Volumetric performance and energy-storage mechanism of high-density Ni-based coordination polymer electrodes for hybrid capacitors

To enhance the performances of energy storage and conversion devices, understanding their charge-storage mechanisms is essential. Research on energy-storage mechanisms for addressing the problem of low energy density in supercapacitors is lacking, which hinders their widespread adoption. This study introduces a novel high-density two-dimensional layered coordination polymer (CP) electrode material (i.e., [Ni(II)6Ni (II)2Li3(mba2- )6(S2- )2(OH- )3]n (Ni-mba-Li; H2mba = 2-mercaptobenzoic acid)) for use in aqueous hybrid capacitors. This electrode exhibits battery-type electrode behavior with a high volumetric capacity. Moreover, the asymmetric device achieves a high volumetric energy density and demonstrates outstanding capacitance retention and Coulombic efficiency over 13,000 charge/discharge cycles. A range of analytical techniques is used to explore the energy-storage mechanism of the CP at the molecular level. Electrochemical studies reveal that in KOH solution, Ni-mba-Li decomposes into Ni(OH)2 and the corresponding ligands. However, under an electric field, these decomposition reactions are converted into reversible redox reactions, enabling rapid charge storage. Moreover, the electric field triggers new irreversible reactions, and effectively controlling these processes is key to enhancing the performance and lifespan of the electrode material. This study highlights the potential of rational molecular design to develop advanced electrode materials for energy-storage applications.