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

Abstract

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.