Sol-gel-processed amorphous lithium ion electrolyte thin films: Structural evolution, theoretical considerations, and ion transport processes

• Amorphous lithium lanthanum titanium oxide (LLTO) thin films were synthesized.
• Structural evolution of film formation was examined.
• Induction time was theoretically determined.
• Ion transport processes in amorphous LLTO was investigated for the first time.

Amorphous lithium lanthanum titanium oxide (LLTO) is a promising inorganic solid electrolyte for all-solid-state lithium ion batteries. However, preparation of amorphous LLTO by sol-gel process is rare. There is a pressing need for experimental and theoretical investigation for sol-gel-processed amorphous LLTO. In this study, amorphous LLTO thin films were successfully prepared by sol-gel process with a partially hydrolyzed sol. Microstructure evolution from dried gel film to fired one to annealed one was examined. The microstructure of the annealed film, amorphous or crystalline, depends on annealing temperature and time. Theoretical analysis based on nucleation and growth theory was carried out to understand the microstructure evolution at various annealing temperatures. Induction time determines the longest annealing time without transformation from amorphous to crystalline state. The induction time decreases with annealing temperature until the time approach a minimum, and after that, the time increases with the temperature. The frequency and temperature dependence of the ac conductivity of amorphous LLTO thin film was analyzed by the jump relaxation model. The plateau at low frequencies results from lithium ion long-range diffusion which contributes to dc conductivity, while the observed frequency dispersion at high frequencies is attributed to short-range forward–backward hopping motion of lithium ions.