Fig. 3: DFT crystal structure of Na2/3Ni1/3Mn2/3O2. aThe honeycomb configuration of the transition metals in the P2 phase of Na2/3Ni1/3Mn2/3O2.bThe DFT-optimized supercell with two different Na sites: Na in a face with the Mn and Ni octahedra Na\({}_{{f}_{{{\rm{Ni-Mn}}}...