A Sodium Ion Battery Separator with Reversible Voltage Response Based on Water-Soluble Cellulose Derivatives.

The development of efficient, safe and environmental friendly energy storage systems plays a pivotal role towards a more sustainable society. Sodium ion batteries (NIBs) have garnered considerable interest for grid energy storage applications because of the abundance of sodium, low cost and suitable redox potential. However, NIB technology is still in its infancy, especially with regard to separators. Here we develop a novel separator based on renewable water-soluble cellulose derivatives. Carboxymethyl cellulose (CMC) and hydroxyethyl cellulose (HEC) are cross-linked to afford large specific surface area membranes upon non-solvent induced phase separation (NIPS). Long-term galvanostatic cycling in a symmetric Na/Na cell configuration shows an impressive reversible voltage response with square wave shape of the polarization even after 250h of cycling, indicating remarkably stable Na plating and stripping with Na dendrite growth suppression. This novel membrane is evaluated as a separator in Na3V2(PO4)3/Na half cells. After 10 cycles at C/10 cellulosic separator delivers a capacity of 74 mA•h•g-1 with a 100% Coulombic efficiency compared to 61 mA•h•g-1 and 96% obtained for Whatman GF/D as commercially available separator. Our work provides novel cues for the development of biomass-derived porous membranes to function as battery separators, surpassing the performance of commercially available separators based on fossil resources in terms of capacity retention, Coulombic efficiency, homogeneous plating/stripping of Na and dendrite growth suppression. These separators, which may be extended to other battery systems, are expected to play a significant role towards sustainable energy storage systems.