High-voltage P2-type manganese oxide cathode induced by titanium gradient modification for sodium ion batteries

•P2-type manganese oxide cathode combines high operating voltage and anionic redox.•Ti gradient modification achieves Ti-enriched surface and Ti-substituted interior.•Gradient modification enhances interfacial stability to reduce side reactions.•Gradient modification mitigates phase transition and irreversible oxygen activity.•Excellent electrochemical performance is obtained at 4.7 V high operating voltage. Constructing layered high-voltage P2-type manganese oxide with working voltage above 4.5 V incorporating into Ni2+/4+ redox and high capacity based on anionic redox has become trend to develop high energy density sodium ion batteries. However, capacity fading and undesired rate capability caused by serious phase transition, irreversible oxygen activity and electrolyte/electrode side reactions would occur especially under high operating voltage besides the Jahn-Teller effect. Here, based on anionic and cationic redox, a novel titanium gradient modification coupling with the synergic Ti-enriched surface and Ti-substituted interior is proposed to achieve excellent stability and rate capability. As a result, the high-voltage P2-Na0.66Mn0.54Ni0.13Co0.13O2 cathode induced by titanium gradient modification can deliver a large reversible discharge capacity of 133.2 mA h g−1 with 77.9% capacity retention after 100 cycles under 4.7 V cut-off voltage at 1C (200 mA g−1), and achieve a high energy density of 456.4 Wh kg−1. These outstanding results are attributed to the facts that Ti gradient modification not only is beneficial for suppressing the side reactions and enhancing the interfacial conductivity, but also can modify the lattice structure to improve the structural stability, leading to mitigate the phase transition and irreversible oxygen activity.