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Gel‐polymer electrolyte (GPE) is a pragmatic choice for high‐safety sodium batteries but still plagued by interfacial compatibility with both cathode and anode simultaneously. Here, salt‐in‐polymer fibers with NaF salt inlaid in polylactide (PLA) fiber network was fabricated via electrospinning and subsequent in situ forming gel‐polymer electrolyte in liquid electrolytes. The obtained PLA‐NaF GPE achieves a high ion conductivity (2.50×10−3 S cm−1) and large Na+ transference number (0.75) at ambient temperature. Notably, the dissolution of NaF salt occupies solvents leading to concentrated‐electrolyte environment, which facilitates aggregates with increased anionic coordination (anion/Na+ >1). Aggregates with higher HOMO realize the preferential oxidation on the cathode so that inorganic‐rich and stable CEI covers cathode' surface, preventing particles' breakage and showing good compatibility with different cathodes (Na3V2(PO4)3, Na2+2xFe2‐x(SO4)3, Na0.72Ni0.32Mn0.68O2, NaTi2(PO4)3). While, passivated Na anode induced by the lower LUMO of aggregates, and the lower surface tension between Na anode and PLA‐NaF GPE interface, leading to the dendrites‐free Na anode. As a result, the assembled Na || Na3V2(PO4)3 cells display excellent electrochemical performance at all‐climate conditions. [ABSTRACT FROM AUTHOR]