Site-specific gradient Y/F co-doping modulating electronic structure for durable O3-type sodium layered oxide cathodes

Abstract

O3-type transition metal oxides (TMOs) are among the most promising cathodes for sodium-ion batteries (SIBs) due to their high capacity. However, the local electronic structure arrangement around transition metals (TM) cations directly affects the phase transition behavior and oxygen redox chemistry upon cycling. Herein, we demonstrate that site-specific gradient Y/F co-doping in O3-NaNi1/3Fe1/3Mn1/3O2 (NFM) cathode can effectively modulate electron density between TM and O to enhance the structural stability. Extra negative charge density is introduced near the oxygen ligands, resulting in the change of TM valence states, confirmed by comprehensive characterizations and theoretical calculations. Through coordinated Y/F regulation, unfavorable OP2 phases and lattice oxygen release at high voltages is effectively inhibited. Furthermore, gradient distribution of Y/F ions mitigates surface degradation by passivating reactive sites and strengthening the electrode–electrolyte interface. Therefore, the Y/F co-modified NFM cathode demonstrates a high capacity of 99.5 mAh g⁻1 at 10C (1C = 150 mA g⁻1) and capacity retention of 80% within the voltage range of 2.0–4.2 V after 150 cycles at 1C, which are higher than their counterpart (67.3% retention, 70.1 mAh g⁻1 at 10C). This work establishes a universal modification strategy for enhancing the structural stability of O3-type sodium-ion battery cathodes.