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

Tuning electrolyte salt concentration has emerged as a powerful lever to enhance the performance of rechargeable batteries by mitigating parasitic side reactions. Here, we introduce a comprehensive thermodynamic framework based on a modified Nernst equation that explicitly incorporates electrolyte-specific parameters-namely, salt and solvent activities and the salt hydration number-to rationalize and predict how electrode potentials vary with electrolyte concentration during reversible ion insertion. We demonstrate this generic methodology for reversible Li⁺ insertion into anatase TiO₂ from aqueous LiCl electrolytes. By combining tabulated water and LiCl activities with concentration-dependent hydration numbers, obtained from both experimental and theoretical approaches, we achieve quantitative agreement between predicted and observed 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.