Elucidating Individual Impedance Contributions via a Versatile Reference Electrode for Lithium-Ion Batteries

The foremost tool for the study of electrochemical processes across a variety of electrochemical devices 1–5 is the reference electrode which enables a clear understanding of the electrode materials within a full cell. 6 When the reference electrode potential is known, and the reference electrode itself does not impact the underlying electrochemical processes, the anode and cathode potentials can be attributed to individual reactions and captured by the fundamentals of electrochemistry. 7 In our previous work 8 we described a versatile thin film LiFePO 4 reference electrode 9–12 for integration into lithium ion batteries, as well as the related details of its use in single-layer laboratory cells in order to facilitate resistance deconvolution 13 and training of electrochemical models 14,15 . Due to the design and nature of the reference electrode, the resistances and individual potentials can be deconvoluted 13 , and it has been shown that the reference electrode only impacts the overall cell signal to the extent of adding one separator with the associated ohmic and liquid phase transport resistance having minimal impact even for rates up to 1C 8 . These benefits are realized due to the relatively thin nature of the versatile reference electrode, as well as the electrode possessing the same or similar porosity compared to the underlying separator. Additionally, due to the uniformity in the versatile reference electrode utilized in this work compared to various dot-type or wire-type reference electrodes, the presence of current and electrolyte flux non-uniformities as well as their underlying artefacts is minimized or removed entirely. Here, we demonstrate how this reference electrode can further be utilized to deconvolute the impedance of the cathode and anode contributions to the cell level impedance. This can enable diagnostics and calibration of models at the beginning of life, as well as an understanding of changes in a battery cell as the state-of-health progresses towards the end of battery life. Because the reference electrode is placed on the surface of a separator and covers the entire electrochemically active interface between the anode and the cathode, use of the results to drive the development and calibration of battery models is improved, and can be reduced to a pseudo-two-dimensional (P2D) set of equations, rather than necessitating a P3D or P4D solution as is the case for side mounted or lithium dot references. References M. W. Verbrugge et al., J. Electrochem. Soc. , 172 , 030529 (2025). C. Vann et al., ECS Adv. , 4 , 040501 (2025). C. M. Hangarter et al., ECS Adv. , 4 , 011501 (2025). C. Glynn et al., ECS Adv. , 4 , 030501 (2025). A. Badalbayli et al., ECS Adv. 4 043001 (2025) A. J. Bard et al., Electrochemical Methods: Fundamentals and Applications , p. 1112, John Wiley & Sons, (2022). J. Newman et al., Electrochemical Systems , p. 608, John Wiley & Sons, (2021). B. J. Koch et al., Journal of The Electrochemical Society , 172 , 013507 (2025). J. Gao, B. J. Koch, and T. R. Garrick, “Electrochemical Systems, Methods, and Devices Using Stacked Electrode Assemblies with In-Stack Sensor Arrays”, U. S. Patent Application No. 17/479,558 B. J. Koch, J. Gao, T. R. Garrick, and W. Zeng, “System and Method for In-Situ Mapping on Electrode Potential and Thermal Distribution”, U. S. Patent Application No. 17/891,120 T. R. Garrick and B. J. Koch, “Battery Cell with Laminated Reference Electrode Core and Z-Folding Stack”, U. S. Patent Application No. 18/754,518 T. R. Garrick and B. J. Koch, “In-Vehicle Cell Diagnostics with Resistance Split Outlier Detection”, U. S. Patent Application No. 19/310,242 B. J. Koch et al., J. Electrochem. Soc. , 171 , 123505 (2024). T. R. Garrick et al., J. Electrochem. Soc. , 172 , 043511 (2025). T. R. Garrick et al., J. Electrochem. Soc. , 168 , 010530 (2021). Figure 1