Boosting zinc metal anodes performance via interface engineering : reaction kinetics, morphology control and electrochemical reversibility

Abstract

Aqueous Zn metal batteries with economic, nontoxic, and intrinsically nonflammable superiority are regarded as the reliable energy storage candidates to complement the conventional Li-ion batteries. A variety of cathode materials have been developed with decent electrochemical performance. Turning to the anode side, despite a high volumetric capacity of 5851 mAh/cm3 for Zn metal, their commercial application is highly plagued by the poor reversibility primarily related to the dendrite growth. This thesis aims to achieve reversible Zn metal anodes through interface engineering. First, we examine the potential effect of desolvation kinetics on Zn deposition/stripping behavior. Acetonitrile (AN) co-solvent with a strong solvation ability is introduced into the electrolytes. The enhanced intermolecular interactions between Zn2+ and the mixed H2O/AN solvents lead to the supersaturating of adatoms on the electrode. Consequently, homogeneous nucleation and smooth growth of Zn are enabled for achieving a high average Coulombic efficiency of ~99.64%. Similarly, an oligomer poly(ethylene glycol) dimethyl ether is developed as a competitive solvent to further regulate the electrode/electrolyte interface for homogenous Zn nucleation. It can shift the water-occupied interface into oligomer one through preferential Zn surface adsorption, enabling increased nucleation sites and dendrite-free Zn morphologies. Furthermore, it also weakens the water/water and water/Zn2+ interaction through rich ether groups and strong solvation ability, alleviating the parasitic reactions. Thanks to these synergistic features, the Zn deposition/stripping lifetime is over tenfold increased at both low and high temperatures. Along with electrolyte optimization, a simple electrochemical method, pulsed cycling protocol, is proposed to control Zn nucleation. Specifically, we demonstrate the dual and contradictory roles of current density (J) in kinetics and thermodynamics using Zn metal anode as a model system. The well-known former renders decreased Sand's time (τ) and deteriorative cycling stability, while the commonly overlooked latter provides larger extra energy that accelerates nucleation rate (νn). Based on the discoveries, an initial high J (IHJ) cycling protocol is proposed to form sufficient nuclei at a high J for guiding subsequent metal deposition at the lower J, achieving high-performance Zn, Li, and K metal batteries. Besides the Zn nucleation optimization, we also regulate the subsequent Zn growth process by constructing a metallic tin-coated separator. Its conductivity helps homogenize Zn2+ flux and delays the initiation of Zn dendrites, while its zincophilicity enables face-to-face Zn growth and eliminates the inevitably formed Zn dendrites. Accordingly, a dramatically improved cycle life of 1000 h (5 mA/cm2, 5 mAh/cm2) is realized on Zn/Zn symmetric cells. Furthermore, we demonstrate that the approach could be readily extended to Na/K metal anodes.