Hybrid electrolyte engineering enables reversible Zn metal anodes at ultralow current densities

Abstract

Parasitic reactions in aqueous Zn metal batteries tend to be intensified along with the decrease of current density, deteriorating the cycling stability of Zn anodes, which unfortunately have been overlooked in previous studies. Herein, an organic solvent N-Methylacetamide (NMA) is introduced into ZnSO4 electrolyte for improving the reversibility of Zn anodes. Combined with theoretical and experimental investigations, it is uncovered that NMA additives are capable of entering not only the pristine Zn2+ solvation structure ([Zn(H2O)x]2+) but also the electrical double layer structure, reducing active H2O molecule proportions and thus suppressing the corrosion reactions. Meanwhile, nucleation and growth processes are also constrained with NMA additive for its strong coordination with Zn2+, inducing dendrite-free Zn depositions. Consequently, an average Coulombic efficiency (CE) of 98.5% for 600 cycles in Zn|Cu half-cells with NMA/ZnSO4 electrolyte is obtained at an ultralow current density of 0.3 mA cm−2. Moreover, Zn|Zn symmetric cells utilizing 20 vol% NMA additive exhibit stable cycle lifespan of 1000 h. Benefitting from the enhanced reversibility of Zn anodes in NMA/ZnSO4 electrolytes, full-cells assembled with carbon cloth as cathodes deliver high performance of 5200 cycles at the current density of 64 mA g−1 with 10 μm thin Zn anodes, favoring the practical applications.