Plasma chemistry enables compositionally tunable artificial SEI for solid polymer electrolyte sodium batteries

Sodium metal anodes paired with solid polymer electrolytes offer a compelling route to safe, high-energy-density batteries, yet uncontrolled interfacial reactivity and dendrite formation remain critical barriers. Here, CO₂-mediated non-thermal plasma technology is used to engineer ex situ artificial solid electrolyte interphases (ASEIs) on sodium metal anodes using fluorinated precursors - NaF, FEC, and PFHxA - spanning a systematic range of inorganic-to-organic fluorine character. Gas-phase characterization by OES, FTIR, and GC-MS reveals that each precursor generates distinct reactive intermediates in the plasma, while XPS confirms precursor-dependent divergence in interphase fluorine speciation: NaF yields an inorganic-rich NaF/organic-carbon hybrid, whereas PFHxA produces a fluoropolymer-dominated film. This compositional divergence directly governs performance - the NaF-derived ASEI delivers the lowest interfacial charge-transfer resistance, sustains symmetric cell cycling beyond 1800 h (18-fold improvement over bare sodium), and maintains stable half-cell operation beyond 3000 cycles at 3C. These results establish that optimal ASEI design requires a deliberate balance of inorganic fluoride for Na⁺ transport and organic carbon functionality for SPE interfacial compatibility, and demonstrate CO₂-mediated NTP as a versatile, ambient-condition platform for rational interphase engineering in solid-state alkali metal batteries.