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 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 Jahn-Teller distortion respectively. According to density functional theory calculations, Fe with slightly larger radius is introduced into NaNiMnO with the intention of enlarging transition metal layers and facilitating electronic delocalization. The obtained NaFeNiMnO exhibits a reversible phase transition of O3P3 without any monoclinic transitions in striking contrast with the complicated phase transitions (O3-O'3-P3-P'3-P3') of NaNiMnO, 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.