Efficient synthesis of kilogram-scale high-performance Na ₃ V ₂ O ₂ (PO ₄ ) ₂ F as a cathode material for sodium-ion batteries

Na₃V₂O₂(PO₄)₂F (NVOPF) has garnered significant attention in the field of sodium-ion battery cathode materials due to its high operating voltage and exceptional cycling stability. However, the practical application of NVOPF is hindered by its low synthesis efficiency, difficulties in large-scale production, and the environmentally unfriendly preparation process. In this study, we have designed a large-scale, one-step hydrothermal synthesis method for NVOPF, which demonstrates impressive high efficiency (completed within 2 hours), large-scale production capability (yielding more than 1 kg per batch), and environmental friendliness (using ultra-pure water or/and ethanol as solvents). The obtained samples exhibit consistent morphology and stable structure, indicating the good reproducibility of our proposed preparation method. Furthermore, the as-synthesized NVOPF using three solvents, including ultra-pure water, ethanol, and their mixture, all presented good electrochemical performance. Notably, the sample obtained with ultra-pure water as the solvent provided the most outstanding sodium storage performance, with a high specific capacity (106.5 mAh g^-1 at a current density of 0.1 A g^-1) and extraordinary stability (capacity retention as high as 85.9% after 5000 cycles at a 1 A g^-1). When integrated with hard carbon as the anode, the full cell delivered an energy density up to 257.2 Wh kg^-1 at a total power density of 123.7 W kg^-1, manifesting that the kilogram-scale NVOPF produced through our method holds significant commercial potential. This study offers a viable pathway for the efficient synthesis of cathode materials for sodium-ion batteries.