Nanocellulose, as a nano-scale product obtained by processing the most abundant natural polysaccharide, has excellent thermal stability, mechanical strength and wettability. Thus nanocellulose-based membrane is a good candidate for separator in battery system. . In this study, nanocellulose was prepared by TEMPO oxidation method and assembled into membranes usinga bottom-up strategy. Nanocellulose suspensions were first vacuum filtrated to form wet membranes. The wet membranes were then subjected to freeze-drying (TOCNF-F), ethanol/tert-butanol solvent exchange (TOCNF-E),or solvent exchange combined with freeze-drying (TOCNF-EF) to obtain three different types of separators. The fourth type of separator was prepared by first protonation of the nanocellulose suspensionwith hydrochloric acid prior to film formation followed by solvent exchange and freeze-drying (TOCNF-HEF). All the separators were assembled into sodium/hard carbon half-cells. The TOCNF-EF separator has comparable cycling performance as glass fiber separator, with a reversible specific capacity of 295 mAh/g at 25 mA/g discharge/charge rate. Higher reversible specific capacity was obtained for the TOCNF-HEF separator with protonation processing (320 mAh/g). SEM imaging show that the TOCNF-EF separator has a three-dimensional mesh-like structure suitable for sodium-ion batteries, while the TOCNF-HEF separator has a more porous and open pore structure. In addition, the separator with high surface polarity helps the formation of SEI (solid electrolyte interphase) film and promotes stable battery cycling. The assembled nanocellulose-based separators are expected to replace the separator currently used in sodium ion battery, such as polyolefin-based separators with poor wettability and expensive glass fiber separators with poor mechanical properties, accelerating the commercialization of sodium-ion batteries.

TEMPO oxidation,nanocellulose,sodium-ion batteries,separators,assembly method