Breaking the capacity-stability trade-off in layered oxide cathodes for Na-ion batteries via phase regulation

Sodium-ion layered oxide cathodes must operate above 4.0 V to achieve per-unit-energy cost advantages over lithium iron phosphate (LFP). However, the associated irreversible phase transitions and electrolyte decomposition present a significant challenge, leading to a trade-off between high capacity and cycling stability. Here, we demonstrate that Ca and Zn co-doping in NaNi₁_/₃Fe₁_/₃Mn₁_/₃O₂ (CZNFM) simultaneously enhanced the specific capacity (154.4 mAh g⁻¹ and 476.9 Wh kg_cathode⁻¹ at 0.1C) and maintained excellent cycling stability (88 % capacity retention after 200 cycles at 1C) under a cut-off voltage of 4.0 V. Our investigation reveals that although Zn doping can facilitate the early activation of the P3–OP2 phase transition, it does not enhance the discharge capacity, whereas Ca plays a critical role in improving the reversibility of this transition. The modified cathode exhibits reduced variation in the c-axis lattice parameter, even at deep states of charge, indicating improved structural robustness. In addition, a thin, inorganic-rich cathode–electrolyte interphase (CEI) forms on the particle surface, which effectively shields it from electrolyte degradation, thereby suppressing surface phase transitions and internal lattice dislocations. These findings highlight a promising strategy for designing sodium-layered oxide cathodes with both high energy density and long-term structural stability.