Sodium-based insertion materials are of interest for low-cost and large-scale Na- and Li-ion batteries. Substantial research effort has been invested during the previous years to produce electrode materials for sodium batteries that will allow for facile intercalation of Na-ions at suitable potentials. Amongst the cathode materials investigated, a variety of layered oxides (e.g. Na_xCoO₂, NaCrO₂, NaVO₂, Na_x[Fe_0.5Mn_0.5]O₂) and polyanionic compounds (e.g. NaFePO₄, Na₃V₂(PO₄)₃, Na₂FePO₄F, NaFeSO₄F) have been reported. The ability of these materials for facile Na⁺/Li⁺ ion exchange has been also revealed [1]. Recently, sodium vanadium fluorophosphates [2-5] and sodium iron pyrophosphate [6,7] have been examined and showed good electrochemical properties in Na-cells. The aim of the present work was to investigate electrochemical and chemical Na⁺/Li⁺ ion exchange in these materials.

Sodium vanadium fluorophosphates Na_1+yVPO₄F_1+y (0≤y≤0.5) and sodium iron pyrophosphate Na₂FeP₂O₇ were prepared by mechanochemically assisted solid state synthesis using high-energy AGO-2 planetary mill. The activated mixtures were subsequently heat treated at 600-700 °C in Ar flow. Structure and morphology of the as-prepared materials were analyzed by XRD with Rietveld refinement using GSAS and TOPAS software packages, SEM, FTIR, Mössbauer, ²³Na and ³¹P MAS NMR spectroscopy. Electrochemical Na⁺/Li⁺ ion exchange was studied by galvanostatic cycling in Li⁺ cells with Li as an anode and LiPF₆ dissolved in EC+DMC as an electrolyte. Non-oxidative chemical ion exchange was carried out in the solution of LiBr in acetonitrile for 24 h. Ex situ XRD, EDX, Mössbauer and NMR spectroscopy studies were carried out to control the phase and structural transformations. Structure of the as-obtained mixed Na/Li compositions was compared with that of the correspondent lithium compounds [8].

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