Hydrothermal Synthesis and Electrochemical Properties of Layered Vanadates

Abstract

ABSTRACTThe synthesis of vanadates from aqueous solutions leads to a large variety of crystalline materials. The nature of molecular precursors mainly depends on pH and three different pH ranges can be distinguished. Cationic precursors are formed at low pH. Precipitation is obtained via the addition of anions leading to materials in which [VO6] octahedra are linked through anions. V2O5,nH2O gels are formed around the Point of Zero Charge (pH=2). They are made of ribbon-like particles and exhibit interesting properties as cathodic materials. Electrochemical properties depend on the drying procedure. Moreover, intercalation reactions lead to a whole range of new inorganic and organic bronzes. Chain metavanadates are precipitated from anionic precursors in basic aqueous solutions. Vanadium coordination decreases and they are made of corner sharing [VO4] tetrahedra. Layered structures are formed in the pH range where negatively charged species such as [VO(OH)4(OH2)]− should behave as precursors. Condensation occurs via V-OH groups in the equatorial plane leading to layered structures in which cations are inserted between the vanadate planes. Some reduction occurs when organic cations such as tetramethyl ammonium [N(CH3)4]+ are used leading to mixed valence vanadates. Electron delocalization is observed in these compounds but Li insertion is hindered by large organic cations. Better electrochemical properties are observed with smaller inorganic cations. Fibrous crystals of NaV3O8,H2O are formed in the presence of NaOH whereas the mixed valence bronze α'-NaxV2O5 is obtained when both NaOH and TMAOH react with V2O5. This fibrous morphology appears to improve diffusion processes at the electrode-electrolyte interface.