Charge polarity inversion and zincophilicity improvement for chitosan separator towards durable aqueous zinc-ion batteries

Aqueous zinc-ion batteries encounter enormous challenges such as Zn dendrites and parasitic reactions. Separator modification is a highly effective strategy to address these issues. With the advantages of low cost, nontoxicity, biodegradability, good film-forming ability, superior hydrophilicity, and rich functional groups, chitosan is an ideal matrix for constructing separators. However, the presence of positive charges within chitosan in weakly acidic electrolytes is unfavorable for dendrite inhibition. Herein, Schiff base reaction is introduced to modify chitosan matrix, transforming its charge polarity from positive to negative. Additionally, NbN with excellent zincophilicity is coated onto chitosan matrix, forming a Janus separator with low thickness of 19 μm and considerably improved mechanical properties. The resultant separator can promote the transport of Zn²⁺ ions while triggering a repulsive shielding effect against anions, therefore dramatically enhancing Zn²⁺ ion transfer number from 0.28 to 0.49. This separator can also facilitate desolvation process, improve exchange current density, restrict two-dimensional Zn²⁺ ion diffusion, and enhance electrochemical kinetics, contributing to significantly inhibited dendrite growth, by-product formation, and hydrogen evolution. Consequently, stable and reversible Zn stripping/plating process is enabled for over 2500 h at 2 mA cm⁻² and 2 mAh cm⁻². And great rate capability and excellent cyclability can be achieved for full batteries even under harsh conditions. This work provides new insights into separator design for Zn-based batteries.