Polypropylene oxide-modified cellulose separators for high-rate Na+ transport via a dual-pathway mechanism in sodium-ion batteries

Sodium-ion batteries (SIBs) offer compelling advantages for grid-scale energy storage, owing to their safety and cost competitiveness. However, the implementation of economically competitive cellulose separators in SIBs is hindered by oversized pore structures and insufficient electrochemical stability. Here, we develop a high-performance cellulose separator through in situ polymerization of a Bis[(3-methyldimethoxysilyl)propyl]-polypropylene oxide (BSPPO) coating on commercial substrates. The BSPPO modification synergistically optimizes pore architecture for homogeneous Na + deposition while expanding the electrochemical stability window to 4.87 V through functional group engineering. Crucially, the polymer coating enables a dual-pathway Na + transport mechanism, integrating high-rate ion conduction through pore channels with polymer chain-mediated transport, resulting in superior ionic conductivity (1.33 mS cm −1 ) and Na + transference number (0.69). Hard carbon||Na cells incorporating the 20 % BSPPO-modified separator demonstrate exceptional cycling stability, maintaining 99.5 % coulombic efficiency and 90.7 % capacity retention over 100 cycles at 2C. This work introduces a dual-pathway ion transport strategy, advancing cellulose separators toward practical high-performance SIB applications.