Electrochemical activity of reference electrodes in Li-ion batteries: Mechanisms, heterogeneities, and measurement artifacts

Reference electrodes (REs) are widely used in lithium-ion cells for characterization, potential monitoring, and electrochemical investigations. They are often assumed to be electrochemically inert. This work challenges this assumption by focusing on REs made of a metallic mesh coated with LiFePO 4 (LFP), a material commonly selected for the apparent stability of its potential. Using finite-element simulations, we demonstrate that the blocking effect of the RE mesh induces simultaneous lithiation and delithiation on opposite sides of the RE. This creates a non-uniform lithiation state of the RE, and gives rise to measurement artifacts originating from local deviations in the LFP open-circuit voltage and from overpotential asymmetries. These simulation-predicted artifacts are experimentally confirmed, revealing a potential deviation of 100 mV at C/2 between two REs in the same cell that have different initial lithiation states. Furthermore, the electrochemical behavior of the RE is shown to be history-dependent and influenced by prior cycling conditions. Overall, this study provides deeper insights into the careful use of REs in three-electrode battery experiments.