Lithium diffusion coefficient in amorphous lithium phosphate thin films measured by secondary ion mass spectroscopy with isotope exchange methods

Lithium diffusion coefficients in amorphous lithium phosphate (a-Li₃PO₄) thin films were determined by secondary ion mass spectroscopy (SIMS), using ⁷Li and ⁶Li stable isotopes. The diffusion couples were prepared by an ion-exchange method using liquid electrolyte and a mask method based on thin-film deposition. The tracer diffusion coefficient, D_Li ^⁎, was evaluated by analyzing the isotope profiles obtained by SIMS in the temperature range of 25–160 °C. The diffusion coefficient of a-Li₃PO₄ was 6.0 × 10^− 13 cm²/s at 25 °C, while the activation energy was 0.58 eV, as measured for the samples prepared by using the ion-exchange method. The conductivity diffusion coefficient, D_σ, was calculated from the thin film ionic conductivity, which was determined by impedance spectroscopy. The correlation factor, H_R, was 0.55 ± 0.20 in the measured temperature range, which indicated the cooperative motion of lithium ions in lithium phosphate glasses.