Nanocrystallization as a method of improvement of electrical properties and thermal stability of V2O5-rich glasses

Vanadia-rich glasses of the Li2O–V2O5–P2O5 system (predominantly electronic conductors) and V2O5–P2O5 one (purely electronic conductors) – potential cathode materials for lithium ion batteries – were investigated by impedance spectroscopy (IS), differential scanning calorimetry (DSC), X-ray diffractometry (XRD) and scanning electron microscopy (SEM). It was shown that the electrical conductivity of the original glasses of both systems can be considerably enhanced by appropriate annealing at temperatures close to crystallization temperature Tc. Moreover, the resulting materials are thermally stable up to ca. 350 °C, i.e., some 100 °C higher than the glass transition of the initial glasses. Increase in conductivity arises from the modification of the microstructure. It was found out by XRD and SEM studies, that by appropriate heat treatment glasses of both systems can be turned into nanomaterials consisting of crystalline grains of V2O5 of ca. 30 nm average size embedded in the glassy matrix. It was postulated that the major role in the conductivity enhancement of these nanomaterials is played by the developed interfacial regions between crystalline and amorphous phases, in which the concentration of V4+–V5+ pairs responsible for electron hopping, is higher than inside the glassy matrix. The annealing at temperatures exceeding Tc leads to massive crystallization and to a conductivity drop.