Sodium‐ion batteries (SIBs) have great potential for large‐scale energy storage. Cellulose is an attractive material for sustainable separators, but some key issues still exist affecting its application. Herein, a cellulose‐based composite separator (CP@PPC) was prepared by immersion curing of cellulose‐based separators (CP) with poly(propylene carbonate) (PPC). With the assistance of PPC, the CP@PPC separator is able to operate the cell stably at high voltages (up to 4.95 V). The "pore‐hopping" ion transport mechanism in CP@PPC opens up extra Na+ migration paths, resulting in a high Na+ transference number (0.613). The separator can also tolerate folding, bending and extreme temperature under certain circumstances. Full cells with CP@PPC reveal one‐up capacity retention (96.97 %) at 2C after 500 cycles compared to cells with CP. The mechanism highlights the merits of electrolyte analogs in separator modification, making a rational design for durable devices in advanced energy storage systems. [ABSTRACT FROM AUTHOR]