Properties of the "Z"-Phase in Mn-Rich P2-Na 0.67 Ni 0.1 Mn 0.8 Fe 0.1 O 2 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 Na 0.67 Ni 0.1 Mn 0.8 Fe 0.1 O 2 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 -OP 4 -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. 57 Fe-Mössbauer spectroscopy revealed that no migration of Fe ions is detected. The O-Ni-O-Mn-Fe-O bond formed in the transition metal MO 6 (M = Ni, Mn, Fe) octahedron can inhibit the elongation of the Mn-O bond and improve the electrochemical activity so that P2-Na 0.67 Ni 0.1 Mn 0.8 Fe 0.1 O 2 has an excellent capacity of 172.4 mAh g -1 and a coulombic efficiency close to 99% at 0.1C.