Suppression of Monoclinic Phase Transitions of O3-Type Cathodes Based on Electronic Delocalization for Na-Ion Batteries

As high capacity cathodes, O3-type Na-based oxides always suffer from a series of monoclinic transitions upon sodiation/desodiation, mainly caused by Na⁺/vacancy ordering and Jahn-Teller (J-T) distortion, leading to rapid structural degradation and serious performance fading. Herein, a simple modulation strategy is proposed to address this issue based on refrainment of electron localization in expectation to alleviate the charge ordering and change of electronic structure, which always lead to Na⁺/vacancy ordering and J-T distortion, respectively. According to density functional theory calculations, Fe³⁺ with slightly larger radius is introduced into NaNi_0.5Mn_0.5O₂ with the intention of enlarging transition metal layers and facilitating electronic delocalization. The obtained NaFe_0.3Ni_0.35Mn_0.35O₂ exhibits a reversible phase transition of O3_hex-P3_hex without any monoclinic transitions in striking contrast with the complicated phase transitions (O3_hex-O'3_mon-P3_hex-P'3_mon-P3'_hex) of NaNi_0.5Mn_0.5O₂, thus excellently improving the capacity retention with a high rate kinetic. In addition, the strategy is also effective to enhance the air stability, proved by direct observation of atomic-scale ABF-STEM for the first time.