The Relationship between Initial Coulombic Efficiency and Transition Metal Ion Redox in P2-Na0.85[Cu0.1FexMn1–x]O2 Cathodes

Low-cost P2-type Cu/Fe/Mn-based layered oxide is considered a promising cathode material of sodium ion batteries (SIBs) for large-scale application. However, the abnormal initial Coulombic efficiency (ICE) of a P2-type Cu/Fe/Mn-based cathode is always much higher than 100% due to the difference of Na+ extraction and insertion content in the first cycle, which becomes an obstacle to its commercialization. Herein, we reveal that increasing the charge capacity contribution of Mn3+/4+ from an open circuit voltage to a high cutoff voltage by adjusting the initial valence state of Mn ions can effectively solve this problem. The optimized P2-type Na0.85Cu0.1Fe0.25Mn0.65O2 cathode demonstrates a high energy density of 452 Wh kg–1 and an ICE of 100.9%. This work provides a regulated strategy for optimizing ICE in the Na-defective layered oxides cathode in SIBs.