Weak solvation effects and molecular-rich layers induced water-poor Helmholtz layers boost highly stable Zn anode

Abstract

The advancement of aqueous Zn-based energy storage systems encounters major challenges due to the occurrence of side reactions and the growth of dendrites caused by the highly active nature of water in the aqueous electrolyte. Herein, a zwitterion additive named sulfonated amphoteric betaine (referred to as DMAPS) regulates the Zn²⁺ electrolyte environment through a weak solvation effect to promote highly stable Zn anodes. The inherently occurring anionic (-SO₃^-) and cationic (-NR₄⁺) counterions in the DMAPS chain can be effectively separated under an external electric field to enable the creation of distinct ion migration pathways, thereby significantly enhancing the transportation of electrolyte ions. Moreover, DMAPS can be adsorbed onto the surface of Zn anode to form a H₂O-poor Helmholtz layer, which can serve as a protective barrier to suppress corrosion of the Zn anode by free H₂O and inhibits the formation of differential electric field to facilitate the directional deposition of Zn²⁺. In addition, density functional theory (DFT) and molecular dynamics (MD) further assisted in demonstrating the intrinsic mechanism of the reconstruction of the solvated sheath structure of Zn²⁺ by DMAPS. As a result, the Zn//Zn symmetric cell in ZnSO₄+DMAPS solution can be cycled steadily for 2100 h at a current density of 1 mA cm^-2 and 1 mAh cm^-2. The assembled Zn ion hybrid capacitor with ZnSO₄+DMAPS electrolyte was stably cycled for 20,000 cycles with 90% capacity retention at 5 A g^-1.