Enabling anion-anchoring and anti-degradation cellulose separators for sodium metal batteries

Renewable cellulose biopolymers have emerged as sustainable and efficient separator materials for sodium metal batteries, owing to their capacity to regulate ion transport and sodium deposition. In this study, we engineer nanocellulose separators by introducing a Zr-O complex coating layer, which negates the intrinsically strong hydrogen bond interactions in conventional nanocellulose, endowing the separator with anti-degradation and anion-anchoring properties. The Zr-O complex not only preserves the nanofibrous network and improves the electrochemical stability of nanocellulose but also expands and stabilizes ion transport channels. More importantly, this Zr-O complex coating transforms the predominantly Na⁺ ions adsorption on carboxyl groups into preferentially ClO₄^- anions immobilization, leading to a high Na⁺ transference number of 0.76. Meanwhile, both experimental and calculation results demonstrate that the intrinsic ferroelectric properties of the Zr-O complex promote the reduction of ClO₄^- anions, and facilitate the formation of a NaCl/NaF-enriched solid electrolyte interphase. These enhancements significantly improved cycle stability, with the Na||Na cells operating over 1100 h at 0.25 mA cm^-2/0.25 mAh cm^-2, about 2.6 times longer than traditional cellulose separators. This research offers new insights into the development of sustainable, and high-performance rechargeable sodium metal batteries through the innovative engineering of natural biopolymers.