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Using Rietveld-refined X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR) and electrochemical cycling, it was established that among sodium vanadium fluorophosphate compositions Na 1+y VPO 4 F 1+y (0 ≤ y ≤ 0.75), the single-phase material Na 1.5 VPO 4 F 1.5 or Na 3 V 2 (PO 4 ) 2 F 3 with a tetragonal structure (the P4 2 /mnm S.G.) is formed only for y = 0.5. The samples with y < 0.5 and y > 0.5 possessed different impurity phases. Na 3 V 2 (PO 4 ) 2 F 3 could be considered as a multifunctional cathode material for the fabrication of lithium-ion and sodium-ion high-energy batteries. The reversible discharge capacity of 116 mAh·g −1 was achieved upon cycling Na 3 V 2 (PO 4 ) 2 F 3 in a hybrid Na/Li cell. Decrease in discharge capacity for the other samples was in accordance with the amount of the electrochemically active phase Na 3 V 2 (PO 4 ) 2 F 3 . Na 3 V 2 (PO 4 ) 2 F 3 showed good cycleability and a high rate of performance, presumably due to operation in the mixed Na/Li electrolyte. The study of the structure and composition of charged and discharged samples, and the analysis of differential capacity curves showed a negligible Na/Li electrochemical exchange, and a predominant sodium-based cathode reaction. To increase the degree of the Na/Li electrochemical exchange in Na 3 V 2 (PO 4 ) 2 F 3 , it needs to be desodiated first in a Na cell, and then cycled in a lithium cell. In this case, the electrolyte would be enriched with the Li ions.

In this work, XRD, EDX, Mössbauer and NMR spectroscopy were used to study chemical and electrochemical Na + /Li + ion exchange in the sodium iron pyrophosphate Na 1.56 Fe 1.22 P 2 O 7 with a triclinic symmetry, S. G. P-1, and sodium vanadium fluorophosphate Na 3 V 2 (PO 4 ) 2 F 3 with a tetragonal symmetry, S. G. P4 2 /mnm, cathode materials. Electrochemical Na + /Li + ion exchange was performed in hybrid-ion cells with Li metal anode and LiPF 6 -based electrolyte, while chemical ion exchange was realized in the solution of LiBr in acetonitrile. A facile electrochemical and chemical Na + /Li + ion exchange was observed for both cathode materials, resulting in the formation of the mixed Na-Li compositions: ∼Na Sodium-ion batteries have been considered as potential candidates for stationary energy storage, because of the low cost and wide availability of sodium sources. Sodium is more abundant and less expensive than lithium. However, Na-ion based systems have a lower energy density compared to Li-ion systems since a reduction potential of sodium metal vs. SHE is 0.3 V higher than that of lithium. Sodium has a larger ionic radius and bigger molecular weight. Therefore, sodium-based insertion materials are of interest for low-cost and large-scale Na-ion batteries. Substantial research efforts have been invested during the previous years to produce electrode materials for sodium batteries allowing facile intercalation of the Na ions at suitable potentials. Amongst the cathode materials investigated, various layered oxides (e.g. Na 0.6 CoO 2 , Na 2/3 Fe 1/3 Mn 2/3 O 2 , Na 2/3 Ni 1/3 Mn 2/3 O 2 , NaNi 1/3 Co 1/3 Mn 1/3 O 2 , etc.) and polyanionic compounds (e.g. NaFePO 4 , Na 2 FePO 4 F, Na 3 V 2 (PO 4 ) 3 , NaFeSO 4 F, etc.) are the most favorite cathode materials.1 To enhance the electrochemical performance of sodium-based cathode materials, Barker et al. first proposed a concept of a hybrid-ion battery whereby a non-lithium containing cathode material is used in conjunction with metallic Li, graphite, Li 4 Ti 5 O 12 , etc., and Li-based or mixed Li/Na electrolytes.2-5 The Na + /Li + ion exchange is an effective way to produce new metastable mixed Na/Li phases for Li-ion batteries without causing many structural changes to the pristine sodium-based materials. A new design concept of the simultaneously energy storage of multi-metal ions may overcome respective disadvantages of singlemetal ions and bring to a synergy effect. 6,7 Since the operation of hybrid-ion batteries depends on the reactivity of the selected electrodes in respect of Li + and Na + ions, two Na-based cathode materials, differing by crystal structure, triclinic Na 1.56 Fe 1.22 P 2 O 7 and tetragonal Na 3 V 2 (PO 4 ) 2 F 3 , were selected in this work to study electrochemical and chemical Na + /Li + ion exchange and their performance in hybridion cells.Sodium iron pyrophosphates 8,9 with a triclinic symmetry (S.G. P-1) and sodium vanadium fluorophosphates with tetragonal symmetry (S.G. P4 2 /mnm) [10][11][12] were recently examined and showed good e...