Vanadium fluorophosphates: advanced cathode materials for next-generation secondary batteries

Next-generation secondary batteries including sodium-ion batteries (SIBs) and potassium-ion batteries (PIBs) are considered the most promising candidates for application to large-scale energy storage systems due to their abundant, evenly distributed and cost-effective sodium/potassium raw materials. The electrochemical performance of SIBs (PIBs) significantly depends on the inherent characteristics of the cathode material. Among the wide variety of cathode materials, sodium/potassium vanadium fluorophosphate (denoted as MVPF, M = Na and K) composites are widely investigated due to their fast ion transportation and robust structure. However, their poor electron conductivity leads to low specific capacity and poor rate capacity, limiting the further application of MVPF cathodes in large-scale energy storage. Accordingly, several modification strategies have been proposed to improve the performance of MVPF such as conductive coating, morphological regulation, and heteroatomic doping, which boost the electronic conductivity of these cathodes and enhance Na (K) ion transportation. Furthermore, the development and application of MVPF cathodes in SIBs at low temperatures are also outlined. Finally, we present a brief summary of the remaining challenges and corresponding strategies for the future development of MVPF cathodes.