Non-Monotonical Ion Association Equilibria in Low-Temperature Sodium-Metal Battery Electrolytes

Intermolecular distance is a critical parameter governing electrostatics and solvation in aprotic electrolytes. Governed by the power-law relationship between interaction energy and distance, the attenuation of molecular Brownian motion at low temperatures leads to a sharp reduction in intermolecular distance, which in turn amplifies dipole-dipole interactions and impacts the transport kinetics of sodium (Na) ions in both bulk and interface domains. How the low-temperature regime influences the association and dissociation of Na solvation equilibrium is rather elusive. In this study, an electrophilicity/nucleophilicity-based descriptor is proposed to quantify solvent self-association, which is primarily driven by dipole-dipole interactions. Based on this framework, the solvent pairs are generated during non-monotonical temperature-dependent Na⁺ solvation evolution, which contributes to excessive ion aggregation to tame redox kinetics at low temperature. In line with this consideration, a facile strategy using a dilute electrolyte was developed to establish a favorable solvation equilibrium between dissociated and associated species. This work refreshes the understanding of thermodynamic equilibrium between ion-dipole and dipole-dipole interactions, and challenges the conventional approach of learning from the lithium batteries that encourage weakly solvating electrolyte, arguing the critical importance of strengthening cation solvation chemistry in Na battery field due to the temperature-dependent dipole-dipole interactions.