Ion Sensing in Concentrated Lithium-Ion Solutions by Oxide Solid Electrolytes and Activity-Coefficient Correction

Abstract The accurate detection of lithium ions (Li+) is crucial for therapeutic drug monitoring and lithium resource recovery and extraction, yet current electrochemical sensors based on polymeric membranes suffer from signal drift and short lifetimes in concentrated solutions. To address this, we report a novel potentiometric sensor employing oxide solid electrolytes (OSEs) from solid-state batteries as the ion-selective membrane (ISM). This all-solid-state design enables highly stable and selective lithium sensing across an unprecedentedly wide concentration range. Specifically, the perovskite-type Li0.33La0.56TiO3 (LLTO) presents the best performance, covering a concentration range from 10–4 mol/L up to 10 mol/L (near-saturated conditions). The LLTO electrode exhibited excellent selectivity, with selectivity coefficients for all major interfering ions exceeding 10–2.8. To account for the increased response slope at high concentrations, we introduce a Li+-corrected (LC) model based on the Debye–Hückel equation and its extended variations to address the enhanced interionic behavior. To decouple the liquid junction potential (ELJP) contribution from the measured potential (Emeas), we combine the Type 1 liquid junction model with our LC activity coefficient model and successfully obtain reliable activity-corrected potential responses across the entire concentration range. This work provides a robust and reliable Li-sensing platform pivotal for both clinical diagnostics and industrial applications.