A Stable Voltage Is Not Enough: Evaluating Reference Electrodes in Solid-State Batteries

Abstract Three-electrode (3E) configurations, which ideally leverage an inert reference electrode (RE) as a passive probe, are important for separating electrode-specific electrochemical processes in solid-state batteries (SSB). Here, by systematically evaluating different embedded RE configurations (Li+/Li wire, AuLi/AuLi3 mesh, and In/InLi mesh) in sulfide-based SSB, we show that practical RE can diverge substantially from this ideal and suggest criteria for robust validation of RE configurations. RE insertion itself is shown to introduce increased cell-to-cell variability and impedance artifacts. Further analysis using time-resolved impedance measurements during cell cycling reveals that voltage stability and impedance fidelity represent two distinct validation criteria that can diverge over time. Certain RE designs maintain stable potentials while distorting impedance responses, whereas others show higher AC fidelity but reduced long-term voltage stability. Notably, we find that common evaluation criteria—stable RE voltage and additive impedance consistency—are insufficient to guarantee impedance validity, and we suggest an alternative asymmetric 3E cell test for robust impedance validation. This study highlights the importance of informed RE selection: RE design should be guided by the targeted electrochemical application and measurement objective.