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.

Automated summary

This study proposes a novel design strategy for sodium-ion battery (SIB) cathodes by modifying NaCrO2 cathode with oxygen vacancies. The oxygen vacancy-modified NaCrO2 cathode exhibits higher reversible discharge capacity and excellent cycling performance at high voltages. Moreover, the oxygen vacancies expedite ion diffusivity, resulting in superior rate capability. This finding provides insights for the structural design of O3-type layered oxide cathodes for SIBs.