Introduction: Anode-free sodium metal batteries have emerged as promising alternatives to lithium-ion batteries (LIBs) due to their high energy density and cost-effectiveness. Nevertheless, their practical implementation is hindered by sodium dendrite growth, an unstable solid electrolyte interphase (SEI), and a limited sodium reservoir, resulting in poor Coulombic efficiency and a short cycle life. High-concentration electrolytes (HCEs) can stabilize Na metal by suppressing solvent-related parasitic reactions and promoting an inorganic-rich SEI, though their high costs and viscosity constitute significant challenges. In this work, an HCE consisting of NaClO4 in ethylene carbonate (EC)–propylene carbonate (PC) diluted with fluoroethylene carbonate (FEC) is proposed to regulate the solvation structure and induce an anion-derived and F-rich SEI.
Methods: Two HCE formulations with NaClO4:EC:PC:FEC molar ratios of 1:2.4:2.4:1 (HELF1) and 1:2.4:2.4:2 (HELF2) were prepared, and a low-concentration electrolyte (1 M NaClO4 in PC-FEC, 98:2 vol%) was used as the baseline.
Results: Raman spectroscopy revealed that FEC increases the fraction of coordinated Na+ species and dissociated ClO4⁻. HELF1 and HELF2 exhibit good oxidative stability up to 4.8 V vs. Na+/Na and improved Na plating/stripping reversibility. However, excessive FEC leads to increased polarization, which is attributed to the formation of a thick, ion-transport-limited, and mechanically fragile SEI. Despite an improvement in the initial Coulombic efficiency (~85%), the Al||NFZC full cell retains only 17.2% of its capacity after 25 cycles.
Conclusion: FEC modulates the solvation structure and participates in SEI formation, effectively enhancing the stability of Na metal and the Coulombic efficiency. However, excessive FEC leads to an unstable SEI and poor long-term performance, highlighting the need to optimize its content to balance interfacial stability and ion transport.
