Stable P2/O3 biphasic layered oxide cathodes with enhanced cycling stability via Mg/Fe co-doping and high sodium content

Traditional cathode materials for sodium-ion batteries, such as P2-Na 0.67 Ni 0.33 Mn 0.67 O 2 , operate at relatively high voltages but suffer structural degradation upon cycling, resulting in unstable electrochemical performance. Here, we report a newly designed layered oxide, Na 0.80 Ni 0.14 Mg 0.14 Mn 0.58 Fe 0.14 O 2 , synthesized through an economical co-doping strategy with Mg and Fe combined with high sodium content. This approach integrates the structural advantages of both P2- and O3-type frameworks. The increased sodium content promotes O3-phase formation and alleviates interlayer O-O repulsion, while Mg/Fe co-doping suppresses Jahn-Teller-active Mn 3 + and increases the Ni 3+ fraction, thereby facilitating Na + transport and enhancing structural stability. Electrochemical analyses (GITT and EIS) confirm accelerated Na + transport, and in-situ XRD reveals a highly reversible P2/O3 biphasic layered oxide with minimal lattice variation. The material delivers a reversible capacity of 125 mAh g⁻ 1 at 0.1 C and retains 88.5 % of its capacity after 100 cycles at 0.2 C. These results highlight that P2/O3-Na 0.80 Ni 0.14 Mg 0.14 Mn 0.58 Fe 0.14 O 2 combines excellent electrochemical performance with cost-effective metal doping, providing a promising strategy for the development of stable and economically viable sodium-ion battery cathodes.