Li+ conduction in Li-Nb-O films deposited by a sol-gel method

We fabricated amorphous Li-Nb-O films by a sol-gel method. The Li+ conductivities of Li-Nb-O films were analyzed by ac impedance spectroscopy after annealing at 300 to 450 degrees C. The highest Li+ conductivity and lowest activation energy were measured in Li-Nb-O films annealed at 400 degrees C. Films annealed at 450 degrees C were crystallized to form the non-Li+-conductive trigonal LiNbO3 structure. Raman spectroscopy measurements revealed that Li-Nb-O films annealed at 400 degrees C had aligned NbO6 octahedra in the structures, although LiNbO3 crystals were undetectable by X-ray diffraction. Additionally, it was found that Li-Nb-O films were preferentially crystallized near the Pt electrode surfaces rather than the quartz surfaces. Hence, Pt nanoparticles (NPs) were pre deposited on quartz substrates by pulsed laser deposition (PLD) before spin-coating of Li-Nb-O films to investigate the effect of small LiNbO3 crystals on the Li+ conductivity. After annealing, the Li-Nb-O films fabricated on Pt-decorated quartz substrates exhibited higher Li+ conductivity and lower activation energy in the in-plane direction compared to films on quartz substrates. Thus, it was considered that the amorphous/LiNbO3 crystal interfaces played an important role in fast Li+ conduction. Although the mechanism of Li+ conduction in the amorphous/crystal interfacial regions was not well understood, we suggest that reduced attraction between Li+ and the terminal oxygen of NbO6 octahedra in the amorphous/crystal interfacial regions contributed to fast Li+ conduction. (C) 2015 Elsevier B.V. All rights reserved.