Interfacial Kinetics, Not Solvation Thermodynamics, Govern the Reversibility of Sodium Metal Batteries

Achieving reversible sodium metal plating and stripping is essential for enabling practical Na metal batteries but remains limited by unstable electrolyte–metal interphases. Here, we quantitatively examine how solvation thermodynamics, interfacial kinetics, ion transport, and solid electrolyte interphase (SEI) composition govern Na metal reversibility in sodium bis(fluorosulfonyl)imide (NaFSI) electrolytes with 1,2-dimethoxyethane (DME), fluoroethylene carbonate (FEC), and N,N-dimethylsulfamoyl fluoride (DMFSA). Unlike Li systems, Na metal Coulombic efficiency (CE) shows no correlation with either the Na+/Na redox potential or the interfacial reaction entropy. Instead, increased CE in electrolytes like 1 M sodium hexafluorophosphate in DME corresponds to faster interfacial kinetics relative to ion diffusivity (j0SEI/FcD). X-ray photoelectron spectroscopy highlights the importance of balancing the inorganic and organic SEI phases to optimize interfacial kinetics and CE. These results establish interfacial kinetics, rather than solvation thermodynamics, as a governing descriptor of Na metal reversibility, providing an electrolyte design framework for improving Na metal batteries.