We have developed a high performance supercapacitor cathode electrode composed of well dispersed MnCO₃ quantum dots (QDs, ∼1.2 nm) decorated on nickel hydrogen carbonate–manganese carbonate (Ni(HCO₃)₂–MnCO₃) hedgehog-like shell@needle (MnCO₃ QDs/NiH–Mn–CO₃) composites directly grown onto a 3D macro-porous nickel foam as a binder-free supercapacitor electrode by a facile and scalable hydrothermal method. The MnCO₃ QDs/NiH–Mn–CO₃ composite electrode exhibited a remarkable maximum specific capacitance of 2641.3 F g⁻¹ at 3 A g⁻¹ and 1493.3 F g⁻¹ at 15 A g⁻¹. Moreover, the asymmetric supercapacitor with MnCO₃ QDs/NiH–Mn–CO₃ composites as the positive electrode and graphene as the negative electrode showed an energy density of 58.1 W h kg⁻¹ at a power density of 900 W kg⁻¹ as well as excellent cycling stability with 91.3% retention after 10 000 cycles, which exceeded the energy densities of most previously reported nickel or manganese oxide/hydroxide-based asymmetric supercapacitors. The ultrahigh capacitive performance is attributed to the presence of the high surface area core–shell nanostructure, the well dispersed MnCO₃ quantum dots, and the high conductivity of MnCO₃ quantum dots as well as the synergetic effect between multiple transition metal ions. The superior supercapacitive performance of the MnCO₃ QDs/NiH–Mn–CO₃ composites makes them promising cathode materials for high energy density asymmetric supercapacitors.