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 x 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 deg. C to 100 deg. C with 25 deg. 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, Li6GeS5, and Li8GeS6 amorphous thin films at 25 deg. C were found to be 1.1 x 10-4 S cm-1, 7.5 x 10-4 S cm-1, 1.7 x 10-3 S cm-1, and 7.0 x 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.