Optimized Surface Alkali Conversion and Regulate the Near-Surface Structure to Enable High-Performance Sodium-Ion Batteries

Lithium-ion batteries (LIBs) are limited by high costs and sustainability issues due to their cobalt content, despite their advantages in energy storage. Sodium-ion batteries (SIBs) emerge as a viable alternative because of their cost-effectiveness and abundant sodium, which is especially suitable for large-scale applications. The O3-type sodium-layered transition-metal oxide (Na_xTMO₂) cathode is pivotal for enhancing energy density, cost-effectiveness, and stability of SIBs. However, these cathodes are affected by poor air stability and irreversible phase transitions that degrade their electrochemical performance. To address these issues, we propose a near-surface structural modulation strategy to convert surface alkali residues into the fast ionic conductor Na₂CaMg(PO₄)₂ (NCMP). This method relieved air sensitivity by forming a protective shield around the cathode and enhanced discharge capacity and cycling stability, demonstrating a significant increase in specific capacity, reaching 159.7 mAh/g at 0.1 C and 106.2 mAh/g at 5 C. This study demonstrates the effectiveness of NCMP coatings in improving the lifetime and performance of SIBs and proposes their general applicability in enhancing sodium-ion-layered oxide cathodes.