23 Na NMR Chemical Shift as a Descriptor of Sodium Electrode Potential

The electrode potential of alkali metals is a critical parameter for battery performance, but its dependence on electrolyte compositions remains poorly understood, particularly for sodium metal, because of its high reactivity with electrolytes. Here, we systematically evaluated the sodium electrode potentials (ENa) in various electrolytes composed of sodium bis(fluorosulfonyl)imide, which enables stable measurements by forming a robust solid electrolyte interphase. The coordination state around Na+ in each electrolyte was analyzed by using Raman spectroscopy and 23Na NMR. In a low-concentration regime, ENa strongly correlates with 23Na chemical shifts that reflect the electron-donating ability of solvents, indicating that the solvating ability is the primary determinant of ENa. At a higher concentration, ab initio molecular dynamics simulations revealed that the 23Na chemical shifts reflected the ion-pairing state, as well as solvation. Leveraging these insights, we developed a simple yet powerful predictive model for the ENa. By using only 23Na chemical shifts and salt-to-solvent molar ratios, the model can accurately predict ENa across diverse electrolytes. This work identifies a new descriptor of ENa and provides a platform for rational electrolyte design for advanced sodium batteries.