A Redox-Active 2-D Covalent Organic Framework as a Cathode in an Aqueous Mixed-Ion Electrolyte Zn-Ion Battery: Experimental and Theoretical Investigations

We demonstrate here a simple alternative strategy of developing a stable and long-lived aqueous Zn-ion battery. The battery comprises a redox-active anthraquinone-based covalent organic framework (COF) and a graphene oxide composite (COF-GOPH) as the cathode, zinc metal as the anode, and a mixed-ion electrolyte with varying proportions of zinc and lithium ions. This cell configuration contrasts with those of conventional organic batteries with aqueous electrolytes having a single type of cation. Our findings convincingly show that an optimal Li+ to Zn2+ ion ratio is beneficial for Zn2+-ion diffusion into the COF. The energy storage mechanism is found to be due to the Zn2+-ion intercalation/deintercalation into the COF with simultaneous reversible redox activity of the framework carbonyl and imine moieties. Additionally, a theoretical analysis of the radial distribution function reveals the preferential insertion of Zn2+-ions along with its partial solvation shell into the framework, leading to an optimal coordination of Zn2+ with oxygen and nitrogen moieties of the COF network. On the other hand, the Li+ ions preferentially reside in solution. Irrespective of the electrolyte composition, the composite electrode COF-GOPH performs better than the COF. The best battery performance is obtained with the COF-GOPH in the presence of 0.5 M ZnSO4 and 0.5 M Li2SO4 electrolyte. The cell shows excellent cyclability and superior capacity with 82% retention even after 500 cycles (from the second cycle onwards). Our studies also reveal a Li+-ion-assisted pseudocapacitance mechanism that is partially responsible for the enhancement in the electrochemical performance in the mixed-ion electrolytes.