Self-supporting NiCo carbonate hydroxyl nanosheet for ultra-high energy density and stable supercapacitors

•Ultrathin nanoscale NiCo carbonate hydroxide nanosheets.•CO32− and OH− groups interact better with electrolyte.•NCCH-130-6 has specific capacity of 11.61 C cm−2 at 1 mA cm−2.•Capacity remains 90 % after 12,000 cycles.•ASC exhibits 9.69 Wh L−1 at 43.29 W L−1. Carbonates and hydroxides are common hydrophilic oxygen-containing functional groups, which are conducive to complete contact with aqueous solution, thus expanding the active area and improving the electrochemical performance of materials. Here, we propose a new synthesis method using nickel foam as the sole nickel source to prepare dense and ultra-thin self-supporting cobalt carbonate hydroxide nanosheets, which are rich in many hydrophilic functional groups. This method goes beyond the traditional layered double hydroxides by combining CO32− and OH− groups, thus promoting more efficient interaction with the electrolyte and creating a larger contact area between the electrode and the electrolyte. When applied in an asymmetric supercapacitor, this material attains an ultrahigh energy density of 9.69 mWh cm−3 at a power density of 43.29 mW cm−3 (even at elevated power densities, e.g., 4.14 mWh cm−3 at 425.52 mW cm−3), with 90 % capacity retention over 5000 cycles at 100 mA cm−2. Nickel cobalt carbonate hydroxide rich in hydrophilic functional groups were synthesized through a one-step hydrothermal synthesis method. Asymmetric supercapacitor exhibits the excellent energy density and cycling stability. [Display omitted]