Enhancing sodium plating efficiency in a carbonate-based electrolyte—A mechanistic study

AbstractThe instability of the Solid Electrolyte Interphase (SEI) in carbonate-based electrolytes hinders the development of high-energy-density sodium batteries. This study elucidates the stabilization mechanism provided by a C65-coated aluminum electrode, representing the nucleation host in an anode-less sodium battery architecture, within a half-cell configuration against a sodium-metal counter electrode, using a carbonate-based electrolyte (1 M NaPF6 in EC:DMC with 5% FEC). We demonstrate that the high-surface-area C65-coating homogenizes the local current density, thereby facilitating the selective reduction of FEC prior to bulk solvent decomposition, fostering the formation of a flexible, NaF-rich SEI. Conversely, the high local current density on the bare aluminum electrode drives nonselective electrolyte decomposition, resulting in a brittle, inorganic-rich SEI and severe accumulation of "dead sodium". Consequently, the C65-coated electrode enhances the average coulombic efficiency to 85%, representing a significant relative improvement over the bare substrate. Furthermore, X-ray photoelectron spectroscopy reveals that the SEI composition on the bare aluminum eventually evolves to resemble the stable SEI composition on the coated electrode, yet electrochemical performance remains inferior. This underscores the long-term impact of the early-stage SEI composition on cell cyclability.