A comparative study of structure, air sensitivity and electrochemistry of sodium iron pyrophosphates Na2−xFe1+x/2P2O7 (x = 0; 0.44)

Sodium iron pyrophosphates Na2−xFe1+x/2P2O7 (x = 0; 0.44) have been prepared by mechanochemically assisted solid-state synthesis. The materials were comprehensively characterized and compared by XRD with Rietveld refinement, FTIR, Mössbauer spectroscopy, 23Na and 31P MAS NMR, SEM, EDX, galvanostatic cycling, and GITT. The structure of the as-prepared pyrophosphates was refined in a triclinic symmetry (the space group P-1) differing by the amount and the occupancy of the Fe and Na sites and structural disordering. Both samples contain a considerable amount of the Fe3+ impurities: ∼23% for Na2FeP2O7 (x = 0) and ∼7% for Na1.56Fe1.22P2O7 (x = 0.44) and the correspondent amount of the Na2CO3 impurity, thus indicating that Na2FeP2O7 is less stable in air atmosphere than Na1.56Fe1.22P2O7. Both pyrophosphates showed good electrochemical activity in hybrid Na/Li-cells upon cycling within the 2.0–4.3 V range. Na1.56Fe1.22P2O7 demonstrated superior discharge capacity due to higher porosity, structural disordering and air stability. The formation of new electrochemically active mixed Na/Li-phases with approximate compositions of NaLiFeP2O7 and Na1.2Li0.36Fe1.22P2O7, respectively, and a pristine triclinic structure was identified by ex situ XRD and EDX as a result of the electrochemical Na+/Li+ ion exchange.