Binary NaCl–NaF and NaCl–LiF Flux-Mediated Growth of Mixed-Valence (V^3+/4+) NASICON-Type Na₃V₂(PO₄)₂F_2.5O_0.5 and Na_2.4Li_0.6V₂(PO₄)₂F_2.5O_0.5 for Highly Reversible Na- and Li-Ion Storage

Development of robust electrode materials that can work for both Li- and Na-ion batteries received enormous interest in recent times. Especially, exploratory crystal growth offers much reward in realizing new chemical compositions and the opportunity to uncover their intrinsic properties. Herein, we report a NASICON-type sodium vanadium oxy-fluorophosphate solid solution, Na₃V₂(PO₄)₂F_2.5O_0.5 (NVPF), and its partial Li-exchanged Na_2.4Li_0.6V₂(PO₄)₂F_2.5O_0.5 (NLVPF) crystals by one-pot solid-state flux techniques using binary NaCl–NaF and NaCl–LiF fluxes, respectively, for the first time. Controlled experiments with the nature of flux, flux ratios, and temperatures reveal that NaCl and NaF/LiF salts play a critical role, not only as cation/anion sources but also as a structure-directing agent from a morphological perspective. X-ray diffraction (XRD) Rietveld refinement, X-ray photoelectron spectroscopy (XPS), and ²³Na solid-state NMR confirm the crystal structure, phase purity, lattice parameter, and mixed valency of Na₃V₂(PO₄)₂F_2.5O_0.5 and its Li-substituted analogue. Investigation of the electrochemical properties of as-synthesized NVPF and NLVPF constructed cathode delivered initial discharge capacities of 117 and 118 mAh g^–1 vs Li/Li⁺, respectively, while in Na-ion configuration provided capacities of 107 and 110 mAh g^–1 (vs Na/Na⁺) at 0.1C rate. Both the NVPF and NLVPF electrodes demonstrate long-term stability by retaining 75 and 86 mAh g^–1 (vs Li/Li⁺), with over 94 and 95% retentions, respectively, after 400 cycles at 1C rate. The present findings strongly suggest that the flux method is one of the potential approaches to synthesize the cathode materials for hybrid-ion batteries in a scalable manner.