Facilitating reversible transition metal migration and expediting ion diffusivity via oxygen vacancies for high performance O3-type sodium layered oxide cathodes

Currently, O3-type layered sodium transition metal oxides (NaxTMO2, TM = transition metal) are the most competitive cathodes for sodium-ion batteries (SIBs). However, affected by the scourge of irreversible TM migration when more than 0.5 mol Na+ is deintercalated, the practical capacities of this class of cathodes fall short of their theoretical values and the cyclabilities in the high-voltage range are significantly deteriorated. To mitigate the above issues, NaCrO2, one of the typical O3-type layered oxides, has been investigated as a model system and the oxygen vacancy strategy instead of the commonly used chemical substitution is audaciously proposed in this study. The oxygen vacancy-modified NaCrO2-x cathode exhibits an extended reversible discharge capacity of 134.5 mA h g(-1) corresponding to similar to 0.7 mol Na+ de/intercalation and excellent capacity retention after 400 cycles. The mechanistic link between oxygen vacancies and stable high-voltage operation (charged up to 3.8 V) via reversible TM migration is expounded through a combined X-ray diffraction, electron diffraction and electrochemistry study. In addition, preeminent rate capability benefiting from the oxygen vacancy-expedited ion diffusivity has also been demonstrated. Furthermore, the as-constructed pouch cell model that paired with a Na metal anode exhibits high capacity and cycling endurance as well, guaranteeing the practical applicability. Our finding provides a novel view of structural design in O3-type layered oxide cathodes for SIBs and will provoke more in-depth thoughts on the usage of oxygen vacancies in other sodium cathodes.