Medium-entropy P2-type layered metal oxide cathode demonstrating complete solid-solution behavior for improved Na-ion battery performance

P2-type layered metal oxides are promising cathode materials for Na-ion batteries (NIBs), but they suffer from Na + /vacancy ordering and P2-O2 phase transitions during charge–discharge processes, resulting in rapid capacity decay and poor rate performance. In this study, a non-equimolar five-component medium-entropy strategy was employed to design a Na 0.67 Ni 0.19 Cu 0.07 Zn 0.07 Mn 0.60 Ti 0.07 O 2 (NNCZMT) material. Unlike equimolar high-entropy designs, Ni and Mn remain as the main components, with Ni providing high capacity through the Ni 2+ /Ni 4+ redox couple and Mn stabilizing the P2 structure. The introduction of Cu 2+ , Zn 2+ , and Ti 4+ , which have slightly larger ionic radii and the same valence states as Ni 2+ and Mn 4+ , respectively, allows for mutual substitution, synergistically promoting a solid-solution reaction and excellent Na + diffusion kinetics. The NNCZMT electrode delivers a high reversible capacity of 110 mAh g −1 at 10 mA g −1 , an initial Coulombic efficiency of 96.8 %, remarkable cycling stability (94.7 % capacity retention after 100 cycles at 200 mA g −1 ), and excellent rate performance (57.2mAh g −1 at 2000 mA g −1 ).