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.