Advanced liquid electrolytes, such as locally-highly concentrated electrolytes (LHCEs), hold significant potential for enabling next-generation battery chemistries like lithium metal. Despite their potential, current approaches for developing LHCEs rely heavily on empirical trial-and-error methods, leaving fundamental questions unanswered regarding their formation mechanisms, solvent interactions, and property optimization. In this work, we introduce a thermodynamics-centered framework to systematically investigate and rationally design novel LHCEs. Using advanced calorimetry, we quantify the enthalpic contributions driving the interactions between cation-coordinating (C-) solvents, non-cation-coordinating (NC-) solvents, and lithium salts. Complementary Raman spectroscopy provides detailed insights into the molecular coordination environments. Together with measurements of ionic conductivity, viscosity, and electrochemical stability, this approach enables us to elucidate critical relationships between solvent structure, coordination chemistry, and the resulting electrolyte performance. Leveraging these insights, we successfully create previously unexplored LHCEs with C-solvents that tend to be immiscible with typical NC-solvents, thus expanding the design space for future electrolyte innovations.
