Upcycling Spent Tea Leaves into a Multifunctional Separator for High-Performance and Eco-Friendly Aqueous Zinc-Ion Batteries

Aqueous zinc-ion batteries (AZIBs) suffer severe limitations from uncontrollable Zn dendrite growth and parasitic side reactions. Separator functionalization is a promising strategy for addressing these challenges. However, modifying cellulose-based separators often involves complex, costly procedures. Herein, we repurpose low-cost spent tea leaves (STLs) as functional fillers to fabricate cellulose nanofiber (CNF)/STL) separators via simple wet cogrinding and vacuum filtration. Tea polyphenols in STL interact with Zn2+, while nitrogen/oxygen-containing groups provide zincophilic sites that, with the interconnected porous CNF network, collectively regulate Zn2+ flux and stabilize the electrode/electrolyte interface. This synergy promotes uniform Zn plating/stripping and suppresses the dendrite formation. Electrochemical evaluations demonstrate that Zn||Zn cells equipped with the CNFs/STL separator cycle stably for over 2900 h at 1 mA cm–2/1 mAh cm–2. Moreover, full cells retain 73.5% of initial capacity after 2000 cycles at 5 A g–1, proving this green, cost-effective, and scalable biomass separators for high-performance AZIBs.