance, cost, and reliability of a li ion battery are intrinsically linked to its manufacturing processes. One of the most critical and defining steps in electrode manufacturing is the calendering or rolling process. This step densifies the coated electrode film by reducing its thickness and porosity, thereby enhancing the volumetric energy density, improving the electronic contact between active material particles, and strengthening the adhesion to the current collector. Traditionally, this is performed at or near ambient temperature (cold calendering). However, recent industrial and research trends are exploring heated calendering, where the calender rolls are maintained at an elevated temperature. This study investigates the impact of roll temperature during the calendering process on the physical, electrical, and electrochemical properties of Lithium Iron Phosphate (LFP) cathodes for li ion battery applications, aiming to identify an optimized thermal processing window.
The electrode fabrication for this study was designed to mirror industrial production conditions. A slurry was formulated using Lithium Iron Phosphate (LiFePO₄) as the active material, conductive carbon black, polyvinylidene fluoride (PVDF) binder, and a dispersant in an N-Methyl-2-pyrrolidone (NMP) solvent. This slurry was coated onto carbon-coated aluminum foil using a double-sided coating machine and dried. The key variable introduced was the temperature of the main calender rolls during the densification step. The roll temperature was systematically varied across five setpoints: 25°C (ambient, control), 60°C, 80°C, 100°C, and 120°C. Crucially, for a controlled comparison, the final calendered thickness and the linear pressure applied were kept constant for all temperature conditions, targeting a fixed electrode density. Electrodes from each condition were then characterized, and CR2032 coin cells were assembled with lithium metal as the counter electrode for electrochemical evaluation.
Physical and Electrical Properties of Calendered Electrodes
The initial analysis focused on the fundamental properties imparted by the calendering process. A primary goal of calendering is to achieve uniform electrode thickness, which is vital for consistent cell assembly and performance. While all processes achieved the target thickness, the data indicated that heated calendering at moderate temperatures (60°C and 100°C) could improve thickness homogeneity compared to the higher 120°C condition, where potential roll temperature non-uniformity might have a negative effect. The electrical resistance of the electrode coating, a critical parameter for cell power and efficiency, was measured directly. The results showed a clear trend of decreasing film resistance with increasing roll temperature, reaching a minimum at the 100°C condition. The reduction can be attributed to the enhanced thermal plasticity of the PVDF binder, which flows more readily at elevated temperatures, creating better conductive bridges between active material and carbon particles. The adhesion strength between the active material coating and the current collector, quantified by the peeling force, exhibited a non-monotonic relationship with temperature. It increased up to 60°C, likely due to improved binder mobility and bonding, but decreased at higher temperatures, possibly due to over-softening or degradation of the binder or the carbon coating interface. Another practically important parameter is thickness rebound or spring-back after calendering. Electrodes calendered at 60°C and 100°C showed significantly reduced rebound (approximately 50% less) compared to the ambient condition. This is a major practical advantage as it leads to more stable and predictable electrode dimensions during subsequent cell stacking or winding.
The fundamental relationship governing the calendering process can be linked to the densification of a porous medium. The relative density ($\rho_r$) after calendering can be expressed in terms of the initial porosity ($\epsilon_0$), the applied pressure ($P$), and the temperature-dependent compaction behavior of the constituent materials. While a complete model is complex, a simplified view considers the role of temperature in modifying the yield stress of the binder phase. The effective compaction stress ($\sigma_{eff}$) experienced by the particle-binder matrix can be thought of as:
$$\sigma_{eff} = f(P, T, \eta_{binder}(T))$$
where $\eta_{binder}(T)$ is the temperature-dependent viscosity of the binder. Higher $T$ reduces $\eta_{binder}$, allowing for densification ($\rho_r \uparrow$) and improved particle rearrangement at a given applied pressure $P$, which manifests as lower final film resistance ($R_{film} \downarrow$).
| Roll Temperature (°C) | Film Resistance (Ω) | Peeling Force (N) | 7-Day Thickness Rebound (%) |
|---|---|---|---|
| 25 | 1.572 | 0.283 | 1.28 |
| 60 | 1.550 | 0.298 | 0.60 |
| 80 | 1.565 | 0.274 | 1.05 |
| 100 | 1.539 | 0.277 | 0.89 |
| 120 | 1.548 | 0.262 | 1.15 |
Electrochemical Performance of Assembled Cells
The ultimate test of any electrode manufacturing process is the performance of the finished li ion battery. Electrochemical Impedance Spectroscopy (EIS) conducted on the coin cells provided insights into the internal resistances. The Nyquist plots typically consist of a high-frequency intercept related to ohmic resistance ($R_s$), a semicircle in the mid-frequency range corresponding to charge-transfer resistance ($R_{ct}$) at the electrode-electrolyte interface, and a low-frequency Warburg tail associated with lithium-ion diffusion. The fitted EIS data revealed that heated calendering significantly reduced both $R_s$ and $R_{ct}$. The reduction in $R_s$ aligns with the direct film resistance measurements, indicating better bulk electronic conduction. The more substantial reduction in $R_{ct}$ suggests that the thermally-assisted calendering created more favorable interfacial conditions for the charge transfer reaction, likely through more intimate contact between all solid phases (active material, conductive agent, binder) and a more uniform electrode surface.
The charge-transfer resistance is a key kinetic parameter in the Butler-Volmer equation, which describes the current density ($i$) at an electrode:
$$ i = i_0 \left[ \exp\left(\frac{\alpha_a F \eta}{RT}\right) – \exp\left(-\frac{\alpha_c F \eta}{RT}\right) \right] $$
Here, $i_0$ is the exchange current density, which is inversely related to $R_{ct}$. A lower $R_{ct}$ implies a higher $i_0$, meaning the electrochemical reaction proceeds more readily. The observed decrease in $R_{ct}$ with heated calendering points to an electrode microstructure that fosters a higher effective exchange current density.
| Roll Temperature (°C) | Ohmic Resistance, $R_s$ (Ω) | Charge-Transfer Resistance, $R_{ct}$ (Ω) |
|---|---|---|
| 25 | 3.696 | 208.4 |
| 60 | 2.331 | 183.3 |
| 80 | 2.353 | 141.9 |
| 100 | 2.210 | 132.6 |
| 120 | 2.173 | 135.9 |
The benefits of reduced impedance translated directly to enhanced rate capability. Cells with electrodes calendered at 100°C delivered the highest discharge capacities at all tested C-rates (0.1C to 2C). This is a critical metric for li ion battery applications requiring high power, such as acceleration in electric vehicles. The improved kinetics from lower $R_{ct}$ allow the cell to deliver more capacity under high current drains without excessive polarization. The specific capacities and capacity retention rates are summarized below.
| Roll Temp. (°C) | Discharge Capacity @ 0.1C (mAh/g) | Discharge Capacity @ 1C (mAh/g) | Capacity Retention (1C/0.1C) | Discharge Capacity @ 2C (mAh/g) | Capacity Retention (2C/0.1C) |
|---|---|---|---|---|---|
| 25 | 154.0 | 137.9 | 89.5% | 130.4 | 84.7% |
| 60 | 155.0 | 142.3 | 91.8% | 135.1 | 87.2% |
| 80 | 154.0 | 142.5 | 92.5% | 133.7 | 86.8% | 100 | 157.5 | 145.5 | 92.4% | 137.6 | 87.4% |
| 120 | 153.8 | 142.2 | 92.5% | 135.4 | 88.0% |
While the rate performance showed clear advantages, the short-term (100 cycle) cycling stability at 1C showed negligible difference between cells made with electrodes from different calendering temperatures. All cells exhibited similar capacity fade trends, indicating that within this cycle range, the heated calendering process did not induce any negative degradation mechanisms that would accelerate capacity loss. This is a positive finding, confirming that the performance gains are not achieved at the expense of cycle life. The overall energy output of a li ion battery is also related to the average discharge voltage. Electrodes processed with heated calendering, particularly at 100°C, exhibited higher discharge voltage plateaus, especially under higher currents. This reduced polarization directly increases the energy (Wh) delivered by the cell, as energy is the integral of voltage over capacity.
Microstructural Considerations and Optimization
The improvements in electrical and electrochemical properties stem from beneficial changes in the electrode microstructure induced by heated calendering. At elevated temperatures, the polymer binder (PVDF) undergoes a transition, becoming more viscoelastic. This allows it to flow and redistribute more effectively under the calender pressure. This enhanced flow achieves several key microstructural outcomes: 1) It creates a more continuous and robust conductive network by improving the contact points between carbon black particles and between carbon and active material. 2) It fills microscopic voids and crevices more completely, leading to a denser and more homogeneous structure with fewer isolated particles. 3) It improves the wetting and adhesion at the interface between the coating and the carbon-coated aluminum current collector. Importantly, scanning electron microscopy (SEM) analysis confirmed that these benefits were achieved without causing cracking or fracture of the brittle LFP primary particles, which can be a risk if excessive pressure or malformed rolls are used.
The process optimization can be framed as seeking the temperature that maximizes the desired property function ($F$) while satisfying constraints. For a li ion battery electrode, $F$ could be a composite metric of rate capability and adhesion:
$$ F(T) = w_1 \cdot \left(\frac{1}{R_{ct}(T)}\right) + w_2 \cdot (Peeling Force(T)) $$
Subject to: $T_{min} \le T \le T_{max}$, and $\Delta Thickness(T) \le \Delta_{spec}$.
Where $w_1$ and $w_2$ are weighting factors, and $\Delta Thickness$ is the thickness uniformity. The data from this study suggests that $F(T)$ is maximized in the vicinity of 100°C for the LFP-based system studied. Beyond this point, diminishing returns or negative effects like binder over-softening or roll temperature instability (as hinted at by the 120°C thickness data) begin to counterbalance the gains.
Conclusion
This comprehensive investigation into heated calendering for LFP cathodes demonstrates that it is a highly effective process modification for enhancing li ion battery performance. Compared to traditional ambient temperature calendering, applying controlled heat during the densification step yields significant benefits: a reduction in electrode electrical resistance, a substantial decrease in charge-transfer impedance, superior rate capability, higher discharge voltage, and significantly reduced thickness rebound. These improvements are attributed to the thermally-assisted reorganization of the electrode microstructure, leading to better particle-to-particle contact and a more robust conductive network. Among the temperatures evaluated, 100°C emerged as the optimal condition, providing the best balance of low impedance, high rate performance, good adhesion, and dimensional stability. While short-term cycle life was not affected, the demonstrated advantages in power and energy output make heated calendering a compelling strategy for manufacturing high-performance electrodes, particularly for li ion battery applications demanding high power density and efficient manufacturing with tight tolerances. The findings provide a clear process window and mechanistic understanding that can be directly applied to advance the production of next-generation energy storage devices.