Influence of Interlayer Cation Ordering on Na Transport in P2-Type Na 0.67- x Li y Ni 0.33- z Mn 0.67+ z O 2 for Sodium-Ion Batteries.

P2-type Na2/3 Ni1/3 Mn2/3 O2 (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 (Na2/3 Li0.05 Ni1/3 Mn2/3 O2 , 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. Na2/3 Ni1/4 Mn3/4 O2 (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.