Aqueous anion insertion into a hydrocarbon cathode via a water-in-salt electrolyte

[Display omitted] •We demonstrate the reversible aqueous insertion of anions into a hydrocarbon cathode.•During the first charge, coronene exhibits a capacity of 88.2 mAh g−1 at 100 mA g−1.•Ex situ structural characterization elucidates the storage mechanism of anions.•A capacity of ~40 mAh g−1 was observed even at 1000 mA g−1 (11.2 C rate).•The polymerization of the coronene helps raise the on-set of OER allowing oxidation. We report coronene, a polycyclic aromatic hydrocarbon (PAH), exhibits reversible oxidative anion insertion properties in a LiTFSI based water-in-salt electrolyte. The coronene electrode delivers a reversible capacity of ~65 mAh g−1 with the first-cycle Coulombic efficiency of ~73%. The results demonstrate that the high concentrations of LiTFSI lower the anion-insertion potential in coronene, which allows to reveal the capacity below the onset of oxygen evolution reaction. Furthermore, evidence points to the polymerization of the coronene cathode surface, which contributes to the reversible anion-storage in coronene as well. Ex situ XRD, FTIR, and SEM are carried out to reveal the structural evolution of the coronene electrode and storage mechanism of the TFSI– anions. FTIR studies reveal that upon anion insertion the interactions between the TFSI– ion and coronene molecules resemble that of covalent bonding.