First-principles investigation on crystal, electronic structures and Diffusion barriers of NaNi1/3Co1/3Mn1/3O2 for advanced rechargeable Na-ion batteries

A new type of layered transition–metal mixture oxides such as NaFe1/2Mn1/2O2, NaFe1/2Co1/2O2 and NaNi1/3Fe1/3Mn1/3O2 are recently proposed for advanced cathode materials of rechargeable sodium-ion batteries. In this work, the crystal structure, electronic structures and diffusion barrier of NaNi1/3Co1/3Mn1/3O2 are studied by means of density functional theory (DFT) calculation with generalized gradient approximation (GGA) method. The theoretical calculations indicate that the NaNi1/3Co1/3Mn1/3O2 with a 3×3R30°-type superlattice in Wood’s notation based on the triangular basal net of sites in α-NaFeO2-type structure is suggested to be stable. The calculated formation energy is −122 meV per a formula unit, which implies that NaNi1/3Co1/3Mn1/3O2 is energetically favorable to form homogeneous solid matrix from NaNiO2(Ni3+), NaMnO2(Mn3+) and NaCoO2 (Co3+). According to the calculation results, the formal valence states of Ni, Co and Mn in the NaNi1/3Co1/3Mn1/3O2 are +2, +3 and +4, respectively. The existence of Jahn–Teller inactive Ni2+ and Mn4+ entails the structural stability of NaNi1/3Co1/3Mn1/3O2. Furthermore, our calculations on the solid-state redox reaction on basis of a superlattice model during sodium ions deintercalation processes indicate that the reactions in the range of 2/3 ⩽ x ⩽ 1, 1/3 ⩽ x ⩽ 2/3 and 0 ⩽ x ⩽ 1/3 in NaxNi1/3Co1/3Mn1/3O2 are the inter-conversion of Ni2+/Ni3+, Ni3+/Ni4+ and Co3+/Co4+, respectively. Due to the lower diffusion energy barrier, the Na+ diffusion rate in NaNi1/3Co1/3Mn1/3O2 is about 10 times faster than that in pristine layered NaCoO2 at room temperature.