Layered transition metal oxides Na_xMO₂ (M = Fe,Co,Ni, and Mn) have been extensively researched as one kind of prospective cathode material for NIBs. Na_xMO₂ compounds are composed of sheets of MO₆ octahedra and they function similarly to Lithium-ion batteries. The 2D channels for the extraction/insertion of Na⁺ ions between them. According to the environment of Na and the amount of oxygen stacking sequences, typical Na_xMO₂ may be separated into two categories. In essence, the latter identifies the location of Na-ions, while the number shows the number of times transition metal layers are repeated inside a unit cell. For the P₂ and O₃-phases, Na-ion is found in trigonal prismatic (edge-sharing or face-sharing) sites with an AB BA oxygen sequence, and at octahedral (edge-sharing) sites including an AB CA BC oxygen arrangement.