Highly Symmetrical Six-Transition Metal Ring Units Promising High Air-Stability of Layered Oxide Cathodes for Sodium-Ion Batteries

Layered oxides are the most prevalent cathodes for sodium-ion batteries (SIBs), but their poor air stability significantly limits their practical application owing to the rapid performance degradation of aged materials and the cost increase for material storage and transportation. Here, an effective strategy of constructing stable transition metal (TM) layers with a highly symmetrical six-TM ring is suggested to enhance structure stability, thus hindering ambient air corrosion. The density functional theory calculations reveal that the higher symmetry ensures a higher thermodynamic energy for H₂O insertion into Na layer. The combined analyses of selected area electron diffraction, transmission electron microscopy, X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy, thermogravimetric analysis, and chemical titration indicate that the six-TM ring structure can effectively suppress the series of aging processes including water insertion, the spontaneous loss of lattice sodium, TM valence increment and residual alkali formation. Benefiting from the overall suppression of aging process, the strategy results in an excellent improvement in capacity retention after air exposure from 13.57% to 95.59%, and exhibits a good universality for both P2- and O3-cathodes, which are the two most common structures of Na-based layered oxides with different aging mechanism. These findings provide new insight to design high-performance cathodes for SIBs.