Azopyridine Aqueous Electrochemistry Enables Superior Organic AZIBs

Azo compounds (AZO), such as azobenzene, are classic organic electrode materials featuring a redox potential close to Zn/Zn²⁺. Recent studies show that azobenzene could work as a cathode in aqueous zinc-ion batteries (AZIBs), providing a voltage output of around 0.7 V. However, the energy storage mechanism of AZO cathodes in AZIBs remains unclear, and their practical usage in AZIBs is hindered by the low voltage. In this study, azopyridine isomers, the hydrophilic analogues of azobenzene, were adopted as cathodes for AZIBs, and the energy storage mechanism was unveiled through aqueous electrochemical studies. Through in situ electrochemical characterizations and theoretical computations, we reveal that both the electron-withdrawing effect of the pyridyl group and the H⁺-involved -N = N-/-NH-NH- redox reaction uplift the redox potential of the azopyridine cathodes. These findings led to the first AZO-based AZIB, providing a voltage output of 1.4 V. The proposed air-stable AZIBs deliver a high energy/power density and a capacity of around 200 mAh g^-1. This work discovers different azopyridine electrochemistry in aqueous and organic electrolytes and enabling AZIBs to outperform its competitors from the AZO family.