NaVPO4X (X = O, F) as cathodes for advanced high-energy Na-ion batteries

Na-ion batteries (NIBs), which possess the same working mechanism as Li-ion batteries (LIBs), have attracted extensive attention and delivered great application potential because of their abundant raw material and excellent comprehensive electrochemical performances. The development of cathodes is one of the key factors for the further industrialization of NIBs. Polyanionic phosphates are a class of cathode materials for NIBs with great research value and application prospect because of their stable 3D-frame structure, suitable working voltage and facilitated Na+ diffusion path. Among them, NaVPO4X (X = O, F) compounds have high theoretical capacities of over 140 mAh/g and deliver an average voltage of over 3.6 V, corresponding to an energy density of more than 500 Wh/kg, which are regarded as cathode candidates with great potential for high-energy NIBs. So far, some of NaVPO4X (X = O, F) materials have been reported as cathodes for NIBs and demonstrated completely different crystal structures and Na+-storage performance. In this manuscript, we firstly summarized the current research progress and related challenges of NaVPO4X (X = O, F) cathodes. Furthermore, a representative KTiOPO4-type NaVOPO4 was successfully prepared and its electrochemical performances were also systematically investigated. Based on these, we committed to emphasizing the relationships between crystal structures and electrochemical performances, and further proposed corresponding guidelines for the design of the advanced cathodes. It is believed that this work can present a profound inspiration for the development of polyanionic cathodes for high-energy NIBs.