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Abstract Vital need to reduce battery cost inspired intensive research in the field of sodium-storage systems, which required extending the range of perspective electrode materials. Herein, we present a new member of the electrochemically active NASICON material family, rhombohedral NaMo 2 (PO 4) 3 , which demonstrates both Mo+4/Mo+5 and Mo+4/Mo+3 redox activities towards sodium (de)intercalation. Desodiation of the initial NaMo 2 (PO 4) 3 material with subsequent discharge within the 1.2–4.0 V range leads to a multi-electron redox transition with reversible capacity of 130 mAh g−1. However, the single Mo+4/Mo+3 redox transition at 2.45 V (vs. Na/Na+) with capacity of 95 mAh g−1 was found to be more stable during cycling, and operando X-ray diffraction confirmed its two-phase mechanism. Sodiation of the initial phase results in Na 2.5 Mo 2 (PO 4) 3 , which maintains the parent NASICON structure (R -3c, a = 8.90532(7) Å, c = 22.2379(3) Å, V = 1527.30(3) Å3), with two Na-positions being almost equal in occupancy and energy. Graphical abstract Image 1 Highlights • NASICON-type NaMo 2 (PO 4) 3 electrode material was prepared by freeze-drying technique. • NaMo 2 (PO 4) 3 shows partially reversible Na-deintercalation above 3.6 V vs. Na/Na+. • Reversible insertion of 1.8 Na occurs at 2.45 V vs. Na/Na+ via two-phase mechanism. • Na-rich Na 1+x Mo 2 (PO 4) 3 phase maintains the NASICON-type framework. [ABSTRACT FROM AUTHOR]

The carbon-coated single-phase samples of the pyrophosphates AMoP 2 O 7 (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 MoO 6 octahedra and P 2 O 7 groups with A-cations located in the open tunnels running along the [100] (LiMoP 2 O 7 ) and [101] (NaMoP 2 O 7 ) 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 LiMoP 2 O 7 and 4.5 eV for NaMoP 2 O 7 ) anticipating LiMoP 2 O 7 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 NaMoP 2 O 7 and more than 70 mA h g −1 for LiMoP 2 O 7 , being in a good agreement with the BVEL results. [ABSTRACT FROM AUTHOR]