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

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.