Air Corrosion of Layered Cathode Materials for Sodium-Ion Batteries: Cation Mixing and a Practical Suppression Strategy

Layered oxide cathodes of sodium-ion batteries (SIBs) are considered promising candidates due to their fascinating high capacity, good cyclability, and environmental friendliness. However, the air sensitivity of layered SIB cathodes causes high electrode manufacturing costs and performance deterioration, hampering their practical application. Herein, a commercial O3-type layered Na(Ni_1/3Fe_1/3Mn_1/3)O₂ (NNFM) material is adopted to investigate the air corrosive problem and the suppression strategy. We reveal that once the layered material comes in contact with ambient air, cations migrate from transition metal (TM) layers to sodium layers at the near surface, although Na⁺ and TM ions show quite different ion radii. Experimental results and theoretical calculations show that more Ni/Na disorder occurs in the air-exposed O3-NNFM materials, owing to a lower Ni migration energy barrier. The cation mixing results in detrimental structural distortion, along with the formation of residual alkali species on the surface, leading to high impedance for Na⁺ diffusion during charge/discharge. To tackle this problem, an ultrathin and uniform hydrophobic molecular layer of perfluorodecyl trimethoxysilane is assembled on the O3-NNFM surface, which significantly suppresses unfavorable chemistry and structure degradation during air storage. The in-depth understanding of the structural degradation mechanism and suppression strategy presented in this work can facilitate high-energy cathode manufacturing from the perspective of future practical implementation and commercialization.