Determination of the Lithium Ion Transference Number of a Battery Electrolyte by VLF-EIS

Significance of the Topic

Understanding the lithium ion transference number is essential for accurate modeling of battery performance and lifetime. This parameter describes the fraction of ionic current carried by lithium ions in an electrolyte, influencing charge/discharge kinetics, transport limitations, and overall cell efficiency. A reliable and efficient method to determine this value across temperatures supports the development and optimization of advanced battery systems.

Objectives and Study Overview

This work demonstrates a one-step, very‐low‐frequency electrochemical impedance spectroscopy (VLF-EIS) method to determine the temperature-dependent lithium ion transference number of a commercial binary electrolyte. The study covers a temperature range from –10 °C to +50 °C and compares results with literature values to validate the approach.

Methodology and Instrumentation

A symmetric lithium|separator|lithium cell was assembled using metallic lithium electrodes and a porous polyethylene separator soaked with 1 M LiPF6 in EC:DMC (1:1 v/v). All cell components were handled under inert atmosphere. Temperature control (±0.1 °C) was achieved with a Peltier-based Microcell HC setup. Electrochemical measurements employed a Metrohm Autolab PGSTAT204 potentiostat/galvanostat with FRA32M module, managed by NOVA 2 software. Impedance spectra were recorded:

• Pre-stability check: AC amplitude 1 mV RMS, 100 kHz to 1 Hz until interface stabilization.
• VLF-EIS: AC amplitude 1 mV RMS, 100 kHz to 10 mHz (10 points per decade) after 900 s equilibration at each temperature.

Data fitting used an equivalent circuit with a bulk resistance (Rbulk), a combined interfacial R–CPE element (merging SEI and charge‐transfer contributions), and a Warburg short element (Ws) to model diffusion impedance.

Main Results and Discussion

All resistive elements decreased with increasing temperature: Rbulk dropped from ~147 Ω at –10 °C to ~50 Ω at +50 °C, while interfacial and diffusion resistances showed similar trends. Calculated transference numbers rose from 0.09 at –10 °C to 0.49 at +50 °C. These values align with published data for similar electrolytes, confirming method accuracy. The merging of SEI and charge‐transfer contributions into a single circuit element was justified by overlapping time constants.

Benefits and Practical Applications

The VLF-EIS method offers a rapid, one-step approach to obtain reliable lithium ion transport parameters across a wide temperature range. Practical applications include:

• Input for battery performance simulations and thermal management models.
• Quality control of electrolyte batches in manufacturing.
• Screening of novel electrolyte formulations and additives.

Future Trends and Opportunities

Further advances may include automated high-throughput VLF-EIS screening of multiple electrolyte compositions, integration with in-situ cell testing to monitor aging effects on transference number, and extension to multicomponent electrolytes. Combining VLF-EIS data with molecular dynamics or thermodynamic factor measurements could yield deeper insight into ion transport mechanisms.

Conclusion

The one-step VLF-EIS protocol provides a straightforward, reproducible route to determine the lithium ion transference number of binary battery electrolytes over a broad temperature range. Results agree well with alternative techniques, underlining the method’s suitability for research and industrial applications.

Reference

• Wohde F., Balabajew M., Roling B., J. Electrochem. Soc. 163 (5) A714–A721 (2016).
• Landesfeind J., Gasteiger H. A., J. Electrochem. Soc. 166 (14) A3079–A3097 (2019).
• Hou T., Monroe C. W., Electrochimica Acta 332 135085 (2020).