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 Na0.67Ni0.19Cu0.07Zn0.07Mn0.60Ti0.07O2 (NNCZMT) material. Unlike equimolar highentropy designs, Ni and Mn remain as the main components, with Ni providing high capacity through the Ni2+/Ni4+ redox couple and Mn stabilizing the P2 structure. The introduction of Cu2+, Zn2+, and Ti4+, which have slightly larger ionic radii and the same valence states as Ni2+ and Mn4+, 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).