Crystallization behavior of sodium iron phosphate glass Na2 − xFe1 + 0.5xP2O7 for sodium ion batteries

Crystallization behavior of precursor glass in the system of sodium iron phosphate Na2 − xFe1 + 0.5xP2O7 was examined. We obtained homogeneous precursor glasses in the composition range between x = 0 and 0.77. The formation of triclinic P1¯ phase, which plays active material for sodium ion batteries, was confirmed in whole composition. Precipitation of Na4P2O7 was confirmed except for the P1¯ phase by means of XRD analysis in x = 0 that means Na2FeP2O7 is thermally unstable, meanwhile a pure P1¯ phase was obtained in x = 0.44, which corresponds to stoichiometric Na3.12Fe2.44(P2O7)2. We propose that the optimal composition exists around x = 0.44 not x = 0 (Na2FeP2O7) to form the P1¯ phase with high volume fractions.