Dual Enhancement of Air Stability and Na ⁺ Diffusion Kinetics in O3-Type High-Entropy Cathode Materials via Na Vacancy Engineering

Sodium-ion battery O3-type layered oxides typically experience sluggish kinetics and detrimental phase transitions at deep desodiation states (i.e., >4.0 V), resulting in poor rate performance and severe capacity degradation. To tackle these handicaps, we leveraged the high-entropy strategy and regulated the Na⁺ ion content to create a specific number of Na vacancies and optimized their concentration. The results indicate that reducing the Na content to form Na vacancies can enhance Na⁺ diffusion kinetics and structural stability, thereby improving the rate performance and cycling stability. Meanwhile, the introduction of Na vacancies increases the valence states of various transition metals, thereby enhancing the antioxidative capability of the material. Notably, the O3-type Na_0.8 cathode exhibits a discharge capacity of 137.8 mAh g^-1 at 0.1 C, much higher than that of the Na₁ cathode (124.5 mAh g^-1). Additionally, the Na_0.8 cathode delivers an initial discharge capacity of 110.9 mAh g^-1 at 0.5 C and retains 82% of its capacity after 200 cycles. In situ diffraction analysis demonstrated that the formation of Na vacancies significantly suppresses the phase transition from O3 to P3 during desodiation and sodiation processes. These results suggest that Na-deficient O3-type cathodes are promising candidates for large-scale applications in sodium-ion batteries.