Edge-Plane-Selective Formation of Mn Vacancies in β-Na_0.7MnO_2+y for Air-Stable Cathode Materials in Sodium-Ion Batteries

Manganese-based P2-type Na_xMnO_2+y, particularly β-Na_0.7MnO_2+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 β-Na_0.7MnO_2+y cathodes is achieved through a selective transition of their edge planes to α-Na_0.7MnO_2+z. When β-Na_0.7MnO_2+y particles are oxidized at a relatively low temperature (350 °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 α-Na_0.7MnO_2+z on the edge surface. The α-Na_0.7MnO_2+z nanolayers on the edge planes effectively suppress H₂O insertion during air exposure, eventually mitigating the phase transition of β-Na_0.7MnO_2+y to Na_γMnO₂·δH₂O birnessite during storage. Moreover, this plane-selective formation of α-Na_0.7MnO_2+z enhances the electrochemical performance of β-Na_0.7MnO_2+y, such as stable capacity retention.