Cation-Anion Redox Active Organic Complex for High Performance Aqueous Zinc Ion Battery

Organic redox compounds are attractive cathode materials in aqueous zinc-ion batteries owing to their low cost, environmental friendliness, multiple-electron-transfer reactions, and resource sustainability. However, the realized energy density is constrained by the limited capacity and low voltage. Herein, copper-tetracyanoquinodimethane (CuTCNQ), an organic charge-transfer complex is evaluated as a zinc-ion battery cathode owing to the good electron acceptation ability in the cyano groups that improves the voltage output. Through electrochemical activation, electrolyte optimization, and adoption of graphene-based separator, CuTCNQ-based aqueous zinc-ion batteries deliver much improved rate performance and cycling stability with anti-self-discharge properties. The structural evolution of CuTCNQ during discharge/charge are investigated by ex situ Fourier transform infra-red (FT-IR) spectra, ex situ X-ray photoelectron spectroscopy (XPS), and in situ ultraviolet visible spectroscopy (UV-vis), revealing reversible redox reactions in both cuprous cations (Cu+) and organic anions (TCNQ(x-1)), thus delivering a high voltage output of 1.0 V and excellent discharge capacity of 158 mAh g(-1). The remarkable electrochemical performance in Zn//CuTCNQ is ascribed to the strong inductive effect of cyano groups in CuTCNQ that elevated the voltage output and the graphene-modified separator that inhibited CuTCNQ dissolution and shuttle effect in aqueous electrolytes.