Fast lithium ion conducting solid state thin-film electrolytes based on lithium thio-germanate materials

In this study, lithium thio-germanate thin-film electrolytes for lithium rechargeable batteries have been successfully prepared by radio-frequency (RF) magnetron sputtering deposition in Ar gas atmospheres. The targets for RF sputtering were prepared by melting, milling and pressing the appropriate amounts of the starting materials in the nLi₂S + GeS₂, n = 1-4, binary system. 2 mm wide × 18 mm long Au electrodes with a parallel configuration of 2 mm spacing were sputtered to ∼100 nm thick at a sputtering rate at ∼5 nm min^-1 on Al₂O₃ single crystal substrates. Thin-film electrolytes were grown on this electrode assembly at 50 W power and 25 mtorr in Ar gas pressure. The ionic conductivities of the thin-film electrolytes were measured from-25 °C to 100 °C with 25 °C increments over the frequency range 0.1 Hz-1 MHz. The d.c. ionic conductivities determined from complex plane plots of the impedance of the Li₂GeS₃, Li₄GeS₄, Li ₆GeS₅, and Li₈GeS₆ amorphous thin films at 25 °C were found to be 1.1 × 10^-4 S cm ^-1, 7.5 × 10^-4 S cm^-1, 1.7 × 10^-3 S cm^-1, and 7.0 × 10^-5 S cm ^-1, respectively. As the Li₂S content in the thin film increases, the ionic conductivities of the thin films increase from n = 1 to n = 3. However, for the n = 4 Li₈GeS₆ thin film, the ionic conductivity decreased and the activation energy increased. The maximum in the conductivity for the n = 3 film is among the highest ever reported for an amorphous solid state Li⁺ ion conductor.