Properties of the "Z"-Phase in Mn-Rich P2-Na0.67Ni0.1Mn0.8Fe0.1O2 as Sodium-Ion-Battery Cathodes

P-2 layered oxides have attracted more and more attention as cathode materials of high-power sodium-ion batteries (SIBs). During the charging process, the release of sodium ions leads to layer slip, which leads to the transformation of P-2 phase into O-2 phase, resulting in a sharp decline in capacity. However, many cathode materials do not undergo P-2-O-2 transition during charging and discharging, but form a "Z" phase. It is proved that the iron-containing compound Na0.67Ni0.1Mn0.8Fe0.1O2 formed the "Z" phase of the symbiotic structure of the P phase and O phase during high-voltage charging through ex-XRD and HAADF-STEM. During the charging process, the cathode material undergoes a structural change of P-2-OP4-O-2. With the increase of charging voltage, the O-type superposition mode increases to form an ordered OP4 phase, and the P-2-type superposition mode disappears after further charging to form a pure O-2 phase. Fe-57-Mossbauer spectroscopy revealed that no migration of Fe ions is detected. The O-Ni-O-Mn-Fe-O bond formed in the transition metal MO6 (M = Ni, Mn, Fe) octahedron can inhibit the elongation of the Mn-O bond and improve the electrochemical activity so that P2-Na0.67Ni0.1Mn0.8Fe0.1O2 has an excellent capacity of 172.4 mAh g(-1) and a coulombic efficiency close to 99% at 0.1C.