Syntheses and nanocrystallization of Na2O-M2O3-P2O5 alluaudite-like phosphate glasses (M = V, Fe, Mn)

In this decade, an interest in alluaudites as potential cathode materials for sodium batteries revived. However, only crystalline compounds have been studied so far. In this work, alluaudite-like glasses of nominal composition Na2M3(PO4)(3) (where M-3 = Fe-3, VFe2, VFeMn) were successfully synthesized using standard melt-quenching technique. Temperatures of glass transition and crystallization were determined at various heating rates in DTA experiments and the activation energies of these processes were calculated using Kissinger's model. XRD studies confirmed that heat-treatment of the glasses led to nanocrystalline materials with alluaudite structure. Glasses exhibited modest, predominant electronic conductivity. After their thermal nanocrystallization, the conductivity increased approximately 2-4 orders of magnitude and the activation energy decreased noticeably. The ionic counterpart of the conductivity became much more pronounced. These preliminary electrical results show that there is a room for further optimization, which may result in highly-conducting nanocrystalline alluaudite-like cathode materials for sodium-ion batteries.