Boosting fast ionic transport and stability of O3-NaNi1/3Fe1/3Mn1/3O2 cathode via Al/Cu synergistically modulating microstructure for high-rate sodium-ion batteries

As one of the most promising sodium-ion cathodes, O3-type transition metal layered oxides (NaxTMO2) with high specific capacity and low cost have received intensive attention. However, they still face the issues of slow Na+ transfer kinetics and undesirable phase transitions. In this study, we report a novel Al and Cu dual substitution strategy to prepare a NaNi1/3Fe1/3Mn1/3O2 cathode using a spray drying method. Specifically, Al serves to stabilize the TM-O layer structure, mitigating adverse phase transitions, while Cu contributes additional capacity and enhances air stability. Consequently, the NaNi0.32Fe0.32Mn0.32Al0.02Cu0.02O2 cathode exhibits enlarged Na+ transfer channels, modulated particle morphology and strengthened layer framework. With the enhanced structure, the John-Teller distortion, adverse phase transitions of O'3 and OP2, and intragranular fatigue cracks are effectively suppressed, leading to improved cycle and air stability. The NaNi0.32Fe0.32Mn0.32Al0.02Cu0.02O2 cathode can maintain 81% of its initial specific capacity at 1C rate after 200 cycles and can obtain a specific capacity of 113 mAh/g even at 5C rate. Furthermore, we find that strengthened Na+ transport kinetics promote homogenous Na+ distribution, which can reduce the formation of unstable interphases to achieve more reversible phase transition. This work helps to better unveil the relationship between Na+ transfer kinetics and phase transition, provides new insight for designing high-performance sodium-ion cathodes.