Development of a Wire Reference Electrode for Lithium All-Solid-State Batteries with Polymer Electrolyte: FEM Simulation and Experiment

Electrochemical impedance spectroscopy (EIS) is a powerful technique to study interface kinetics in lithium ion batteries. In order to separate the contributions of the working and counter electrode, a reference electrode (RE) is crucial. However, size and shape of the RE strongly influence the quality of the measurement data. In this study we define two criteria that enable the theoretical quality assessment of a wire-shaped RE as a function of its diameter. With help of a simplified Newman-type battery model we show that small wire diameters are essential in order to obtain meaningful EIS data, in particular when using polymer electrolytes, which have comparably low ionic conductivity. Therefore, a gold plated tungsten wire with 10 μm diameter is chosen as preferred RE. It is placed between two layers of electrolyte and lithiated before use to ensure a long-term stable reference potential. Using this setup, artifact-free EIS spectra are obtained for lithium/lithium and lithium/LFP cells with polymer electrolyte. Finally, two applications are shown for which a RE is indispensable: (I) the symmetry of anodic and cathodic lithium dissolution and deposition kinetics is characterized; (II) the growth of the anode and cathode impedance of a lithium/LFP cell during cycling is monitored.