Rational design of ZnO-based aqueous batteries for safe, fast, and reliable energy storage: Accomplishment of stable K+ storage/release

Rechargeable aqueous batteries are attractive for grid-scale applications. Yet, their broad adoption is hindered by the unsatisfactory durability, capacity, and Coulombic efficiency (CE). To alleviate these problems, we report a new Ni-Zn system composed of our newly developed concentrated CH3COOK (KAc) gel electrolyte, a ZnO@C anode, and a commercial Ni(OH)2 cathode. In this system, the ZnO@C anode demonstrates significantly improved stability than bare ZnO electrode (or ZnO@C in liquid KOH), and displays a reversible capacity of 400 mAh/g after 50 cycles at 1 A/g. Most importantly, the full cell demonstrates an ultrahigh midpoint discharge voltage of about 2.1 V, which is ∼0.5 V higher than that of traditional Ni-Zn battery with KOH as electrolyte. With Zn(OH)42- source in the gel, capacity and durability of the anode are largely enhanced, achieving 540 mAh/g for ∼450 cycles at 1 A/g. Surprisingly, the reversibility is increased with the increase of discharge current, achieving a capacity of 640 mAh/g at 4–8 A/g while maintaining CE as high as 97.3 % for the whole cell. After carefully analysis, it is found that stable and reversible K+ insertion but not Zn/Zn(OH)42- dominates the reaction mechanism and takes main contribution to the high voltage; While, the concentrated gel electrolyte, the C coating layer, and the solid electrolyte interphase (SEI) formed during cycling play the major role on durability and CE.