Stability and electronic properties of layered NaMnO₂ using the SCAN(+U)

Considering electron correlation appropriately is important to predict the properties of layered transition metal oxides, which have drawn a lot of attention as cathode materials for sodium-ion batteries. Here, we explore the phonon and electronic properties of layered NaMnO₂ using the recently developed strongly constrained and appropriately normed (SCAN) functional. We also introduce the Coulomb interaction U to find an accurate description of Mn 3d orbitals. The phonon dispersion curves show the structural stability with the SCAN, which is consistent with prior experimental stability at high Na concentrations. On the other hand, imaginary phonon frequencies were observed by applying U, which indicates structural instability. Namely, SCAN properly describes the phonon properties of layered NaMnO_2, whereas SCAN + U does not. We further explore the Jahn–Teller (J–T) stability and magnitude of J–T distortion depending on U and find that SCAN results are consistent with Perdew–Burke–Ernzerhof functional, PBE + U results. Our results suggest that SCAN itself properly describes the physical properties of NaMnO₂ without adding U explicitly.