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 N-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.