Use of quinhydrone as a promising cathode material for aqueous zinc-ion battery

Aqueous zinc ion batteries have attracted attention in recent years due to growing demand for energy storage devices, cost effectiveness coupled with high energy densities. They are attractive for large-scale storage, longevity, and inherent safety. However, their development is relatively slow due to limited availability of solid inorganic frameworks wherein diffusion of Zn2+ is known to be sluggish. In the present study, traditional pH electrode redox system based on quinhydrone (QH) is proposed as a host cathode, which offers reversible and effective Zn2+ storage due to its reversible electron-donor acceptor characteristics. An attractive specific capacity of 232 mAh g-1 at 50 mA g-1 with a small polarization of 80 mV and a safe operating voltage of 1.0 V is observed. Unprecedently, the QH electrode exhibits a life span of over 2000 cycles with a fairly good capacity retention. Physicochemical characterization using X-ray diffraction (XRD), Fourier transform infrared (FTIR), Raman, nuclear magnetic resonance (NMR), and electron microscopy reveal structural evolution and reversibility of active material during charge - discharge. Density functional theory (DFT) is used to understand the highest occupied molecular orbital (HOMO)-lowest unoccupied molecular orbital (LUMO) levels to support the Zn2+ storage mechanism of QH.