Li-ion battery electrolytes play a crucial role in enabling electrochemical energy storage and conversion, where the solvation of Li+ ions strongly influences the battery performance and stability. Understanding how salt concentration and counteranion chemistry affect both the enthalpic and entropic contributions to Li+ solvation could enable new design principles for next-generation electrolytes. In this work, we seek to rationalize the composition dependence of ionic Seebeck coefficients in dimethyl sulfoxide (DMSO) and 1,2-dimethoxyethane (DME) electrolytes based on independent measurements of the entropy of mixing, bulk configurational entropy (derived from heating the solidified electrolyte to the measurement temperature), ion pairing, and temperature dependence of Li+ solvation enthalpy. In DMSO electrolytes with negligible ion pairing, the measured ionic Seebeck coefficients were governed solely by entropy through the combined influence of the entropy of mixing and the configurational entropy of Li+. On the other hand, in DME electrolytes where ion pairing was significant, enthalpic contributions due to ion pairing, as well as the temperature dependence of solvation enthalpy, dominated. These findings provide new molecular-level insights into how electrolyte composition and structure drive Li+ solvation thermodynamics, informing future strategies for designing advanced electrolytes with improved performance.

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