Modulation of Electron Push–Pull by Redox Non‐Innocent Additives for Long Cycle Life Zinc Anode

Abstract

Application of an aqueous Zn‐ion battery is plagued by a water‐induced hydrogen evolution reaction (HER), resulting in local pH variations and an unstable electrode–electrolyte interface (EEI) with uncontrolled Zn plating and side reactions. Here, 4‐methyl pyridine N‐oxide (PNO) is introduced as a redox non‐innocent additive that comprises a hydrophilic bipolar N+–O− ion pair as a coordinating ligand for Zn and a hydrophobic ─CH3 group at the para position of the pyridine ring that reduces water activity at the EEI, thereby enhancing stability. The N+–O− moiety of PNO possesses the unique functionality of an efficient push electron donor and pull electron acceptor, thus maintaining the desired pH during charging/discharging. Intriguingly, replacing ─CH3 (electron pushing +I effect) by ─CF3 group (electron pulling ─I effect), however, does not improve the reversibility; instead, it degrades the cell performance. The electrolyte with 2 m ZnSO4 + 15 mm PNO enables symmetric cell Zn plating/stripping for a remarkable > 10 000 h at 0.5 mA cm−2 and exhibits coulombic efficiency (CE) ≈99.61% at 0.8 mA cm−2 in Zn/Cu asymmetric cell. This work showcases the immense interplay of the electron push–pull of the additives on the cycling.