The Mystery from Tetragonal NaVPO4F to Monoclinic NaVPO4F: Crystal Presentation, Phase Conversion, and Na-Storage Kinetics

NaVPO4F is regarded as one of the most competitive cathodes in high performance sodium ion batteries (SIBs). Two polymorphs with the tetragonal and monoclinic NaVPO4F have been reported, while the true presentation of the fluorophosphates is still poorly understood. Herein, fluorophosphates with a molar ratio of 1:1:1:1 for Na, V, P, F are fabricated at different temperatures. The presentation of tetragonal and monoclinic NaVPO4F is strikingly confirmed. For the first time, the accurate atomic arrangement in the crystal, and the irreversible phase transition from tetragonal to monoclinic with temperature variation are unveiled by combining series of advanced in/ex situ characterizations. From both the thermodynamic and kinetic perspectives, the sodium storage behavior and the electron/Na+ conduction, contributing to notably different electrochemical performance for the two fluorophosphates, are elaborately studied based on experimental and density functional theory calculations. The comprehensive expositions indicate that the tetragonal and monoclinic NaVPO4F have the potential to be employed as high-energy-density and high-power-density cathodes, respectively. This research answers the question of the true state of the fluorophosphates and reveals some of their previously unexplored mysteries, which provides an instructive selection principle for NaVPO4F compounds for serving in well-defined application scenarios and is expected to become an accelerator for the next generation of high-performance SIBs.

Automated summary

This study investigates the structural characteristics and electrochemical performance differences between tetragonal and monoclinic NaVPO4F through fabrication of fluorophosphates at different temperatures. By combining advanced characterizations and theoretical calculations, the accurate atomic arrangement and irreversible phase transition from tetragonal to monoclinic are unveiled. This research provides insight into the potential of tetragonal and monoclinic NaVPO4F as high-energy-density and high-power-density cathodes, respectively, and offers a selection principle for their application in specific scenarios.