Determination of the MacMullin number

Importance of the Topic

In lithium-ion batteries, the separator is a porous, electronically insulating membrane that critically influences ion transport and overall cell performance. Assessing its effective ionic conductivity via the MacMullin number provides key insight into how separator microstructure reduces electrolyte conductivity in practical cells.

Objectives and Study Overview

This study applies the stacking method combined with electrochemical impedance spectroscopy (EIS) to determine the MacMullin number of a typical tri-layer polyethylene/polypropylene separator soaked in a 1 M LiPF6/EC:DMC electrolyte. By varying the number of separator layers and measuring ionic resistance, the work aims to quantify the conductivity attenuation introduced by the separator.

Methodology and Instrumentation

Sample Preparation and Cell Assembly:

• Electrolyte: 1 mol/L LiPF6 in 1:1 EC/DMC, handled in an argon glove box.
• Separator: Tri-layer PE/PP, 21.5 μm thickness, punched into 12 mm discs and wetted for 24 h.
• Cell: TSC Battery standard cell with stainless steel disc electrodes (8 mm), assembled in a Microcell HC stand.

Instrumentation:

• Autolab PGSTAT204 potentiostat/galvanostat with FRA32M EIS module.
• NOVA software controlling EIS and Peltier temperature module.
• Temperature control at 20.0 °C via Peltier element and Pt100 sensor (0.1 °C accuracy).

EIS Measurements:

• Frequency range: 1 MHz–100 Hz, amplitude 10 mV.
• Stacking method: impedance recorded for 1 to 5 separator layers, 300 s equilibration at 20.0 °C.

Main Results and Discussion

Impedance spectra show a high-frequency intercept corresponding to bulk ionic resistance (Rion), which increases linearly with the number of separator layers. Fitting with an equivalent circuit (inductance for cables, Rion for ion migration, and CPE for polarization) yields ΔRion/ΔN=2.49 Ω per layer. Calculated ionic conductivity of the soaked separator is 1.7 mS·cm−1 at 20 °C versus 9.9 mS·cm−1 for the pure electrolyte. Thus, the MacMullin number NM=σelectrolyte/σseparator is 5.8, consistent with literature values (4–20).

Benefits and Practical Applications

This stacking-EIS approach offers a reproducible and straightforward protocol for evaluating separator performance in QA/QC and research settings. Quantifying NM helps guide material selection, optimize separator design, and predict cell impedance in battery development.

Future Trends and Opportunities

• Investigating the influence of electrolyte composition, salt concentration, and solvent systems on MacMullin number.
• Extending to novel separator materials (e.g., ceramic composites, nanofibers) and next-generation electrolytes.
• Integrating operando and high-throughput EIS techniques for in situ separator characterization.
• Modeling microstructure-conductivity relationships by coupling imaging and simulation.

Conclusion

The stacking method combined with EIS provides an effective means to determine the MacMullin number and thus the ion transport performance of battery separators. The measured NM of 5.8 underscores the method’s validity and applicability for separator evaluation.

Reference

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4. Raccichini R., Furness L., Dibden J.W., Owen J.R., García-Araez N.; J. Electrochem. Soc. (2018) 165(11):A2741–A2749