How Solvent Activity and Solute Solvation Control Electrode Potentials: Quantitative Insights from Li⁺ Insertion into TiO₂

Abstract

Tuning electrolyte salt concentration has emerged as a powerful strategy to enhance the performance of rechargeable batteries by mitigating parasitic side reactions. Here, we introduce a unified and quantitative validation of an electrolyte thermodynamic framework incoporated into a modified Nernst equation that explicitly includes electrolyte-specific parameters—namely, salt and solvent activities, as well as the salt hydration number—to rationalize and predict how electrode potentials vary with electrolyte concentration in reversible ion-insertion systems. We demonstrate this generic methodology for reversible Li⁺ insertion into anatase TiO₂ using aqueous LiCl electrolytes. By combining tabulated water and LiCl activities with concentration-dependent hydration numbers derived from both experimental and theoretical approaches, we achieve quantitative agreement between predicted and measured potential shifts. This work establishes a reliable and broadly applicable framework for deciphering and controlling electrolyte-concentration effects in batteries and, more generally, in electrochemical systems involving concentrated electrolytes.