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

Abstract

AbstractP2 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 P2 phase into O2 phase, resulting in a sharp decline in capacity. However, many cathode materials do not undergo P2‐O2 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 P2‐OP4‐O2. With the increase of charging voltage, the O‐type superposition mode increases to form an ordered OP4 phase, and the P2‐type superposition mode disappears after further charging to form a pure O2 phase. 57Fe‐Mössbauer 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.