Lithium-Ion Partial Molar Entropies in Liquid, Composite, and Solid-State Electrolytes

Electrolyte composition, concentration, and structure can affect reaction kinetics, ion transport, and heat generation during operation. In liquid electrolyte systems, the properties of ion–solvent and ion–ion interactions strongly affect thermodynamics and electrochemistry. The change in total entropy of the electrolyte when one mole of an ion is added to an electrolyte at a constant temperature and pressure is known as the partial molar entropy of ion solvation and describes broadly how the solvation environment changes with the addition or removal of an ion. Herein, we measure the partial molar entropies of lithium-ion solvation in solid-state systems using potentiometric temperature coefficient measurements. For solid-state systems, we measure positive partial molar entropies of lithium-ion solvation of 43.3 J mol^–1 K^–1 for Li₆PS₅Cl and 26.4 J mol^–1 K^–1 for Li_6.5La₃Zr_1.5T_0.5O₁₂. In contrast, the liquid electrolyte 1 M LiPF₆ in 1:1 vol % EC:DEC exhibits a negative partial molar entropy of −76.0 J mol^–1 K^–1. A one-dimensional analytical model is developed to elucidate how the temperature coefficient is influenced by the multiple interfaces formed when an inorganic single-ion conductor is combined with a binary liquid electrolyte, providing a unified framework for understanding the partial molar entropies of ion solvation across a wide range of electrochemical systems.