Sodium iron pyrophosphates Na-2 xFe(1+ x/2)P(2)O(7) (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, Mossbauer spectroscopy, Na-23 and P-31 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: similar to 23% for Na2FeP2O7 (x=0) and similar to 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. (C) 2017 Elsevier Ltd. All rights reserved.