Ion replacement to construct coral-like montmorillonite/manganese-cobalt bimetallic oxide for high-performance supercapacitors with aqueous electrolyte

With the growth demand of new energies and the limitations of traditional fossil fuels, the development of efficient and environmentally friendly energy storage technologies for the new energies have become crucial. In this study, montmorillonite (MON) was used as the substrate to prepare a novel composite electrode material with manganese-cobalt oxide (HSMnCo), which is in-situ grown on the surface of MON. The manganese-cobalt oxide grown on the surface of MON fills the voids between the MON lamellae, preventing the agglomeration of MON lamellae and keeping it in a relatively loose structure. Meanwhile, MON has a high ion exchange capacity, and the Si⁴⁺ between its layers can undergo exchange reactions with Mn²⁺ and Co²⁺. Moreover, there is a synergistic and competitive effect between Mn²⁺ and Co^2+, which changes the nucleation sites, nucleation number and growth process of manganese-cobalt oxide on the surface of MON. As a result, HSMnCo with a unique coral-like structure with multiple wrinkles and layers is obtained, significantly increasing the specific surface area (212 m²∙g^-1) and porosity of the composite electrode material. Electrochemical tests showed that HSMnCo has a specific capacitance as high as 1235 F∙g^-1 at a current density of 0.5 A∙g^-1 and still retains 91.6 % of its initial capacity after 5000 cycles, demonstrating excellent cycling stability. The synergistic effect of manganese-cobalt bimetals optimizes the electron transport path, reduces the charge transfer resistance (Rct = 3.58Ω), and improves the power density (279.83 W∙kg⁻¹) and energy density (43.53 Wh∙kg^-1). In addition, when it was applied to zinc-ion supercapacitors, the long cycle capacity retention rate is further increased to 94.7 %, and when the power density is 374.78 W∙kg^-1, the energy density reaches 177.50 Wh∙kg^-1. This study not only provides new ideas for the high value utilization of waste resources such as silicate minerals but also promotes the development of green and low cost electrode materials for supercapacitors.