Cu/Mg substitution enables O3-type NaNi_0.4Fe_0.2Mn_0.4O₂ cathode material with simpler phase transition process for sodium-ion batteries

O3-type layered oxides are considered ideal cathode materials for sodium-ion batteries (SIBs) owing to their high theoretical capacities, but these materials undergo complex phase transitions during the Na⁺ extraction and insertion, resulting in serious structural degradation and reduced cycling stability. Herein, we employ “Cu/Mg co-doping strategy” to synthesize a stable O3-type layered transition metal oxide NaNi_0.25Cu_0.1Mg_0.05Fe_0.2Mn_0.4O₂ (NaNCMFMO) with excellent structural reversibility. The resulting NaNCMFMO achieves reversible specific capacity of 123 mAh g⁻¹ at 0.1 C and remarkable improvement in capacity retention of 79.4 % over 300 cycles at 1.0 C, compared to the pristine NaNi_0.4Fe_0.2Mn_0.4O₂ cathode (42.1 % retention after 300 cycles). Additionally, ex situ X-ray diffraction pattern of NaNCMFMO indicates a highly reversible phase evolution of O3-P3-O3 during charge/discharge cycles, and GITT measurements reveal faster Na⁺ diffusion kinetics. Moreover, a NaNCMFMO//hard carbon full-cell battery exhibits superior electrochemical performance: a high reversible specific capacity of 105.1 mAh g⁻¹ at 1.0 C and capacity retention of 81.1 % over 200 cycles. This work presents a facile and effective strategy to suppress structural degradation in O3-type layered cathodes materials of high-performance SIBs.