Syntheses and nanocrystallization of NaF-M2O3-P2O5 NASICON-like phosphate glasses (M = V, Ti, Fe)

Six NASICON-type phosphate glasses with a wide variety of compositions (Na3M2(PO4)(2)F-3, where M-2 = V-2, Ti-2, Fe-2, TiV, FeV, and FeTi) were synthesized using melt-quenching and double-crucible techniques. Their glass transition and crystallization temperatures were determined from differential thermal analysis experiments. The electrical properties were studied with impedance spectroscopy. We found that depending on temperature and composition the studied materials exhibit predominant electronic, ionic, or mixed conduction. This observation is interesting from both fundamental and application point of view (eg, in all-solid-state batteries). In general, the conductivity of glasses ranged from 3 center dot 10(-13) to 10(-10) S/cm at room temperature, with activation energies varying from 0.65 to 0.73 eV. After crystallization at 600 degrees C, the values of conductivity noticeably increased. For nanocrystalline materials, they were between 10(-11) and 10(-7) S/cm (at room temperature). The values of the activation energy spread from 0.53 to 0.70 eV. Most of the glasses exhibited predominant electronic conductivity. After nanocrystallization, the ionic transference number considerably increased in almost all samples. This study proves that thermal nanocrystallization can be used to synthesize nanocrystalline NASICON-like cathode materials for Na-ion batteries from their glassy analogs. We believe that this method can be adopted also to other interesting sodium compounds in the future.