Edge-Plane-Selective Formation of Mn Vacancies in β-Na0.7MnO2+y for Air-Stable Cathode Materials in Sodium-Ion Batteries

초록

Manganese-based P2-type Na x MnO2+y , particularly beta-Na0.7MnO2+y , exhibit high reversible capacity but are prone to stability issues, especially hydration-induced degradation when exposed to air. Herein, a significant improvement in the air stability of beta-Na0.7MnO2+y cathodes is achieved through a selective transition of their edge planes to alpha-Na0.7MnO2+z . When beta-Na0.7MnO2+y particles are oxidized at a relatively low temperature (350 degrees C), oxygen is selectively inserted at the edge planes due to the higher energy barrier for oxygen insertion at the basal planes compared with the edge planes. This mild oxidation selectively locally creates manganese vacancies near the edge plane surface, promoting the exclusive formation of alpha-Na0.7MnO2+z on the edge surface. The alpha-Na0.7MnO2+z nanolayers on the edge planes effectively suppress H2O insertion during air exposure, eventually mitigating the phase transition of beta-Na0.7MnO2+y to Na gamma MnO2<middle dot>delta H2O birnessite during storage. Moreover, this plane-selective formation of alpha-Na0.7MnO2+z enhances the electrochemical performance of beta-Na0.7MnO2+y , such as stable capacity retention.

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