What Electrochemical Voltages Can-and Cannot-Tell Us about Electrolyte Non-Ideality

Electrochemical voltages depend on ionic activities and are therefore intrinsically sensitive to electrolyte non-ideality. Inverse electrochemical approaches leverage this dependence to infer thermodynamic information directly from measured voltages rather than predicting voltages from assumed activity models. A central challenge is the interpretation and identifiability of inferred quantities when both the electrolyte and the membrane phase are strongly non-ideal. Here, we show that voltage measurements necessarily constrain effective thermodynamic descriptors that integrate electrolyte, membrane, and interfacial non-ideality. We further establish identifiability through sensitivity analysis linking perturbations in descriptor components to the non-ideal voltage contribution. These results define fundamental limits on what voltage observables can-and cannot-reveal about electrolyte non-ideality.