Mechanism of Zn storage in polyaniline in deep eutectic solvent–water mixtures: effect of anion and interfacial phenomena

Abstract

Zn-ion batteries (ZIBs) are promising energy storage devices, wherein both the electrode and electrolytes play a pivotal role. Aqueous electrolytes have a limited electrochemical window that reduces the energy density of the battery. In comparison, deep eutectic solvents (DES) have a wider electrochemical window that can offer a higher energy density. In this paper, the performance of Zn-ion batteries in formamide-based DES containing Zn salts of different anions (Cl⁻, SO₄²⁻, Ac⁻ and TfO⁻) was investigated. The study revealed that anions significantly influence Zn solvation, hydrogen evolution reaction (HER), charge storage mechanism and stability of the battery. X-ray photoelectron spectroscopy (XPS), Raman spectroscopy and charge–discharge analyses showed that both chlorine and triflate anions store charge by anion exchange followed by Zn storage in polyaniline. For acetate and sulfate anions, the storage mechanism is by direct interaction of Zn with Zn-polyaniline (PANI). Among the four anions studied, the dual-storage mechanism in ZnCl₂- and ZnTfO-based DES electrolytes resulted in a more stable Zn–PANI battery performance. However, ZnCl₂-based DES electrolytes led to corrosive issues that affected the long-term stability. The study provides useful insights on novel electrolyte development through manipulating the solvation chemistry of the molecules and the electrode/electrolyte interface.