Controlled Synthesis of Na₃(VOPO₄)₂F Cathodes with an Ultralong Cycling Performance

Na-ion batteries (NIBs) are increasingly studied as a low-cost technology for revolutionizing stationary energy storage applications. Many positive electrode materials have been extensively studied, mainly including layered oxides and polyanionic compounds. In particular, sodium vanadium fluorophosphates (NVPFs) are more attractive owing to their high theoretical capacity and energy density. This work reports on a significant increase in rate capability and cycling stability for Na3(VOPO4)2F (NVOPF). The improvement is achieved by mixing Ketjenblack (KB) with NVOPF combined with the addition of sodiation agent Na2C4O4, based on a controllable synthesis of NVOPF hollow microspheres. It is a green and facile hydrothermal synthetic method by simply adjusting the phosphorus source and temperature. The nanosized NVOPF@KB framework formed by high-energy ball milling provides better rate capability, which exhibits a capacity of 118.3 mA h g–1 at 20 C, while it is only 69.2 mA h g–1 for NVOPF at 20 C. To be interested, the addition of Na2C4O4 could greatly prolong the cycling stability. As an example, the capacity retention of NVOPF@KB with Na2C4O4 increases to 99.41% from 33.42% compared with that without Na2C4O4 at 30 C. To our best knowledge, NVOPF@KB-Na2C4O4 shows the best Na-storage performance in terms of both superior rate capability (up to 150 C rate) and outstanding long cycling stability over 12000 cycles with a capacity retention of 71% reported so far. The current developed microsphere synthetic method and the improved strategies for cathodes would boost the development of positive electrode materials for ion batteries.