A New Type of Reference Electrode for Three-Electrode Setups: Choice of Material and Geometry

Electrochemical Impedance Spectroscopy (EIS) is highly supportive for analyzing electrode processes in lithium-ion cells. Commonly, a three-electrode setup is used, with a lithium metal counter electrode and/or a lithium metal point-like reference electrode. But its usability is restricted, as lithium metal as a counter electrode often dominates the impedance response, and in general, causes a stochastic component in the EIS data. Furthermore, a point-like lithium metal reference electrode may cause errors such as quantitative factors, cross contaminations and inductive artifacts, as shown by experiment and FEM-simulation [1]. Therefore, material choice and geometric arrangement of a reference electrode seems crucial for obtaining reliable, reproducible and long-term stable measurements.

Li₄Ti₅O₁₂ (LTO) was already proposed in literature as a most promising candidate, showing a stable potential plateau at 1.55 V versus lithium, because of a two-phase equilibrium [2]. This study compares LTO versus lithium metal and examines various geometries for reference electrodes by EIS measurements and FEM-simulations.

Our results recommend a reference electrode in a mesh-type design, centered between two separators, as most reliable in terms of symmetry. Furthermore, an aluminum mesh slurry coated by fine LTO particles delivered a stable potential over more than hundred hours, as proven by potential measurements.

Finally, the suitability of this new type of reference electrode is validated for electrochemical impedance spectroscopy and C-rate tests.

References

[1]        M. Ender, A. Weber, E. Ivers-Tiffée, Journal of the Electrochemical Society, 159 (2012), A128-A136

[2]        F. La Mantia, C.D. Wessells, H.D. Deshazer, Y. Cui, Electrochemistry Communication, 31 (2013), 141-144