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In this paper, sodium and nickel ion dynamics in layered cathode materials of NaNi1/2Mn1/2O2 and NaNi1/3Mn1/3Co1/3O2 for Na-ion battery applications were investigated using first principles based on density functional theory. The diffusion pathways for sodium and nickel migration inside the layered cathode materials were optimized and their energy barriers were calculated based on the nudged elastic band method. The energy barrier of Na diffusion in NaNi1/2Mn1/2O2 and NaNi1/3Mn1/3Co1/3O2 is about 1.0 eV. However, when the Ni is located at the Na layer, i.e. occupying the Na position, the energy barrier for Ni diffusion inside the layered cathode materials is about 0.36 eV, which indicates that Ni will diffuse to the surface of electrode materials, thus will block or slow down the sodium diffusion, limiting the battery performance. [ABSTRACT FROM AUTHOR]