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 °C. The highest Li+ conductivity and lowest activation energy were measured in Li-Nb-O films annealed at 400 °C. Films annealed at 450 °C were crystallized to form the non-Li+-conductive trigonal LiNbO3 structure. Raman spectroscopy measurements revealed that Li-Nb-O films annealed at 400 °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.