Methylurea as Both Promoter to Dissolve Cellulose and Regulator of Solvation Structure of Zinc Ions for Quasi-Solid Dendrite-Free Zinc Hybrid Supercapacitors

Abstract

Flexible zinc-ion hybrid supercapacitors (FZHSCs) with high energy and power density with long cycle life are promising. However, dendrites growth and unsatisfactory cycle stability limits their application. Here, 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 quasi-solid electrolyte, and high-performance FZHSCs were prepared. The presence of Mu molecules not only contributed to the dissolution of cellulose, but also changed the solvation structure of zinc ions in CMZ-gel. The network structure of hydrophilic cellulose chains broke the hydrogen bond between water molecules, inhibiting proton transport and reducing HER. The Zn//Zn symmetric cells with CMZ-gel delivered an extended lifespan of over 1000 h at 1 mA cm−2 and 1 mAh cm−2. The Zn//CMZ-gel//N-rGO ZHSC showed a high specific capacitance of 173 F g-1 (0.5 A g-1), a superior power density of 4287 W kg-1 (13.1 Wh kg-1) and a high energy density of 81.2 Wh kg-1 (562 W kg-1). Furthermore, after 10000 charge-discharge cycles (10 A g-1), the Zn//CMZ-gel//N-rGO ZHSC demonstrated excellent cycle stability with a high capacitance retention of 93%.

Keywords: Flexible zinc-ion hybrid supercapacitors (FZHSCs), Hydrogel electrolyte, cellulose hydrogel, methylurea, suppressed dendrite growth