Copper hybrid cluster-engineered cellulose hydrogels enabling coupled ion and water regulation for highly reversible zinc anode

Cellulose-based hydrogels hold great promise as electrolytes for aqueous Zinc-ion batteries (ZIBs), yet their performance is hindered by chain aggregation, overly strong Zn²⁺-cellulose coordination that slows ion transport, and parasitic reactions triggered by free water and sulfate species. Here, we design a multifunctional cellulose nanofiber (CNF) hydrogel electrolyte incorporating copper hybrid cluster Cu₄I₄(L)₄ (L = 3-Fluoropyridine (3-FPy)) to address these intrinsic limitations. Strong interactions between the clusters and cellulose chains prevent molecular aggregation and introduce additional crosslinking, reorganizing the network into a more continuous and uniform 3D architecture. Unexpectedly, the hydrogel develops a sandwiched structure consisting of dense outer layers that homogenize Zn²⁺ flux and shield the anode from free water, and a porous intermediate layer that serves as an electrolyte reservoir to facilitate long-range ion migration. The clusters exhibit weaker affinity toward Zn²⁺ than cellulose, mitigating excessive Zn²⁺-polymer coordination and enabling faster ion movement. Their spatially polarized charge distribution also provides sequential Zn²⁺ hopping sites, further promoting directional ion transport. Meanwhile, strong binding with H₂O and SO₄^2- converts free water into bound water and immobilizes anions, thereby suppressing H₂O- and SO₄^2--driven parasitic reactions. Benefiting from this cooperative regulation of microstructure, ion coordination, and water chemistry, the CNF@Cu₄I₄(L)₄ hydrogel exhibits high ionic conductivity, stable Zn plating/stripping, and effectively suppressed side reactions. As a result, Zn anodes achieve a long-term cycling stability up to 1200 h, and capacity retention of 99.5% over 2000 cycles for Zn||Polyaniline full cells. This work establishes a robust strategy for engineering cluster-modified hydrogels toward safe, durable, and high-performance aqueous ZIBs.