Fast‐Charging Single‐Crystalline O3‐NaNi _1/3 Fe _1/3 Mn _1/3 O ₂ via Molten Salt Synthesis for Sodium‐Ion Batteries

ABSTRACT O3‐type cathodes in sodium‐ion batteries (SIBs) are the most promising cathodes with higher sodium content and energy density than the P2‐type cathodes. Along with standard battery performance metrics, fast‐charging cathodes with high‐rate kinetics and cycle stability are crucial for advancement in SIB technologies. The single‐crystalline materials show improved cycling stability, and when combined with particle size control, result in high‐rate performing and long cycle life cathodes. Herein, we report a molten salt‐synthesized single‐crystalline O3‐NaNi 1/3 Fe 1/3 Mn 1/3 O 2 (NFM) cathode with control over particle size/distribution through optimized molten salt ratios. The NFM cathode obtained from a 40:1 salt ratio shows rate capabilities of 102@1C, 93@2C, 75@5C (1C = 240 mAh g −1 ), improving fast‐charging kinetics. NFM showed impressive cycling stability at 68% retention after 200 cycles at 1C, 83% at 2C, and 76% at 5C after 100 cycles. This work offers a simple molten‐salt synthesis for ∼1–2 µm‐sized single‐crystalline particles to achieve good rate capabilities through effective charge transfers and high ionic diffusion kinetics, which holds the future promise for fast charging and durable SIBs.