Self-assembled monolayer boosts the air-stability and electrochemical reversibility of O3-type layered oxides for sodium-ion batteries†

Abstract

O3-NaNi_0.33Fe_0.33Mn_0.33O₂ (NFM) is a promising cathode material for sodium ion batteries (SIBs), yet its instability in ambient air and transition metal dissolution during cycling hinder its practical applications. In this study, we introduce a self-assembled layer of C₃₂H₆₇O₄P (DHP) to boost the air-stability and electrochemical reversibility of NFM. The DHP can form robust chemical bonds with the surface hydroxyl group of NFM, yielding a thin (∼6 nm), robust, and hydrophobic coating layer that effectively protects the electrode from air degradation and transition metal dissolution. Consequently, the DHP-coated NFM (NFM@DHP) cathode exhibits a capacity retention of 87.3% after 330 cycles at 1C, far surpassing the unmodified sample's retention of 54.5%. Furthermore, after 7 days exposure to humid air (30 °C, 50% relative humidity), NFM@DHP maintains a specific capacity of 118 mA h g⁻¹ at 0.1C, whereas the capacity of unmodified NFM decreases to 49 mA h g⁻¹. Our work offers a novel approach for developing stable layered oxide cathodes for practical applications in SIBs.