Structure and electrochemical properties of mixed transition-metal pyrophosphates Li2Fe1-yMnyP2O7 (0≤y≤1)

A family of lithium mixed transition-metal pyrophosphates Li2Fe1-yMnyP2O7/C (0 <= y <= 1) has been prepared by mechanochemically assisted solid state synthesis using a high-energy planetary mill AGO-2. The samples were studied by X-ray powder diffraction (XRD) including Rietveld refinement, Fourier transform infrared spectroscopy (FTIR), Mossbauer spectroscopy, scanning electron microscopy (SEM), Li-6 nuclear magnetic resonance spectroscopy (NMR), galvanostatic cycling, and galvanostatic intermittent titration technique (GITT). All samples were refined in a monoclinic framework (space group P2(1)/c) with M ions occupying MO6 and MO5 sites and Li ions occupying LiO4 and LiO5 sites. The cell volume of Li2Fe1-yMnyP2O7 increases vs. Mn content with a slight deviation from the linearity. M2+ ion fractions in different coordination polyhedra and structural disordering among the Li and M sites in the as-prepared pyrophosphates were studied in detail by a multi-technique approach. According to galvanostatic cycling, only the Fe2+/Fe3+ redox couple operates upon the cycling of Li2Fe1-yMnyP2O7 in the 2.0-4.3 V range, while the Mn2+/Mn3+ redox pair becomes active at higher voltage (similar to 4.8V). The Li2Fe0.75Mn0.25P2O7/C sample shows the highest discharge capacity of 87 mA h g(-1), close to the theoretical capacity delivered by the Fe2+/Fe3+ redox couple. A single-phase mechanism of lithium deintercalation was confirmed. Lithium diffusion coefficient DLi+ during the charge-discharge processes Li2Fe0.75Mn0.25P2O7/C is within the range from 10(-13) to 10(-15) cm(2) s(-1) which is slightly higher than that of Li2FeP2O7/C. (C) 2015 Elsevier Ltd. All rights reserved.