Free-Energy Profile of Li-Ion Insertion Reaction in Highly Concentrated Electrolytes: Impact of Destabilized Li-Ion on Electrode Reaction Thermodynamics.

Understanding the Li-ion insertion reaction at graphite electrodes as a chemical reaction requires a thermodynamic perspective, namely elucidating the underlying free-energy profile. In this study, we quantitatively constructed the free-energy profile of the Li-ion insertion reaction in highly concentrated electrolytes by combining activation thermodynamics derived from electrochemical impedance measurements with experimentally determined solvation free energies of Li+. The resulting free-energy profile revealed that the activation free energy (ΔG⧧) remains nearly constant across different solvent systems, whereas the solvation free energy of Li+ (ΔGLi) varies significantly depending on solvent properties and dominates the thermodynamics of the electrode reaction. Furthermore, high-energy X-ray total scattering and molecular dynamics simulations showed that Li+ forms ion-aggregate structures with anions and solvent molecules, where short-range Li+-Li+ configurations give rise to electrostatic repulsion, leading to destabilization of Li+. These results establish a direct link between solution structure and electrode reaction thermodynamics, providing new insights into the role of Li+ solvation in highly concentrated electrolytes.