Thermodynamic Descriptors for the Design of Liquid Electrolytes for Sodium Metal Batteries

Sodium-based batteries are a promising, sustainable alternative to lithium-based batteries due to the high abundance and low cost of sodium resources, providing an attractive pathway to large-scale grid energy storage. While significant advances have been made in the development of high-performance cathode materials, the electrolyte remains the critical limitation for the practical viability of high energy density Na-metal batteries. Conventional sodium liquid electrolytes form poor electrode-electrolyte interfaces due to the high reactivity of Na metal, causing continuous electrolyte decomposition that results in uncontrolled solid electrolyte interphase (SEI) growth, Na dendrite formation, poor Coulombic efficiencies, and irreversible capacity loss. In order to achieve high-performance sodium liquid electrolytes, a fundamental understanding of the correlation between the solvation environment of Na⁺ions in the electrolyte and the reversibility of Na plating and stripping is necessary. In this study, we aim to probe the salt- and solvent-dependent Na⁺solvation energy by measuring the Na electrode redox potential and the electrolyte entropy to correlate with the Coulombic efficiencies and ionic conductivities of diverse electrolyte compositions. We investigate a series of sodium salts such as NaFSI, NaTFSI, and Na FAST-B in various carbonate, ether, and sulfonamide solvents at different concentrations. Our preliminary analysis reveals that changing the electrolyte solvent can alter the redox potential by > 1 V and that weakly solvating solvents with low donor numbers demonstrate an upshift in their redox potentials, implying that the reducing ability of Na metal is weakened. We also observe that the anion identity does not vary the redox potential as significantly as the solvent identity does. We further correlate the electrolyte-dependent redox potential with the ion solvation structure via Raman spectroscopy. This work reveals how the electrode potential and the electrolyte entropy can serve as descriptors for Na metal reversibility, which will guide the design of high-performing liquid electrolytes for practical Na-metal batteries.