Elucidation of Battery Electrolyte Coordination Sphere Thermodynamics via Calorimetric and Potentiometric Titrations

Abstract

Rechargeable metal-anode batteries are a promising post Li-ion battery development. However, the high reactivity of metallic anodes with the electrolyte results in the formation of a solid-electrolyte interphase (SEI). Electrolyte design is a key handle in controlling the SEI composition in metal-anode batteries, but our understanding of the electrolyte—specifically the cation’s first coordination sphere—is limited. In this thesis, the study of ion solvation and complexation techniques are brought into the context of battery electrolytes. Relevant data from literature is summarized and supplemented with enthalpy of solution (ΔsolH) and enthalpy of transfer (ΔtrH) measurements for the Li-battery relevant salts, LiPF6 and LiTFSI, in a set of polar aprotic solvents. The trends observed are rationalized by consideration of solvent and anion properties, particularly the solvent donicity and anion size. To achieve a finer picture of the Li+ coordination sphere, isothermal titration calorimetry (ITC) and potentiometric titrations (PT) were employed with a set of exemplar electrolytes to probe the thermodynamic evolution of the Li+ coordination complex as weak solvent is displaced by a stronger solvent in the first coordination sphere. Raman spectroscopy is used to confirm that solvent displacement occurs as expected, and the effect of the anion on ITC measurements is investigated. A statistical binding model is developed which is fit to the experimental titration data to extract an average change in Gibbs free energy (ΔG), enthalpy (ΔH), and entropy (ΔS) of solvent displacement. Preferential solvation tendencies are quantified for EC:DMC and EC:PC electrolyte using this methodology, and compared with preferences observed by other workers. This thesis provides the framework for future studies on the thermodynamics of more complex battery electrolyte coordination environments and its connection with the SEI composition.