MVO5(M = Nb, Ta) mixed oxides: sol–gel synthesis, structural and thermal characterization and electrochemical Li+ insertion

Abstract

Our studies on the preparation, via the sol–gel techniques, of MVO₅(M = Nb, Ta) oxides have been rewarded here with an increased understanding of the synthesis process. The sol–gel method is at the moment the only procedure for producing NbVO₅ as a pure phase. We present results concerning the influences of different parameters on the nature of the final products in the V–Nb–O system. Compounds NbVO₅ and TaVO₅ are isostructural with orthorhombic unit cells. The structural parameters have been refined using the Rietveld method with space group Pnma. Thermal analyses of the precursor xerogels show that the thermal effects depend on the nature of the M cations. The crystallization and decomposition temperatures are lower for NbVO₅ than for TaVO₅. The electrochemical insertion of lithium ions into the new MVO₅(M = Nb, Ta) mixed oxides has also been investigated. For both materials, this process is complex and consists of three main steps. The maximum insertion degrees (x in Li_xMVO₅) achieved after the reduction process are x= 1.95 for NbVO₅ and x= 1.57 for TaVO₅. The galvanostatic cycling studies show the good rechargeability of the Li⁺/e^– insertion/deinsertion process in the MVO₅ oxides.