First-principles computational studies on Na+ diffusion in Li-doped P3-type NaMnO2 as cathode material for Na-ion batteries

Na-ion diffusion kinetics is a key factor that decided the charge/discharge rate of the electrode materials in Na-ion batteries. In this work, two extreme concentrations of NaMnO2 and Na2/3Li1/6Mn5/6O2 are considered, namely, the vacancy migration of Na ions in the fully intercalated and the migration of Na ions in the fully de-intercalated. The Na-vacancy and Na+ distribution in NaMnO2 migrated along oxygen dumbbell hop (ODH) and tetrahedral site hop (TSH), and the migration energy barriers were 0.374 and 0.296 eV, respectively. In NaLi1/6Mn5/6O2, the inhomogeneity of Li doping leads to the narrowing of the interlayer spacing by 0.9% and the increase of the energy barrier by 53.8%. On the other hand, due to the alleviation of Jahn-Teller effect of neighboring Mn, the bonding strength of Mn-O was enhanced, so that the energy barrier of path 2–3 in Mn-L1 and Mn-L2 was the lowest, which was 0.234 and 0.424 eV, respectively. In Na1/6Li1/6Mn5/6O2, the migration energy barriers of Na-L2 and Na-L3 are 1.233 and 0.779 eV, respectively, because Li+ migrates from the transition (TM) layer to the alkali metal (AM) layer with Na+ migration, which requires additional energy.