Sodium and mixed sodium/lithium iron ortho-pyrophosphates: Synthesis, structure and electrochemical properties

Na4Fe3(PO4)2P2O7 (NFPP) was synthesized by the fast and facile mechanochemically assisted solid-state route using iron oxalate as a precursor. By means of XRD, FTIR, and Mӧssbauer spectroscopy, it was found that NFPP prepared by annealing the activated mixture at 450 °C is the most homogeneous phase and shows the best electrochemical behavior when cycling in both Na and Li cells. During its cycling in a Li cell and chemical treatment with LiBr the Na+/Li+ ion exchange occurs. It has been shown that in both cases the process is not fully complete and leads to the formation of the mixed Na/Li compounds. Phase composition, crystal and local structure, and electrochemistry of the as-prepared mixed Na/Li iron ortho-pyrophosphates were investigated. We concluded that the remaining Na+ ions stabilize the structure of Na4-xLixFe3(PO4)2P2O7 (NLFPP) during cycling, and the simultaneous Na+/Li+ (de)intercalation at the cathode side is realized in the hybrid Na/Li cells. Mixed NLFPP can be considered as promising cathode materials for Li-ion batteries with high energy and power characteristics.