Methylurea as a Promoter of Cellulose Dissolution and a Regulator of Solvation Structures of Zinc Ions for Quasi-Solid Dendrite-Free Zinc Hybrid Supercapacitors

Flexible zinc-ion hybrid supercapacitors (FZHSCs) with high energy and power densities and long cycle lives are promising. However, dendrite growth and unsatisfactory cycle stability limit their applications. By adopting Zn foil as the anode, N-doped reduced graphene oxide (N-rGO) as the cathode, and cellulose-methylurea (Mu) hydrogel (CMZ-gel) as the electrolyte, we prepared high-performance FZHSCs. The presence of Mu molecules contributes to cellulose dissolution, alters zinc-ion solvation in the CMZ-gel, and guides uniform zinc deposition. The network structure of hydrophilic cellulose chains disrupts the hydrogen bonds between water molecules, hindering proton transport and reducing the HER. Simultaneously, the porous hydrogel structure facilitates three-dimensional zinc-ion diffusion. Zn//Zn symmetric cells with CMZ-gel exhibit an extended lifespan exceeding 1000 h at 1 mA cm –2 and 1 mAh cm –2 . The Zn//CMZ-gel//N-rGO ZHSC displays a high specific capacitance of 173 F g –1 (0.5 A g –1 ), superior power density of 4287 W kg –1 (13.1 Wh kg –1 ), and high energy density of 81.2 Wh kg –1 (562 W kg –1 ). After 10,000 charge–discharge cycles (10 A g –1 ), the Zn//CMZ-gel//N-rGO ZHSC shows excellent cycle stability with a high capacitance retention of 93%.