Influence of Interlayer Cation Ordering on Na Transport in P2-Type NaLi NiMnO for Sodium-Ion Batteries.

P2-type NaNiMnO (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 (NaLiNiMnO, 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. NaNiMnO (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.