Pyrophosphates AMoP2O7 (A = Li and Na): Synthesis, structure and electrochemical properties

Abstract The carbon-coated single-phase samples of the pyrophosphates AMoP2O7 (A = Li and Na) were synthesized using freeze-drying followed by annealing in a sealed silica tube with the Fe/FeO getter. The Rietveld refinement of the X-Ray powder diffraction data proved both structures are built up as a network of corner-linked MoO6 octahedra and P2O7 groups with A-cations located in the open tunnels running along the [100] (LiMoP2O7) and [101] (NaMoP2O7) directions. The bond valence energy landscapes (BVEL) analysis revealed three-dimensional pathways of alkali-ion migration with significantly different activation energies (0.8 eV for LiMoP2O7 and 4.5 eV for NaMoP2O7) anticipating LiMoP2O7 to exhibit faster Li+ diffusion and better electrochemical characteristics. Both materials displayed electrochemical activity in Li- and Na-cells with reversible capacities of ∼20 mA h g−1 for NaMoP2O7 and more than 70 mA h g−1 for LiMoP2O7, being in a good agreement with the BVEL results.