Properties of the “Z”‐Phase in Mn‐Rich P2‐Na_0.67Ni_0.1Mn_0.8Fe_0.1O₂ as Sodium‐Ion‐Battery Cathodes

Abstract 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.