Single‐Ion‐Functionalized Nanocellulose Membranes Enable Lean‐Electrolyte and Deeply Cycled Aqueous Zinc‐Metal Batteries

Aqueous cells with zinc‐metal anodes featuring safety and low cost, are beneficial for diversifying energy‐storage technologies, while their energy density and cyclability have been long limited by side‐reactions and dendrite issues, especially at the practical device level. Though sustained efforts are underway to renovate electrodes and electrolytes, the roles of other indispensable components, such as separators, in the cell operation have been not fully unexplored thus far. Here, it is demonstrated that both the reversibility and utilization of aqueous zinc anodes can be improved by using a single‐ion Zn2+‐conducting nanocellulose membrane as the separator. Even without any treatments to the electrodes and thereof interfaces, this functional membrane marked by synergetic optimization on key required properties regarding mechanical strength, preferred Zn2+ conduction and hydrophilicity, mitigates H2 evolution, corrosion, and dendrite growth on zinc anodes, thus enabling >80% depth‐of‐discharge stable cycling under practically feasible lean electrolyte (electrolyte‐to‐capacity ratio = 1.0 g Ah−1) and high areal capacity (8.0 mAh cm−2) conditions. These findings translate to an excellent capacity retention of exceeding 95% after 150 cycles for full cells with practically high‐loading cathodes (17 mg cm−2). This work provides a simple yet practical avenue to high‐energy, long‐cycling aqueous zinc‐metal batteries. To address the poor reversibility of aqueous Zn batteries due to hydrogen evolution, corrosion, and dendrite, it is presented that using a rationally functionalized nanocellulose membrane as a separator mitigates most irreversible issues in aqueous zinc anodes, enabling >80% depth‐of‐discharge stable cycling under practically feasible lean‐electrolyte (electrolyte‐to‐capacity ratio, E/C = 1.0 g Ah−1) and high‐areal‐capacity (>8 mAh cm−2) conditions.