A Uniform Solid Electrolyte Interface Enabling Long Cycling in Sodium-Metal Batteries with Fast Charging

The high reactivity of sodium leads to significant safety challenges, while the unstable solid electrolyte interphase (SEI) further complicates its use in sodium-metal batteries (SMBs), collectively impeding their path to commercialization. A deep eutectic electrolyte (DEE) is introduced, which addresses these challenges by balancing high ionic conductivity with stable SEI formation. The introduction of <i>N</i>-methylacetamide enhances the nonflammability of the solvent and adjusts the SEI composition. The DEE accelerates the interfacial kinetics and promotes the formation of a uniform, inorganic-rich SEI with Na₂S and NaF on the sodium anode. It also extends the electrochemical stability window to 4.8 V. Additionally, the DEE exhibits high ionic conductivity, enabling fast charging of the SMBs. In a Na₃V₂(PO₄)₃||Na cell, the electrolyte achieved a capacity retention of 96% after 1,500 cycles at a current density of 5 C. These findings offer important insights for advancing the commercialization of SMBs with enhanced safety features and superior electrochemical performance.