Probing thermodynamics of Li-ion solvation through reversible heat

<h2>Summary</h2> The reversible heat generated during lithium electrochemical reactions—known as the Peltier heat—offers a direct probe of Li-ion solvation thermodynamics in electrolytes. Using temperature difference metrology (TDM) in a coin cell geometry, we systematically explore how lithium-ion solvation varies with the solvent, the choice of anion, and the salt concentration. We find that ether-based solvents produce larger entropy changes upon Li-ion solvation than carbonate-based solvents. As the salt concentration in the electrolyte increases, fewer solvent molecules coordinate with Li ions, reducing the entropy changes. In binary mixture electrolytes, Li ions exhibit preferential interactions with one component of the solvent over the other, causing deviations from a linear dependence on solvent composition. We also extract salt diffusion coefficients and heat of transport from transient features in the reversible heat. Our approach provides fundamental insights into electrolyte solvation thermodynamic, structure, and transport properties that will aid the design of electrolytes and electrode/electrolyte interfaces with improved performance.