An Ab Initio Study of the Origin of Structural Stability in P2-NaMnO2 with Li Doping at High Voltage

P2-type sodium cathode materials generally exhibit a P2-O2 phase transition upon deintercalation at high voltage, which is detrimental to their cycling performance. Herein, using first-principles calculations, we investigate the structural stability and phase transition of P2-Na₀MnO₂ upon substitution of Li for Mn as a model of a high-voltage phase. The phonon of P2-Na₀MnO₂ shows an imaginary phonon frequency, indicating instability, which is consistent with the experimental P2-O2 transformation. On the contrary, the phonon of P2-Na₀Li_0.25Mn_0.75O₂ shows dynamic stability. We demonstrate that the substitution of the Li ion induces the redistribution of charge from the out-of-plane to in-plane orbitals along with a reduced charge of oxygen. Furthermore, we consider the various Li doping compositions and suggest that the density of the next-nearest-neighbor Li-ion pairs also plays an important role in stabilizing the P2 phase. On the basis of our findings, we propose a minimum of ∼20% Li doping to stabilize P2-NaLi_xMn_1-xO₂ at high voltage.