Architecting O3/P2 layered oxides by gradient Mn doping for sodium-ion batteries

O3 phase layered oxides are highly attractive cathode materials for sodium-ion batteries because of their high capacity and decent initial Coulombic efficiency. However, their rate capability and long cycling life are unsatisfactory due to the narrow Na⁺ transfer channel and irreversible phase transitions of O3 phase during sodiation/desodiation process. Constructing O3/P2 multiphase structures has been proven to be an effective strategy to overcome these challenges. In this study, we synthesized bi-phasic structured O3/P2 Na(Ni_2/9Fe_1/3Cu_1/9Mn_1/3)_1-xMn_xO₂ (x = 0.01, 0.02, 0.03, 0.04, 0.05) materials through Mn doping during sodiation process. Benefiting from surface P2 phase layer with the enhanced Na⁺ transfer dynamics and high structural stability, the Na(Ni_2/9Fe_1/3Cu_1/9Mn_1/3)_0.98Mn_0.02O₂ (NFCM-M2) cathode delivers a reversible capacity of 139.1 mA h g^-1 at 0.1 C, and retains 71.4 % of its original capacity after 300 cycles at 1 C. Our work provides useful guidance for designing multiphase cathodes and offers new insights into the structure-performance correlation for sodium-ion cathode materials.