Assessing the Impact of Drying Time on Electrode Tortuosity

1. Introduction

In lithium-ion battery production, controlling electrode moisture is critical, typically managed through parameters like temperature and time during drying to minimize its impact on performance. However, the internal physical and chemical properties of the electrode may undergo significant restructuring during this drying process.

This study evaluates the effect of drying time on electrode tortuosity under constant temperature. Identical calendered NCM cathode sheets were subjected to vacuum drying at 80°C for 0 hours, 6 hours, and 12 hours. Subsequently, symmetric cells were assembled, and their electrochemical impedance spectra (EIS) were measured for analysis.

Tortuosity (τ) was characterized using the MacMullin number (Nm), derived from the following equation:

Where: Rion is the ionic resistance; A is the electrode area; ε is the electrode porosity; σ is the electrolyte conductivity; d is the electrode thickness.

2. Testing Equipment and Procedure

1. Calendered LiFePO4 (LFP) cathode sheets were cut into consistent circular discs.

2. The cut cathode electrodes were placed in a vacuum oven and dried for the specified durations (0h, 6h, 12h at 80°C).

3. Immediately after drying, the electrodes were transferred to the Electrode Tortuosity Test System (Figure 1, EIC Equipment) and assembled into symmetric cells with corresponding PP separators.

4. The system’s automatic electrolyte injection function was used to add a consistent volume of electrolyte to each cell. Equal pressure was applied to ensure rapid and uniform wetting.

5. Electrochemical Impedance Spectroscopy (EIS) was performed. The ionic resistance (Rion) of the electrode coating was determined from the Nyquist plot: the difference between the low-frequency intercept and the high-frequency intercept with the real impedance axis (Z’) was multiplied by 3.

6. Data was analyzed to calculate the MacMullin number and assess electrode tortuosity.

Figure 1. IEST Electrode Tortuosity Test System(EIC 2400)

3. Results and Data Analysis

Table 1. Tortuosity and Related Material Information for Electrodes Subjected to Different Vacuum Drying Times

Figure 2. (a) Impedance spectra and (b) MacMullin numbers for electrodes subjected to different drying times.

As shown in Table 1 and Figure 2, the results demonstrate a clear trend: longer drying times lead to a higher electrode tortuosity. While reduced moisture content from extended drying is generally considered beneficial for performance, these findings reveal a more complex interaction.

Prolonged exposure to elevated temperature during drying likely causes reorganization of the binder at the particle surfaces within the electrode. This microstructural change can alter the surface charge characteristics of the active material particles, potentially increasing their interaction strength with lithium ions in the electrolyte. This intensified interaction manifests as an increased ionic resistance and, consequently, a higher MacMullin number, indicating a more tortuous ion transport path.

4. Mechanistic Interpretation

Although removing residual moisture typically benefits electrochemical stability, extended drying at elevated temperature can also drive subtle physicochemical rearrangements in the electrode microstructure:

• Binder redistribution / surface aggregation. The data and process observations indicate PTFE / polymer binder migration and surface aggregation under extended thermal/vacuum exposure. These changes can coat particle surfaces and alter pore throat geometry.

• Increased ion–surface interaction. Surface chemical changes (e.g., increased exposure of binder or additive-rich domains) may enhance local interactions with Li⁺, effectively slowing free-ion transport and increasing measured Rion.

• Reduced effective porosity or increased tortuosity. Even if bulk porosity is similar, pore connectivity can degrade (more dead-ends, narrower throats), raising the path-length factor represented by Nm.

As a result, a drying protocol that simply minimizes moisture can unintentionally compromise ionic transport by increasing electrode tortuosity; this produces a trade-off engineers must manage when specifying drying schedules.

5. Conclusion

This investigation demonstrates that moisture control processes for battery electrodes can significantly influence the restructuring of the electrode’s tortuosity. This, in turn, affects the lithium-ion diffusion impedance during normal cell operation. These findings have important implications for both process parameter design and for R&D activities where tortuosity is used as a metric for material screening.

Furthermore, the results indicate that the tortuosity of a material within an electrode coating, influenced by its formulation, can be modulated by optimizing vacuum drying parameters. This provides a potential lever to fine-tune electrode performance and better achieve specific cell design goals.

6. Recommended Testing Equipment: IEST Electrode Tortuosity Tester(EIC2400)

Features:

1. Calculate electrode tortuosity measurement from electrochemical impedance spectroscopy (EIS) of symmetric cells.
2. Streamlined cell assembly, automated testing and analysis, simplified operation workflow, enhanced testing throughput
3. Four-channel synchronous measurement
4. High-purity argon gas atmosphere
5. Rapid EIS testing module
6. McMullin Number and Tortuosity Measurement

Application:

• Electrode tortuosity test
• Separator ion conductivity test