Influence of Interlayer Cation Ordering on Na Transport in P2-Type Na_{0.67–<i>x</i>}Li_<i>y</i> Ni_{0.33–<i>z</i>}Mn_{0.67+<i>z</i>}O₂ for Sodium-Ion Batteries

Abstract

P2-type Na_2/3Ni_1/3Mn_2/3O₂ (PNNMO) has been extensively studied because of its desirable electrochemical properties as a positive electrode for sodium-ion batteries. PNNMO exhibits intralayer transition-metal ordering of Ni and Mn and intralayer Na⁺/vacancy ordering. The Na⁺/vacancy ordering is often considered a major impediment to fast Na⁺ transport and can be affected by transition-metal ordering. We show by neutron/X-ray diffraction and density functional theory (DFT) calculations that Li doping (Na_2/3Li_0.05Ni_1/3Mn_2/3O₂, LFN5) promotes ABC-type interplanar Ni/Mn ordering without disrupting the Na⁺/vacancy ordering and creates low-energy Li-Mn-coordinated diffusion pathways. A structure model is developed to quantitatively identify both the intralayer cation mixing and interlayer cationic stacking fault densities. Quasielastic neutron scattering reveals that the Na⁺ diffusivity in LFN5 is enhanced by an order of magnitude over PNNMO, increasing its capacity at a high current. Na_2/3Ni_1/4Mn_3/4O₂ (NM13) lacks Na⁺/vacancy ordering but has diffusivity comparable to that of LFN5. However, NM13 has the smallest capacity at a high current. The high site energy of Mn-Mn-coordinated Na compared to that of Ni-Mn and higher density of Mn-Mn-coordinated Na⁺ sites in NM13 disrupts the connectivity of low-energy Ni-Mn-coordinated diffusion pathways. These results suggest that the interlayer ordering can be tuned through the control of composition, which has an equal or greater impact on Na⁺ diffusion than the Na⁺/vacancy ordering.