Experimental Pathways for Unlocking Thermodynamics of Ion Coordination in Battery Electrolytes

Despite the critical role of the microscopic local coordination environment within liquid battery electrolytes, our ability to predict and control these environments remains limited. While conventional spectroscopic techniques have advanced our understanding of average cation coordination, preferential solvation, and ion pairing, they often fall short of revealing the underlying thermodynamic forces that govern coordination sphere formation and electrolyte stability. This Perspective calls for a renewed focus on coordination thermodynamics to deepen our fundamental knowledge and empower researchers to more precisely engineer electrolyte systems for next-generation batteries. The Gibbs free energy, enthalpy, and entropy of ion transfer are the most fundamental yet underutilized metrics for assessing the relative stability of coordination environments in battery research. Solution calorimetry, solubility product measurements, and specialized electrochemical methods are introduced as practical tools to obtain transfer thermodynamic data for diverse electrolyte systems. Additionally, complexometric titration techniques─including spectrophotometric titrations, potentiometric titrations, and isothermal titration calorimetry (ITC)─are emphasized for their unparalleled but underexplored capability to examine the stepwise coordination of solvent molecules and anions, yielding valuable stability constants that quantify the formation energy of a cation coordination sphere. Statistical thermodynamic models, such as Covington’s preferential solvation model and Bjerrum’s stepwise coordination model, provide a theoretical foundation to contextualize these thermodynamic parameters and inspire the development of new models for complex electrolytes. By shifting focus from compositional analysis of the coordination sphere toward a quantitative thermodynamic framework, this Perspective seeks to bridge the gap between theoretical understanding and practical electrolyte design, driving more deliberate advancements in battery technology.