Superconductivity in nickelates apparently takes place in two different Ni oxidation regimes, namely either for infinite-layer-type compounds close to ${\mathrm{Ni}}^{+}$, or for Ruddlesden-Popper materials close to ${\mathrm{Ni}}^{2+}$. The reduced ${\mathrm{La}}_3{\mathrm{Ni}}_2{\mathrm{O}}_6$ bilayer with a nominal ${\mathrm{Ni}}^{1.5+}$ oxidation state may therefore serve as a normal-state mediator between the two known families of $3d^8$-like and $3d^9$-like superconducting nickelates. Using first-principles many-body theory, we explain its experimental 55 meV charge gap as originating from a different type of correlated (quasi-)insulator. Flat-band electrons of $\mathrm{Ni}\text{−}{d}_{z^2}$ character become localized from scattering with orbital-selective Mott-localized $\mathrm{Ni}\text{−}{d}_{x^2-y^2}$ electrons, by trading in residual hopping energy for a gain in local exchange energy in a ferromagnetic Kondo-lattice scenario. Most importantly, the flat-band electrons offer another route to unconventional superconductivity in nickelates at ambient pressure.

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• Superconductivity
• Strongly correlated systems
• Electron-correlation calculations
• First-principles calculations
• Kondo lattice model