A Cross-Linked Polymer Coating Strategy to Enhance the Performance of NaNi_0.33Fe_0.33Mn_0.33O₂ Cathode for Sodium-Ion Batteries

O3-type layered oxide materials are regarded as optimal cathode candidates for sodium-ion batteries (SIBs) on account of their exceptional energy density. Nevertheless, the rapid decline in capacity resulting from the instability of the interface structure represents a significant challenge to the practical implementation of these materials. In this study, we propose an innovative method to modify the O3-type NaNi_0.33Fe_0.33Mn_0.33O₂ (NFM) cathode material by applying a cross-linked polymer (CLP) coating. X-ray photoelectron spectroscopy (XPS) analysis demonstrates that the CLP coating effectively inhibits the decomposition of the cathode electrolyte interface (CEI) membrane in the course of cycling, leading to a substantial improvement in the stability of the electrode material's interface. Moreover, the oxygen-containing groups within the coating can compete with propylene carbonate (PC) solvent molecules in the electrolyte for Na⁺ coordination, reducing the coordination between Na⁺ and PC molecules. This process facilitates more efficient diffusion of Na⁺, thereby enhancing the rate performance. Consequently, CLP-coated NFM (NFM@CLP) materials exhibit enhanced electrochemical performance. After 300 cycles at 25 °C, NFM@CLP retains 72.36% of its capacity, compared to 62.59% for pristine NFM. Even at elevated temperatures (65 °C), the capacity retention of NFM@CLP remains high at 63.84% after 200 cycles, whereas pristine NFM drops to 3.65%. In full-cell tests (vs hard carbon), the NFM@CLP also exhibits better capacity retention (85.07% after 150 cycles). This study offers an effective and simple approach to enhancing the capacity retention and rate performance of O3-type layered materials in SIBs, providing unique insights into advanced energy storage materials.