The rich redox chemistry of vanadium oxides has meant that they have been and continue to be the subject of exhaustive study in many areas and particularly in energy materials. High energy densities reported for vanadium oxides are related to the electrochemical activity associated with multiple V(V) to V(III) redox state changes. The large variety of stable and metastable structures are a challenge to synthesize vanadium oxides with high purity and controlled stoichiometry.

V₄O₉ is an intermediate crystal phase along the line from V₂O₅ to VO₂. It is halfway in composition, although not structurally since it is not directly related to them; it is not a “shear structure” but a 3D tunnelled structure instead. Defined as a metastable phase, it has been little investigated. The use of reducing agents either in solvothermal conditions or in solid-solid or solid-gas reactions requires an accurate control of reaction conditions and usually leads to mixed phases.

In this sense and taking advantage of the exceptional catalytic properties of vanadium oxides, we have successfully prepared flake-shaped nanoparticles of V₄O₉ from nanoparticulate V₂O₅ and acetonitrile as solvent/reducer by a direct novel solvothermal method, which has proven to be very efficient and versatile. The as-prepared sample consisted of 5–20 nm average size V4O9 nanoparticles exhibiting an average oxidation state of 4.65 vanadium ions as deduced from XPS. The enhanced electronic conductivity of V₄O₉ is reflected in its electrochemical performances. Against Li anode upon discharge to 1.5 V, the V₄O₉ could intercalate ~6 Li⁺ corresponding to 464.8 mA h g^-1 at 10 mA g^-1 and exhibited reversible Coulombic efficiency of 75.2% upon charging to 4.0 V [1]. The V₄O₉ displayed superior rate capability at various current densities for 110 cycles and excellent long cycle stability delivering a reversible capacity of 160.8 mA h g^-1 at 100 mA g^-1 even after 2000 cycles. Preliminary electrochemical results against Na anode showed that V4O9 could intercalate 4 Na⁺ with highest first discharge capacity of 318.9 mA h g⁻¹ and delivered a reversible capacity of 55.9 mA h g^-1 at 250 mA g^–1 even after 10000 cycles, being the first to report for sodium cells. The investigation of charge-storage mechanisms [1] showed that the electrochemical performance of V₄O₉ is surface controlled at very high rate but can still be considered an intercalation material when used at lower or medium rate providing a total high capacity. Of special interest for V₄O₉ application is that it stands very long cycling making it a challenging/ competitive cathode with high capacity and cyclability to replace expensive electrode compounds