Development of sodium anodes, both hard carbon (HC) and metallic, is dependent on the discovery of electrolyte formations and additives able to stabilize the interphase and support Na+ transport. Halogen salt additives are known to lower the energy barrier for the Na-ion charge transfer at the interface and facilitate stable Na plating/stripping in a symmetric cell configuration. Here, a halogen-rich additive for the sodium-ion battery electrolyte, 2-chloro-1,1,2-trifluoroethyl difluoromethyl ether (enflurane), is reported. Enflurane offers a simple molecular alternative to salt-based additives. The additive is also shown to improve the cycling performance of sodium metal electrodes. Our analysis demonstrates that enflurane is preferentially reduced at the HC electrode over propylene carbonate and is incorporated into the solid electrolyte interphase (SEI). The result is a thin, halogen-rich SEI that offers better charge transport properties and stability during cycling compared to that formed in the additive-free electrolyte. Additionally, enflurane inhibits polarization of metallic sodium electrodes, and when included in HC half-cells at 10 v/v %, it improves the reversible specific capacity and stability.
Keywords: electrolyte additive; enflurane; hard carbon; sodium metal; sodium-ion battery; solid electrolyte interphase.