Determination of the Through-Plane Tortuosity of Battery Electrodes by EIS in a symmetric Lithium-iron-phosphate cell

Importance of the Topic

The through-plane tortuosity of porous electrodes influences ion transport and determines power capability. In lithium ion cells, accurate quantification of this structural parameter is essential for optimizing electrode design and ensuring high performance in applications demanding rapid charge–discharge cycles.

Objectives and Study Overview

This study presents a straightforward electrochemical impedance spectroscopy (EIS) method to quantify the through-plane tortuosity of a commercial lithium–iron–phosphate (LFP) cathode. By measuring symmetric cells with known porosity and thickness, the approach aims to extend the method introduced by Landesfeind et al. and validate its reproducibility under different electrolyte conductivities.

Methodology and Instrumentation

• Cell assembly: Symmetric LFP electrodes (114 μm thick, 37% porosity) were placed around a Celgard 2340 polypropylene separator soaked for 48 h with EC/DMC mixture (1:1 w/w) containing nBu₄NPF₆ at 0.01 and 0.10 mol/L.
• Instrumentation:
  • Metrohm Autolab Microcell HC with Peltier temperature control (±0.1 °C accuracy).
  • Metrohm Autolab PGSTAT204 potentiostat/galvanostat with FRA32M module for EIS.
  • NOVA software for data acquisition and temperature control.
  • RelaxIS 3® software for fitting impedance spectra.
• Measurement parameters: Potentiostatic EIS with 1 mV RMS amplitude over 100 kHz–10 mHz (20 points per decade) at 20 °C.
• Data analysis: Fitting to a simplified transmission line open element (TMLqo) model to extract ionic resistance (R_ion) and non-ideal capacitive elements (Q, α).

Main Results and Discussion

• Bulk electrolyte conductivities (σ_dc) at 20 °C: 0.4 mS/cm (0.01 M) and 2.7 mS/cm (0.10 M).
• Equivalent circuit fits yielded R_ion of 367.4 Ω and 42.7 Ω for 0.01 M and 0.10 M, with α ≈ 0.84–0.87.
• Calculated tortuosities: 2.6 (0.01 M) and 2.2 (0.10 M), showing minor dependence on electrolyte conductivity and confirming negligible electronic film resistance.
• Agreement with literature values supports the method’s validity and recommends low-conductivity electrolytes to ensure measurement accuracy.

Benefits and Practical Applications

This impedance-based technique offers:

• Non-destructive evaluation of electrode microstructure without elaborate imaging.
• Rapid assessment of tortuosity under realistic cell conditions.
• Guidance for electrode formulation and processing in high-power battery development.

Future Trends and Potential Applications

• Extension to other electrode chemistries and multicomponent blends.
• In situ EIS during cell cycling to monitor tortuosity evolution.
• Integration with modeling tools for predictive electrode design.
• Automation in standardized quality-control workflows.

Conclusion

The described EIS approach reliably determines through-plane tortuosity of porous battery electrodes using a simplified transmission line model. Its sensitivity to structural properties, combined with ease of implementation, makes it a valuable tool for research and quality assurance.

Reference

1. J. Landesfeind, A. Eldiven, H. A. Gasteiger, J. Electrochem. Soc. 165(5) A1122–A1128 (2018).
2. J. Landesfeind, J. Hattendorff, A. Ehrl, W. A. Wall, H. A. Gasteiger, J. Electrochem. Soc. 163(7) A1373–A1387 (2016).
3. J. Landesfeind, M. Ebner, A. Eldiven, V. Wood, H. A. Gasteiger, J. Electrochem. Soc. 165(3) A469–A476 (2018).
4. AN-EC-010 ‘In-temperature Ionic Conductivity Measurements with the Autolab Microcell HC Setup’ (Metrohm, 2020).