Mg substitution regulates high-voltage P3-OP2 phase transition in O3-phase layered oxide cathodes for sodium-ion batteries

O3-phase layered oxide material NaNi_0.33Fe_0.33Mn_0.33O₂ (NFM) exhibits high energy density and low cost. However, its practical application for achieving higher energy density is hindered by inherent complex phase transitions, particularly the unfavorable P3-OP2 phase transition above 4 V. In this work, an O3-NaNi_0.28Fe_0.33Mn_0.33Mg_0.05O₂ (NFMM0.05) cathode material was synthesized by Mg substitution. Based on in-situ electrochemical X-ray diffraction analysis, Mg substitution effectively suppressed P3-OP2 phase transition, reduced the diffraction peak shift and regulated the structural evolution of the material at high voltage. Mg substitution formed the strong Mg-O bonds, which can reinforce the TM-O framework and suppress layer-gliding behavior above 4 V. NFMM0.05 delivered a high discharge capacity of 144.2 mA h g⁻¹, maintained 75.4 mA h g⁻¹ at a rate of 5 C (1 C = 150 mA g^− 1) significantly better than the pristine NFM (46.7 mA h g^− 1) and exhibited outstanding cycling stability with 81% capacity retention at 1 C after 100 cycles. Besides, the full cell assembled with commercial hard carbon exhibited a specific capacity of 114.02 mA h g⁻¹ and maintained 80.4% capacity retention at 1 C after 100 cycles. The outstanding electrochemical performance convincingly demonstrated both the effectiveness of the Mg substitution strategy and the potential practical value of NFMM0.05.