Multiphase Spherical O3/P2 Core-shell Heterojunction Cathode Material for High-Performance Sodium-Ion Batteries

Among sodium-ion battery (SIB) layered cathode materials, O3-type cathodes are attractive due to their high capacity, high initial efficiency, and low cost. However, their commercial application is limited by structural instability, complex phase transitions during cycling, high sensitivity to moisture and CO ₂ , and poor rate performance due to sluggish ion diffusion. To overcome these challenges, we designed a multiphase core-shell heterojunction consisting of an O3-NaNi _0.33 Fe _0.33 Mn _0.33 O ₂ core and a P2-Na _0.67 Ni _0.3 Mn _0.7 O ₂ shell. This structure effectively isolates the O3 phase from moisture and electrolyte exposure, combining high capacity with structural stability. The half-cells with the optimized heterojunction cathode delivered 129.1 mAh g ^-1 at 0.1 C and 103.4 mA g ^-1 at 10 C, with 80.1% capacity retention at high rates. After 300 cycles under ambient air exposure, it maintains 78.2% capacity, compared to only 27.7% for the pure O3 counterpart. The P2 shell enhances structural reversibility, air/thermal stability, electrochemical kinetics, and lowers charge-transfer resistance. This rational design demonstrates a promising strategy for developing high-performance SIB cathodes.