High electronic conductivity in nanostructured materials based on lithium-iron-vanadate-phosphate glasses

Highly conducting olivine-like materials have been prepared by thermal nanocrystallization of a series of glassy lithium-iron-vanadate-phosphates, of the compositions close to that of the LiFePO4 olivine, with only small amounts of vanadium additive. It was found out that their thermal treatment up to a certain temperature, from the 460 and 500 degrees C range, optimized for each composition separately, led to: i) a considerable and irreversible conductivity increase by a factor of up to 10(6) (at room temperature) and ii) a change in the microstructure, from purely amorphous to nanocrystalline (for samples with lower vanadium contents) or polycrystalline (for a sample with higher vanadium content). The conductivity enhancement was accompanied by a decrease in the activation energy from the initial value of ca 0.70 eV to ca 0.11 eV after nanocrystallization. STEM and HRTEM microscopy studies showed, that in the materials with lower vanadium content, the heat treatment leads to formation of very small (ca 10 nm) crystallites densely distributed inside the glassy matrices. For the material with higher vanadium content similar processing leads to crystallization with larger grains (over 100 nm). The increase in conductivity of the latter material was smaller, than in the former ones. The observed correlation between the improvement of the electrical properties and a microstructure of annealed samples was attributed to a substantial role of the interfacial regions in the electrical conduction. These defective interfacial regions contain an increased concentration of transition metal aliovalent ions, which can provide multiple channels for electronic hopping between: Fe2+ and Fe3+, V4+ and V5+, or V3+ and V4+ centers. As the volume fraction of the interfacial regions in the nanostructured (with 10 nm grains) material is much higher than in a coarse-grained one (over 100 nm), the effective conductivity of the former material is much higher than that of the latter one.(1) (C) 2015 Elsevier B.V. All rights reserved.